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Colour changes of Octopus rubescens during attacks on unconditioned and conditioned stimuli
Anita. Behav., 1974, 22, 211-219
COLOUR
CHANGES
OF OCTOPUS RUBESCENS DURING
UNCONDITIONED
AND CONDITIONED
ATTACKS
ON
STIMULI
BY LINDA R. WARREN, MICHAEL F. SCHEIER & DONALD A. RILEY
Department of Psychology, University of California, Berkeley, California Abstract. Octopus rubescens exhibits a sequence of skin colour changes when it attacks and captures prey. These displays were observed during attacks on (1) free-swimming crabs, (2) crabs of different sizes impaled on thin rods, and (3) positive and negative discriminative stimuli. Animals were housed separately in light-blue aquaria which also served as experimental apparatuses. Animals in each condition showed the same typical sequence of colour displays. This sequence is (1) before detection of crab: various colours, (2) on detection and during a free-swimming attack: colours ranging from light orange to grey, (3) on landing: colourless and nearly transparent, (4) on seizing the crab: spotted or mottled, and (5) afterward: various colours. The colour changes may be tied to locomotor acts and postural adjustments. The fact of adaptive colouration in members of the octopus family is well known and has been described by a number of writers (Cowdry 1911; Holmes 1955; Packard 1963; Packard & Sanders 1969). These colour changes, remarkable for their detail and variety, have been interpreted as background matching and as serving a cryptic function (Wells 1962). Recent evidence for such an interpretation is to be found primarily in photographs of octopuses displaying patterns markedly similar to the rocks on which they sit (Packard & Sanders 1969). Adaptive as such behaviour might be, it is not clear that all colour change can be attributed to background matching. Packard (1963) has described colour changes that occur during the attack of Octopus vulgaris on its prey, and we have observed similar colour changes in Octopus rubescens, a small Pacific octopus of the coastal shelf. Preliminary observations suggested that as O. rubescens first sees its prey (a crab), its skin becomes a uniform orangish colour that persists through the attack until the moment of landing upon the crab. At that moment, there is usually a dramatic bleaching out followed almost immediately by a change to a patterned display. Such behaviour might, under natural circumstances, result in background matching, for octopuses frequently jet from rock to rock across sand in hunting crabs (Yarnall 1969). While changing from mottled to uniform to mottled in accord with the background might reflect the animal's ability to select appropriate colouration independent of the behavioural sequence, we thought this an unlikely explanation of colour changes during attack for several reasons.
These initial observations of octopus colour changes were gathered rather casually in the course of investigating some hypotheses about multiple-reversal learning. However, the sequence of colour change during attack, while systematic, appeared to bear no particular relation to the semi-natural rock and sand background of the aquaria in which the animals were observed. In addition, it appeared that the same sequence of colour changes that occurred during attacks on crabs also occurred during attacks on a conditioned stimulus, in this case a black or white vertical rectangle. The possibility that a series of systematic colour changes occurred independent of background colouration, and that this sequence could come under the control of a previously neutral stimulus, suggested that the colour changes were mediated entirely by the elicited attack sequence which became conditioned to the cues, or that the colour changes themselves were conditioned responses, modifiable by reward contingencies. It was to investigate questions of this sort that the present research was initiated. The first section asks, what are the colour changes that occur when an octopus attacks an unconditioned stimulus, in the present case always a crab (Hemigrapsus oregonensis)? This procedure when repeated a number of times on each octopus allows an assessment of the reliability of the colour change within an octopus and the generality of this colour change series from one octopus to another. The experiments also examine some possible factors determining the colour change such as the nature of the unconditioned stimulus. When the unconditioned stimulus in a conditioning experiment was a crab, it was gener211
212
ANIMAL
BEHAVIOUR,
ally presented to the octopus on the end of a sharply pointed rod. Consequently, in these situations the crab was not free-moving, and colour changes that the octopus made in attacking the crab under such circumstances could be quite different from those occurring in a more natural setting. To assess this possible restrictive effect, we observed octopuses attacking both impaled and freely moving crabs. In addition to inquiring into the effects of free as opposed to restricted prey, we inquired whether the size of the crab used as the unconditioned stimulus affected the nature of the colour change. (We thought we had seen such an effect in preliminary observations.) Also, our conditioned stimuli were larger than crabs, so some estimate of the effect of variation in target size was desired. The second section examines questions of simple discrimination learning and asks whether the colour changes observed with the unconditioned stimulus also occur to conditioned stimuli.
Methods Animals
Octopus rubescens is a species of small octopus found in shallow waters of the northern California coast. Animals collected in Monterey Bay ranged in size from 2 to 100 g. The young animals, initially weighing 2 to 10 g, are most suitable for purposes of prolonged behavioural observation, because females of 30 to 50 g frequently develop and lay eggs, during which period they become inactive and refuse food. The small animals can be maintained with ease in the laboratory for at least 6 months, on a diet of fifteen to twenty medium-sized rock crabs (shell width, 10 to 12 mm; weight, 0.2 to 0.5 g) per week. Weight gain on this regimem is about 15 to 20 g a month. Body weights of the otopuses used for the observations reported here varied from 10 to 20 g at the initiation of testing to 25 to 40 g at the end of testing. Laboratory Conditions The work was performed in a closed seawater system with a capacity of approximately 1000 litres. Water was continuously recirculated through a central gravel and oyster-shell filter and refrigeration unit, and tanks were individually aerated. Approximately 500 litres were removed each month and replaced with fresh seawater. The mean temperature in the
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system was kept at 17~ W h e n animals were brought into the laboratory, they were placed in a large holding tank, and then transferred when needed to individual tanks where they remained for the duration of observations and training. The individual tanks were epoxied fibreglass with a glass front panel and clear Plexiglass tops. Each one held 57 litres and measured 60 x 45 x 30 cm. Unlike the rock and sand backgrounds of the tanks used in the initial observations, these tanks were uniformly coloured aquamarine. Neither direct observation nor motion-picture records revealed any difference in octopus displays in these different aquara. Animals were maintained on a light-dark cycle, with light from 09.00 hours to 19.00 hours; and were tested in the afternoon. During testing the tank-top was removed and the tank illuminated by a 100-W bulb, 60 cm above the tank. This continuous over-tank lighting was incorporated into the testing procedure to enable motion-picture and still-photography to be carried out at intervals and with minimal disruption. The movements of observers were partially hidden by a large screen which had a rectangular hole cut to expose the glass front panel of the tank. The procedure followed in presenting crabs (or cues) and recording behaviour was as follows. One of the authors ran the equipment and made all observations and classifications of behaviour. He lowered in the cue or the bait, timed the animal's response, and immediately recorded the attack and corresponding colour changes according to the system described below. On many of the trials, at the same time, the other two experimenters were taking coloured motionpictures and still-photographs of the same sequences. The purpose of the motion-pictures was to allow a continuous record of the changes so that the recordings of the experimenter could subsequently be checked against an objective record. The stills recorded details of the colour displays at higher resolution than the motionpictures allowed and were used for later comparisons to Munsell colour patches to specify colour components of the displays (Munsell 1943). All motion-pictures were taken with a Bolex 16 mm camera from a distance of approximately one metre. Still-pictures were taken with a Nikon camera from approximately half a metre. Subsequently, these other experimenters made judgments of the colour changes from the film~ These judgments agreed 96 per cent of the time with those of the experimenter who made the
WARREN ET AL.: COLOUR CHANGES OF OCTOPUS RUBESCENS DURING ATTACK initial observations. The observations reported are those of the initial observer.
Classification of Colour Displays On the basis of our preliminary observations a colour and pattern code was developed which characterized in a gross way all of the displays observed by us in the laboratory. These can be roughly divided into uniform colour displays, spotted displays, and complex mottled patterns. Under natural feeding conditions, these displays are typically associated with certain behaviours which we will identify. Approximate counterparts to the colours described were identified in the Munsell colour series and are indicated in parentheses. 1. Greys or octopus colour displays. The greys vary from light to medium to dark grey and represent a characteristic colouration of animals sitting or moving about in the tanks during the lighted portion of the day. They also occur with some frequency during feeding sessions (Munsell 7.5 YR 6/4). 2. Blush displays. The term blush refers to colours that are close to a coral pink. Blushes also vary from light to dark, and seem to be confined solely to feeding sessions, occurring most frequently just prior to and during swimming attacks (Munsell 2.5 YR 6/10). 3. Red displays. The reds differ from the blushes in that they are always darker and less yellow. Although the reds vary in intensity, the range is considerably smaller than the greys or blushes. Reds occur infrequently during feeding sessions but are a predictable response when the animal is frightened. Red is the characteristic colour as an initial response to sudden increase in illumination, when pursued by the experimenter's net in the tank, or when an animal has escaped from a tank and is crawling out of the water (Munsell 7-5 R 3/8). 4. Transparent displays. These displays are a complete loss of colour and a simultaneous loss of the opaque quality that characterizes the grey displays. Internal organs are more clearly visible and the areas around the interbrachial web are virtually transparent. This condition occurs very briefly when the octopus lands by or on its prey at the end of jetting (Munsell 10.0 Y R 8/2). 5. Spotted displays. These displays appear to be on a continuum between the uniform displays and the more complex patterns, occurring most frequently in the feeding sessions as a fairly brief interruption of a uniform or mottled
213
display or as a transition between uniform and mottled displays. They are characterized by irregular dark, nearly black areas, over a uniform grey or blush, and also vary from a few small dark areas on the body sac to larger dark areas all over the body. (Munsell 10-0 R 2/2 approximates the colour of the black spots). 6. Mottled and complex displays. The complex patterns are extremely varied and difficult to classify. These patterns are usually composed of some combination of reddish browns and whites. They range from an overall marbled effect to patterns such as white circles surrounded by dark areas, irregular white stripes on a dark background, or a particularly distinct lacy white on the head and body sac, over a uniform, very dark, red on the legs and web. This last we recorded separately as white over red. These patterns occur when the animal is in contact with a surface and are especially prevalent during the capture of and withdrawal with prey. We have never observed an animal displaying a mottled pattern when free swimming.
Classification of Steps in Behaviour Sequence To discuss the colour changes that occur during attacks, it is necessary to describe the attack in sequential order. Ideally, one would wish to present a description of the eolour changes on a strict time dimension from the first presentation of a stimulus until some later time when the sequence presumably is completed. The behaviour sequences, from the onset of the attack to retreat by the animal with a crab, take, however, anywhere from 5 to 20 s. The critical variable, at least at this stage of the research, seems to be what the animal is doing rather than time since the presentation of the cue. In the work that follows, therefore, we have related colour change to phases in the behaviour sequence. They are as follows: Phase I: Animal's state prior to presentation of the stimulus. The animal was typically sitting on the bottom or the side of the tank, or slowly crawling along the walls. Phase H: Detection. When the stimulus was lowered into the tank, the animal usually drew the legs closer to the body, raised and oriented the eyes toward the prey, and bobbed the head once or twice. Phase HI: Attack. Attacks were almost always performed by jetting through t h e water toward the prey, funnel pointed behind, and arms trailing. On rare occasions, an animal
214
ANIMAL
BEHAVIOUR,
would combine such swimming with crawling when approaching the target. These occasions were noted but included under attacks. Phase IV: Landing. Octopuses typically landed on or beside the prey with interbrachial web outstretched and arms loosely wrapped around the web. Phase V: Capture. The target was seized and pulled under the web. Phase VI: Withdrawal. Withdrawal took a variety of forms of crawling, either with arms extended or with arms tucked under, or jetting, but involved moving away from the point of capture usually in the direction from which the animal came. Phase VII: After capture. A final observation was recorded when the animal was again resting on the tank bottom or wall. Colour Changes During Attacks on Crabs Free and Impaled Conditions Four octopuses were observed attacking medium-sized crabs (shell width approximately 10 to 12 ram) under free and impaled conditions. In the free condition, octopuses attacked a freeswimming crab dropped into the home tank at the end remote from the octopus. This condition allowed the observation of behaviour and colour changes under conditions that roughly approximated natural attack conditions. In the impaled conditon, octopuses attacked a crab impaled on the end of a pointed transparent plastic rod. We wanted these observations of attack on a crab under conditions identical to those to be used later during conditioning, except that in conditioning the crab would be replaced by a discriminative stimulus, thus allowing direct comparison of behaviour to unconditioned and conditioned stimuli.
Procedure Animals were tested 5 days a week and fed only on test days. Each presentation of a free crab or impaled crab was considered a trial. There were three trials per day. If the octopus did not attack within 30 s, the trial was terminated by removal of the crab from the tank. The intertrial interval was in all cases approximately 5 min. On trials for which an attack occurred, latency of the attack (time from presentation of a crab to capture) was recorded as well as the observed colour changes. If an animal did not attack during test trials, it was fed at the end of the day's testing session.
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To impose standardization on the impaledcrab procedures, we modified the procedure from that used with free-swimming crabs. Prior to the start of a trial, the octopus was retained in one-half of the tank by a barrier with a large vertically sliding door in it. The barrier was clear Plexiglass, with black outlines so as to be clearly visible. The impaled crab was always lowered at the far end of the tank not occupied by the octopus. Consequently, the barrier served to divide the tank into a starting box and a target chamber. A small black box suspended above the end of the tank served to hide the impaled crab from the octopuses's view until the start of a trial. At the start of a trial the door was raised and the crab, impaled on a thin plastic rod, was lowered into the tank. The latency of attack was recorded from presentation of the crab to capture (seizing of the crab by the octopus). We used this method of modifying the animal's living tank into an experimental apparatus rather than the technique described by Walker, Longo & Bitterman (1970), in which they transported the animal to the experimental apparatus to minimize the disruptive effects that occur when the octopus is transferred from one tank to another.
Results of Observations with Free and Impaled Crabs The colour displays of four octopuses attacking free crabs for a series of 12 days and the same octopuses attacking impaled crabs for a series of 12 days were compared. Originally there were six animals, with three receiving the free condition first and three receiving the impaled condition first. Two of the animals receiving free crabs first died in the course of the experiment. Consequently, three had impaled first and one had free first. The results, however, were sufficiently consistent that this lack of counterbalancing did not seem to create a bias in the results. The data presented are only for those trials on which octopuses attacked. The octopuses had thirty-six trials under each condition and the frequencies of attack were distributed among the four animals as follows: free-swimming 35, 34, 33 and 33; impaled 18, 29, 27 and 9. These octopuses were less willing to attack a crab on a stick than they were a freeswimming crab; the mean latency of response for attacks was, however, approximately the same despite the free movement of the crab in the free condition. In the case of the free-swimming crabs the mean latency was 10-6 s and in the
W A R R E N ET AL.: COLOUR CHANGES OF OCTOPUS RUBESCENS D U R I N G ATTACK Free
mm
~
215
mpoled m
__
m
50 plush
o/n. HI o/HI $ o
nm
50 t Trons 0 50 l Sp~
nm
m
N
m
NI N,,
N
oir I
n|
50 ~Mottle
[~1
m
wm
nm
~
m
m
iN
Before Detect Attack Land Capt. W/draw After Temporal category Fig. ]. Distribution of colour displays during attacks on free and impaled crabs. Entries in each column add to 100 per cent.
case of the impaled crabs the mean latency was 10.25 s. Figure 1 shows the distribution of skin patterns at each of the seven time classifications. The figure should be read by looking first at the left column, which shows the display conditions for the octopus prior to the lowering of the crab into the tank ('before'). The first bar in each square shows the mean per cent of the total displays belonging in that colour class in the free crab condition. The second bar shows the same information for the impaled condition. Thus, before the crab was lowered, a mean of 8 per cent of the observations showed a uniform red octopus in the free condition, 5 per cent in the impaled. A mean of 32 per cent of the cases in the free condition blushed, and so on. The two conditions, free and impaled, should be the same in the 'before' phase for it is a pre-trial observation. In most o f these observations the skin colour was uniform, but all of the animals spotted on some trials.
Immediately on 'detection' (column 2) the skin display changes. In each condition, a mean of 75 per cent or more of the responses were blush or grey. This concentration of displays becomes even more marked in the 'attack' phase (column 3) with more than 90 per cent of the responses being in the blush and grey categories. On 'landing' (column 4) the animals show a marked switch to the category designated as 'transparent' (row 4). With 'capture' (column 5) there is a return to grey on some trials, the emergence of spots and mottled patterns on other trials. During 'withdrawal' (column 6) and the retreat to a corner of the tank (column 7), patterns are quite evenly distributed among a variety of classifications with somewhat less tendency to show the uniform blush pattern, somewhat more tendency to show more complex patterns than in the 'before' phase. In addition to this overall description, which shows this change from a varied condition prior to the appearance o f the crab to a uniform skin
ANIMAL
216
BEHAVIOUR,
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1
Table I. Mean Per Cent Agreement Among the Distributions of Individual Animals' Colour Displays
Condition Free crab
Impaled crab
Large crab
Small crab
Before
60
49
48
68
47
43
34
Detect
61
57
92
84
69
46
56
Attack
50
59
84
79
76
52
56
Stage in sequence
Pretraining S-tS--
Training S§
Land
89
86
86
68
65
48
35
Capture
62
65
55
60
39
42
52
Withdrew
75
38
58
57
57
--
41
After
60
38
61
52
60
--
42
display during orientation and attack, a transparent pattern on landing, and then a variety of displays again, two questions are of interest. First, is the distribution of displays consistent from animal to animal? Second, are there any reliable differences that appear between the free and impaled conditions ? The first question is of interest because summary statements such as that given above imply that there are regular patterns of change that the octopuses characteristically exhibit. Figure 1 shows, however, that there is considerable variability of the patterns at some points in time. While this variability reflects some idiosyncratic behaviour, it largely shows the variability characteristic of each octopus. Three different indices illustrate the extent to which the animals are similar. First, in 48 per cent of the cells with entries in Fig. 1 all four animals are represented. Second, only one entry with a value greater than 15 per cent has less than four animals, that one has three. A third: measure of inter-animal agreement is the mean per cent agreement score shown for each t i m e and each experimental condition in Table I. These scores are the m e a n per cent of the trials on which each animal's responses were in the same display class as each other animal's. Consider now only the entries for free a n d impaled. The highest value, 89 per cent for free, "land', reflects a condition in which nil animals responded with a transparent display on almost all trials, Also, here, even less frequent responses, grey and spots, had four and three animals represented. The lowest of the agreement values in free is 'attack' at 50 per cent. Here two of the animals gave mostly blush responses and two
gave mostly grey responses. Only when the agreement index drops substantially below 40 per cent is there a clear lack of agreement between animals. For example, m impaled, 'after' (38 per cent) each animal has a different modal response. In these data such disagreement is the exception. In general the distribution shown in the bar graphs represent each animal quite well. We know of no completely acceptable statistic for assessing the reliability of experimental differences with such a small number of animals and such a large number of Classifications. We can only suggest a rough-and-ready measure that we have used in thinking about this problem. Our practice has been to run t-tests of significance, but only in those cells in which all four animals have a higher per cent of occurrences in one condition than in the other. This procedure is more conservative than the indiscriminate use of t-tests, since it assesses only those differences to which all animals contribute. On the other hand, the use of such statistics renders the probabilities uninterpretable in any precise sense. We used the tests only as general guides in thinking about the data, and present below only those results which were reliable (P<0.02). Using this procedure, there were reliable differences between free and impaled conditions during 'attack' and during 'capture'. During attack, the difference was in the cell attack-blush. In this case the animals blushed more often when attacking a crab on the stick than they did when attacking a free-swimming crab. We have no satisfactory interpretation of this effect. One possibility is that, in general,
WARREN ET AL.: COLOUR CHANGES OF OCTOPUS ,RUBESCENS DURING ATTACK
217
octopuses were obliged to swim farther for a crab on a stick than for a free-swimming crab because the free-swimming crab sometimes approached the octopus. During 'capture', there was a reliable effect for grey displays favouring the animals under the free condition, whereas the spot displays occurred more frequently in the impaled condition. When the animal attacks a free crab, it is merely required to scoop it up into its web. On the other hand, when it attacks a crab on a stick, it must seize the crab, tear it off the stick, and take it under its web. The spotting and mottling are responses accompanying the struggle to take the crab off the stick. The simplest interpretation of these data is that this spotting is a skin display accompanying a certain kind of effort on the part of the animal.
blushed more when c o n f r o n t e d with a large crab than with a small crab in the 'detect' category. This reliable difference, of over 25 per cent in frequency of this response, is the sort of effect that we were looking for when we began this research; that is, stimulus control of: colour changes not directly associated with attack or withdrawal behaviours. However, there is a possible association of colour change with move= ment change here also. When an octopus detects a crab it not only orients toward the crab but bobs its head up and down. Whether this-movement is sufficient to produce the coMur changes and whether the animal is more likely to make such head-bobbing movements to a large crab is a question that we have not yet been able to answer.
Large Crab and Small Crab Conditions To determine the effect of crab size, another group of four octopuses was observed during a series of attacks on relatively large impaled crabs (shell width approximately 15 to 20 ram) and during a series of attacks on relatively small impaled crabs (shell width approximately 6 to 8 mm). The animals were tested with large or small crabs in "/-day blocks. Two animals were tested in the order large-small-large-small and two animals were tested in the reverse order. Trials per day varied with crab size in an attempt to equalize food intake (large crabs: 2 trials/day; small crabs: 4 trials/day).
A third group of four octopuses was trained to attack a positive cue and not to attack a negative cue. We wished to know whether the colour displays observed during attacks on conditioned stimuli would differ from those observed during attacks on crabs.
Results of Observations with Large and Small Crabs The colour-change data reproduced with different animals the pattern shown in Fig. 1. As before, there was considerable variability during the period before dropping of the crab ('before'). Both during 'detection' and during 'attack' the largest percentage fell in blush. The relative frequency of blushing during attack was somewhat larger than for the previous groups, but the distribution of displays during landing was strikingly similar. 'Capture' showed substantially more spotting and mottling; 'withdraw' and 'after' showed considerable variability as they did before. For purposes of comparing the effects of variation in crab size, only attacks on the first two trials per day of the first two blocks were analysed statistically. With one exception, the variations in crab size had no discernible effect. The octopuses all
Colour Changes During Learning
Apparatus and Procedure Aside from the learning aspects of the present experiment, the procedures were the same as those already described in impaled crab conditions. The stimuli used as cues were pieces of polished black or white lucite plastic, 55 x 5 • 5 mm, glued in a horizontal orientation onto the end o f a long, narrow, transparent plastic rod. Each cue had a small, thin spike protruding from the bottom side. Medium-sized (shell width, 10 to 12 mm) crabs, the reward for attack on a positive cue, were delivered either on the bottom of the cue or on an independent delivery stick identical to the stick described in the preceding section. Direct-current electric shock, the punishment for attack on a negative cue, was delivered b y means of a separate stick with a shocker on the end, of j u s t about :enough intensity to force the animal to abandon the cue when it had seized it. This shock intensity was approximately eight volts; All octopuses were given minimal initial training with a crab on a stick just to insure that they w o u l d readily attack a n impaled crab. During the conditioning series, there were eight trials per day with S + and S - - presented equally often i n n random sequence. At the start of the series (pretraining), S + was baited with a crab, and the baiting of S + continued until animals
218
ANIMAL
BEHAVIOUR,
reached a criterion of 7/8 correct responses for three consecutive days. Thereafter (training) the crab was delivered, on the end o f a separate stick, only when the animals touched the S + cue. Training was continued until the animal again reached the criterion of 7/8 correct responses for three consecutive days. Learning Results The mean number of trials to reach the final criterion was 224. The conditioning series yielded means of forty-six attacks per animal on the baited S + , twenty-six attacks per animal on the unbaited S + , and twenty-six attacks per animal on S--. There are three principal questions concerning learning. First, did the general distributions of colour displays during attacks under conditioning procedures resemble those for attacks in the impaled condition? Second, were there any systematic differences in the colour displays attributable to the conditioning procedures? Third, were there any differences in colour displays during attacks on the baited S § unbaited S + , and S - - ? The general pattern of colour changes during the conditioning series was the same for these four animals as it was in the previous analyses. Again, as before, there was a general grouping of responses in the blush and grey categories during before, detect and attack; a strong clustering of responses in the transparent category during landin.g, but less strong than previously; and an increase in mottle during capture and withdraw. Using statistical criteria comparable to those previously described, several reliable effects attributable to conditioning procedures were present. During landing, animals under conditioning procedures were less likely to make transparent responses and more likely to blush or mottle than in the impaled condition. Again the difference is associated with changes in motor pattern. During conditioning, animals often landed directly upon the cue, rather than landing on the tank bottom and engulfing the cue or cue plus crab. The transparent display appears to be associated with a fiat umbrella landing in which the prey or cue is overlaid by the interbrachial web. During conditioning there was also a general reduction in the frequency of spotting respones in all segments of the attack sequence, with a statistically reliable reduction at capture. Animals under conditioning procedures tended to display uniform colour or complex patterns, but we have
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no satisfactory explanations of this effect. In comparing attack sequences on the baited S + , unbaited S + , and S - - there was only one reliable difference. During attacks on S--, there was less tendency to mottle at capture than during attacks on either S + . This difference was paralleled by an increased tendency to maintain a uniform display at capture when attacking S--. This difference may reflect a direct effect of negative reinforcement on colour display, but it is also associated with a less vigorous assault upon the S - - cue and probably with the maintenance of a readiness to flee the shock. The early and late trials in the three cue conditions were examined separately to see whether there were any systematic changes in the distribution of colour responses as a function of the stage of learning. No such changes were evident. Thus, throughout learning, attacks resulted in the same display pattern. Finally, the mean per cent agreement scores, especially in the discrimination task, are lower than in the previously described tasks (see Table I). It is also true that the attacks on cues are more variable and more tentative than attacks on crabs. Probably the two facts are related. Discussion The purpose in undertaking this research was to determine whether the colour changes that were observed during attacks on crabs were systematic and to determine whether they were transferred to a conditioned stimulus. To assess this, it was necessary first to describe carefully the nature of the colour changes that occurred in the attacks on crabs; second, to observe whether these changes were at all affected by our experimental conditions; and third, to examine the changes that occurred during conditioning. It appears quite clear that in fact the colour changes which occurred during attacks on the crabs also occurred during attacks on the conditioned stimulus, but we think that this occurs not because both the colour reactions and the attack behaviour are learned responses to the conditioned stimulus but because the colour changes are inevitable given the changes in locomotor activity which occur during attacks. That is, the response which is transferred is the attack: the display sequence is a component of this attack sequence. The evidence for such a conclusion is not wholly to be found in the statistical comparisons, but also in incidental observations that came up
WARREN ET AL.: COLOUR CHANGES OF OCTOPUS RUBESCENS DURING ATTACK along the way. For example, in attack there was a very small percentage of responses in which the animal was not of uniform colour (blush or grey) but instead showed spotting or mottling. In every single instance where spotting or mottling occurred during attack, the animal touched either the side or bottom of the tank, or was walking rather than jetting. Or again, a strong mottling during capture invariably occurred during the animal's efforts to manipulate the cue or to pull the crab off of the stick on which it was impaled. Finally, during conditioning, we found no evidence that colour changes may be learned independently of attack behaviour. Learning produced a change in the probability of attack. Given an attack, however, display changes followed. It should be emphasized that this account of the determinants of display changes does not assert that the locomotor activity produces the change (Steinach 1901; Cowdry 1911). We can as yet take no position on this issue. We only assert that under these experimental conditions, display changes are so associated with the animals' motor activity that the correlation suggests a causal relation of some sort. Also the wide variety of activity-related displays that occurred make it desirable to treat other interpretations o f display change with caution. Thus, although colour displays of the octopus may on occasion serve a cryptic function (Packard & Sanders 1969), our data suggest the possibility that even those displays which so obviously hide the animal may not be independent of the animals' locomotor adjustments, and that the relation between background and cryptic display may be complex.
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Acknowledgments Supported in part by USPHS grants MH17801 and MH12485 to Donald A. Riley. We also wish to acknowledge with thanks the support of the Bodega Marine Laboratory, University o f California; Steinhart Aquarium, California Academy of Science; and the Institute of Human Learning, University of California, Berkeley. Linda R. Warren is a USPHS pre-doctoral fellow in experimental psychology. REFERENCES Cowdry, E. V. (1911). Color changes of Octopus vulgaris Lmk. Univ. Toronto Stud. bioL Ser., 10, 1-53. Holmes, W. (1955). The colour changes of cephalopods. Endeavour, 14, 78--82. Munsell Book of Color, pocket ed. (1943). Baltimore, Md.: Munsell Color Co., Inc. Packard, A. (1963). The behaviour of Octopus vulgaris.
Bulletin de I'Institut Oceanographique, Foundation Albert Ier, Prince de MonaCo, Numero Speciale 1D
Packard, A. & Sanders, G. (1969). What the octopus shows the world. Endeavour, 28, 92-99. Steinach, E. (1901). Studien uber die Hautfarburg und uber den Farbenwechsel der Cepalopoden. Arch. ges. PhysioL Bd., 87, as cited in Cowdry (1911). Walker, J., Longo, N. & Bitterman, M. E. (1970)o The octopus in the laboratory: Handling, maintenance, training. Behav. Res. Meth. and lnstr., 2(I), 15-18. Wells, M. J. (1962). Brain and Behavior in Cephalopods.
Stanford, Cal.: Stanford University Press. Yarnall, J. L. (1969). Aspects of the behaviour of Octopus cyanea Gray. Anita. Behav., 17, 747-754. (Received 5 February 1972; revised 26 October 1972; MS. number: A1291)
COLOUR
CHANGES
OF OCTOPUS RUBESCENS DURING
UNCONDITIONED
AND CONDITIONED
ATTACKS
ON
STIMULI
BY LINDA R. WARREN, MICHAEL F. SCHEIER & DONALD A. RILEY
Department of Psychology, University of California, Berkeley, California Abstract. Octopus rubescens exhibits a sequence of skin colour changes when it attacks and captures prey. These displays were observed during attacks on (1) free-swimming crabs, (2) crabs of different sizes impaled on thin rods, and (3) positive and negative discriminative stimuli. Animals were housed separately in light-blue aquaria which also served as experimental apparatuses. Animals in each condition showed the same typical sequence of colour displays. This sequence is (1) before detection of crab: various colours, (2) on detection and during a free-swimming attack: colours ranging from light orange to grey, (3) on landing: colourless and nearly transparent, (4) on seizing the crab: spotted or mottled, and (5) afterward: various colours. The colour changes may be tied to locomotor acts and postural adjustments. The fact of adaptive colouration in members of the octopus family is well known and has been described by a number of writers (Cowdry 1911; Holmes 1955; Packard 1963; Packard & Sanders 1969). These colour changes, remarkable for their detail and variety, have been interpreted as background matching and as serving a cryptic function (Wells 1962). Recent evidence for such an interpretation is to be found primarily in photographs of octopuses displaying patterns markedly similar to the rocks on which they sit (Packard & Sanders 1969). Adaptive as such behaviour might be, it is not clear that all colour change can be attributed to background matching. Packard (1963) has described colour changes that occur during the attack of Octopus vulgaris on its prey, and we have observed similar colour changes in Octopus rubescens, a small Pacific octopus of the coastal shelf. Preliminary observations suggested that as O. rubescens first sees its prey (a crab), its skin becomes a uniform orangish colour that persists through the attack until the moment of landing upon the crab. At that moment, there is usually a dramatic bleaching out followed almost immediately by a change to a patterned display. Such behaviour might, under natural circumstances, result in background matching, for octopuses frequently jet from rock to rock across sand in hunting crabs (Yarnall 1969). While changing from mottled to uniform to mottled in accord with the background might reflect the animal's ability to select appropriate colouration independent of the behavioural sequence, we thought this an unlikely explanation of colour changes during attack for several reasons.
These initial observations of octopus colour changes were gathered rather casually in the course of investigating some hypotheses about multiple-reversal learning. However, the sequence of colour change during attack, while systematic, appeared to bear no particular relation to the semi-natural rock and sand background of the aquaria in which the animals were observed. In addition, it appeared that the same sequence of colour changes that occurred during attacks on crabs also occurred during attacks on a conditioned stimulus, in this case a black or white vertical rectangle. The possibility that a series of systematic colour changes occurred independent of background colouration, and that this sequence could come under the control of a previously neutral stimulus, suggested that the colour changes were mediated entirely by the elicited attack sequence which became conditioned to the cues, or that the colour changes themselves were conditioned responses, modifiable by reward contingencies. It was to investigate questions of this sort that the present research was initiated. The first section asks, what are the colour changes that occur when an octopus attacks an unconditioned stimulus, in the present case always a crab (Hemigrapsus oregonensis)? This procedure when repeated a number of times on each octopus allows an assessment of the reliability of the colour change within an octopus and the generality of this colour change series from one octopus to another. The experiments also examine some possible factors determining the colour change such as the nature of the unconditioned stimulus. When the unconditioned stimulus in a conditioning experiment was a crab, it was gener211
212
ANIMAL
BEHAVIOUR,
ally presented to the octopus on the end of a sharply pointed rod. Consequently, in these situations the crab was not free-moving, and colour changes that the octopus made in attacking the crab under such circumstances could be quite different from those occurring in a more natural setting. To assess this possible restrictive effect, we observed octopuses attacking both impaled and freely moving crabs. In addition to inquiring into the effects of free as opposed to restricted prey, we inquired whether the size of the crab used as the unconditioned stimulus affected the nature of the colour change. (We thought we had seen such an effect in preliminary observations.) Also, our conditioned stimuli were larger than crabs, so some estimate of the effect of variation in target size was desired. The second section examines questions of simple discrimination learning and asks whether the colour changes observed with the unconditioned stimulus also occur to conditioned stimuli.
Methods Animals
Octopus rubescens is a species of small octopus found in shallow waters of the northern California coast. Animals collected in Monterey Bay ranged in size from 2 to 100 g. The young animals, initially weighing 2 to 10 g, are most suitable for purposes of prolonged behavioural observation, because females of 30 to 50 g frequently develop and lay eggs, during which period they become inactive and refuse food. The small animals can be maintained with ease in the laboratory for at least 6 months, on a diet of fifteen to twenty medium-sized rock crabs (shell width, 10 to 12 mm; weight, 0.2 to 0.5 g) per week. Weight gain on this regimem is about 15 to 20 g a month. Body weights of the otopuses used for the observations reported here varied from 10 to 20 g at the initiation of testing to 25 to 40 g at the end of testing. Laboratory Conditions The work was performed in a closed seawater system with a capacity of approximately 1000 litres. Water was continuously recirculated through a central gravel and oyster-shell filter and refrigeration unit, and tanks were individually aerated. Approximately 500 litres were removed each month and replaced with fresh seawater. The mean temperature in the
22,
1
system was kept at 17~ W h e n animals were brought into the laboratory, they were placed in a large holding tank, and then transferred when needed to individual tanks where they remained for the duration of observations and training. The individual tanks were epoxied fibreglass with a glass front panel and clear Plexiglass tops. Each one held 57 litres and measured 60 x 45 x 30 cm. Unlike the rock and sand backgrounds of the tanks used in the initial observations, these tanks were uniformly coloured aquamarine. Neither direct observation nor motion-picture records revealed any difference in octopus displays in these different aquara. Animals were maintained on a light-dark cycle, with light from 09.00 hours to 19.00 hours; and were tested in the afternoon. During testing the tank-top was removed and the tank illuminated by a 100-W bulb, 60 cm above the tank. This continuous over-tank lighting was incorporated into the testing procedure to enable motion-picture and still-photography to be carried out at intervals and with minimal disruption. The movements of observers were partially hidden by a large screen which had a rectangular hole cut to expose the glass front panel of the tank. The procedure followed in presenting crabs (or cues) and recording behaviour was as follows. One of the authors ran the equipment and made all observations and classifications of behaviour. He lowered in the cue or the bait, timed the animal's response, and immediately recorded the attack and corresponding colour changes according to the system described below. On many of the trials, at the same time, the other two experimenters were taking coloured motionpictures and still-photographs of the same sequences. The purpose of the motion-pictures was to allow a continuous record of the changes so that the recordings of the experimenter could subsequently be checked against an objective record. The stills recorded details of the colour displays at higher resolution than the motionpictures allowed and were used for later comparisons to Munsell colour patches to specify colour components of the displays (Munsell 1943). All motion-pictures were taken with a Bolex 16 mm camera from a distance of approximately one metre. Still-pictures were taken with a Nikon camera from approximately half a metre. Subsequently, these other experimenters made judgments of the colour changes from the film~ These judgments agreed 96 per cent of the time with those of the experimenter who made the
WARREN ET AL.: COLOUR CHANGES OF OCTOPUS RUBESCENS DURING ATTACK initial observations. The observations reported are those of the initial observer.
Classification of Colour Displays On the basis of our preliminary observations a colour and pattern code was developed which characterized in a gross way all of the displays observed by us in the laboratory. These can be roughly divided into uniform colour displays, spotted displays, and complex mottled patterns. Under natural feeding conditions, these displays are typically associated with certain behaviours which we will identify. Approximate counterparts to the colours described were identified in the Munsell colour series and are indicated in parentheses. 1. Greys or octopus colour displays. The greys vary from light to medium to dark grey and represent a characteristic colouration of animals sitting or moving about in the tanks during the lighted portion of the day. They also occur with some frequency during feeding sessions (Munsell 7.5 YR 6/4). 2. Blush displays. The term blush refers to colours that are close to a coral pink. Blushes also vary from light to dark, and seem to be confined solely to feeding sessions, occurring most frequently just prior to and during swimming attacks (Munsell 2.5 YR 6/10). 3. Red displays. The reds differ from the blushes in that they are always darker and less yellow. Although the reds vary in intensity, the range is considerably smaller than the greys or blushes. Reds occur infrequently during feeding sessions but are a predictable response when the animal is frightened. Red is the characteristic colour as an initial response to sudden increase in illumination, when pursued by the experimenter's net in the tank, or when an animal has escaped from a tank and is crawling out of the water (Munsell 7-5 R 3/8). 4. Transparent displays. These displays are a complete loss of colour and a simultaneous loss of the opaque quality that characterizes the grey displays. Internal organs are more clearly visible and the areas around the interbrachial web are virtually transparent. This condition occurs very briefly when the octopus lands by or on its prey at the end of jetting (Munsell 10.0 Y R 8/2). 5. Spotted displays. These displays appear to be on a continuum between the uniform displays and the more complex patterns, occurring most frequently in the feeding sessions as a fairly brief interruption of a uniform or mottled
213
display or as a transition between uniform and mottled displays. They are characterized by irregular dark, nearly black areas, over a uniform grey or blush, and also vary from a few small dark areas on the body sac to larger dark areas all over the body. (Munsell 10-0 R 2/2 approximates the colour of the black spots). 6. Mottled and complex displays. The complex patterns are extremely varied and difficult to classify. These patterns are usually composed of some combination of reddish browns and whites. They range from an overall marbled effect to patterns such as white circles surrounded by dark areas, irregular white stripes on a dark background, or a particularly distinct lacy white on the head and body sac, over a uniform, very dark, red on the legs and web. This last we recorded separately as white over red. These patterns occur when the animal is in contact with a surface and are especially prevalent during the capture of and withdrawal with prey. We have never observed an animal displaying a mottled pattern when free swimming.
Classification of Steps in Behaviour Sequence To discuss the colour changes that occur during attacks, it is necessary to describe the attack in sequential order. Ideally, one would wish to present a description of the eolour changes on a strict time dimension from the first presentation of a stimulus until some later time when the sequence presumably is completed. The behaviour sequences, from the onset of the attack to retreat by the animal with a crab, take, however, anywhere from 5 to 20 s. The critical variable, at least at this stage of the research, seems to be what the animal is doing rather than time since the presentation of the cue. In the work that follows, therefore, we have related colour change to phases in the behaviour sequence. They are as follows: Phase I: Animal's state prior to presentation of the stimulus. The animal was typically sitting on the bottom or the side of the tank, or slowly crawling along the walls. Phase H: Detection. When the stimulus was lowered into the tank, the animal usually drew the legs closer to the body, raised and oriented the eyes toward the prey, and bobbed the head once or twice. Phase HI: Attack. Attacks were almost always performed by jetting through t h e water toward the prey, funnel pointed behind, and arms trailing. On rare occasions, an animal
214
ANIMAL
BEHAVIOUR,
would combine such swimming with crawling when approaching the target. These occasions were noted but included under attacks. Phase IV: Landing. Octopuses typically landed on or beside the prey with interbrachial web outstretched and arms loosely wrapped around the web. Phase V: Capture. The target was seized and pulled under the web. Phase VI: Withdrawal. Withdrawal took a variety of forms of crawling, either with arms extended or with arms tucked under, or jetting, but involved moving away from the point of capture usually in the direction from which the animal came. Phase VII: After capture. A final observation was recorded when the animal was again resting on the tank bottom or wall. Colour Changes During Attacks on Crabs Free and Impaled Conditions Four octopuses were observed attacking medium-sized crabs (shell width approximately 10 to 12 ram) under free and impaled conditions. In the free condition, octopuses attacked a freeswimming crab dropped into the home tank at the end remote from the octopus. This condition allowed the observation of behaviour and colour changes under conditions that roughly approximated natural attack conditions. In the impaled conditon, octopuses attacked a crab impaled on the end of a pointed transparent plastic rod. We wanted these observations of attack on a crab under conditions identical to those to be used later during conditioning, except that in conditioning the crab would be replaced by a discriminative stimulus, thus allowing direct comparison of behaviour to unconditioned and conditioned stimuli.
Procedure Animals were tested 5 days a week and fed only on test days. Each presentation of a free crab or impaled crab was considered a trial. There were three trials per day. If the octopus did not attack within 30 s, the trial was terminated by removal of the crab from the tank. The intertrial interval was in all cases approximately 5 min. On trials for which an attack occurred, latency of the attack (time from presentation of a crab to capture) was recorded as well as the observed colour changes. If an animal did not attack during test trials, it was fed at the end of the day's testing session.
22,
1
To impose standardization on the impaledcrab procedures, we modified the procedure from that used with free-swimming crabs. Prior to the start of a trial, the octopus was retained in one-half of the tank by a barrier with a large vertically sliding door in it. The barrier was clear Plexiglass, with black outlines so as to be clearly visible. The impaled crab was always lowered at the far end of the tank not occupied by the octopus. Consequently, the barrier served to divide the tank into a starting box and a target chamber. A small black box suspended above the end of the tank served to hide the impaled crab from the octopuses's view until the start of a trial. At the start of a trial the door was raised and the crab, impaled on a thin plastic rod, was lowered into the tank. The latency of attack was recorded from presentation of the crab to capture (seizing of the crab by the octopus). We used this method of modifying the animal's living tank into an experimental apparatus rather than the technique described by Walker, Longo & Bitterman (1970), in which they transported the animal to the experimental apparatus to minimize the disruptive effects that occur when the octopus is transferred from one tank to another.
Results of Observations with Free and Impaled Crabs The colour displays of four octopuses attacking free crabs for a series of 12 days and the same octopuses attacking impaled crabs for a series of 12 days were compared. Originally there were six animals, with three receiving the free condition first and three receiving the impaled condition first. Two of the animals receiving free crabs first died in the course of the experiment. Consequently, three had impaled first and one had free first. The results, however, were sufficiently consistent that this lack of counterbalancing did not seem to create a bias in the results. The data presented are only for those trials on which octopuses attacked. The octopuses had thirty-six trials under each condition and the frequencies of attack were distributed among the four animals as follows: free-swimming 35, 34, 33 and 33; impaled 18, 29, 27 and 9. These octopuses were less willing to attack a crab on a stick than they were a freeswimming crab; the mean latency of response for attacks was, however, approximately the same despite the free movement of the crab in the free condition. In the case of the free-swimming crabs the mean latency was 10-6 s and in the
W A R R E N ET AL.: COLOUR CHANGES OF OCTOPUS RUBESCENS D U R I N G ATTACK Free
mm
~
215
mpoled m
__
m
50 plush
o/n. HI o/HI $ o
nm
50 t Trons 0 50 l Sp~
nm
m
N
m
NI N,,
N
oir I
n|
50 ~Mottle
[~1
m
wm
nm
~
m
m
iN
Before Detect Attack Land Capt. W/draw After Temporal category Fig. ]. Distribution of colour displays during attacks on free and impaled crabs. Entries in each column add to 100 per cent.
case of the impaled crabs the mean latency was 10.25 s. Figure 1 shows the distribution of skin patterns at each of the seven time classifications. The figure should be read by looking first at the left column, which shows the display conditions for the octopus prior to the lowering of the crab into the tank ('before'). The first bar in each square shows the mean per cent of the total displays belonging in that colour class in the free crab condition. The second bar shows the same information for the impaled condition. Thus, before the crab was lowered, a mean of 8 per cent of the observations showed a uniform red octopus in the free condition, 5 per cent in the impaled. A mean of 32 per cent of the cases in the free condition blushed, and so on. The two conditions, free and impaled, should be the same in the 'before' phase for it is a pre-trial observation. In most o f these observations the skin colour was uniform, but all of the animals spotted on some trials.
Immediately on 'detection' (column 2) the skin display changes. In each condition, a mean of 75 per cent or more of the responses were blush or grey. This concentration of displays becomes even more marked in the 'attack' phase (column 3) with more than 90 per cent of the responses being in the blush and grey categories. On 'landing' (column 4) the animals show a marked switch to the category designated as 'transparent' (row 4). With 'capture' (column 5) there is a return to grey on some trials, the emergence of spots and mottled patterns on other trials. During 'withdrawal' (column 6) and the retreat to a corner of the tank (column 7), patterns are quite evenly distributed among a variety of classifications with somewhat less tendency to show the uniform blush pattern, somewhat more tendency to show more complex patterns than in the 'before' phase. In addition to this overall description, which shows this change from a varied condition prior to the appearance o f the crab to a uniform skin
ANIMAL
216
BEHAVIOUR,
22,
1
Table I. Mean Per Cent Agreement Among the Distributions of Individual Animals' Colour Displays
Condition Free crab
Impaled crab
Large crab
Small crab
Before
60
49
48
68
47
43
34
Detect
61
57
92
84
69
46
56
Attack
50
59
84
79
76
52
56
Stage in sequence
Pretraining S-tS--
Training S§
Land
89
86
86
68
65
48
35
Capture
62
65
55
60
39
42
52
Withdrew
75
38
58
57
57
--
41
After
60
38
61
52
60
--
42
display during orientation and attack, a transparent pattern on landing, and then a variety of displays again, two questions are of interest. First, is the distribution of displays consistent from animal to animal? Second, are there any reliable differences that appear between the free and impaled conditions ? The first question is of interest because summary statements such as that given above imply that there are regular patterns of change that the octopuses characteristically exhibit. Figure 1 shows, however, that there is considerable variability of the patterns at some points in time. While this variability reflects some idiosyncratic behaviour, it largely shows the variability characteristic of each octopus. Three different indices illustrate the extent to which the animals are similar. First, in 48 per cent of the cells with entries in Fig. 1 all four animals are represented. Second, only one entry with a value greater than 15 per cent has less than four animals, that one has three. A third: measure of inter-animal agreement is the mean per cent agreement score shown for each t i m e and each experimental condition in Table I. These scores are the m e a n per cent of the trials on which each animal's responses were in the same display class as each other animal's. Consider now only the entries for free a n d impaled. The highest value, 89 per cent for free, "land', reflects a condition in which nil animals responded with a transparent display on almost all trials, Also, here, even less frequent responses, grey and spots, had four and three animals represented. The lowest of the agreement values in free is 'attack' at 50 per cent. Here two of the animals gave mostly blush responses and two
gave mostly grey responses. Only when the agreement index drops substantially below 40 per cent is there a clear lack of agreement between animals. For example, m impaled, 'after' (38 per cent) each animal has a different modal response. In these data such disagreement is the exception. In general the distribution shown in the bar graphs represent each animal quite well. We know of no completely acceptable statistic for assessing the reliability of experimental differences with such a small number of animals and such a large number of Classifications. We can only suggest a rough-and-ready measure that we have used in thinking about this problem. Our practice has been to run t-tests of significance, but only in those cells in which all four animals have a higher per cent of occurrences in one condition than in the other. This procedure is more conservative than the indiscriminate use of t-tests, since it assesses only those differences to which all animals contribute. On the other hand, the use of such statistics renders the probabilities uninterpretable in any precise sense. We used the tests only as general guides in thinking about the data, and present below only those results which were reliable (P<0.02). Using this procedure, there were reliable differences between free and impaled conditions during 'attack' and during 'capture'. During attack, the difference was in the cell attack-blush. In this case the animals blushed more often when attacking a crab on the stick than they did when attacking a free-swimming crab. We have no satisfactory interpretation of this effect. One possibility is that, in general,
WARREN ET AL.: COLOUR CHANGES OF OCTOPUS ,RUBESCENS DURING ATTACK
217
octopuses were obliged to swim farther for a crab on a stick than for a free-swimming crab because the free-swimming crab sometimes approached the octopus. During 'capture', there was a reliable effect for grey displays favouring the animals under the free condition, whereas the spot displays occurred more frequently in the impaled condition. When the animal attacks a free crab, it is merely required to scoop it up into its web. On the other hand, when it attacks a crab on a stick, it must seize the crab, tear it off the stick, and take it under its web. The spotting and mottling are responses accompanying the struggle to take the crab off the stick. The simplest interpretation of these data is that this spotting is a skin display accompanying a certain kind of effort on the part of the animal.
blushed more when c o n f r o n t e d with a large crab than with a small crab in the 'detect' category. This reliable difference, of over 25 per cent in frequency of this response, is the sort of effect that we were looking for when we began this research; that is, stimulus control of: colour changes not directly associated with attack or withdrawal behaviours. However, there is a possible association of colour change with move= ment change here also. When an octopus detects a crab it not only orients toward the crab but bobs its head up and down. Whether this-movement is sufficient to produce the coMur changes and whether the animal is more likely to make such head-bobbing movements to a large crab is a question that we have not yet been able to answer.
Large Crab and Small Crab Conditions To determine the effect of crab size, another group of four octopuses was observed during a series of attacks on relatively large impaled crabs (shell width approximately 15 to 20 ram) and during a series of attacks on relatively small impaled crabs (shell width approximately 6 to 8 mm). The animals were tested with large or small crabs in "/-day blocks. Two animals were tested in the order large-small-large-small and two animals were tested in the reverse order. Trials per day varied with crab size in an attempt to equalize food intake (large crabs: 2 trials/day; small crabs: 4 trials/day).
A third group of four octopuses was trained to attack a positive cue and not to attack a negative cue. We wished to know whether the colour displays observed during attacks on conditioned stimuli would differ from those observed during attacks on crabs.
Results of Observations with Large and Small Crabs The colour-change data reproduced with different animals the pattern shown in Fig. 1. As before, there was considerable variability during the period before dropping of the crab ('before'). Both during 'detection' and during 'attack' the largest percentage fell in blush. The relative frequency of blushing during attack was somewhat larger than for the previous groups, but the distribution of displays during landing was strikingly similar. 'Capture' showed substantially more spotting and mottling; 'withdraw' and 'after' showed considerable variability as they did before. For purposes of comparing the effects of variation in crab size, only attacks on the first two trials per day of the first two blocks were analysed statistically. With one exception, the variations in crab size had no discernible effect. The octopuses all
Colour Changes During Learning
Apparatus and Procedure Aside from the learning aspects of the present experiment, the procedures were the same as those already described in impaled crab conditions. The stimuli used as cues were pieces of polished black or white lucite plastic, 55 x 5 • 5 mm, glued in a horizontal orientation onto the end o f a long, narrow, transparent plastic rod. Each cue had a small, thin spike protruding from the bottom side. Medium-sized (shell width, 10 to 12 mm) crabs, the reward for attack on a positive cue, were delivered either on the bottom of the cue or on an independent delivery stick identical to the stick described in the preceding section. Direct-current electric shock, the punishment for attack on a negative cue, was delivered b y means of a separate stick with a shocker on the end, of j u s t about :enough intensity to force the animal to abandon the cue when it had seized it. This shock intensity was approximately eight volts; All octopuses were given minimal initial training with a crab on a stick just to insure that they w o u l d readily attack a n impaled crab. During the conditioning series, there were eight trials per day with S + and S - - presented equally often i n n random sequence. At the start of the series (pretraining), S + was baited with a crab, and the baiting of S + continued until animals
218
ANIMAL
BEHAVIOUR,
reached a criterion of 7/8 correct responses for three consecutive days. Thereafter (training) the crab was delivered, on the end o f a separate stick, only when the animals touched the S + cue. Training was continued until the animal again reached the criterion of 7/8 correct responses for three consecutive days. Learning Results The mean number of trials to reach the final criterion was 224. The conditioning series yielded means of forty-six attacks per animal on the baited S + , twenty-six attacks per animal on the unbaited S + , and twenty-six attacks per animal on S--. There are three principal questions concerning learning. First, did the general distributions of colour displays during attacks under conditioning procedures resemble those for attacks in the impaled condition? Second, were there any systematic differences in the colour displays attributable to the conditioning procedures? Third, were there any differences in colour displays during attacks on the baited S § unbaited S + , and S - - ? The general pattern of colour changes during the conditioning series was the same for these four animals as it was in the previous analyses. Again, as before, there was a general grouping of responses in the blush and grey categories during before, detect and attack; a strong clustering of responses in the transparent category during landin.g, but less strong than previously; and an increase in mottle during capture and withdraw. Using statistical criteria comparable to those previously described, several reliable effects attributable to conditioning procedures were present. During landing, animals under conditioning procedures were less likely to make transparent responses and more likely to blush or mottle than in the impaled condition. Again the difference is associated with changes in motor pattern. During conditioning, animals often landed directly upon the cue, rather than landing on the tank bottom and engulfing the cue or cue plus crab. The transparent display appears to be associated with a fiat umbrella landing in which the prey or cue is overlaid by the interbrachial web. During conditioning there was also a general reduction in the frequency of spotting respones in all segments of the attack sequence, with a statistically reliable reduction at capture. Animals under conditioning procedures tended to display uniform colour or complex patterns, but we have
22,
1
no satisfactory explanations of this effect. In comparing attack sequences on the baited S + , unbaited S + , and S - - there was only one reliable difference. During attacks on S--, there was less tendency to mottle at capture than during attacks on either S + . This difference was paralleled by an increased tendency to maintain a uniform display at capture when attacking S--. This difference may reflect a direct effect of negative reinforcement on colour display, but it is also associated with a less vigorous assault upon the S - - cue and probably with the maintenance of a readiness to flee the shock. The early and late trials in the three cue conditions were examined separately to see whether there were any systematic changes in the distribution of colour responses as a function of the stage of learning. No such changes were evident. Thus, throughout learning, attacks resulted in the same display pattern. Finally, the mean per cent agreement scores, especially in the discrimination task, are lower than in the previously described tasks (see Table I). It is also true that the attacks on cues are more variable and more tentative than attacks on crabs. Probably the two facts are related. Discussion The purpose in undertaking this research was to determine whether the colour changes that were observed during attacks on crabs were systematic and to determine whether they were transferred to a conditioned stimulus. To assess this, it was necessary first to describe carefully the nature of the colour changes that occurred in the attacks on crabs; second, to observe whether these changes were at all affected by our experimental conditions; and third, to examine the changes that occurred during conditioning. It appears quite clear that in fact the colour changes which occurred during attacks on the crabs also occurred during attacks on the conditioned stimulus, but we think that this occurs not because both the colour reactions and the attack behaviour are learned responses to the conditioned stimulus but because the colour changes are inevitable given the changes in locomotor activity which occur during attacks. That is, the response which is transferred is the attack: the display sequence is a component of this attack sequence. The evidence for such a conclusion is not wholly to be found in the statistical comparisons, but also in incidental observations that came up
WARREN ET AL.: COLOUR CHANGES OF OCTOPUS RUBESCENS DURING ATTACK along the way. For example, in attack there was a very small percentage of responses in which the animal was not of uniform colour (blush or grey) but instead showed spotting or mottling. In every single instance where spotting or mottling occurred during attack, the animal touched either the side or bottom of the tank, or was walking rather than jetting. Or again, a strong mottling during capture invariably occurred during the animal's efforts to manipulate the cue or to pull the crab off of the stick on which it was impaled. Finally, during conditioning, we found no evidence that colour changes may be learned independently of attack behaviour. Learning produced a change in the probability of attack. Given an attack, however, display changes followed. It should be emphasized that this account of the determinants of display changes does not assert that the locomotor activity produces the change (Steinach 1901; Cowdry 1911). We can as yet take no position on this issue. We only assert that under these experimental conditions, display changes are so associated with the animals' motor activity that the correlation suggests a causal relation of some sort. Also the wide variety of activity-related displays that occurred make it desirable to treat other interpretations o f display change with caution. Thus, although colour displays of the octopus may on occasion serve a cryptic function (Packard & Sanders 1969), our data suggest the possibility that even those displays which so obviously hide the animal may not be independent of the animals' locomotor adjustments, and that the relation between background and cryptic display may be complex.
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Acknowledgments Supported in part by USPHS grants MH17801 and MH12485 to Donald A. Riley. We also wish to acknowledge with thanks the support of the Bodega Marine Laboratory, University o f California; Steinhart Aquarium, California Academy of Science; and the Institute of Human Learning, University of California, Berkeley. Linda R. Warren is a USPHS pre-doctoral fellow in experimental psychology. REFERENCES Cowdry, E. V. (1911). Color changes of Octopus vulgaris Lmk. Univ. Toronto Stud. bioL Ser., 10, 1-53. Holmes, W. (1955). The colour changes of cephalopods. Endeavour, 14, 78--82. Munsell Book of Color, pocket ed. (1943). Baltimore, Md.: Munsell Color Co., Inc. Packard, A. (1963). The behaviour of Octopus vulgaris.
Bulletin de I'Institut Oceanographique, Foundation Albert Ier, Prince de MonaCo, Numero Speciale 1D
Packard, A. & Sanders, G. (1969). What the octopus shows the world. Endeavour, 28, 92-99. Steinach, E. (1901). Studien uber die Hautfarburg und uber den Farbenwechsel der Cepalopoden. Arch. ges. PhysioL Bd., 87, as cited in Cowdry (1911). Walker, J., Longo, N. & Bitterman, M. E. (1970)o The octopus in the laboratory: Handling, maintenance, training. Behav. Res. Meth. and lnstr., 2(I), 15-18. Wells, M. J. (1962). Brain and Behavior in Cephalopods.
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