Individual variation in learning by foraging pumpkinseed sunfish, Lepomis gibbosus: the influence of habitat

Anim. Behav., 1991, 41,603-611

Individual variation in learning by foraging pumpkinseed sunfish, Lepomis gibbosus: the influence of habitat J A M E S D. K I E F F E R & P A T R I C K W. C O L G A N Department o f Biology, Queen's University, Kingston, Ontario K7L 3N6, Canada (Received 4 May 1990; initial acceptance 25 June 1990; final acceptance 30 August 1990; MS. number: A5794)

Abstract. Individual variation in the foraging behaviour of the pumpkinseed sunfish was investigated

under laboratory conditions in feeding arenas containing structured and open-water habitats. In general, fish learned to feed on a novel prey item over a 1O-day period with both inter-capture interval and total time to capture prey decreasing over the period. With respect to habitat differences, pumpkinseed sunfish fed faster in an open-water habitat. Individuals varied in such measures as foraging efficiency in the different habitats. Although fish acted differently in the two habitats, the degree of flexibility varied between individuals. The order of habitat presentation had an effect on the overall behaviour of the fish.

Two of the most important aspects of foraging behaviour are individual variation and learning abilities (Kamil et al. 1987). Learning is usually defined in terms of changes in individual behaviour as a function of experience (Dill 1983). Inferences about learning are based upon observations of the behaviour of individual animals over time, particularly in response to environmental change (Kamil & Yoerg 1982). Individual differences in behaviour have been traditionally ignored, treated as white noise (as discussed by Ringler 1983), or considered to be maladaptive deviations from optimal strategies (reviewed by Magurran 1986). Rather than noise, this variation may represent an adaptive flexibility in the foraging behaviour offish because most natural environments vary both spatially and temporally (Dill 1983). Because there is variability in environments, fish have developed mechanisms (one of which includes learning) that allow them to adjust a generalized foraging pattern to the current environmental situation (Vinyard 1980; Dill 1983). Kamil (1983) suggested that learning is important in the recognition of novel food items and, to some extent, the development of efficient foraging behaviour. Although there are some examples of individual variation of foraging behaviour of fish (e.g. Ware 1971; Ringler 1983; Paszkowski & Olla 1985), few studies have contributed any data on individual variation and learning abilities (Gotceitas & Colgan 1988; Ehlinger 1989, 1990). Of these studies, only Ehlinger (1989, 1990) attempted to describe 0003-3472/91/040603 + 09 $03.00/0

individual variation in foraging behaviour and learning with respect to differences in habitats. The source and consequence of this variation is seldom known in either the proximate or ultimate sense. In the proximate sense, flexible patterns of foraging are the product of behavioural, physiological or morphological variation (Ringler 1983). With respect to behavioural sources, experience with prey and habitat will influence individual variation in learning rate. The rate at which individuals learn to forage may be important. For example, 'fast learning' individuals may be able to take greater advantage of prey that are only temporarily available. Further, fast learners may gain an advantage by using resources more efficiently resulting in an increased body size, which may decrease predation risk (Mittelbach 1981). Ultimately, if individuals learn to forage at different rates, intraspecific competition may be reduced (Rubenstein 1981) and the differences between individuals may have adaptive significance (see Clark & Ehlinger 1987 for an extensive review). Our aim was to examine the foraging patterns of the pumpkinseed sunfish feeding in different habitats. We addressed a series of questions regarding foraging behaviour. (1) Do pumpkinseed sunfish learn to forage on a novel food item as reflected in increases in their foraging efficiency over days? (2) How is foraging behaviour influenced when the fish are exposed to a new habitat? (3) Is foraging efficiency influenced by the order in which the habitats are experienced?

9 1991 The Association for the Study of Animal Behaviour 603

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To answer these questions two approaches were used. First, we looked at behavioural changes based on measurements such as latency (time to the first capture), inter-capture intervals (time between captures), and total time spent in the feeding area (as noted by Gotceitas & Colgan 1988). We also observed hover times, capture rates and number of searches, and how these changed with exposure to different habitats (as in Ehlinger 1989, 1990). Second, we looked at how morphological differences in parameters such as pectoral fin length and body depth related to fish mobility (Webb 1984).

METHODS

AND MATERIALS

Pumpkinseed sunfish were seined from Lake Opinicon, Ontario, Canada (40~ 76~ where the ecology of the species has been extensively studied (see Keast 1978). Fish were brought back to the laboratory at Queen's University where they were housed in flow-through tanks measuring 1 x 0.75 • 0,38 m at 19_+ I~ under a 12:12 h light: dark cycle. While they were in the holding tanks, the fish were fed frozen adult brine shrimp, Artemia salina, or Tetramin, daily. White worms, Enchytraeus albidus, 13-15mm in length, were selected as the experimental food because these worms are somewhat cryptic on white sand and were novel to the fish. White worms were kept in plastic containers in a 2:1 mixture of moist peat moss and fine sand, and fed white bread moistened with milk. Experiments were carried out in a flow-through tank measuring 1-8 x 1 • 0.6 m, divided into four holding areas measuring 0.23 • 0.22 • 0.36 m and two foraging arenas measuring 0.46 x 0.64 x 0.36 m. Each arena represented either a 'vegetated' or an 'open' habitat. The 'vegetation' consisted of 'stems' of green nylon rope at a density of 800 stems/m 2. The open water habitat contained no such vegetation. The bottom of the entire arena was covered with 2-3 cm of washed white silica. Four 40-W soft spectrum fluorescent lights were placed above the tank to provide even illumination. Each chamber had a sliding door (9.5 • 7 cm), operated by a pulley system, to allow access into the foraging arena. A single pumpkinseed sunfish was housed in each chamber. Water depth was 35 cm, temperature was maintained at 20_+ I~ and a 12:12h

light:dark cycle was maintained throughout the study. Individual pumpkinseed sunfish (total length = 75-80 mm) were randomly selected and placed into one of two groups. Fish in treatment 1 ( N = 8) were fed for 10 days in the vegetation on white worms and then for a second block of 10 days in the open water. Fish in treatment 2 (N= 8) faced the same prey conditions but in the reverse order of habitat presentation. Before the experiment was carried out, the fish were coaxed from the holding area to the test arena by presenting them with brine shrimp as the door of the holding area was opened, ensuring that the fish would readily come into the test arena. The procedure for each treatment consisted of placing 20 white worms in the test arena. After a few minutes, once the prey were distributed on the bottom of the tank, the door of the holding area was opened and one fish was allowed to swim into the arena. The door was left open during the trial, so that the fish was free to leave the arena. Each fish was tested once per day and fed only while in the test arena. All trials were run between 0900 and 1200 hours to reduce any effect of the time of day. Activities were recorded using a video recorder mounted on a support rack 1.5 m above the tank while verbal recordings were made via an external microphone on the video recorder. The following activities were measured. (1) Latency to first capture: time (s) for the fish to capture the first prey item. (2) Inter-capture interval: time between successive captures. This was recorded for the first 10 captures to minimize any prey depletion or satiation effects. (3) Total time feeding: time that a fish spent in the foraging arena (maximum 300 s). The trial began when the fish came into the arena and ended when the fish left the arena. (4) Search: swimming around the arena but showing no signs of pursuing a prey item, or the activity between two hovers when no prey is pursued. (5) Hover: remaining motionless. From a lateral view, hovering was detected by the raising of the dorsal fin at the onset of the hover and the lowering of the dorsal fin at its completion. From above, hovering was detected by the jerky slowing of the fish and the movement of the pectoral fins. When the experiments were completed, each fish was measured for pectoral fin length and body depth, which reflect mobility in swimming (Webb 1984). To analyse the behavioural activities, a repeatedmeasures ANOVA was used (SAS 1985, PROC GLM) against a behavioural measure with factors

Kieffer & Colgan: Foraging behaviour in sunfish

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each point represents the measure for that fish on that day. (d) X__+s~for eight fish with their first experience in the structured habitat. of day (indicating learning), habitat, individuals nested within treatment and treatment. A treatment effect indicates the differences in behaviour in the same habitat due to the order of experience. Log transformations were performed to achieve better approximations to normality. In all statistical analyses the level of significance was set at 0.05.

RESULTS Prey

On introduction into the foraging arena, the white worms sank to the bottom and either remained motionless, wiggled about or attached themselves to a stem or its base (in the case of a vegetated habitat). At no time did the worms bury themselves in the sand or remain at the water surface. Predator

Days and individuals The term for days was significantfor each of three temporal behavioural measures: latency to first capture (F1.253= 124.6, P<0.0001), inter-capture interval (F~,2s 3 = 104.6, P<0-0001) and total time

feeding (F1,253=188-1, P<0.0001). All three measures began to level off after about the sixth exposure to the novel prey item (Figs 1-3; see sample for eight fish). The days effect indicated that learning took place during the experiment. There was a significant effect due to individual fish with respect to latency to first capture (F14,253=4.56, P<0.0001) and to total time feeding (F14,25a= 5"17, P<0'0003) but not inter-capture interval (F14.253=1-45, P>0.10). For example, the temporal pattern and final level of latency until the first capture in fish 1 was lower than for fish 6 and similar to that of the overall group (Fig. 1). Similarly, total time feeding in fish 13 was lower than that of the overall group while fish 1 was higher than and fish 6 was similar to the overall group (Fig. 3). An interactive term for days and individual fish was significant for all three behavioural measures: latency to first capture ( F I , 2 s 3 = 3.92, P < 0.0001), inter-capture interval (F1.253=2.35, P<0.005) and total time feeding (F~,253= 3-54, P<0.0001), reflecting differences across fish in their ability to learn.

Habitat effects Habitat structure had a significant effect on hovering (Table I). Pumpkinseed sunfish learned to

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Figure 2. The change in the inter-capture interval over days. Fish I (a), 6 (b) and 13 (c) are representative individual fish, and each point represents the measure for that fish on that day. (d) X + SE for eight fish with their first experience in the structured habitat.

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Figure 3. The total time feeding by pumpkinseed sunfish over days. Fish 1 (a), 6~b) and 13 (c) are representative individual fish, and each point represents the measure for that fish on that day. (d) X_SE for eight fish with their first experience in the structured habitat.

Kieffer & Colgan."Foraging behaviour in sunfish

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Table I. The comparison of various parameters (X4- SE) of efficiency within different habitats Habitat

Totat time (s) Inter-capture interval (s) Capture rate (no./s) Hover duration (s) Number of searches

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75.74-4.4 2.74- 0.2 0.3 + 0.01 0.9 4- 0.02 11.2+0.9

116.54-7.1 4.1 +0.3 0.2 +0-01 1.3 4- 0.1 20.74-1.3

0-029 0.011 0.003 0.021 0.012

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I-0 I'l 1'2 1'5 }.4 t'5 1'6 I-7 1'8 Hover durafion in open-water (s) Figure 5. Hover durations of individual pumpkinseed sunfish in the structured and open habitat. Data are .Y+ SEfor the final 4 days of each treatment. The diagonal line indicates equal hover durations used in both habitats.

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Figure 6. The influence of the order of presentation on the (a) mean latency to first capture (b) mean capture rate (c) mean total time feeding and (d) mean number of searches. Fish were either exposed to a structured habitat for 10 days (O) and then transferred to an open-water habitat ( 9 for another 10 days or they were exposed to an open-water habitat for 10 days ( 9 and then transferred to a structured habitat (A) for another 10 days. use slightly longer hovers a n d m o r e searches w h e n feeding on white w o r m s in the structured habitat; b u t learned over days to search fewer times a n d use shorter h o v e r d u r a t i o n s when feeding o n w o r m s in the o p e n water (Table I; Fig. 4a, b). A l t h o u g h 13 o f 16 fish used longer h o v e r d u r a t i o n s in the vegetation c o m p a r e d to the open water, individual fish

differed greatly in the extent o f their h o v e r adjustm e n t to h a b i t a t s (Fig. 5). Individuals t h a t hovered for shorter periods o f time in one h a b i t a t t e n d e d to do the same in the o t h e r habitat. M a n y of the points in Fig. 5 lie a b o v e the diagonal line, indicating that some fish were flexible in their h o v e r d u r a t i o n s w h e n going from the open water into the vegetated

Kieffer & Colgan: Foraging behaviour in sunfish habitat. Habitat complexity also affected total feeding time, inter-capture interval and capture rate (Table I). When fish fed in a simpler habitat, they had a higher capture rate, lower inter-capture rate and lower total time spent feeding.

Order effect Fish exhibited habitat specificity in their ability to hover. Figure 4a indicates that when we transferred fish from an open-water habitat to a structured habitat, hover duration increased and then levelled off after a few days. Figure 4b reveals, on the other hand, that when fish from a vegetated habitat were transferred to an open-water habitat, there was some carry-over in the first day after the fish were exposed to a new habitat. After the second day, the hover duration dropped. This brief carry-over may reflect the flexibility of the fish when exposed to various habitats or the effect of presentation of habitat. The order in which fish received the different habitats had an effect on the four behavioural measures of efficiency. These included latency to the first capture (F1.253= 109-1, P<0.0001, Fig. 6a), the capture rate (F 1,253 = 7" 15, P < 0"008; Fig. 6b), the total time feeding (F1,253=180.96, P<0.0001, Fig. 6c) and the number of searches (F1,253=39.99, P<0.0001, Fig. 6d). When fish were exposed to the more complex habitat first and then transferred to the simple habitat, they performed more efficiently than if they were presented with the open-water habitat first.

Morphology and behaviour There was a positive correlation between hover duration and the length of the pectoral fin and body depth (standardized for total length) in both habitats (Table II). Generally, fish that'had longer pectoral fins also had deeper bodies (r 2 = 0.74, pectoral fin = 0.68 (body depth) + 0.397, P = 0-0001). DISCUSSION Learning

Our results show that learning plays a role in the increase of prey capture. This was shown over a course of 10 days in which after 6 days, on average, behavioural measures of learning to forage on a novel food item appeared to stabilize. These results are consistent with behavioural measures reported in other species of fish (e.g. rainbow trout, Salmo

609

Table II. Correlations between hover duration versus

pectoral fin length and body depth in both open-water and structured habitats Open water habitat Hover duration=7.03 0'006 Hover duration=3.87 0-045 Structured habitat Hover duration=7.44 0.003 Hover duration=4.36 0.030

PL/TL--0.58, r2=0.43, P= BD/TL-0.27, r2=0.25, P= PL/TL--0.45, r2=0.49, P= BD/TL-0.22, r2=0.32, P=

PL: pectoral fin length (ram); TL: total length (mm); BD:

body depth (mm). N= 16, df= 1,15.

gairdneri, Ware 1971; bluegill sunfish, Lepomis macrochirus, Gotceitas & Colgan 1988; Ehlinger 1990) and may indicate a common time constraint on the rate of learning. One obvious consequence of learning is an increased capture rate with experience (Figs 1-3). For example, Werner et al. (1981) showed that bluegill sunfish feeding on Daphnia and Chironomus larvae increased their foraging efficiency up to four-fold over the course of six to eight foraging bouts. Individual Differences

Our results showed that individuals differed significantly from one another in their learning rates and the level of efficiency (Figs 1-3). Ware (1971) and Ringler (1979) suggested that attributes of the prey such as movement and colour may contribute to individual differences in foraging. Colgan (1989) suggested that motivational differences between fish may play a role in the variability. Even though our fish were not fed for 24 h prior to subsequent trials, some differences in physiological processes between individualfish (i.e. stomach size and rate of digestion) may be expected. Godin (1978) also suggested that physiological differences between fish could explain some of the variation between fish. This was the case in his study using pink salmon, Oneorhynehus gorbuseha, where the decrease in the latency of feeding may have been accounted for by changes in physiological conditions of the fish, which may reflect motivational differences. Habitat Effects

f

Werner et al. (1981) proposed that learning is important particularly in different habitats. Our

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results showed that habitat structure had a significant effect on the efficiency of individuals with respect to inter-capture interval and total time feeding (Table I). Gotceita~s (1990) found a similar result with bluegill sunfish: increased stem density significantly affected the inter-capture interval and total time spent feeding. Werner et al. (1981) suggested that individual specialization was indicative in the use of habitats by bluegills. Our study suggests that individual pumpkinseed sunfish have some behavioural pliancy when they are exposed to different habitats (Fig. 4a, b). This could allow fish to move between habitats. Order Effect The order of habitat presentation had an effect on overal feeding efficiency (Fig. 4a, b, 6a-d). This result could be explained from both proximate and ultimate viewpoints. With respect to the proximate viewpoint, the observed results could be explained by contrast effects. Given two stimuli (vegetated and open-water habitats), the contrast effect suggests that the performance of the fish would vary as a result of the order of presentation (see Mackintosh 1974 for a review; Crawford 1983). For instance, when pumpkinseed sunfish were exposed first to the vegetated habitat (i.e. to a smaller reward per unit effort) and then transferred to the open-water habitat, performance was enhanced (Fig. 6a-c). This is what is referred to as a positive successive contrast (cf. Mackintosh 1974). On the other hand, when pumpkinseed sunfish were exposed to the reverse condition, their performance only improved slightly rather than decreasing as predicted by a negative successive contrast effect (Fig. 6a-c; see Mackintosh 1974). Positive behavioural contrast effects were similarly found in goldfish but it has been suggested that negative successive contrast is difficult or impossible to demonstrate in fish (Bottjer et al. 1977). Our results also suggest that a time lag is associated with a switch in habitats (Fig. 4b). This supports the interpretation that memory plays a role in the assessment of habitat quality (see Heinrich et al. 1977). There are some functional implications of learning and the order effect. For instance, if individual fish were exposed to a prey type before other individuals, and if this experience led it to more efficient foraging, it might have a major impact on the allocation and partitioning of food resources and in the

size distribution and diversity of prey organisms (Ringler 1983). Food organisms also vary with seasonal succession (see Werner et al. 1981). Therefore, if an organism can learn about the food type and subsequently retain this information over some period of time, it may improve its chances to forage efficiently during that time. This may be important at latitudes where the growing season is short and prey items are short-lived. Learning and order effects also raise important questions about the efficiency of sampling other habitats, i.e. do animals compensate for inexperience while sampling a new habitat? If previous experience has an effect on the foraging efficiency in a new situation, it may reflect how quickly a fish may switch habitats (see Werner et al. 1981). Morphology and Behaviour Although our results show that pumpkinseed sunfish may be using different tactics in different habitats, these behavioural differences (including individual differences) may be the result of underlying morphological variation. Generally, fish that have longer hover durations have longer pectoral fins and deeper bodies. This may have important ecological implications because fish with longer pectoral fins and deeper bodies may hover longer and presumably manoeuvre more efficiently (Webb 1984). Individual fish may be specializing for feeding in either the littoral zone or open-water habitat. Ehlinger & Wilson (1988) proposed that in bluegills, this feeding specialization may be occurring and may be important in reducing intraspecific and interspecific competition. ACKNOWLEDGMENTS We greatly acknowledge the advice and support of Bob Lavery, Stephan Reebs, Ian Jamieson and Janice Frame. Also, special thanks go to Milton Suboski and Timothy Ehlinger for their input into the study. This research was supported by an Operating Grant from the National Sciences and Engineering Research Council of Canada to P.W.C. and a Queen's Graduate Fellowship to J.D.K. REFERENCES Bottjer, S. W., Scobie, S. R. & Wallace, J. 1977. Positive behavioural contrast, autoshaping, and omission responding in the goldfish (Carassius auratus). Anim. Learn. Behav., 5, 33~342.

Kieffer & Colgan: Foraging behaviour in sunfish Clark, A. B. & Ehlinger, T. J. 1987. Patterns and adaptation in individual behavioral differences. In: Perspectives in Ethology (Ed. by P. P. G. Bateson & P. H. Klopfer), pp. 1-47. New York: Plenum Press. Colgan, P. W. 1989. Animal Motivation. London: Chapman & Hall. Crawford, L. I. 1983. Local contrast and memory windows as proximate foraging mechanisms. Z. Tierpsychol., 63, 283~93. Dill, L. W. 1983. Adaptive flexibility in the foraging behavior of fishes. Can. J. Fish. Aquat. Sci., 40, 398-408. Ehlinger, T. J. 1989. Learning and individual variation in bluegill foraging: habitat-specific techniques. Anim. Behav., 38, 643-658. Ehlinger, T. J. 1990. Habitat choice and phenotype-limited feeding efficiency in bluegill: individual differences and trophic polymorphism. Ecology, 71,886-896. Ehlinger, T. J. & Wilson, D. S. 1988. Complex foraging polymorphism in bluegill sunfish. Proc. natn. Acad. Sci. U.S.A.,85, 1878 1882. Godin, J. G. 1978. Behavior of juvenile pink salmon (Oncorhynchus gorbuscha Walbaum) toward novel prey: influence of ontogeny and experience. Environ. Biol. Fish., 3, 261-266. Gotceitas, V. 1990. Variation in plant stem density and its effects on foraging success of juvenile sunfish. Environ. Biol. Fish., 27, 63-70. Gotceitas, V. & Colgan, P. W. 1988. Individual variation in learning by foraging juvenile bluegill sunfish (Lepomis rnacrochirus). J. comp. Psychol., 102, 294-299. Heinrich, B., Mudge, P. & Deringis, P. 1977. A laboratory analysis of flower constancy in foraging bumblebees: Bombus ternarnius and B. terricola. Behav. Ecol. Sociobiol., 2, 247-266. Kamil, A. C. 1983. Optimal foraging theory and the psychology of learning. Am. Zool., 23, 291 302. Kamil, A. C. & Yoerg, S. I. 1982. Learning and foraging behavior. In: Perspectives in Ethology (Ed. by P. P. G. Bateson & P. H. Klopfer), pp. 325-364. New York: Plenum Press.

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Kamil, A. C., Krebs, J. R. & Pulliam, H. R. (Eds). 1987. Foraging Behavior. New York: Plenum Press. Keast, A. 1978. Feeding interactions between age-groups of pumpkinseed (Lepomis gibbosus) and comparisons with bluegill (L. macrochirus). J. Fish. Res. Bd Can., 35, I~27. Mackintosh, N. J. 1974. The Psychology of Animal Learning. London: Academic Press. Magurran, A. E. 1986. Individual differences in fish behavior. In: The Behaviour of Teleost Fishes (Ed. by T. J. Pitcher), pp. 338 365. London: Croom Helm. Mittelbach, G. G. 1981. Foraging efficiency and body size: a study of optimal diet and habitat use by bluegills. Ecology, 62, 1370-1386. Paszkowski, C. A. & Olla, B. L. 1985. Foraging behavior of hatchery-produced coho salmon (Oncorhynchus kisutch ) smolts on live prey. Can. J. Fish. Aquat. Sci., 42, 1915-1921. Ringler, N. H. 1979. Selective predation by drift feeding brown trout (Salmo trutta). J. Fish. Res. Bd Can., 36, 392-403. Ringler, N. H. 1983. Variation in foraging tactics of fishes. In: Predators' andPrey in Fishes (Ed. by D. L. G. Noakes, D. G. Linquist, G. S. Helfman & J. A. Ward), pp. 159-171. The Hague: Dr W. Junk. Rubenstein, D. L. 1981. Individual variation and competition, in Everglades pygmy sunfish. J. Anita. Ecol., 50, 337-350. SAS Institute. 1985. Users Guide: Statistics. Version 5 Edition. Cary, North Carolina: SAS Institute. Vinyard, G. L. 1980. Differential prey vulnerability and predator selectivity: effects of evasive prey on bluegills (Lepomis macrochirus) and pumpkinseed (L. gibbosus) predation. Can. J. Fish. Aquat. Sci., 39, 208-211. Ware, D. M. 1971. Predation by rainbow trout (Salmo gairdneri): the effect of experience. J. Fish. Res. Bd Can., 28, 1847-1852. Webb, P. W. 1984. Body form, locomotion, and foraging in aquatic vertebrates. Am. Zool., 24, 107-120. Werner, E. E., Mittelbach, G. G. & Hall, D. J. 1981. The role of foraging profitability and experience in habitat use by the bluegill sunfish. Ecology, 62, 116-125.