- Email: [email protected]
The effect of tunnel depth and of working in pairs on the speed of excavation in ants (Formica lemani bondroit)
Anita. Behav., 1971, 19, 677-686
THE EFFECT OF TUNNEL DEPTH AND OF WORKING IN PAIRS ON THE SPEED OF EXCAVATION IN ANTS (FORMICA LEMANI BONDROIT) BY J. H. SUDD The University, Hull Abstract. The time taken to transport spoil in longer tunnels significantly reduced the speed at which tunnels deeper than 8 mm were extended by single ants. The number of loads removed however was not significantly affected because ants removed fewer loads from the ends o f deep tunnels. When ants worked in pairs the tunnels were extended significantly more slowly than by a single ant at the same depth. The number of loads was not significantly less than with single ants; either of the ants might work at the end of the tunnel while its fellow worked higher up. Group working need not be in all ways less efficient than single working. This finding is discussed in relation to 'social facilitation'. The rate at which ants excavate soil is fairly easily measured either as mass, number of loads or tunnel extension, and has been used in interspecific comparisons by Sudd (1969, 1970a), and by Chen (1937) and Sakagami & Hayashida (1962) to compare ants working singly with ants in groups. The method by which ants dig, described briefly by Sakagami & Hayashida (1962) and in more detail by Sudd (1969, 1970b) has two main components, the removal of material from a workface by a 'grab', and its transport to the surface. The time an ant spends on these two components determines the number o f loads it removes in a given time, and this in turn, with the mean load size (or the mean tunnel extension per load) determines the mass of spoil (or the tunnel extension) produced. The time spent on the first component is very variable because of the inability of the ant to remove every grain it attempts (Sudd 1969). Transport time on the other hand should increase with the distance of transport so that deeper tunnels should have a lower yield and lower rate o f growth. The relationship with depth cannot be simple, as some 20 per cent of loads are collected from the walls of the tunnel, some distance above its end (Sudd 1970b). Observations on the effect of tunnel depth on the rate of digging and on the rate of'tunnel extension when single ants dug singly in sand are decribed below. In addition, as an obvious effect of placing ants to dig in pairs was an increase in the amount o f digging above the tunnel end, some observations on the effects of this simple interaction are described.
The cells were cut in a sheet of opaque plastic (Tufnol) 3 mm thick, and clamped vertically between a 10 mm Tufnol sheet and a sheet of plate glass. Parallax-free measurements of the depth at which each grab was made with reference to a millimetre scale stuck to the glass, were recorded at about 4 x magnification by closed circuit television (Sudd 1970b). To avoid differences in fatigue ants were given preformed tunnels of different depths. These tunnels were made by moulding straight lengths of wire (3 mm diameter) into the sand and removing them to leave tunnels of various lengths. In a tunnel 2-mm wide ants could not move easily and attacked the walls or mouth of the tunnel. Their behaviour in a tunnel 4-mm wide or more differed from that in an ant-made tunnel in that the ants were able to turn round and walk headfirst up the tunnel. They also attacked the wall while turning. The ants were placed singly or in pairs in the space above the sand. To increase the chance of the ant's digging in the tunnel strips of card were placed over the surface of the sand with their ends embedded in the sand near the tunnel mouth. About 20 per cent of ants set up in this way yielded usable records; the rest failed to dig, dug very sporadically, dug outside the tunnel or escaped.
Formica lemani Bondroit was chosen for these experiments because of its medium size and dark colour, and because of its strong attraction to preformed tunnels (Sudd 1970b) and its habit of digging straight tunnels (Sudd 1970a). Ants were collected from Broxa Forest, North Yorkshire and stored in jars of soil. 'Aggressive' ants (which attacked rather than avoided capture) were used in all experiments.
Methods
Ants were observed digging in cells 30 mm wide filled with moist sand to a depth of 70 mm. 677
678
ANIMAL
BEHAVIOUR,
Results The Rate of Digging of Single Ants in Tunnels of Various Depths Ants were placed singly in cells with tunnels designed to be 8, 16, 24 and 32 mm deep, with three replicates at each depth. Because of sandfalls and other faults these depths were not exact; the actual depth of each tunnel at the Start of observations is shown to the nearest 1 mm in Table I. The depth below the sand surface at which each grab was delivered was recorded to the nearest 1 mm for the first 30 min of digging, as well as the length of any pause when the ant remained outside the tunnel for 5 s or more. The number of these pauses and the total of their durations are shown in Table I. The remaining part of Table I was calculated as follows (Sudd 1970b). The depth of each grab was compared with a running maximum which at the start of the calculation was set to the initial depth of the tunnel. A grab whose depth was equal to or greater than the current running maximum was counted as an 'extension grab' and its depth replaced the current running maximum. A grab whose depth was less than the current running maximum was counted as a "deficit grab'. In extension grabs the ant's digging increases the depth of the tunnel; in deficit grabs it works on the tunnel walls. The distances from the points of delivery of deficit grabs to the end of the tunnel were totalled as the total deficit and averaged for all grabs as the mean deficit. The depths below the surface of all grabs were accumulated as the
19,
4
total run, which represents half the total distance moved by the ant in digging (or its transport work in load-mm). These totals are given in two forms: crude and corrected for pauses to the number or distance in 1 hour of actual digging. The mean values of each quantity at the different depths were compared by simple analysis of variance, followed by detailed comparisons using Tukey's t test. Two additional series of observations of ants in very short tunnels have been included as the last two lines of Table I. At these depths transport does not always occur (Sudd 1969) and these records are not comparable with those in the body of the table. They have not been included in the statistical analysis. The total of pauses differs significantly between depths (F=5.02, P<0.05). The significance arises from the high total pause in tunnels 8 and 32 mm deep compared with tunnels 16 and 24 mm deep. The number of pauses (longer than 5 s) also shows significant differences ( F = 19.22, P<0.001), with the number of pauses in 32-mm tunnels significantly higher than in any other tunnel depth; tunnels 24 mm deep also show significantly more pauses than tunnels 16 mm deep. The extension in length (crude growth) of the tunnels in 30 min observation differs significantly between depths ( F = 7 . 2 , P < 0 . 0 5 ) with growth in 8-mm tunnels signifiantly faster than in any other depth and no other significant differences. The difference in growth is not merely due to differences in pauses, for correction of the crude growth figures for varying amounts of pause only reduces the overall
150u y - 2.1071x -I- 2 . 8 6 9 4
100-
I--
U O3 f-
50-
O)
~o
i /
0
o
*
r = 0"4489
/
~ 9
lb
2b
Depthof Grab(rnm) Fig. 1. The time taken to descend the tunnel, collect a load and bring it to the surface (cycle time), in relation to the depth from which the load was brought.
24
27
25
33
32
31
24
24
24
32
32
32
6
8
9
4
2
1
1
2
60
60
5-0*
385
660
470
165
45
20
15
15
0
0
360
600
5
6
7"2*
2
2
3
3
10-3
12"4
4-1"
5
6-3
7.9
6.6
6'2
8"1
4 3
6"1
6"1
6
8
15
21
4
4
3
4
6
7
53
54
5"9*
8
23
18
13
20
29
32
26
19
42
41
33
110
112
6"1"
21
73
48
29
41
59
65
52
38
84
103
99
89
81
1"1
45
32
52
49
69
114
84
79
71
124
63
41
184
168
0'8
114
101
139
108
142
233
169
159
142
248
158
123
0"595
0"666
1-6
0"117
0.718
0.346
0.265
0.289
0"254
0.380
0.329
0.267
0.338
0.650
0.804
76
73
415
53
767
483
508
952
555
363
259
372
52
9
205
617
1.8
807
973
885
711
1225
1555
1126
1283
1321
1179
655
430
Total run (mm)
0-85
0"9
2'9
9.20
1"66
14"75
9.86
7.36
8"35
6-61
4.59
3-65
3.0
0'83
0.22
Mean deficit (ram)
'Corrected' quantities have been adjusted for the time the ant Spent in not digging. F is the variance ratio under the hypothesis that there are no differences between means at each depth. The last two lines summarize experiments without preformed tunnels.
8
12
33
34
36
28
30
28
24
1
3
20
16
24
0
0
21
16
25
0
5
22
16
16
2
6
12
8
17
2
0
11
8
17
No. of No. of No. of Proportion Total No. of Total Crude Corrected No. of extension all grabs all grabs of deficit pauses pause(s) growth growth extension grabs------crude ----tort- extension (mm) (mm) (mm) grabs---- corrected ected grabs crude (mm) (mm/hr)
19-2"
10
8
Final depth (ram)
F
Initial depth (ram)
Designed depth (mm)
Table I. The Effect of Tunnel Depth on the Digging Performance of Single Ants
XO
r~
O
~
680
ANIMAL
BEHAVIOUR,
level of significance (F----4-13, P<0.05). The difference in growth is not due to a greater crude total number of grabs, for these too do not differ significantly between depths (F:0.83, P>0.05), and this is little altered by correction for pauses (F--1.19, P>0.05). The crude number of extension grabs does however differ significantly between depths (F----5.93, P<0.05) and its significance is slightly raised by correction for pauses. Table I suggests that the growth is even faster in very short tunnels. The total run is not significantly affected by tunnel depth. The variation in the actual initial depth of tunnels at each designed depth must have increased the error mean squares. Analysis by correlation with the actual initial depths suggests some interesting points. The significant simple negative correlation between tunnel growth (corrected for pauses) and initial depth (r=--0.7100, P<0.01) disappears in partial correlation when the effect of the correlation of number of extension grabs and depth is eliminated (r= --0-3261, P>0.05). Elimination of the slight correlation of total grabs with depth has no effect. The proportion of extension grabs to all grabs is more important, though the partial correlation coefficient between growth and depth is still significant (r------0.6479, P<0.05) with this factor eliminated, and so is the higher partial with total grabs and the proportion of extension grabs both eliminated (r---- --0.6506, e<0.05). Thus there is a relationship between the rate of work and tunnel depth, but it applies only to work done at the full depth of the tunnel and apparently only in tunnels less than 16 mm deep. The effect is not accounted for by the difference in time spent at work as opposed to pauses, so the time spent in collecting each load and carrying it to the surface must vary with depth. This should produce an inverse relation between growth and depth, rather than a negative linear one. Recalculations of the correlations using reciprocal depth, and orthogonal subdivision of the between-depths sumof-squares show only a marginal advantage however, so that this relation could not be demonstrated. It was possible in some cases to measure the time an ant took to go down the tunnel, collect a load and return to the surface (cycle time), and in other cases the tim~ it took simply to transport a load it had already collected to the surface (transport time). Figures 1 and 2 show the relation of these times to the
19, 4
depth from which each load was carried. Although the calculated equation for transport time agrees with other observations in showing that transport time becomes zero at a depth of about 7 mm the 95 per cent confidence interval of the regression coefficient is 0.7815 and the corresponding limits of the x-intercept therefore about --19 to about 12. Similarly the equation of cycle time, which suggests a positive yintercept (equivalent to a depth-independent collection-time) has a true 95 per cent confidence interval at (x=0) of more than 30 s and so might have either a positive or negative y-intercept. The Rate of Digging of Pairs of Ants Preliminary observations with pairs of ants suggested that the two ants often dug at different depths in the tunnel. As working depth had been shown to be a significant factor in the rate of tunnel extension, the rate of working of pairs of ants was studied by methods very similar to those which have just been described for single ants. One ant in each pair was marked on the dorsum of its gaster with pale blue quickdrying plastic enamel: this had no adverse effects on its digging performance or survival up to two days. The ants were provided with artificial tunnels of designed depth 16 mm and observations began as soon as both ants were digging consistently. The proportion of trials in which this happened was again about 20 per cent. Sometimes, however, the tunnel had been lengthened appreciably by one ant before the other began to dig. These experiments confirmed the impression from earlier observations that ants dug at different depths. When, as in these experiments, both ants were obliged to dig in a single tunnel, encounters between ants were of several kinds. First the two ants might meet at the tunnel mouth, either when both were going down or both coming up. In seven half-hour periods of observation, during which 718 cycles were observed, this was recorded only six times. When both ants were entering the tunnel the result was always that one ant at least was turned back and did not begin to dig; interference when both were coming up was less severe. Second, the ants might meet during movement within the tunnel, either in opposite directions or both transporting upwards. This occurred forty-nine times. Ants travelling in opposite directions were able to pass and this rarely led to an ant's stopping digging. If both were transporting soil upwards they simply continued
SUDD: SPEED OF EXCAVATION IN ANTS
681
lapse photographs of tunnels dug naturally by F. lemani singly and in pairs gave dimensions appreciably smaller than this perhaps because of light and shade effects. For single ants these ranged from 1 to 2 mm and for pairs from 1.5 to 3 mm; six measurements of each differed significantly at the 0.01 level (Mann-Whitney test). In general, comparisons have been made between paired ants (considered singly) (Table III) and those single ants in groups which did not differ significantly among themselves (usually tunnel depths 16, 24 and 32 mm). The MannWhitney test has been used in these comparisons. The total of pauses is significantly more for ants in pairs than for single ants (P<0.02) and this conclusion is not altered even if the significantly high values for single ants in 8- and 32-mm tunnels are also included. It is surprising that the activity of ants in the pair should be reduced. Further work, to be published shortly shows that when periods of 24 hr or more are considered pauses are not always longer in pairs than in single ants. It must be remembered that if a single ant had paused as much as some of the ants in pairs observations on it might well have been abandoned. There was no significant difference between single ants and the more active ant in each pair, and the lower
to move up in tandem; probably on some occasions the upper ant took its load just as it was touched by the ant from below. The third and most common interaction occurred when an ant travelling downwards found the other ant already digging lower in the tunnel. This happended 255 times. In nearly every case the upper ant dug at the lowest level it could reach without colliding with the lower ant, at a point of grab 4 to 7 mm higher than the lower ant. Figure 3a shows a short extract from records which has some fairly typical features. There was no tendency for one ant to be 'upper' or 'lower' consistently, and the higher ant was often able to 'nest' more than one of its cycles within a single cycle of its workmate. Figure 3b shows a short period during the untypical experiment D1A in which the unmarked ant made only three grabs. These forced the marked ant to make five deficit grabs at points 6 and 14 mm above the end of the tunnel. During the remaining 21.75 min of observation the marked ant made predominantly extension grabs. The five forced deficit grabs accounted for almost half the total deficit for the marked ant. The result of these extra deficit grabs was an irregular widening of the tunnels dug by pairs of ants, to about 4 mm from the 3 mm provided in artificial tunnels. Preliminary time50-
40"G" 30" y = 1.0689x
v
r
E "C
o r~
--
7,2261
= 0.6938
9
20100 .
o .
. .
.
.
.
~
I
/
~
lb 20 Depthof Grab{rnm) I
30
Fig. 2. The time taken to bring a load to the surface from a given depth (transport time) in relation to that depth.
|
16
21
14
15
15
12
17
D1A marked Unmarked Total
D2marked Unmarked Total
D3marked Unmarked Total
D4 marked Unmarked Total
D5 marked Unmarked Total
D6 marked Unmarked Total
Initial depth (mm)
Dlmarked Unmarked Total
Experiment number Ant
20
17
18
17
17
23
18
Final depth (mm)
7 3 10
1 3 4
3 5 8
) ~
/ ~
/ ~
8 7 15
2 6 8
5 7 12
] 4 ~ 13 17
/ ~
/ ~
/ ~
No. of pauses
630 260 890
105 195 300
400 515 915
175 670 845
435 170 605
45 1655 1700
5 605 690
Total pauses (s)
2 1 3
4 1 5
1 2 3
0 2 2
1 2 3
2 0 2'
1 1 2
62 2.3
8.5 2.2
2.6 5.6
0 6.4
2.6 4.4
4.1 0
2.0 3.0
12 10 22
18 15 33
12 20 32
11 5 16
8 5 13
19 2 21
8 2 10
Crude Corrected No. of growth growth extension (mm) (mm/hr) grabsm crude
36.9 23.4
38-2 33.6
30.9 56-0
24.4 15.9
21.1 11.4
38.9 49.6
16.8 6.0
70 66 136
85 66 151
52 45 97
79 41 I20
63 44 107
36 3 39
64 24 88
215.3 154.3
180-5 148.0
133.7 126.0
175.0 130.6
166.1 97.2
73 8 74.5
134-3 79.3
0.171 0.152
0.212 0.227
0.231 0.444
0.139 0.122
0.127 0.114
0.520 0.660
0.125 0-083
513 574
382 320
243 217
622 553
374 241
78 50
505 260
No. of N o . of No. of Proportion Total extension all all of deficit grabs-- grabs-- grabs-extension (mm) corrected crude corrected grabs
Table H. The Digging Performance of Ants Working in Pairs
790 647
835 603
610 527
738 316
636 462
708 59
568 144
Total run (mm)
8.48 1.03
5.70 6.27
6.08 8.68
8.25 6-39
6.80 6.18
2.17 16.6
9-01 11.81
Mean deficit (mm)
4~
7~
,<
r*
Z
S U D D : SPEED O F E X C A V A T I O N IN ANTS
0
O~
oo~
z_~
~.~~
u
io
683
6o
66
66
66
66
66
66
684
ANIMAL
BEHAVIOUR,
activity of the other ant may only reflect the difficulty of selecting, in advance, two ants that are both likely to dig well. The total number of grabs per hour (corrected for pauses) did not differ significantly between paired and single ants ( P > 0 . 0 5 ) but was slightly greater in single ants. The number of extension grabs (corrected in the same way) was however higher in single ants than in ants in pairs ( P < 0.02), and the corrected tunnel extension
19,
4
acbieved by single ants was significantly greater than that of each of the ants in pairs taken singly (/'<0-02). The extension achieved by the paired ants together was not significantly different from that of single ants ( P > 0 . 0 5 ) ; but this pooled extension was significantly less than twice the extension of single ants (P<0.002). Thus interference between the ants in a pair made itself felt in tunnel extension but not in the total of loads removed. Even in tunnel extension it
E fi
8
d2
~t. 02 r~
A
E 8
t=
&.
Fig. 3. The vertical location of successivegrabs in experiments with paired ants. Black spots and whole lines show the grabs and movements of the marked ant; white dots and dotted lines those of the unmarked. Where lines cross one another the ants met during movement up or down tho tunnel. Duration of (a) about 6 rain, of (b) about 10 rain.
SLIDD: SPEED OF EXCAVATION IN ANTS was not possible to show that the tunnel was extended less by two ants than by one. Table III has some interesting features, apart from comparisons with ants working singly. There is a marked degree of correlation between the scores of the two ants in each pair for proportion of extension grabs (r=0.94; P<0.01), total grabs (r=0.76, P<0.05), total deficit (r=0"80, P<0.05) and mean deficit (r=0.93, P<0.01). This co-ordination between the two ants in a pair does not necessarily imply a direct effect of the behaviour of one ant on that of the other; and the absence of negative correlations between these quantities suggests that interference effects are absent, or more probaby balanced, with neither ant dominating the other. The positive correlations might then be the result of similar response by both ants in the pair to details of tunnel form and relief, producing similar proportions of deficit grabs concentrated at the same points in the tunnel. Total pauses, however, show a different relationship with a negative and insignificant correlation between the scores of the ants in a pair ( r = - - 0 - 5 6 , P > 0 . 1 ) . There is therefore no evidence that the greater the activity of one ant the greater that of the other. Discussion The amount of spoil an ant brings to the surface is equal to the number o f transport trips it makes, multiplied by the mean load it carried. The number of transport trips in turn depends on the ratio of the time spent on actual digging (after subtraction of pauses) to the mean time taken to go down the tunnel, collect a load and bring it to the surface. The ratio of digging time to total time changes as digging progresses; this will be treated in more detail in a later paper based on the automatic recording of digging over long periods. As far as observation periods of 30 rain are concerned ants spent less time in digging when their tunnels were 32 mm deep than in shorter tunnels and paused more frequently in 32- and 24-ram tunnels than in shorter ones. This difference accounts for the apparent, but insignificant reduction in tunnel growth in 32-ram tunnels. The second component, time taken to travel up and down the tunnel, does, as would be expected, increase with the distance travelled (Fig. 1). Provided correction is made for the time spent in not digging, therefore, the rate of work should be slower in deep tunnels than in shallow ones. This simple relationship does not
685
exist because, in Formica lemani and Lasius niger (and no doubt in other ants), a proportion o f loads are collected above the tunnel's end (Sudd 1970b). The rate of working can then be measured in two ways: by the total number of loads, related to the mass excavated or by the number of extension grabs, related to the growth of the tunnel. The second of these is significantly correlated with tunnel depth, with over 50 per cent of its variation accounted for by variation in tunnel depth: the first however is little affected. This independence of total loads from tunnel depth is achieved by an increase in the proportion of loads taken from above the end of the tunnel ('deficit grabs') in tunnels 16 mm or more deep. Sudd (1970b), trying to explain differences between Lasius niger and Formica lemani, failed to find a significant effect of tunnel depth on this proportion because his results referred to tunnels less than 16 mm deep. He also recorded much smaller mean deficits and larger proportions of extension grabs: this may have been because he used natural tunnels, not artificial ones. Flaws in the walls of artificial tunnels seemed to attract grabs, and might have been eliminated earlier during the excavation of natural tunnels. Chen (1937) reported that the mass of soil, and the number of loads, excavated by Carnponotus japonicus workers in pairs was greater than the sum of the outputs of the same individuals working singly. Chert ascribed this result to the occurrence of 'social facilitation' in ants, but more recently several authors have suggested that Chen's findings were exceptional, both in the sense that Sakagami & Hayashida (1962) obtained a contrary result with Formica fusea; and in the sense that in many other social tasks the output of groups is inferior to that of single ants (Wilson 1966; Sudd 1967; Chauvin 1969). If the validity o f Chen's experiments is accepted two problems remain: how was the productivity of pairs increased and could this same mechanism operate for other tasks or other species of ant ? Two possible mechanisms seem to exist. The first favoured by Chen himself and by Chauvin (1944), is that the increase in productivity came from the utilization of 'idle time' or pauses. The other possibility is that pairs of ants operate more efficient techniques of digging than single ones, or at least that their techniques are not much less efficient. This would accord with general views of 'co-operation' such as
686
ANIMAL
BEHAVIOUR,
those put forward by Sudd (1960, 1965). Techniques based on the division of labour (e.g. one ant digs and the other carries) cannot increase output for ants of similar powers. Techniques based on the spatial arrangement of ants remain. The observation, that one ant of a pair usually dug at a higher level in their shared tunnel, may be coupled with the finding that ants working singly overcame the effect of tunnel depth on load production (though not on tunnel extension) by an increase in the proportion of deficit grabs. I f pairs of ants collect more loads high in their tunnels, their rate of load production could be similarly protected from the interference of one ant with another. Comparison of Tables I and I I I shows that the tunnel growth produced by each ant in a pair is significantly less than that of single ants in tunnels o f similar depth (i.e. 16 and 24 mm), and the pooled extension of each pair just misses being significantly less than the extensions of these single ants. The corrected number of extension grabs is also significantly less in ants working in pairs. Neither the crude or corrected total of all grabs however differ significantly between paired and single ants. The 'interference' between ants working in pairs thus makes itself felt in tunnel extension, but not, or hardly at all, in total loads. Consequently, if the mean load is the same for both single and paired ants the mass excavated would likewise be little affected. It m a y be concluded that, at worst, interference of this
19,
4
kind would not nullify any other advantages of pair working, for instance, increased working time. Further, this effect arises from special features of the digging situation and could not apply to, for instance, prey transport or brood care. REFERENCES
Chauvin, R. (1944). L'effet du groupe et la regulation de l'activite social chez les fourmis du genre Leptothorax. Bull. biol., 78, 197-205. Chauvin, R. (1969). Le Monde des Fourmis. Paris: Libraire Plon. Cben, S. C. (1937). Social modification of the activity of ants in nest building. Physiol. Zool., 10, 420436. Sakagami, S. F. & Hayashida, K. (1962). Work efficiency in heterospecific ant groups composed of hosts and their labour parasites. Anim. Behav., 10, 96-105. Sudd, J. H. (1960). Transport of prey by an ant, Pheidole crassinoda Em. Behaviour, 16, 295-308. Sudd, J. H. (1965). The transport of prey by ants. Behaviour, 25, 234-271. Sudd, J. H. (1967). Introduction to the Behaviour of Ants. London: Edward Arnold. Sudd, J. H. (1969). The excavation of soil by ants. Z. Tierpsychol., 26, 257-276. Sudd, J. H. (1970a). Specific patterns of excavation in isolated ants. Insectes Sociaux, 27 (4), 253-260. Sudd, J. H. (1970b). The response of isolated digging worker ants (Formica lemani Bondroit and Lasius niger (L.)) to tunnels. Insectes Sociaux, 27, (4), 261-270. Wilson, E. O. (1966). Behaviour of social insects. Syrup. Roy. ent. Soc., London, 3, 81-96. (Received 13 January 1971; revised 2 April 1971; MS. number: 1030)
THE EFFECT OF TUNNEL DEPTH AND OF WORKING IN PAIRS ON THE SPEED OF EXCAVATION IN ANTS (FORMICA LEMANI BONDROIT) BY J. H. SUDD The University, Hull Abstract. The time taken to transport spoil in longer tunnels significantly reduced the speed at which tunnels deeper than 8 mm were extended by single ants. The number of loads removed however was not significantly affected because ants removed fewer loads from the ends o f deep tunnels. When ants worked in pairs the tunnels were extended significantly more slowly than by a single ant at the same depth. The number of loads was not significantly less than with single ants; either of the ants might work at the end of the tunnel while its fellow worked higher up. Group working need not be in all ways less efficient than single working. This finding is discussed in relation to 'social facilitation'. The rate at which ants excavate soil is fairly easily measured either as mass, number of loads or tunnel extension, and has been used in interspecific comparisons by Sudd (1969, 1970a), and by Chen (1937) and Sakagami & Hayashida (1962) to compare ants working singly with ants in groups. The method by which ants dig, described briefly by Sakagami & Hayashida (1962) and in more detail by Sudd (1969, 1970b) has two main components, the removal of material from a workface by a 'grab', and its transport to the surface. The time an ant spends on these two components determines the number o f loads it removes in a given time, and this in turn, with the mean load size (or the mean tunnel extension per load) determines the mass of spoil (or the tunnel extension) produced. The time spent on the first component is very variable because of the inability of the ant to remove every grain it attempts (Sudd 1969). Transport time on the other hand should increase with the distance of transport so that deeper tunnels should have a lower yield and lower rate o f growth. The relationship with depth cannot be simple, as some 20 per cent of loads are collected from the walls of the tunnel, some distance above its end (Sudd 1970b). Observations on the effect of tunnel depth on the rate of digging and on the rate of'tunnel extension when single ants dug singly in sand are decribed below. In addition, as an obvious effect of placing ants to dig in pairs was an increase in the amount o f digging above the tunnel end, some observations on the effects of this simple interaction are described.
The cells were cut in a sheet of opaque plastic (Tufnol) 3 mm thick, and clamped vertically between a 10 mm Tufnol sheet and a sheet of plate glass. Parallax-free measurements of the depth at which each grab was made with reference to a millimetre scale stuck to the glass, were recorded at about 4 x magnification by closed circuit television (Sudd 1970b). To avoid differences in fatigue ants were given preformed tunnels of different depths. These tunnels were made by moulding straight lengths of wire (3 mm diameter) into the sand and removing them to leave tunnels of various lengths. In a tunnel 2-mm wide ants could not move easily and attacked the walls or mouth of the tunnel. Their behaviour in a tunnel 4-mm wide or more differed from that in an ant-made tunnel in that the ants were able to turn round and walk headfirst up the tunnel. They also attacked the wall while turning. The ants were placed singly or in pairs in the space above the sand. To increase the chance of the ant's digging in the tunnel strips of card were placed over the surface of the sand with their ends embedded in the sand near the tunnel mouth. About 20 per cent of ants set up in this way yielded usable records; the rest failed to dig, dug very sporadically, dug outside the tunnel or escaped.
Formica lemani Bondroit was chosen for these experiments because of its medium size and dark colour, and because of its strong attraction to preformed tunnels (Sudd 1970b) and its habit of digging straight tunnels (Sudd 1970a). Ants were collected from Broxa Forest, North Yorkshire and stored in jars of soil. 'Aggressive' ants (which attacked rather than avoided capture) were used in all experiments.
Methods
Ants were observed digging in cells 30 mm wide filled with moist sand to a depth of 70 mm. 677
678
ANIMAL
BEHAVIOUR,
Results The Rate of Digging of Single Ants in Tunnels of Various Depths Ants were placed singly in cells with tunnels designed to be 8, 16, 24 and 32 mm deep, with three replicates at each depth. Because of sandfalls and other faults these depths were not exact; the actual depth of each tunnel at the Start of observations is shown to the nearest 1 mm in Table I. The depth below the sand surface at which each grab was delivered was recorded to the nearest 1 mm for the first 30 min of digging, as well as the length of any pause when the ant remained outside the tunnel for 5 s or more. The number of these pauses and the total of their durations are shown in Table I. The remaining part of Table I was calculated as follows (Sudd 1970b). The depth of each grab was compared with a running maximum which at the start of the calculation was set to the initial depth of the tunnel. A grab whose depth was equal to or greater than the current running maximum was counted as an 'extension grab' and its depth replaced the current running maximum. A grab whose depth was less than the current running maximum was counted as a "deficit grab'. In extension grabs the ant's digging increases the depth of the tunnel; in deficit grabs it works on the tunnel walls. The distances from the points of delivery of deficit grabs to the end of the tunnel were totalled as the total deficit and averaged for all grabs as the mean deficit. The depths below the surface of all grabs were accumulated as the
19,
4
total run, which represents half the total distance moved by the ant in digging (or its transport work in load-mm). These totals are given in two forms: crude and corrected for pauses to the number or distance in 1 hour of actual digging. The mean values of each quantity at the different depths were compared by simple analysis of variance, followed by detailed comparisons using Tukey's t test. Two additional series of observations of ants in very short tunnels have been included as the last two lines of Table I. At these depths transport does not always occur (Sudd 1969) and these records are not comparable with those in the body of the table. They have not been included in the statistical analysis. The total of pauses differs significantly between depths (F=5.02, P<0.05). The significance arises from the high total pause in tunnels 8 and 32 mm deep compared with tunnels 16 and 24 mm deep. The number of pauses (longer than 5 s) also shows significant differences ( F = 19.22, P<0.001), with the number of pauses in 32-mm tunnels significantly higher than in any other tunnel depth; tunnels 24 mm deep also show significantly more pauses than tunnels 16 mm deep. The extension in length (crude growth) of the tunnels in 30 min observation differs significantly between depths ( F = 7 . 2 , P < 0 . 0 5 ) with growth in 8-mm tunnels signifiantly faster than in any other depth and no other significant differences. The difference in growth is not merely due to differences in pauses, for correction of the crude growth figures for varying amounts of pause only reduces the overall
150u y - 2.1071x -I- 2 . 8 6 9 4
100-
I--
U O3 f-
50-
O)
~o
i /
0
o
*
r = 0"4489
/
~ 9
lb
2b
Depthof Grab(rnm) Fig. 1. The time taken to descend the tunnel, collect a load and bring it to the surface (cycle time), in relation to the depth from which the load was brought.
24
27
25
33
32
31
24
24
24
32
32
32
6
8
9
4
2
1
1
2
60
60
5-0*
385
660
470
165
45
20
15
15
0
0
360
600
5
6
7"2*
2
2
3
3
10-3
12"4
4-1"
5
6-3
7.9
6.6
6'2
8"1
4 3
6"1
6"1
6
8
15
21
4
4
3
4
6
7
53
54
5"9*
8
23
18
13
20
29
32
26
19
42
41
33
110
112
6"1"
21
73
48
29
41
59
65
52
38
84
103
99
89
81
1"1
45
32
52
49
69
114
84
79
71
124
63
41
184
168
0'8
114
101
139
108
142
233
169
159
142
248
158
123
0"595
0"666
1-6
0"117
0.718
0.346
0.265
0.289
0"254
0.380
0.329
0.267
0.338
0.650
0.804
76
73
415
53
767
483
508
952
555
363
259
372
52
9
205
617
1.8
807
973
885
711
1225
1555
1126
1283
1321
1179
655
430
Total run (mm)
0-85
0"9
2'9
9.20
1"66
14"75
9.86
7.36
8"35
6-61
4.59
3-65
3.0
0'83
0.22
Mean deficit (ram)
'Corrected' quantities have been adjusted for the time the ant Spent in not digging. F is the variance ratio under the hypothesis that there are no differences between means at each depth. The last two lines summarize experiments without preformed tunnels.
8
12
33
34
36
28
30
28
24
1
3
20
16
24
0
0
21
16
25
0
5
22
16
16
2
6
12
8
17
2
0
11
8
17
No. of No. of No. of Proportion Total No. of Total Crude Corrected No. of extension all grabs all grabs of deficit pauses pause(s) growth growth extension grabs------crude ----tort- extension (mm) (mm) (mm) grabs---- corrected ected grabs crude (mm) (mm/hr)
19-2"
10
8
Final depth (ram)
F
Initial depth (ram)
Designed depth (mm)
Table I. The Effect of Tunnel Depth on the Digging Performance of Single Ants
XO
r~
O
~
680
ANIMAL
BEHAVIOUR,
level of significance (F----4-13, P<0.05). The difference in growth is not due to a greater crude total number of grabs, for these too do not differ significantly between depths (F:0.83, P>0.05), and this is little altered by correction for pauses (F--1.19, P>0.05). The crude number of extension grabs does however differ significantly between depths (F----5.93, P<0.05) and its significance is slightly raised by correction for pauses. Table I suggests that the growth is even faster in very short tunnels. The total run is not significantly affected by tunnel depth. The variation in the actual initial depth of tunnels at each designed depth must have increased the error mean squares. Analysis by correlation with the actual initial depths suggests some interesting points. The significant simple negative correlation between tunnel growth (corrected for pauses) and initial depth (r=--0.7100, P<0.01) disappears in partial correlation when the effect of the correlation of number of extension grabs and depth is eliminated (r= --0-3261, P>0.05). Elimination of the slight correlation of total grabs with depth has no effect. The proportion of extension grabs to all grabs is more important, though the partial correlation coefficient between growth and depth is still significant (r------0.6479, P<0.05) with this factor eliminated, and so is the higher partial with total grabs and the proportion of extension grabs both eliminated (r---- --0.6506, e<0.05). Thus there is a relationship between the rate of work and tunnel depth, but it applies only to work done at the full depth of the tunnel and apparently only in tunnels less than 16 mm deep. The effect is not accounted for by the difference in time spent at work as opposed to pauses, so the time spent in collecting each load and carrying it to the surface must vary with depth. This should produce an inverse relation between growth and depth, rather than a negative linear one. Recalculations of the correlations using reciprocal depth, and orthogonal subdivision of the between-depths sumof-squares show only a marginal advantage however, so that this relation could not be demonstrated. It was possible in some cases to measure the time an ant took to go down the tunnel, collect a load and return to the surface (cycle time), and in other cases the tim~ it took simply to transport a load it had already collected to the surface (transport time). Figures 1 and 2 show the relation of these times to the
19, 4
depth from which each load was carried. Although the calculated equation for transport time agrees with other observations in showing that transport time becomes zero at a depth of about 7 mm the 95 per cent confidence interval of the regression coefficient is 0.7815 and the corresponding limits of the x-intercept therefore about --19 to about 12. Similarly the equation of cycle time, which suggests a positive yintercept (equivalent to a depth-independent collection-time) has a true 95 per cent confidence interval at (x=0) of more than 30 s and so might have either a positive or negative y-intercept. The Rate of Digging of Pairs of Ants Preliminary observations with pairs of ants suggested that the two ants often dug at different depths in the tunnel. As working depth had been shown to be a significant factor in the rate of tunnel extension, the rate of working of pairs of ants was studied by methods very similar to those which have just been described for single ants. One ant in each pair was marked on the dorsum of its gaster with pale blue quickdrying plastic enamel: this had no adverse effects on its digging performance or survival up to two days. The ants were provided with artificial tunnels of designed depth 16 mm and observations began as soon as both ants were digging consistently. The proportion of trials in which this happened was again about 20 per cent. Sometimes, however, the tunnel had been lengthened appreciably by one ant before the other began to dig. These experiments confirmed the impression from earlier observations that ants dug at different depths. When, as in these experiments, both ants were obliged to dig in a single tunnel, encounters between ants were of several kinds. First the two ants might meet at the tunnel mouth, either when both were going down or both coming up. In seven half-hour periods of observation, during which 718 cycles were observed, this was recorded only six times. When both ants were entering the tunnel the result was always that one ant at least was turned back and did not begin to dig; interference when both were coming up was less severe. Second, the ants might meet during movement within the tunnel, either in opposite directions or both transporting upwards. This occurred forty-nine times. Ants travelling in opposite directions were able to pass and this rarely led to an ant's stopping digging. If both were transporting soil upwards they simply continued
SUDD: SPEED OF EXCAVATION IN ANTS
681
lapse photographs of tunnels dug naturally by F. lemani singly and in pairs gave dimensions appreciably smaller than this perhaps because of light and shade effects. For single ants these ranged from 1 to 2 mm and for pairs from 1.5 to 3 mm; six measurements of each differed significantly at the 0.01 level (Mann-Whitney test). In general, comparisons have been made between paired ants (considered singly) (Table III) and those single ants in groups which did not differ significantly among themselves (usually tunnel depths 16, 24 and 32 mm). The MannWhitney test has been used in these comparisons. The total of pauses is significantly more for ants in pairs than for single ants (P<0.02) and this conclusion is not altered even if the significantly high values for single ants in 8- and 32-mm tunnels are also included. It is surprising that the activity of ants in the pair should be reduced. Further work, to be published shortly shows that when periods of 24 hr or more are considered pauses are not always longer in pairs than in single ants. It must be remembered that if a single ant had paused as much as some of the ants in pairs observations on it might well have been abandoned. There was no significant difference between single ants and the more active ant in each pair, and the lower
to move up in tandem; probably on some occasions the upper ant took its load just as it was touched by the ant from below. The third and most common interaction occurred when an ant travelling downwards found the other ant already digging lower in the tunnel. This happended 255 times. In nearly every case the upper ant dug at the lowest level it could reach without colliding with the lower ant, at a point of grab 4 to 7 mm higher than the lower ant. Figure 3a shows a short extract from records which has some fairly typical features. There was no tendency for one ant to be 'upper' or 'lower' consistently, and the higher ant was often able to 'nest' more than one of its cycles within a single cycle of its workmate. Figure 3b shows a short period during the untypical experiment D1A in which the unmarked ant made only three grabs. These forced the marked ant to make five deficit grabs at points 6 and 14 mm above the end of the tunnel. During the remaining 21.75 min of observation the marked ant made predominantly extension grabs. The five forced deficit grabs accounted for almost half the total deficit for the marked ant. The result of these extra deficit grabs was an irregular widening of the tunnels dug by pairs of ants, to about 4 mm from the 3 mm provided in artificial tunnels. Preliminary time50-
40"G" 30" y = 1.0689x
v
r
E "C
o r~
--
7,2261
= 0.6938
9
20100 .
o .
. .
.
.
.
~
I
/
~
lb 20 Depthof Grab{rnm) I
30
Fig. 2. The time taken to bring a load to the surface from a given depth (transport time) in relation to that depth.
|
16
21
14
15
15
12
17
D1A marked Unmarked Total
D2marked Unmarked Total
D3marked Unmarked Total
D4 marked Unmarked Total
D5 marked Unmarked Total
D6 marked Unmarked Total
Initial depth (mm)
Dlmarked Unmarked Total
Experiment number Ant
20
17
18
17
17
23
18
Final depth (mm)
7 3 10
1 3 4
3 5 8
) ~
/ ~
/ ~
8 7 15
2 6 8
5 7 12
] 4 ~ 13 17
/ ~
/ ~
/ ~
No. of pauses
630 260 890
105 195 300
400 515 915
175 670 845
435 170 605
45 1655 1700
5 605 690
Total pauses (s)
2 1 3
4 1 5
1 2 3
0 2 2
1 2 3
2 0 2'
1 1 2
62 2.3
8.5 2.2
2.6 5.6
0 6.4
2.6 4.4
4.1 0
2.0 3.0
12 10 22
18 15 33
12 20 32
11 5 16
8 5 13
19 2 21
8 2 10
Crude Corrected No. of growth growth extension (mm) (mm/hr) grabsm crude
36.9 23.4
38-2 33.6
30.9 56-0
24.4 15.9
21.1 11.4
38.9 49.6
16.8 6.0
70 66 136
85 66 151
52 45 97
79 41 I20
63 44 107
36 3 39
64 24 88
215.3 154.3
180-5 148.0
133.7 126.0
175.0 130.6
166.1 97.2
73 8 74.5
134-3 79.3
0.171 0.152
0.212 0.227
0.231 0.444
0.139 0.122
0.127 0.114
0.520 0.660
0.125 0-083
513 574
382 320
243 217
622 553
374 241
78 50
505 260
No. of N o . of No. of Proportion Total extension all all of deficit grabs-- grabs-- grabs-extension (mm) corrected crude corrected grabs
Table H. The Digging Performance of Ants Working in Pairs
790 647
835 603
610 527
738 316
636 462
708 59
568 144
Total run (mm)
8.48 1.03
5.70 6.27
6.08 8.68
8.25 6-39
6.80 6.18
2.17 16.6
9-01 11.81
Mean deficit (mm)
4~
7~
,<
r*
Z
S U D D : SPEED O F E X C A V A T I O N IN ANTS
0
O~
oo~
z_~
~.~~
u
io
683
6o
66
66
66
66
66
66
684
ANIMAL
BEHAVIOUR,
activity of the other ant may only reflect the difficulty of selecting, in advance, two ants that are both likely to dig well. The total number of grabs per hour (corrected for pauses) did not differ significantly between paired and single ants ( P > 0 . 0 5 ) but was slightly greater in single ants. The number of extension grabs (corrected in the same way) was however higher in single ants than in ants in pairs ( P < 0.02), and the corrected tunnel extension
19,
4
acbieved by single ants was significantly greater than that of each of the ants in pairs taken singly (/'<0-02). The extension achieved by the paired ants together was not significantly different from that of single ants ( P > 0 . 0 5 ) ; but this pooled extension was significantly less than twice the extension of single ants (P<0.002). Thus interference between the ants in a pair made itself felt in tunnel extension but not in the total of loads removed. Even in tunnel extension it
E fi
8
d2
~t. 02 r~
A
E 8
t=
&.
Fig. 3. The vertical location of successivegrabs in experiments with paired ants. Black spots and whole lines show the grabs and movements of the marked ant; white dots and dotted lines those of the unmarked. Where lines cross one another the ants met during movement up or down tho tunnel. Duration of (a) about 6 rain, of (b) about 10 rain.
SLIDD: SPEED OF EXCAVATION IN ANTS was not possible to show that the tunnel was extended less by two ants than by one. Table III has some interesting features, apart from comparisons with ants working singly. There is a marked degree of correlation between the scores of the two ants in each pair for proportion of extension grabs (r=0.94; P<0.01), total grabs (r=0.76, P<0.05), total deficit (r=0"80, P<0.05) and mean deficit (r=0.93, P<0.01). This co-ordination between the two ants in a pair does not necessarily imply a direct effect of the behaviour of one ant on that of the other; and the absence of negative correlations between these quantities suggests that interference effects are absent, or more probaby balanced, with neither ant dominating the other. The positive correlations might then be the result of similar response by both ants in the pair to details of tunnel form and relief, producing similar proportions of deficit grabs concentrated at the same points in the tunnel. Total pauses, however, show a different relationship with a negative and insignificant correlation between the scores of the ants in a pair ( r = - - 0 - 5 6 , P > 0 . 1 ) . There is therefore no evidence that the greater the activity of one ant the greater that of the other. Discussion The amount of spoil an ant brings to the surface is equal to the number o f transport trips it makes, multiplied by the mean load it carried. The number of transport trips in turn depends on the ratio of the time spent on actual digging (after subtraction of pauses) to the mean time taken to go down the tunnel, collect a load and bring it to the surface. The ratio of digging time to total time changes as digging progresses; this will be treated in more detail in a later paper based on the automatic recording of digging over long periods. As far as observation periods of 30 rain are concerned ants spent less time in digging when their tunnels were 32 mm deep than in shorter tunnels and paused more frequently in 32- and 24-ram tunnels than in shorter ones. This difference accounts for the apparent, but insignificant reduction in tunnel growth in 32-ram tunnels. The second component, time taken to travel up and down the tunnel, does, as would be expected, increase with the distance travelled (Fig. 1). Provided correction is made for the time spent in not digging, therefore, the rate of work should be slower in deep tunnels than in shallow ones. This simple relationship does not
685
exist because, in Formica lemani and Lasius niger (and no doubt in other ants), a proportion o f loads are collected above the tunnel's end (Sudd 1970b). The rate of working can then be measured in two ways: by the total number of loads, related to the mass excavated or by the number of extension grabs, related to the growth of the tunnel. The second of these is significantly correlated with tunnel depth, with over 50 per cent of its variation accounted for by variation in tunnel depth: the first however is little affected. This independence of total loads from tunnel depth is achieved by an increase in the proportion of loads taken from above the end of the tunnel ('deficit grabs') in tunnels 16 mm or more deep. Sudd (1970b), trying to explain differences between Lasius niger and Formica lemani, failed to find a significant effect of tunnel depth on this proportion because his results referred to tunnels less than 16 mm deep. He also recorded much smaller mean deficits and larger proportions of extension grabs: this may have been because he used natural tunnels, not artificial ones. Flaws in the walls of artificial tunnels seemed to attract grabs, and might have been eliminated earlier during the excavation of natural tunnels. Chen (1937) reported that the mass of soil, and the number of loads, excavated by Carnponotus japonicus workers in pairs was greater than the sum of the outputs of the same individuals working singly. Chert ascribed this result to the occurrence of 'social facilitation' in ants, but more recently several authors have suggested that Chen's findings were exceptional, both in the sense that Sakagami & Hayashida (1962) obtained a contrary result with Formica fusea; and in the sense that in many other social tasks the output of groups is inferior to that of single ants (Wilson 1966; Sudd 1967; Chauvin 1969). If the validity o f Chen's experiments is accepted two problems remain: how was the productivity of pairs increased and could this same mechanism operate for other tasks or other species of ant ? Two possible mechanisms seem to exist. The first favoured by Chen himself and by Chauvin (1944), is that the increase in productivity came from the utilization of 'idle time' or pauses. The other possibility is that pairs of ants operate more efficient techniques of digging than single ones, or at least that their techniques are not much less efficient. This would accord with general views of 'co-operation' such as
686
ANIMAL
BEHAVIOUR,
those put forward by Sudd (1960, 1965). Techniques based on the division of labour (e.g. one ant digs and the other carries) cannot increase output for ants of similar powers. Techniques based on the spatial arrangement of ants remain. The observation, that one ant of a pair usually dug at a higher level in their shared tunnel, may be coupled with the finding that ants working singly overcame the effect of tunnel depth on load production (though not on tunnel extension) by an increase in the proportion of deficit grabs. I f pairs of ants collect more loads high in their tunnels, their rate of load production could be similarly protected from the interference of one ant with another. Comparison of Tables I and I I I shows that the tunnel growth produced by each ant in a pair is significantly less than that of single ants in tunnels o f similar depth (i.e. 16 and 24 mm), and the pooled extension of each pair just misses being significantly less than the extensions of these single ants. The corrected number of extension grabs is also significantly less in ants working in pairs. Neither the crude or corrected total of all grabs however differ significantly between paired and single ants. The 'interference' between ants working in pairs thus makes itself felt in tunnel extension, but not, or hardly at all, in total loads. Consequently, if the mean load is the same for both single and paired ants the mass excavated would likewise be little affected. It m a y be concluded that, at worst, interference of this
19,
4
kind would not nullify any other advantages of pair working, for instance, increased working time. Further, this effect arises from special features of the digging situation and could not apply to, for instance, prey transport or brood care. REFERENCES
Chauvin, R. (1944). L'effet du groupe et la regulation de l'activite social chez les fourmis du genre Leptothorax. Bull. biol., 78, 197-205. Chauvin, R. (1969). Le Monde des Fourmis. Paris: Libraire Plon. Cben, S. C. (1937). Social modification of the activity of ants in nest building. Physiol. Zool., 10, 420436. Sakagami, S. F. & Hayashida, K. (1962). Work efficiency in heterospecific ant groups composed of hosts and their labour parasites. Anim. Behav., 10, 96-105. Sudd, J. H. (1960). Transport of prey by an ant, Pheidole crassinoda Em. Behaviour, 16, 295-308. Sudd, J. H. (1965). The transport of prey by ants. Behaviour, 25, 234-271. Sudd, J. H. (1967). Introduction to the Behaviour of Ants. London: Edward Arnold. Sudd, J. H. (1969). The excavation of soil by ants. Z. Tierpsychol., 26, 257-276. Sudd, J. H. (1970a). Specific patterns of excavation in isolated ants. Insectes Sociaux, 27 (4), 253-260. Sudd, J. H. (1970b). The response of isolated digging worker ants (Formica lemani Bondroit and Lasius niger (L.)) to tunnels. Insectes Sociaux, 27, (4), 261-270. Wilson, E. O. (1966). Behaviour of social insects. Syrup. Roy. ent. Soc., London, 3, 81-96. (Received 13 January 1971; revised 2 April 1971; MS. number: 1030)