The role of conformity in foraging when personal and social information conflict

Transcription

1 Behavioral Ecology Vol. 15 No. 2: DOI: /beheco/arh008 The role of conformity in foraging when personal and social information conflict Rachel L. Kendal, Isabelle Coolen, and Kevin N. Laland Subdepartment of Animal Behaviour, Department of Zoology, University of Cambridge, Madingley, Cambridge CB3 8AA, UK There is currently considerable interest in the interplay between personal and social information in decision-making processes. Two experiments are presented exploring the relative use of prior personal information and subsequent social information in foraging decisions of guppies. Experiment 1 tested the assumption that when the use of information acquired through personal experience is not costly, conflicting social information will be ignored. The assumption was confirmed because, when given a choice between feeding at two food patches, at one of which they had previously seen conspecifics feed, individual fish with prior experience of feeding at the alternative site chose the alternative, whereas fish with no prior experience chose the site at which their conspecifics had fed. Experiment 2 tested theoretical predictions that when the use of information acquired through personal experience is potentially costly, conflicting social information will be weighed more heavily than will personal information. The prediction was confirmed because, when given a choice between feeding at two food patches, one at which they had previously seen conspecifics feed and one behind a visual barrier, individual fish with prior experience of feeding behind the barrier chose the site at which their conspecifics had fed. These findings suggest that conformity can promote social learning in naïve individuals, but prior experience can insulate individuals from conformity provided the costs of relying on that experience are small. In addition, the experiments highlight the fact that personal and social information are not always weighed equally. Key words: conformity, guppy, personal information, social foraging, social learning. [Behav Ecol 15: (2004)] Acommon feature of social foraging is that the food discoveries of a few lead to the feeding of many, and as a result, group living may enable individuals to forage more efficiently through information transfer and social learning (Heyes and Galef, 1996; Ward and Zahavi, 1973; Zentall and Galef, 1988). This is because when there is temporal or spatial variation in habitat quality, animals must continually assess aspects of their environment (Mangel, 1990; Yoerg, 1991), and social animals commonly have access to a greater amount of relevant information than do solitary ones, as they can observe the activities, successes, and failures of others (Templeton and Giraldeau, 1995a, 1996). In addition, although large groups may attract predators, they may also reduce per capita predation pressures through increased group vigilance, dilution, confusion, and selfish-herd mechanisms (Bertram, 1978; Hamilton, 1971; Pulliam, 1973). Information transfer and social learning has long been of interest to ethologists and behavioral ecologists because such processes appear to enable onlookers to exploit information gained by others and allow animals to learn about their environments rapidly and efficiently, without making costly mistakes or wasting time on exploration. Through social learning, animals may learn how (observational learning) to deal with a resource or where (local enhancement) it is located, whereas through the use of public information (Valone, 1989) animals may learn about the quality of an environmental resource. Although these distinctions are useful, the latter term is arguably overly restrictive as all forms of social learning involve the use of socially acquired information (Giraldeau, 1997; Giraldeau and Caraco, Address correspondence to R. L. Kendal, who is now at the Department of Biological Sciences, Stanford University, Stanford, CA 94305, USA. rachel@ants.stanford.edu. K. Laland and I. Coolen are now at The Centre for Social Learning and Cognitive Evolution, School of Biology, St. Andrews University, UK. Received 11 September 2002; revised 4 April 2003; accepted 29 April ). Here, we use public information in Valone s more restrictive sense and follow Valone and Templeton s (2002) recommendation in using the more general term social information when referring to all information acquired from, or as a consequence of observation of or interaction with, other animals. Under natural conditions the use of social information is not the only means by which animals make decisions. Theoretical analyses suggest that species living in an environment of intermediate variability will be likely to use social learning, whereas those living in a highly variable or in a stable environment, with less to gain from observing others, should rely upon asocial learning or genetic inheritance of information, respectively (Boyd and Richerson, 1985, 1988; Laland et al., 1996). Social learning is thought to be favored at intermediate rates of environmental variability, as individuals can acquire relevant information without bearing the costs of direct interaction with the environment associated with asocial learning, but with greater phenotypic flexibility than if the behavior was unlearned (Boyd and Richerson, 1985, 1988). A common, but untested, assumption of these theoretical analyses is that individuals will be more likely to rely on social information if they lack the requisite personal information to guide their decisionmaking than if they possess relevant personal information (see Boyd and Richerson, 1988). Individual foragers may use personal information, such as (1) prior knowledge, described variously as pre-harvest or prior information that is available before patch exploitation (Valone, 1991, 1992), and (2) harvest or sample information that is acquired during the actual exploitation of a patch. How animals weight personal and social information is unknown, although theoretical analyses commonly assume that personal and social information are weighted equally (Clark and Mangel, 1984; Templeton and Giraldeau, 1995a, 1996; Valone and Giraldeau, 1993). However, there may be many situations in which a forager would benefit by weighting one or other type of information above the other. For example, the value of social information may depend upon Behavioral Ecology vol. 15 no. 2 Ó International Society for Behavioral Ecology 2004; all rights reserved.

2 270 Behavioral Ecology Vol. 15 No. 2 Figure 1 Diagram of the tank showing the partitions used throughout the experiments numbered as in the text (a) (Methods, experiment 1; dashed lines indicate transparent partitions; solid lines, opaque partitions); the division of the tank into zones, where the lettering would be reversed were the feeder on the right to be baited with bloodworm (b); and the set-up during training of the shoals, showing the four feeders at either end of the tank and the partitions used (c). the cost of acquiring or using personal information, owing to missed foraging opportunities or encountering dangers (Boyd and Richerson, 1985, 1988). It is now well established that the use of social information can enhance foraging efficiency in shoaling fish (Day et al., 2001; Laland and Williams, 1997, 1998; Pitcher and House, 1987; Ryer and Olla, 1992). Lachlan et al. (1998) suggest that in fish a tendency to shoal with the largest number of individuals may generate a positive frequency-dependent transmission of foraging information, or conformity (Boyd and Richerson, 1985). Day et al. (2001) found that, in the guppy, conformity hindered the performance (and subsequent social learning) of a foraging innovation that required the loss of visual contact with the shoal. In the current study, the circumstances under which individuals favor personal over social information (or vice versa) are examined by using the previously established effect of opaque barriers upon positive frequency-dependent social learning in guppies (see Day et al., 2001). In contrast to other studies that have compared the use of sample and social information (Smith et al., 1999; Templeton and Giraldeau, 1995a, 1996), here the use of prior personal and social information is compared, using small laboratory populations of fish. It would appear that this is the first study conducted to test hypotheses concerning the relative weighting of prior personal and social information and the assumptions of Boyd and Richerson s (1985, 1988) theoretical models. Two experiments were conducted that each presented individual guppies with a choice between feeding at two food patches, at one of which they had just seen conspecifics feed or not (social information), depending on the condition, and an alternative patch in which fish had prior experience of feeding or not (personal information), depending on the condition. Experiment 1 tests the assumption that individuals are disposed to rely on social information when they lack the requisite personal information to guide decision-making (Boyd and Richerson, 1988). A finding that naïve fish but not those with prior personal information behave according to the social information received would provide the first empirical confirmation of this assumption. In experiment 2, the theoretical prediction that individuals will be more reliant on social information in circumstances in which personal information is costly to acquire or use is tested (Boyd and Richerson, 1985, 1988). In this experiment the use of prior personal information is costly, as it requires fish to swim behind an opaque barrier and lose visual contact with conspecifics. A finding that those fish with personal and social information use the social information more in experiment 2 (visual barrier) than in experiment 1 (no visual barrier) would support the theoretical assumption that reliance upon social information increases when personal information is costly to acquire or use. METHODS The study used laboratory populations of guppies (Poecilia reticulata), purchased from Neil Hardy Aquatica, London. Domestic guppies were used as the diverse coloration of these fish makes identification of both sexes possible without stressful marking procedures. We know of no evidence that the behavior of domestic strains of guppy differs significantly from that of the wild type, and social learning of foraging behavior has been found in both laboratory (Lachlan et al., 1998; Laland and Williams, 1997, 1998) and wild populations of the guppy (Reader et al., 2003). Only females were used to rule out confounding effects of sexual interactions; the experimental evidence for social learning in guppies is more compelling in this sex (Laland and Williams, 1997). At the start of the experiment, all subjects were experimentally naïve. Fish were housed in aquarium tanks of dimensions cm (length 3 width 3 height), with a water level of 25 cm. Tanks were visually isolated from each other by using white Perspex. All tanks were equipped with plastic runners into which sheets of plastic could be placed to partition them (see Figure 1a) and were visually divided into six zones, the two feeders at either end of the tank each comprised a zone, as did each quarter of the tank (see Figure 1b). Boxlike feeders of cm (length 3 width 3 height) were used. They were designed so that subjects could not see whether they contained food until inside them. Fish could only gain access to the food (freeze-dried blood-worm) by swimming through a cm (high) hole at the bottom of the feeder and upward to where it floated on the water surface (see Figure 1c).

3 Kendal et al. Conformity and personal versus social information 271 Figure 2 Procedure of experiment 1, showing the personal experience training procedure in which fish were trained to feed (food designated as F) from either blue (B) or yellow (Y) feeders, in either the upper or lower part of the tank; the social experience procedure in which fish act as both demonstrators (as shoals of 10) and, when transferred into the opposing set up tank, as observers; and the testing procedure in which the demonstration shoal was confined with opaque partitions while the observer was released and its behavior monitored. Statistical methods One sample and independent samples chi-square tests were used to analyze the feeder choices of fish within and between conditions, respectively. All statistical tests were two-tailed. When sample sizes for conditions within an experiment differ, unless stated otherwise, this is owing to the death of a subject part way through the study. Experiment 1: Absence of visual barrier Subjects were allocated at random to three conditions: (1) trained observers, who experienced prior personal and subsequent conflicting social information; (2) naïve observers, who experienced social information only; and (3) controls, who experienced neither prior personal nor social information. At test, all subjects were given a choice between feeding at two food sites, one at which only trained observers had prior personal experience of feeding and a second at which trained observers and naïve observers had observed conspecifics feed. Subjects and apparatus Twenty adult female guppies acted as trained observers, 30 as naïve observers, and 29 as controls. In addition, 20 females acted as shoal mates to create shoals of 10 fish in the trained observer tanks. Two tanks contained naïve observers (in groups of 15), two contained controls (in groups of 15), and four contained shoals of 10 trained observers (of which five from each tank were tested). The short sides of the trained observer tanks were colored either yellow or blue, using paper attached to the outside of the tanks. The use of these colors was counterbalanced across tanks and provided cues to assist training of the fish. During the training period, the naïve observers and controls were exposed to white opaque feeders, whereas the trained observers were exposed to identical feeders colored blue and yellow. Procedure The experiment involved a personal experience (training period), a social experience (demonstration period) and a test (Figure 2). Personal experience training period Personal training of trained observers occurred over a 7-day period. Subjects were given a total of 15 trials, at a maximum of three trials a day. To ensure that the subjects would be motivated to feed during the test period, the number of training sessions per day decreased, such that on days 1, 2, and 3, three were given; on days 4 and 5, two were given; and on days 6 and 7, one was given. To familiarize the trained observers with the test procedure, they were constrained before introduction of the feeders. Trained observers were shepherded into one half of the tank, which was divided lengthwise by using a transparent plastic partition ( cm high; partition 1 (Figure 1a) and then constrained in the central portion of the remaining tank area by using two transparent partitions ( cm high; partitions 2 and 3) placed 15 cm from either short end of the tank. Opaque partitions (partitions 4 and 5) were placed outside of the transparent partitions to ensure subjects could not observe the introduction of the feeders. The feeders were positioned in the appropriate end of the tank according to the color cues, and blood-worm was placed in the feeder to which the fish were to be trained (the experimenter carried out a motion as if to provision the other feeder with food, preventing the fish from using the behavior of the experimenter as a cue). In total, four feeders were placed in the tank, one of each color on either side of the length-way partition (partition 1) (Figure 1c). The partitions constraining the fish centrally (partitions 2, 3, 4, and 5) were simultaneously removed and the fish allowed to feed for a maximum of 10 min. The direction and color to which fish were trained to feed was counterbalanced (left-blue [LB], left-yellow [LY], rightblue [RB], right-yellow [RY]). LB- and LY-trained fish were always constrained (by partition 1) in the portion of the tank

4 272 Behavioral Ecology Vol. 15 No. 2 Figure 3 Procedure of experiment 2, showing the personal experience training of observers to feed (food designated as F) from either blue (B) or yellow (Y) feeders, behind an opaque barrier and demonstrators to feed at the open end of the tank; the social experience procedure for fish in all three conditions; and the identical test period for all conditions, in which observers must lose visual contact with the constrained demonstrator shoal in order to feed at the trained feeder. furthest from the experimenter during training, whereas RBand RY-trained fish were always constrained in the portion of the tank nearest the experimenter during training (Figure 2). This training procedure allowed LB-trained fish to be tested in a physically identical area of the RY-trained tank (and vice versa), and LY-trained fish to be tested in a physically identical area of the RB-trained tank (and vice versa). During this period naive observers and controls were exposed to white feeders placed in random positions within the tank. After 7 days of 10-min presentations, all fish in the naïve observer and control tanks were competent at entering the feeders and appeared to associate them with food. Social experience and test For each trained observer tank in turn, fish were shepherded into the central portion of the tank to act as demonstrators in the trained observer condition. An observation area was created by using transparent partitions (partitions 6 and 7) placed in alignment with partitions 2 and 3 in the empty half of the tank (Figure 1a). A trained observer, now acting as an observer, was removed from a different tank and placed into this area. Fish from the two tanks LY and RB and those from the two tanks LB and RY acted as demonstrators and observers for each other (Figure 2). All fish were allowed a settling period of 2 min before the four feeders were introduced to the tank, as described for training, and blood-worm was placed in all of them. Demonstration began when the shoal of 10 fish were released. To ensure that the demonstrators ate only from the alternative colored feeder to that which the observer had been trained, the transparent partition allowing access to this feeder was left in place. The quality of demonstration was assessed by recording the number of demonstrators that were in each zone of the tank at every 10 s for the duration of the 3-min demonstration (Figure 1b). After demonstration, the shoal was shepherded into the central area of its half of the tank. Opaque partitions prevented the shoal seeing the color cues and thus continuing to demonstrate a preference for one end of the tank by aggregating at one end of the central zone. Once all fish had settled, the observer fish was released from the observation area (Figure 2). The feeder it entered and the latency with which it did so was noted. The demonstrator shoal remained in the tank (constrained but visible) while the observer was tested in order to alleviate stress associated with isolation. Fish in the naïve observer condition were also provided with social experience and tested in the same manner. Four trained and four naive fish were tested each day. A set of four fish was tested from h and from h. Within the naive observers and trained observers, morning and afternoon trials were counterbalanced. Similarly, the order in which trained observers were tested in the pairs of identically color-cued tanks was counterbalanced. The control fish were tested as described above except the demonstrator shoal remained in the central portion of their half of the tank for the 3-min demonstration period. Experiment 2: Presence of visual barrier In this experiment the cost of using prior personal information is increased by placing an opaque barrier in front of the feeder at which fish receive personal experience training. Subjects were allocated at random to three conditions; (1) trained observers, who experienced prior personal and conflicting social information; (2) trained nonobservers, who experienced prior personal information only; and (3) controls, who experienced neither prior personal nor social information (Figure 3). The control condition was to verify that losing visual contact with the shoal by going behind the barrier represented a cost of some kind that fish avoided. The trained nonobserver condition was to establish whether the behavior of trained observers was purely related to avoidance of losing contact with conspecifics or was influenced by the use of social information. Subjects and apparatus Twenty-nine guppies acted as trained observers, 29 as trained nonobservers, and 28 as controls. In addition, 20 fish acted as demonstrators. Six shoals of 15 fish and two shoals of 10 fish were established. The latter two shoals were the demonstrator fish, and the remaining six shoals were trained observers, trained nonobservers, or controls. Color cues to assist training

5 Kendal et al. Conformity and personal versus social information 273 Figure 4 Results of experiment 1, showing the proportion of fish that chose the demonstrator s feeder, for trained observer, naïve observer, and control conditions (* p,.05, ** p,.01, *** p,.001). Figure 5 The difference in latency (median and interquartile range), in experiment 1, of trained observers, naïve observers, and controls to enter a feeder, regardless of whether it was the observer s or demonstrator s trained feeder (* p,.05, *** p,.001). comprised yellow and blue paper placed on the left and right short ends of the tank, respectively. The opaque barrier comprised a plastic partition cm (width 3 height) with a slit vertically through the centre. This was placed (over the length-way transparent partition 1) 15 cm from the end of the tank in front of the feeder to which fish were trained (Figure 3). One side of the partition was colored either yellow or blue (the other being white) so that the color cue of the end of the tank was equally salient across experiments. Procedure Personal experience training period. The training procedure for trained observer and trained nonobserver fish was similar to that of experiment 1. However, to feed from the provisioned feeder (RB/ LY), fish were required to swim behind a barrier, which although the same color as the feeder, shielded the feeder from their view. All fish were constrained in the portion of the tank nearest to the experimenter (by partition 1) during training (Figure 3). Two demonstrator shoals were created in the two test tanks. One shoal was trained to feed from the yellow feeder and the other from the blue feeder. To reach the provisioned feeder demonstrator, fish were not required to swim behind the visual barrier, as it was placed (as described above) in front of the nonprovisioned feeder (Figure 3). Social experience and test. The 10 demonstrator fish were confined in the central portion of the half of the test tank furthest from the experimenter. A fish trained to feed from the opposite colored feeder to the demonstrator shoal was placed into the observation area. Thus, fish from the two LYbarrier training tanks were placed in the RB no barrier test tank, and fish from the two RB barrier training tanks were placed in the LY no barrier test tank (Figure 3). Trained observers saw a 3-min demonstration, whereas trained nonobservers were confined opposite the demonstrators for 3 min. In addition to the measurements taken in experiment 1, the behavior of the trained fish during demonstration was measured by noting which zone (of C and D) it was in at every 10 s. As in experiment 1, demonstrator fish were shepherded back into the central portion of their half of the tank (where necessary), and after a settling period, the subject fish was released. In addition to the feeder entered and the latency taken to do so, the zone the subject was in was noted every 10 s until it entered a feeder or 15 min had elapsed. The controls were tested as described for experiment 1. At the end of each day, training occurred as described above, in the four trained fish holding tanks, to ensure all subjects had equally up-to-date information as to the fact that food could be found in the feeder behind the barrier. RESULTS Experiment 1 Across all demonstration periods, there was a mean 6 SE of demonstrator fish in the feeder (zone A), fish in the zone immediately surrounding the feeder (zone B), fish in zone C, and fish in zone D (the middle zone nearest the observer s trained feeder). All demonstrators were observed to eat at the feeder. The choice of feeder by the fish in the control condition was not significantly different to an even distribution (v 2 ¼ 0.18, df ¼ 1, p ¼ NS). Of the 22 control fish that entered a feeder, 12 went to that to which the demonstrator shoal had been trained, whereas 10 went to the opposite feeder. As several fish did not actually enter a feeder, the zone (B or E) surrounding the feeder in which they spent the most time was used to indicate their choice of feeder. Again, the choice of feeder by the control fish was not significantly different from an even distribution (v 2 ¼ 0.04, df ¼ 1, p ¼ NS), with 14 fish choosing the feeder to which the demonstrator shoal had been trained and 15 the opposite feeder. The random behavior of the control fish suggests that while constrained, the trained demonstrators did not give off cues influencing the choice of feeder by the observer fish (Figure 4). Of the 30 naïve observers, 22 fed from the feeder that had been used by the demonstrator shoal, whereas eight fed from the opposite feeder. This preference for the demonstrated feeder represented a significant departure from that of the controls (v 2 ¼ 4, df ¼ 1, p,.05) (Figure 4). Of the 20 fish in the trained observer condition, 18 fed from the feeder at which they had been trained, whereas two fed from the feeder used by the demonstrator shoal. The preference of the trained fish for the feeder to which they had been trained was significantly different from the behavior of the controls (v 2 ¼ 9.28, df ¼ 1, p,.005) (Figure 4). Trained observer fish fed significantly faster than did the control fish (Mann-Whitney: U ¼ 132.5, N 1 ¼ 20, N 2 ¼ 29, p,.001) (Figure 5). Trained and naïve observers significantly differed in their choice of feeder (v 2 ¼ 20.4, df ¼ 1, p,.0001) (Figure 4). Trained observers entered a feeder more rapidly than did naïve observers (Mann-Whitney: U ¼ 192, N 1 ¼ 20, N 2 ¼ 30, p ¼.032) (Figure 5).

6 274 Behavioral Ecology Vol. 15 No. 2 Figure 6 Results of experiment 2, showing the proportion of fish that chose the demonstrator s feeder, for trained observers, trained nonobservers, and control conditions. The asterisks directly above the bars indicate a significant difference from an even distribution, set at 50% (* p,.05, ** p,.005). Experiment 2 All demonstrator fish were observed to eat. Across all demonstration periods, there was a mean 6 SE of fish in the feeder (zone A), fish in the zone immediately surrounding the feeder (zone B), fish in zone C, and fish in zone D (the middle zone nearest the observer s trained feeder). The choice of feeder by the control fish was significantly different from an even distribution (v 2 ¼ 8, df ¼ 1, p,.005) (Figure 6). Of the 15 fish that entered a feeder, 13 went to that which the demonstrator shoal had been trained, whereas two went to the opposite feeder, behind the visual barrier. As several fish did not actually enter a feeder, the zone (B or E) surrounding the feeder in which they spent the most time was used to indicate their choice of feeder. Again, there was a significant feeder preference in the naïve fish (v 2 ¼ 20.6, df ¼ 1, p,.001), with 26 fish choosing the feeder to which the demonstrator shoal had been trained and two the opposite feeder, behind the visual barrier. Of the 29 fish in the trained observer condition, 19 fed from the feeder used by the demonstrator shoal, whereas 10 fed from the opposite feeder, behind the visual barrier, at which they had received personal experience training. This preference for the demonstrated (nontrained) feeder approached a significant departure from an even distribution (v 2 ¼ 2.8, df ¼ 1,.1. p..05), and was less than expected when compared with the preference of the control fish for this feeder (v 2 ¼ 6.4, df ¼ 1,.05. p..025) (Figure 6). Trained observer fish fed at the demonstrated feeder (without barrier) significantly more quickly than did control fish (Mann-Whitney: U ¼ 94.5, N 1 ¼ 19, N 2 ¼ 26, p,.001) (Figure 7). Of the 29 fish in the trained nonobserver condition, 20 fed from the feeder opposite to that at which they had received personal experience training (no visual barrier), and nine fed from the feeder behind the visual barrier, according to their personal experience training. This preference for the feeder to which they had received no personal experience training was a significant departure from what would be expected given an even distribution (v 2 ¼ 4.2, df ¼ 1,.05. p..025), and was less than expected when compared with the preference of the control fish for this feeder (v 2 ¼ 5.4, df ¼ 1,.05. p..025) (Figure 6). There was no significant difference between trained nonobserver and control fish in Figure 7 The difference in latency (median and interquartile range) in experiment 2, of fish in the trained observer, trained nonobserver, and control conditions, to enter the demonstrators (no barrier) feeder, including those who did not actually enter the feeder (** p,.01, *** p,.001). the latency to feed at the feeder to which they had received no personal experience training (without the barrier; Mann- Whitney: U ¼ 244, N 1 ¼ 20, N 2 ¼ 26, p ¼.720) (Figure 7). The choice of feeder by trained fish in the two conditions was virtually identical and not significantly different (see Figure 6). Five of the fish in the trained nonobserver condition did not enter a feeder within 15 min (their choice being determined by the feeder zone in which they significantly spent the most time). In the trained observer condition, only one fish did not enter a feeder within 15 min, indicating that fish in the trained nonobserver condition may have been slower to choose a feeder. Indeed, trained fish who received a demonstration of the alternative feeder (with no barrier) took significantly less time to feed at this feeder than did those who did not receive such a demonstration of the alternative feeder, both including (Mann-Whitney: U ¼ 80, N 1 ¼ 19, N 2 ¼ 20, p ¼.002) (Figure 7) and excluding fish who did not actually enter a feeder (U ¼ 62, N 1 ¼ 18, N 2 ¼ 16, p ¼.005). There was, however, no significant difference, at test, in the percentage of time fish in either condition spent in the zone immediately surrounding the feeder to which they had not been trained (zone B; t test: t ¼ 0.432, df ¼ 56, p ¼.667) or the feeder to which they had been trained (zone E, behind barrier; t ¼ 0.394, df ¼ 56, p ¼.695). Trained observers and trained nonobservers spent (mean 6 SE) % and % of the test period in zone B and % and % in zone E, respectively. When the analysis was repeated using only fish who entered the nontrained feeder (no barrier), there was no significant difference in the percentage of time, at test, that fish in either condition spent in the zone immediately surrounding the nontrained feeder (zone B; t test: t ¼ 0.310, df ¼ 32, p ¼.759) and the feeder to which the fish had received personal experience training (zone E, behind barrier; t ¼ 1.132, df ¼ 32, p ¼.266). During the observation period, the number of 10-s intervals that trained observers were seen to be in the zone (C) closest to the feeding demonstrators did not significantly differ from an even distribution regardless of which feeder was entered at test (v 2 ¼ 3.47, df ¼ 1,.1. p..05). Fish that chose the feeder to which they had been trained rather than the demonstrated feeder spent (mean 6 SE) s intervals in zone C, whereas those that chose the demonstrated feeder spent intervals in this zone.

7 Kendal et al. Conformity and personal versus social information 275 Comparison of experiments 1 and 2 There was a significant difference in the feeder choices of the two trained observer conditions between experiments 1 and 2 (v 2 ¼ 16.2, df ¼ 1, p,.0001) (cf. Figures 4 and 6). In experiment 1 (feeder in open tank), trained fish fed from the feeder at which they had received personal experience training more than expected, whereas in experiment 2 (feeder behind barrier), trained fish fed at this feeder less than expected, preferring to feed at the demonstrated feeder (with no barrier). DISCUSSION Experiment 1 clearly shows that the fish lacking prior personal information (naïve observers) used the social information provided by the demonstrator shoal, whereas those with prior personal information (trained observers) relied upon this rather than upon social information. In contrast, in experiment 2, fish in the trained observer condition favored the social information provided by the demonstrator shoal over their prior personal information as to the location of food, as use of the latter necessitated loss of visual contact with the shoal, which we interpret as incurring (or potentially incurring) a cost. The preference of naïve observer fish for the demonstrated feeder, in experiment 1, indicates that when guppies have no prior information and cannot obtain sample information without some cost (swimming to and entering feeders), they will use social information. More generally, the findings of the experiment suggest that animals may be disposed to rely on social information when they lack the requisite personal information to guide decision-making (Boyd and Richerson, 1988). Although this experiment was not designed to ascertain the psychological processes that allowed naïve fish to use social information, the most parsimonious explanation is in terms of local enhancement. As local enhancement merely represents an attraction to a particular locality, it is not in itself learning. However, as the naïve observers chose a feeder after observing demonstration, the local enhancement must have led to social learning as they subsequently remembered the location of food. The fact that naïve observers fed at the demonstrated feeder more than expected when compared with naïve fish without a demonstration (controls) provides further support to the argument that the naïve observers had learned the location of food from the one demonstration they received. In comparison, asocial learning processes fully account for the behavior of the trained observers. In experiment 1, the trained observer fish preferred the feeder at which they had prior personal experience, despite conflicting social information. This indicates that they weighted their personal information more strongly than the social information that they received during the demonstration. A similar result was reported by Pongrácz et al. (2003), in which the greater the previous experience of an indirect route to reach a food reward, the longer it took dogs to adopt a socially demonstrated shorter route. Thus, in experiment 1 although the social information was sufficient for learning (as seen with the naïve observers), the reliability of prior personal experience, together with the low cost associated with continued use of the personal information, appeared to insulate these fish against the adoption of the socially demonstrated alternative food patch. In experiment 2, however, we attempted to increase the cost of using personal information. The finding that naive control fish consistently chose the feeder in the open end of the tank rather than behind the visual barrier confirms the proposed cost to use of personal information for trained fish. Indeed, fish in both the trained observer and trained nonobserver conditions tended to ignore their prior personal information (directing them to feed behind the visual barrier) but rather fed at the feeder that their prior personal information indicated never contained food. The proportions of fish in the trained conditions entering the feeder to which they had been trained were virtually identical regardless of the fact that one condition observed a demonstration of fish feeding at the alternative feeder and the other did not. It is possible therefore that fish in the trained observer condition did not use the social information provided but merely ignored their prior personal information (as trained nonobservers did) owing to a shoaling tendency. At test, trained observers and trained nonobservers spent an equal amount of time in the zone surrounding the feeder to which they had been trained. However, we do have evidence that the trained fish that observed conspecifics feeding at the previously nonprovisioned feeder did use this social information. Fish in the trained observer condition ignored their prior personal information and fed at the feeder to which they had not been trained more rapidly than did those fish in the trained nonobserver condition. This result mirrors that of Pongrácz et al. (2003), who found that where dogs were prevented from using their prior personal information to reach a food reward, they used an alternative route more rapidly when provided with social information as to its use. It is also consistent with the suggestion of Valone and Templeton (2002) that public information can be used to make faster, more accurate choices about the environment. When the use of personal information was costly, necessitating loss of visual contact with the shoal, social information was used, confirming the prediction of Boyd and Richerson s (1988) model. Taken together, the findings of experiments 1 and 2 suggest that when personal and social information conflict, animals will be more likely to use social information if the personal information is costly to use (e.g., necessitating loss of contact with conspecifics) than if it is not. This result corresponds to the findings of theoretical analyses regarding the utility of conformist social learning (Day et al., 2001; Boyd and Richerson, 1985; Lachlan et al., 1998). However, in contrast to the Day et al. (2001) study in which the conformity effect hindered the performance (and subsequent social learning) of a foraging innovation, in these experiments conformity promoted social learning. This is because in order to innovate and socially learn in Day et al. s (2001) study, guppies were required to lose visual contact with the shoal, whereas in the current study it was the use of prior personal information that required loss of contact with the shoal. This caused fish to weight the social information more heavily than their prior personal information, and thus conform. Similarly, in marmosets a prior personal aversion to a food was overridden by social information as to its palatability (Queyras et al., 2000), presumably owing to the cost of unnecessarily avoiding a nutritious food source. The present study may shed light on the question as to whether or not individuals weight personal and social information equally, as assumed in some recent analyses (Clark and Mangel, 1984; Templeton and Giraldeau, 1995a, 1996; Valone and Giraldeau, 1993). For example, in studies of

8 276 Behavioral Ecology Vol. 15 No. 2 the use of public information in mate quality assessment, it appears that individuals first rely on their own personal information and only use social information (the mating decisions of others) when their discrimination ability is inadequate (Dugatkin, 1996; Dugatkin and Godin, 1993; Nordell and Valone, 1998). Similarly, in the current study, fish first relied upon their personal information (experiment 1) until such a time as the use of this information became too costly (experiment 2), when social information was used. This result mirrors that of Templeton and Giraldeau (1996), who found that starlings only used public information to assess patch quality when the cost of acquiring accurate personal sample information increased. The findings of experiments 1 and 2 are also consistent with the assumption that, the value of public (social) information may depend upon the cost of acquiring sample (personal) information (Valone and Templeton, 2002, pp. 21, terms in parentheses added). Although in this study we did not manipulate the cost of acquiring personal and social information simultaneously, as suggested by Templeton and Giraldeau (1995b), we did show that when the cost of acquiring sample information (to corroborate prior information) was minimal, social information was ignored. However, when the cost of acquiring sample information increased (necessitating loss of visual contact with the shoal), social information was no longer ignored. Most studies of public information have involved the comparison of social and personal sample information, whereas our study involves social and personal prior information. It is thought that the combination of prior personal and social information is less complicated than the combination of personal sample and social information (Valone and Templeton, 2002). It is however, ecologically feasible that individuals may acquire prior personal and public information without personal sample information. For example, subordinate individuals (in a heterospecific or conspecific scenario) may not have access to defendable resources to acquire sample information but may have previously obtained prior personal information and be able to observe the foraging success of dominant individuals. However, our study differs from the majority of public information foraging studies in that we measured the choices of individuals rather than patch departure decisions. Thus, the study may be more applicable to instances in which decisions involve a choice between two options, such as mates, escape routes, or shelter. In conclusion, the findings of this study show that positive frequency-dependent social learning, or conformity, can promote social learning in naïve individuals, but prior experience can insulate individuals from conformity provided the costs of relying on that experience are small. In contrast to previous assumptions, the current study shows that personal and social information are not always weighed equally, and their relative use depends upon the costs involved. The context-dependent relative weightings of information types could, and arguably should, be incorporated into foraging models. Examination of the possibility that conflicting social information provided by various numbers of individuals would be weighted differently, relative to personal information, owing to conformity effects could extend the current study. We are grateful to J. Kendal and C. Brown for helpful discussion and to Y. Van Bergen for useful comments on the manuscript. 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