Influences of aggregative behaviour on space occupation in the spider Zygiella x-notata (Clerck)

Summary The distribution of the spider Zygiella x-notata was examined using field populations of adult females occupying the outside frames of windows. The structure of the populations was aggregative, and the distribution of individuals on the window frames and the size of the webs were density dependent. Also, the sizes of the webs of neighbouring spiders on the same window alternated. This spatial organization involves interactions between neighbours. If one spider out of two is removed, and if all the webs are destroyed, remaining spiders that previously had small webs significantly increase the size of their construction. In contrast, individuals that previously had large webs do not modify the size of their construction. This shows that individuals of Z. x-notata respond to the presence of neighbours. The influence of intraspecific interactions in such a population is discussed.     

Leading from the front? Social networks in navigating groups

In many group-living animals, leadership by only a fraction of the group members can be important for group navigation. It has been shown that subgroups of informed individuals can steer the remainder of the group without direct communication, resolving conflicts of interest through individual-to-individual interactions. We present a model for the navigation of collectively moving groups that includes preferential interactions between individuals as a way of imposing social network structures, known to be present in many species. We show that effective leadership can occur when leaders do not occupy frontal spatial positions and when navigation tendency is appropriately balanced with social position. Our model also shows that small minorities can dominate movement decisions if they have navigational knowledge combined with influential social network positions. Our findings highlight the mechanistic importance of social networks for the movement decisions of animal groups. We discuss the implications of our research for interpreting empirical observations.     

Social foraging and dominance relationships: the effects of socially mediated interference

In socially foraging animals, it is widely acknowledged that the position of an individual within the dominance hierarchy of the group has a large effect upon its foraging behaviour and energetic intake, where the intake of subordinates can be reduced through socially mediated interference. In this paper, we explore the effects of interference upon group dynamics and individual behaviour, using a spatially explicit individual-based model. Each individual follows a simple behavioural rule based upon its energetic reserves and the actions of its neighbours (where the rule is derived from game theory models). We show that dominant individuals should have larger energetic reserves than their subordinates, and the size of this difference increases when either food is scarce, the intensity of interference suffered by the subordinates increases, or the distance over which dominant individuals affect subordinates increases. Unlike previous models, the results presented in this paper about differences in reserves are not based upon prior assumptions of the effects of social hierarchy and energetic reserves upon predation risk, and emerge through nothing more than a reduction in energetic intake by the subordinates when dominants are present. Furthermore, we show that increasing interference intensity, food availability or the distance over which dominants have an effect also causes the difference in movement between ranks to increase (where subordinates move more than dominants), and the distance over which dominants have an effect changes the size of the groups that the different ranks are found in. These results are discussed in relation to previous studies of intra- and interspecific dominance hierarchies.     

Social cohesion and foraging decrease with group size in fallow deer (Dama dama)

We studied the impact of group size on foraging behaviour and level of movement synchronisation among female herdmates of a fallow deer population in Central Italy. Both proportion of foraging events and movement synchronisation decreased with increasing group size. The proportion of foraging events was higher for animals on the edge of the group than for deer in the centre of the group; hence, there appears to be a trade-off between protection against predators and foraging interference, both of which decrease from the centre to the periphery of the group. This is the first time this type of behaviour has been recorded for wild ungulates. As expected, we also found that the movement of peripheral animals was less synchronised than that of central animals. Consequently, peripheral animals may lose contact with their herdmates and split off the group. We conclude that social inequalities may lead to conflicting requirements among group members and instability of large groups. Movement synchronisation (as a function of group size) appears to interact with habitat openness to produce variations of group size (which appear to be adaptive for individuals) as an emergent property of these aggregations.     

Loud calls as a mechanism of social coordination in a fission–fusion taxon, the white-bellied spider monkey (Ateles belzebuth)

Spider monkeys (Ateles spp.) live in social groups that exhibit high levels of fission–fusion dynamics, in which group members form subgroups of varying sizes and compositions. Within these fluid societies, how individuals establish contact with dispersed group members with whom they might choose to associate remains unclear. Long-range vocalizations might facilitate interactions between group members and provide a means of social coordination in fission–fusion societies. We evaluated this possibility for one spider monkey vocalization, the loud call, by examining calling behavior, the relationship between loud calls and changes in subgroup size, and the response of individuals to distant calls and playback experiments in a single study group. We found that 82 % of loud calls were emitted within 30 min of a call from a different location, suggesting that individuals frequently emit loud calls in response to the calls of distant group members. Subgroups that emitted loud calls, especially those that responded to distant calls, were much more likely to experience an increase in subgroup size within an hour after calling than those that did not. Animals also approached distant loud calls more than they avoided or ignored these calls. Finally, playbacks of male calls demonstrated that females respond preferentially to the calls of some individuals over others. Taken together, these results provide support for the hypothesis that spider monkey loud calls function to facilitate and initiate interactions between dispersed group members and suggest that vocal signals can play an important role in influencing social interactions in fission–fusion societies.     

Group structure in a restricted entry system is mediated by both resident and joiner preferences

The benefits of grouping behaviour may not be equally distributed across all individuals within a group, leading to conflict over group membership among established group members, and between residents and outsiders attempting to join a group. Although the interaction between the preferences of joining individuals and existing group members may exert considerable pressure on group structure, empirical work on group living to date has focussed on free entry groups, in which all individuals are permitted entry. Using the humbug damselfish, Dascyllus aruanus, we examined a restricted entry grouping system, in which group residents control membership by aggressively rejecting potential new members. We found that the preferences shown by joining members were not always aligned with strategies that incurred the least harm from resident group members, suggesting a conflict between the preferences of residents and preferences of group joiners. Solitary fish preferred to join familiar groups and groups of size-matched residents. Residents were less aggressive towards familiar group joiners. However, resident aggression towards unfamiliar individuals depended on the size of the joining individual, the size of the resident and the composition of the group. These results demonstrate that animal group structure is mediated by both the preferences of joining individuals and the preferences of residents.     

Social learning of predators by coral reef fish: does observer number influence acquisition of information?

Prey that are capable of continuously learning the identity of new predators whilst adjusting the intensity of their responses to match their level of risk, are often at a substantive advantage. Learning about predators can occur through direct experience or through social learning from experienced individuals. Social learning provides individuals with an effective means of acquiring information while reducing the costs associated with direct learning. Under a natural setting, social learning is likely to occur between more than two individuals. As such, investigating the effect that group size has on the ability of individuals to acquire information is vital to understanding social learning dynamics. Given the characteristics of coral reefs and the biology of coral reef fishes, these habitats are an ideal medium to test whether group size affects the transmission of information. Using newly settled damselfish (Pomacentrus amboinensis), we examined whether the number of observers present influences transmission of information. We showed that: (1) predator recognition is socially transmitted from predator-experienced to predator-naïve individuals regardless of group size, and that (2) the level of response displayed by the observer does not differ following learning in different sized groups. Our study highlights that information on predator identities is able to be passed onto group members quickly without a dilution of information content.     

The functional significance of the diphasic advertisement call of Geocrinia victoriana (Anura: Leptodactylidae)

Summary The advertisement call of Geocrinia victoriana is markedly diphasic and consists of one introcuctory note (rarely two or three), followed by a series of repeated notes of similar carrier frequency, but much shorter duration and higher pulse rate. In two-choice field discrimination experiments with the two phases of the advertisement call presented as alternative stimuli, reproductively ripe females were attracted only to the repeated notes and contacted the source of these signals (a loudspeaker). Field playback experiments were carried out on residential males, with a complete call, an introductory note, or a series of repeated notes being used as the stimulus, and intensity varied upwards in 10 dB increments. With the complete call or the introductory note as the stimulus, calling behaviour of subjects altered significantly from the pre-stimulatory pattern, with a great reduction in output of repeated notes, and an increase in duration and decrease in pulse rate of introductory notes. The threshold intensity (lowest level for a significant change in calling behaviour) with the introductory note as the stimulus, 90–99.9 dB peak sound pressure level (PSPL), was at least 10 dB lower than that with the complete call as the stimulus (110–125 dB, PSPL). Playback of repeated notes had little effect on calling behaviour until the intensity exceeded 120 dB, PSPL. Playback of all three classes of stimulus at constant intensities below and above the threshold for the complete call (i.e. 99–99.9, and 110–125 dB, PSPL) indicated that changes in calling behaviour were due to the intensity of stimulus rather than to a cumulative effect based on the number of stimuli. Measurements of intensities of advertisement calls of nearest neighbours in natural breeding assemblages (modal class=104–106 dB, PSPL) were consistent with the experimental results. It was thus demonstrated that in G. victoriana the two functions of the advertisement call are partitioned: the repeated notes attract breeding females (hence are equivalent to the mating call), and the introductory notes are directed at other males in a territorial context. The increase in duration of the introductory note is graded rather than discrete, so that this phase of the advertisement call changes progressively from the equivalent of a long-range (first-order) to a short-range (second-order) territorial call, then to an encounter call.     

Assessment and individual recognition of opponents in the pygmy swordtails Xiphophorus nigrensis and X. multilineatus

We examined factors that determine the outcome of agonistic encounters between male pygmy swordtail fish. Xiphophorus nigrensis and X. multilineatus males formed dominance relationships based on body size in staged laboratory encounters. There was a significant negative correlation between size asymmetry and fight intensity, suggesting that males assessed size in the encounters. However, a significant proportion of the variation in fight intensity in contests that escalated to bites could not be explained by size asymmetry. Aggressive motivation may also influence the outcome of contests and could be assessed in agonistic encounters. Theory suggests that signals of aggressive intention will be evolutionarily stable if individuals can recognize opponents and encounter one another repeatedly. In addition, individual recognition is one way that dominance hierarchies can be maintained. Here we demonstrate that males from both species can recognize individuals. In addition, at least some X. nigrensis males were site-faithful in the field, suggesting males encounter the same opponents repeatedly.Communicated by G.M. Klump     

Movement of whelks (Buccinum undatum) towards a baited trap

The movement of whelks, Buccinum undatum L., towards a food source was examined by releasing a large number of tagged individuals at 6 distances in each of four directions from a baited trap and recording their recovery in the trap. Eight such experiments were performed under a variety of conditions. The peak recapture of individuals released downstream at 2, 5, 10, 18, and 30 m from a baited trap was at 6 h, 6 h, 1 d, 2 d and 3 d, respectively. The area of attraction of a trap, the surface over which the whelks perceive and are drawn towards the bait, is usually irregularly shaped and determined mainly by current direction. In strong directional current it is elongated in the direction of the current and its size is much reduced. The area of attraction is also less at 20 m than at 10 m in depth. The size of the area of attraction varies seasonally due to behavioural changes probably associated with temperature and the whelk's reproduction. Isopleths of 25% capture of tagged whelks usually expand and become more rounded with time, probably reflecting the entry of whelks from outside the area of attraction.     

The nature of the photosynthate in natural phytoplankton populations in relation to light quality

The relative concentrations of a variety of organic compounds in phytoplankton from different depths of the photic zone of Saanich Inlet and Indian Arm, British Columbia, Canada, were determined. In surface phytoplankton the greatest proportion of ¹⁴C in newly-formed organic compounds was in the ethanol-soluble fraction irrespective of the light intensity. The proportion of the label in the insoluble fraction increased with depth. Within the ethanol-soluble fraction the relative importance of carbohydrates decreased and amino acids increased with depth. Experiments in Indian Arm indicated that these observed changes were in response to light quality rather than intensity. The percentage release of dissolved organic carbon as a percentage of total fixation decreased with depth and was directly related to the size of the ethanol-soluble fraction.     

Colonial breeding and nest defence in Montagu's harrier (Circus pygargus)

We assessed whether colonial breeding allows individuals to decrease their investment in predator defence, by presenting decoys of owls, foxes and crows to Montagu's harrier, Circus pygargus. Decoy detection increased with colony size, as did the number of individuals mobbing the decoy. The number of mobbers was greater for predators potentially risky for the adults (owl or fox) than for non-dangerous predators (crow). Recruits (breeding neighbours, fledglings and non-breeders) were present a lower percentage of the time, and attacked and alarm called less frequently than tested individuals. Nevertheless, the overall attack rate on the predator increased with the number of mobbers. When the size of the mobbing group increased, individuals were more likely to attack predators that represented a risk for adults, but did so less intensively and with a lower frequency of close dives. Thus, coloniality decreased the individual costs of defence in terms of risk taken, whilst enhancing defence efficacy. Birds alarm called more intensively when presented with dangerous predators than with the crow. The number of recruits significantly increased with increasing alarm rate of the tested individuals, even when taking colony size into account. Furthermore, the alarm rate of the tested birds also had a significant effect on the proportion of recruits that engaged in attacks against dangerous predators but not against the crow. The higher recruitment and attack rates for dangerous predators were thus apparently modulated through alarm calling. We discuss whether tested birds may manipulate recruits' behaviour to lessen their own risk.     

Neighbours, strangers and male-male aggression as a determinant of lek size

Interactions between males on leks may play an influential role in lek formation and the regulation of lek size. In this paper I present the results of a playback experiment that simulated de novo settlement at sites adjacent to currently existing display territories of the ochre-bellied flycatcher, Mionectes oleagineus. In the study population, males displayed both solitarily and at small leks. A large proportion of males held no display territory at all. A stranger's song was played to both solitary and lekking males from 10 m outside their territorial boundaries. In separate playbacks, lekking males were also played neighbour's song. Both lekking and solitary territorial males reacted to the playback by decreasing their song rate, approaching the playback speaker and, on occasion, attacking the model. Solitarily displaying males responded more aggressively to playback of stranger's song than did lek males. Lek males were able to distinguish between their neighbour's and a stranger's song and did so irrespective of whether it was played from the neighbour's territory or from outside the lek. In addition to distinguishing between neighbours and strangers, lek males modified their responses to these different playbacks depending on where the playback originated. These results suggest that male-male interactions can be influential in structuring leks. In M. oleagineus, interactions between males are aggressive and act to limit rather than augment lek size. Received: 6 March 1996 / Accepted after revision: 9 December 1996     

Species-specific influence of group composition on collective behaviors in ants

The success of a social group is often driven by its collective characteristics and the traits of its individuals. Thus, understanding how collective behavior is influenced by the behavioral composition of group members is an important first step to understand the ecology of collective personalities. Here, we investigated how the efficiency of several group behaviors is influenced by the aggressiveness of its members in two species of Temnothorax ants. In our manipulation of group composition, we created two experimentally reconstituted groups in a split-colony design, i.e., each colony was split into an aggressive and a docile group of equal sizes. We found strong species-specific differences in how collective behaviors were influenced by its group members. In Temnothorax longispinosus, having more aggressive individuals improved colony defense and nest relocation efficiency. In addition, source colony identity strongly influenced group behavior in T. longispinosus, highlighting that manipulations of group compositions must control for the origin of the chosen individuals. In contrast, group composition and source colony did not influence collective behaviors in Temnothorax curvispinosus. This suggests that the mechanisms regulating collective behaviors via individual differences in behavior might differ among even closely related species.     

Experimental tests of the information-center hypothesis with black vultures (Coragypsatratus) and turkey vultures (Cathartesaura)

In field tests of the information-center hypothesis (ICH) in south Texas with black vultures (Coragyps atratus) and turkey vultures (Cathartes aura), large carcasses were provided and kept under continuous observation. The use vultures made of these bait sites and their patterns of arrival were recorded to evaluate predictions derived from the ICH. Turkey vultures discovered most bait sites (30 of 31) first, but frequently were displaced from the food by later-arriving black vultures. This competitive exclusion by black vultures limited subsequent feeding opportunities for turkey vultures sufficiently that few (27%) returned on subsequent days to bait sites they had previously visited. I found no evidence that those turkey vultures that did return to bait sites acted as leaders for groups of naive birds and led them to bait sites – knowledgeable and naive turkey vultures did not arrive at bait sites together, and groups arriving at bait sites were not larger on subsequent days than on the first days carcasses were available. In contrast, a significantly larger percentage (47%) of knowledgeable black vultures returned to bait sites they had visited on previous days, and the first groups of black vultures arriving at bait sites on subsequent days were significantly larger than the equivalent groups on first days. Nine flocks of black vultures that arrived on subsequent days at bait sites before sunrise (which suggests the birds had commuted directly from a roost) contained knowledgeable birds, and two of these flocks contained both knowledgeable and naive individuals. Overall, 10 of 54 naive tagged black vultures (18.5%) arrived at bait sites under circumstances that suggested they had followed conspecifics to the food from a roost. However, most black vultures apparently found carcasses through independent search or by using local enhancement. Therefore, I conclude that while following from roosts to food sites is a strategy used by black vultures, at this study site it is one they use relatively infrequently. Received: 20 February 1997 / Accepted after revision: 28 June 1997     

Size-assorted fish shoals and the majority's choice

Similarity among group members may serve as a defence against visually hunting predators that preferentially attack individuals who are phenotypically different from the group majority. The presence of such odd individuals in an otherwise homogeneous group may, however, increase the vulnerability of the other group members as well. Individuals might thus be expected to form uniform groups in order to decrease predation risk, not only in trying to avoid being odd in a group, but also when attempting to avoid being accompanied by odd individuals. This hypothesis was tested with small and large three-spined sticklebacks, Gasterosteus aculeatus. Focal fish were offered the choice between a shoal consisting of conspecifics that were all similar in body length to the focal fish (matching shoal) and a shoal in which one or a few individuals differed in body length from the focal fish (non-matching shoal). In the control experiment, all individuals in the non-matching shoal differed in size from the focal fish. The control confirmed that individuals preferably joined the matching shoal when the alternative option was to be odd in another one. However, when the alternative for size-assortative shoaling was to belong to the majority in a mixed shoal, the shoal choice of individuals appeared on average to be random. Visual contact with a live pike, Esox lucius, did not affect the shoal choice pattern. Furthermore, despite the frequency-dependent nature of the oddity effect, varying the number of odd individuals in the non-matching shoal did not have a significant effect on individual's shoaling decisions. These results suggest that size-assortativeness in fish shoals is not a result of individuals avoiding being among the majority in a mixed group. Received: 2 September 1998 / Received in revised form: 12 May 1999 / Accepted: 29 May 1999     

The social network structure of a wild meerkat population: 2. Intragroup interactions

Knowledge of the structure of networks of social interactions is important for understanding the evolution of cooperation, transmission of disease, and patterns of social learning, yet little is known of how environmental, ecological, or behavioural factors relate to such structures within groups. We observed grooming, dominance, and foraging competition interactions in eight groups of wild meerkats (Suricata suricatta) and constructed interaction networks for each behaviour. We investigated relationships between networks for different social interactions and explored how group attributes (size and sex ratio), individual attributes (tenure of dominants), and ecological factors (ectoparasite load) are related to variation in network structure. Network structures varied within a group according to interaction type. Further, network structure varied predictably with group attributes, individual attributes, and ecological factors. Networks became less dense as group size increased suggesting that individuals were limited in their number of partners. Groups with more established dominant females were more egalitarian in their grooming and foraging competition interactions, but more despotic in their dominance interactions. The distribution of individuals receiving grooming became more skewed at higher parasite loads, but more equitable at low parasite loads. We conclude that the pattern of interactions between members of meerkat groups is not consistent between groups but instead depends on general attributes of the group, the influence of specific individuals within the group, and ecological factors acting on group members. We suggest that the variation observed in interaction patterns between members of meerkat groups may have fitness consequences both for individual group members and the group itself.     

Does supernormal stimulus influence parental behaviour of the cuckoo's host?

The supernormal stimulus hypothesis (SSH) states that a cuckoo chick should obtain more parental care than host young by means of exaggerated sensory signals. We tested the SSH by comparing parental care by reed warblers at parasitized and non-parasitized nests. A comparison of feeding rates to parasite and host chicks of the same size showed that parasitized nests received more food than non-parasitized ones with one host chick. There was an interesting relationship between average prey length and the mass of a cuckoo chick: prey length first increased with chick mass, but decreased after the cuckoo chick outgrew the average-sized host brood (three to four young at fledging). This might be expected if fosterers reduced the selectivity of their foraging behaviour when trying to satisfy the supernormal food demands of the parasitic chick. This suggestion is supported by the finding that the relationship between nestling mass and proportion of less economical small prey is inverse to the relationship between nestling mass and prey size. These results suggest that the parental behaviour of reed warblers is adjusted by selection to the needs of an average-sized brood. The overall proportion of insect orders was significantly different between the parasitic and host chicks. This result probably reflects the opportunistic foraging habits of the host. The qualitative difference (proportion of insect orders) between host and cuckoo nestling diets is partly a by-product of unequal length distribution of members of different taxonomic groups. The results of this study are consistent with the SSH.     

Cooperative prey capture by young subsocial spiders

Matriphagous young of a subsocial spider Amaurobius ferox exhibit collective predation during their post-maternal social period. In this paper, we examine functional mechanisms of collective predation by sibling groups. Predation efficiency increased with increasing number of individuals within each group. Solitary or paired individuals were generally unable to capture a 20 mg cricket. In larger groups, more individuals participated and captured the prey more quickly. Some siblings did not take part in paralyzing prey, but later consumed it. The proportion of these profiteers within a group increased with the group size. Presented with prey of different sizes (1, 5, or 40 mg), siblings were most aggressive towards each other when predating on 5 mg prey. Prey of this size could be captured by a single individual and yet were sufficiently large for more than one individual to eat. Siblings were much less aggressive towards one another during the capture of 40 mg prey, which require the assistance of other individuals to capture. By providing the same mass of prey in different numbers of individuals (a single cricket of 40 or 40 mg of first-instar crickets), we tested the influence of cooperation on the post-maternal social period. We found no difference in the development of young during the social period nor the timing of dispersal and the body mass of dispersing individuals. We conclude that the young of this subsocial animal increased predation efficiency by cooperative hunting after the mother's death.