Safi K. Darden

Behavioural trait assortment in a social network: patterns and implications

The social fine structure of a population plays a central role in ecological and evolutionary processes. Whilst many studies have investigated how morphological traits such as size affect social structure of populations, comparatively little is known about the influence of behaviours such as boldness and shyness. Using information on social interactions in a wild population of Trinidadian guppies (Poecilia reticulata), we construct a social network. For each individual in the network, we quantify its behavioural phenotype using two measures of boldness, predator inspection tendency, a repeatable and reliably measured behaviour well studied in the context of co-operation, and shoaling tendency. We observe striking heterogeneity in contact patterns, with strong ties being positively assorted and weak ties negatively assorted by our measured behavioural traits. Moreover, shy fish had more network connections than bold fish and these were on average stronger. In other words, social fine structure is strongly influenced by behavioural trait. We assert that such structure will have implications for the outcome of selection on behavioural traits and we speculate that the observed positive assortment may act as an amplifier of selection contributing to the maintenance of co-operation during predator inspection.

Adaptive Prolonged Postreproductive Life Span in Killer Whales

Killer whale mothers continue to help their adult male offspring to survive long after ceasing reproduction. Prolonged life after reproduction is difficult to explain evolutionarily unless it arises as a physiological side effect of increased longevity or it benefits related individuals (i.e., increases inclusive fitness). There is little evidence that postreproductive life spans are adaptive in nonhuman animals. By using multigenerational records for two killer whale (Orcinus orca) populations in which females can live for decades after their final parturition, we show that postreproductive mothers increase the survival of offspring, particularly their older male offspring. This finding may explain why female killer whales have evolved the longest postreproductive life span of all nonhuman animals.

Male harassment drives females to alter habitat use and leads to segregation of the sexes

Sexual conflict is ubiquitous across taxa. It often results in male harassment of females for mating opportunities that are costly for females, in some cases reducing reproductive success and increasing mortality. One strategy that females may employ to avoid sexual harassment is to segregate spatially from males. In fact, we do find sexual segregation in habitat use in species that have high levels of sexual conflict; however, the role of sexual harassment in driving such segregation remains poorly understood. Here, we demonstrate experimentally in a population of wild Trinidadian guppies Poecilia reticulata that male sexual harassment drives females into habitats that they otherwise do not prefer to occupy. In support of the social factors hypothesis for sexual segregation, which states that social factors such as harassment drive sexual segregation, this female behaviour leads to segregation of the sexes. In the presence of males, females actively select areas of high predation risk, but low male presence, and thus trade off increased predation risk against reduced sexual harassment.

Social implications of the battle of the sexes: sexual harassment disrupts female sociality and social recognition

Across sexually reproducing species, males and females are in conflict over the control of reproduction. At the heart of this conflict in a number of taxa is male harassment of females for mating opportunities and female strategies to avoid this harassment. One neglected consequence that may result from sexual harassment is the disruption of important social associations. Here, we experimentally manipulate the degree of sexual harassment that wild female guppies (Poecilia reticulata) experience by establishing replicated, semi-natural pools with different population sex ratios. We quantify the effects of sexual harassment on female social structure and the development of social recognition among females. When exposed to sexual harassment, we found that females had more disparate social networks with limited repeated interactions when compared to females that did not experience male harassment. Furthermore, females that did not experience harassment developed social recognition with familiar individuals over an 8-day period, whereas females that experienced harassment did not, an effect we suggest is due to disruption of association patterns. These results show that social network structure and social recognition can be affected by sexual harassment, an effect that will be relevant across taxonomic groups and that we predict will have fitness consequences for females.

Environmental effects on social interaction networks and male reproductive behaviour in guppies, Poecilia reticulata

In social species, the structure and patterning of social interactions have implications for the opportunities for sexual interactions. We used social network analysis to explore the effect of habitat structural complexity on the social and sexual behaviour of male Trinidadian guppies. We used replicated seminatural pools in which we quantified male social network structure and reproductive behaviour under simple and complex habitats. In addition, we compared two populations of guppies that differed in their evolutionary history of predation (one high, one low). The level of habitat complexity did not significantly affect social network structure. However, social networks differed significantly between populations, which we suggest is due to differences in predator experience. Males from the high-predation population had greater overall social network differentiation and fewer male–male associations than their low-risk counterparts. Contrary to our prediction that males would associate more frequently with relatively large (more fecund) females, we observed a negative correlation between female size and the strength of male–female associations. We also found no effect of population or habitat complexity on either harassment or sexual network structures. There was, however, a significant interaction between habitat structure and population on the expression of reproductive strategies, with high-predation males expressing fewer sigmoid displays in the complex habitat and the opposite trend in low-predation males. We suggest this pattern is driven by population differences in male–male competition. We discuss our results in the context of the evolution of social structure and male reproductive strategies.

Development of a Single-Sampling Noninvasive Hair Snare

Abstract Noninvasive hair and fecal DNA sampling provides a means of collecting information on elusive species, while causing little or no disturbance. However, current methods of hair collection do not preclude multiple sampling, thus risking sample contamination. We developed a hair snare that prevents multiple sampling, is cost-effective, easy to construct, and safe for target and nontarget species. Our initial field tests on endangered San Joaquin kit foxes (Vulpes macrotis mutica) and swift foxes (Vulpes velox) suggest that this hair snare may be effective in collecting uncontaminated samples for DNA analysis.

Predation risk as a driving force for phenotypic assortment: a cross-population comparison

Frequency-dependent predation has been proposed as a general mechanism driving the phenotypic assortment of social groups via the ‘oddity effect’, which occurs when the presence of odd individuals in a group allows a predator to fixate on a single prey item, increasing the predators attack-to-kill ratio. However, the generality of the oddity effect has been debated and, previously, there has not been an ecological assessment of the role of predation risk in driving the phenotypic assortment of social groups. Here, we compare the levels of body length assortment of social groups between populations of the Trinidadian guppy (Poecilia reticulata) that experience differences in predation risk. As predicted by the oddity effect hypothesis, we observe phenotypic assortment by body length to be greater under high predation risk. However, we found that a number of low-predation populations were also significantly assorted by body length, suggesting that other mechanisms may have a role to play.

The role of relatedness in structuring the social network of a wild guppy population

The role of relatedness in structuring animal societies has attracted considerable interest. Whilst a significant number of studies have documented kin recognition in shoaling fish under laboratory conditions, there is little evidence that relatedness plays a significant role in structuring social interactions in wild populations that are characterised by fission–fusion dynamics. Previous work has tended to compare relatedness within and among entire shoals. Such an approach however, does not have the ability to detect social sub-structuring within groups, which appears to be a major factor driving the social organisation of fission–fusion animal societies. Here, we use social network analysis combined with DNA microsatellite genotyping to examine the role of relatedness in structuring social relationships in a wild population of guppies (Poecilia reticulata). Consistent with previous findings, female–female dyads formed the strongest social relationships, which were stable over time. Interestingly, we also observed significant co-occurrence of male–male interactions, which is in contrast to previous work. Although we observed social sub-structuring in the population, we found no evidence for relatedness playing a significant role in underpinning this structure. Indeed, only seven first-degree relative dyads were identified among the 180 fish genotyped, indicating that the majority of individuals do not have a first-degree relative in the population. The high genetic diversity observed in this population is indicative of a large effective population size typical of lowland guppy populations. We discuss our findings in the context of the evolution of social organisation and the mechanisms and constraints that may drive the observed patterns in wild populations.

Acoustic territorial signalling in a small, socially monogamous canid

Animals that actively defend all or part of their home range for the exclusive use of members of their social group are considered territorial. Defended areas may contain resources such as dens or nests, key foraging sites, or sexual partners that vary in value by season. We investigated territoriality and the function of long-ranging barking sequences in a wild population of swift foxes, Vulpes velox. We monitored space use and barking behaviour and combined this with experimental acoustic playback during the mating season. Mated male foxes used barking sequences mainly inside or close to the boundary of the pairs home range core (50% kernel contour isoline of estimated home range). Similarly, male resident foxes responded more intensely with barking if a playback simulating intrusion by a rival occurred inside of the core compared to outside of it. However, it was common for home range cores to be partly overlapped by neighbouring home ranges and therefore we cannot arbitrarily define 50% home range cores as territories. Still, pair home ranges had areas that were exclusive to the mated pair and their primary and secondary daytime sleeping dens were usually located inside these areas. These results suggest that the barking sequence is used in territorial defence and we conclude that at least male swift foxes are territorial in the mating season and they use a long-ranging acoustic signal in territory defence.

Current directions in animal social networks

A social network approach provides a framework to study the link between individual behaviour and population-level patterns and processes. Studies have demonstrated how animal social network structure can be influenced by factors ranging from characteristics of the environment to characteristics of the individual, such as developmental experience and personality. At the level of the individual, the patterning of social connections can be an important determinant of fitness, predicting both survival and reproductive success. At the population level, network structure can influence the patterning of ecological and evolutionary processes, such as frequency-dependant selection and disease and information transmission.