Intraspecific Variation In Social Organization Research Articles

Primate social organization evolved from a flexible pair-living ancestor.

Explaining the evolution of primate social organization has been fundamental to understand human sociality and social evolution more broadly. It has often been suggested that the ancestor of all primates was solitary and that other forms of social organization evolved later, with transitions being driven by various life history traits and ecological factors. However, recent research showed that many understudied primate species previously assumed to be solitary actually live in pairs, and intraspecific variation in social organization is common. We built a detailed database from primary field studies quantifying the number of social units expressing different social organizations in each population. We used Bayesian phylogenetic models to infer the probability of each social organization, conditional on several socioecological and life history predictors. Here, we show that when intraspecific variation is accounted for, the ancestral social organization of primates was inferred to be variable, with the most common social organization being pair-living but with approximately 10 to 20% of social units of the ancestral population deviating from this pattern by being solitary living. Body size and activity patterns had large effects on transitions between types of social organizations. As in other mammalian clades, pair-living is closely linked to small body size and likely more common in ancestral species. Our results challenge the assumption that ancestral primates were solitary and that pair-living evolved afterward emphasizing the importance of focusing on field data and accounting for intraspecific variation, providing a flexible statistical framework for doing so.

The evolution of marsupial social organization.

It is generally believed that marsupials are more primitive than placentals mammals and mainly solitary living, representing the ancestral form of social organization of all mammals. However, field studies have observed pair and group-living in marsupial species, but no comparative study about their social evolution was ever done. Here, we describe the results of primary literature research on marsupial social organization which indicates that most species can live in pairs or groups and many show intra-specific variation in social organization. Using Bayesian phylogenetic mixed-effects models with a weak phylogenetic signal of 0.18, we found that solitary living was the most likely ancestral form (35% posterior probability), but had high uncertainty, and the combined probability of a partly sociable marsupial ancestor (65%) should not be overlooked. For Australian marsupials, group-living species were less likely to be found in tropical rainforest, and species with a variable social organization were associated with low and unpredictable precipitation representing deserts. Our results suggest that modern marsupials are more sociable than previously believed and that there is no strong support that their ancestral state was strictly solitary living, such that the assumption of a solitary ancestral state of all mammals may also need reconsideration.

Social organization variation and behavioural flexibility in the facultative cleaning goby Elacatinus prochilos

Social species often show intraspecific variation in the size and composition of their social units, i.e. the social organization. This variation represents a challenge for comparative studies trying to understand evolutionary transitions in social organization complexity. Most studies on intraspecific variation in social organization (IVSO) have focused on the environmental factors selecting for variation in social organization but the mechanisms underlying such variation remain relatively unexplored. Here we investigated the potential mechanisms giving rise to IVSO in the Caribbean cleaning goby Elacatinus prochilos . This small reef fish typically exhibits two types of social organization, pair living or group living, which not only differ in group size but also in feeding strategy and social behaviour. Pair-living gobies cooperatively feed on ectoparasites removed from larger visiting fish, while group-living gobies feed on microinvertebrates living inside sponges and aggressively defend the best feeding spots against each other. We tested how flexible the social organization, social structure and competition over a food source of E. prochilos are by experimentally manipulating group composition, habitat structure and feeding condition of pair-living and group-living wild adult gobies in laboratory. Despite having different original social organizations in the wild, both pair-living and group-living gobies formed dominance hierarchies and exhibited affiliative behaviours in our group manipulations. This indicates that IVSO in this species is not driven by pre-determined or early-developed mechanisms, but rather a consequence of flexibility in social organization, also known as social flexibility. • We investigated the mechanisms of social organization variation in E. prochilos. • We used a controlled group manipulation to test the behavioural flexibility of individuals. • Individuals exhibiting different social organizations in the wild converged in social behaviour. • This indicates that variable social organization is driven by social flexibility.

Geographic intra-specific variation in social organization is driven by population density

Social flexibility enables individuals to switch between group and solitary living and is suggested to be an adaptation to varying environments. Several previous studies on different species compared two populations and hypothesized that observed differences in the social organization were due to differences in population density but lacked the necessary sample size to test this prediction. In a previous 8-year long-term study, we showed that one population of African striped mice (Rhabdomys pumilio) displayed temporal social flexibility, living solitarily in years when population density was low but in groups in years when population density was high. Building on this temporal variation, we now tested whether geographic variation reveals the same pattern. We studied 6 populations in discrete geographical locations simultaneously, predicting more solitary living in populations with lower population density. Population density correlated significantly with the percentage of striped mice living in groups whereas other (environmental) factors were not significant. Moreover, some individuals dispersed over unoccupied habitats between these populations, switching from group to solitary living. Geographic variation in population density could make social flexibility adaptive because it allows individuals to respond quickly to the prevailing conditions they experience post dispersal. Our results suggest that geographic variation drives the evolution of social flexibility in our metapopulation of striped mice, causing intra-specific variation in its social organization, which might also be important in other species, especially in species with a fast life history. Populations of the same species can differ in their social organization. It has often been assumed that this is due to differences in population density. We studied 6 populations of the African striped mice, showing that more mice were solitary living when population density was low. Thus, we demonstrated that population differences in social organization were due to differences in population density.

The evolution of intraspecific variation in social organization

AbstractMany species show intraspecific variation in their social organization (IVSO), which means the composition of their social groups can change between solitary living, pair living, or living in groups. UnderstandingIVSOis important because it demonstrates species resilience to environmental change and can help us to study ultimate and proximate reasons for group living by comparing solitary and group‐living individuals in a single species. It has long been realized that the environment plays a key role in explaining the occurrence ofIVSO.IVSOis expected to have evolved in variable environments and can thus be a key adaptation to environmental change. It has previously been suggested that four different mechanisms relying on the environment exist that can lead toIVSO: environmental disrupters, genetic differentiation, developmental plasticity, and social flexibility. All four mechanisms depend on the environment such that focusing only on environmental factors alone cannot explainIVSO. Importantly, only three represent evolved mechanisms, while environmental disrupters leading to the death of important group members induce nonadaptiveIVSO. Environmental disrupters can be expected to causeIVSOeven in species whereIVSOis also an adaptive response. Here, we focus on the questions of whyIVSOoccurs and why it evolved. To understandIVSOat the species level, it is important to conduct continuous long‐term studies to differentiate between nonadaptive and adaptiveIVSO. We predict thatIVSOevolves in environments that vary in important ecological variables, such as rainfall, food availability, and population density.IVSOmight also depend on life history factors, especially longevity.IVSOis predicted to be more common in species with a short life span and that breed only for one breeding season, being selected to respond optimally to the prevailing environmental situation. Finally, we emphasize the importance of accounting forIVSOwhen studying social evolution, especially in comparative studies, as not every species can be assigned to one single form of social organization. For such comparative studies, it is important to use data based on the primary literature.

Intra-specific variation in social organization of Strepsirrhines.

Strepsirrhines, that is, lemurs, galagos, and lorises, are considered basal primates, making them important to understand the evolution of primate sociality. Apart from some lemurs, they are nocturnal and solitary living, though the view of their sociality nature has changed with field studies being completed. We conducted a review of the primary literature about the social organization (group composition) of strepsirrhines, with the aim to determine whether intra-specific variation in social organization (IVSO) occurs and to determine how many species are pair-living, group-living, or solitary living. We found data in 83 peer-reviewed studies for 43 of the 132 strepsirrhine species and compared our results using two databases on social systems of mammals published in 2011 and 2013. While it is often assumed that primates show relatively fixed social organizations, we found that 60.5% of species for which data exist have IVSO. We found only 7% of the species to be truly solitary living (with 34.9% additional species to be sometimes solitary living), which is in contrast to the other databases, which had reported 60.9% and 37.7% of species to be solitary. We further explored group compositions by designating "functional groups" (e.g., foraging, breeding, and infant care groups). While functional groups might explain IVSO within a single species, this was not consistent over species with IVSO, such that IVSO was poorly explained by functional groups. Our study supports the view that most strepsirrhines are social (58.1% of species with another 34.9% of species sometimes living in pairs or groups) and show complex and often variable social organizations; reinforcing the assumption that the ancestor of all primates was social and not solitary.

Social chromosome variants differentially affect queen determination and the survival of workers in the fire ant Solenopsis invicta.

Intraspecific variation in social organization is common, yet the underlying causes are rarely known. An exception is the fire ant Solenopsis invicta in which the existence of two distinct forms of social colony organization is under the control of the two variants of a pair of social chromosomes, SB and Sb. Colonies containing exclusively SB/SB workers accept only one single queen and she must be SB/SB. By contrast, when colonies contain more than 10% of SB/Sb workers, they accept several queens but only SB/Sb queens. The variants of the social chromosome are associated with several additional important phenotypic differences, including the size, fecundity and dispersal strategies of queens, aggressiveness of workers, and sperm count in males. However, little is known about whether social chromosome variants affect fitness in other life stages. Here, we perform experiments to determine whether differential selection occurs during development and in adult workers. We find evidence that the Sb variant of the social chromosome increases the likelihood of female brood to develop into queens and that adult SB/Sb workers, the workers that cull SB/SB queens, are overrepresented in comparison to SB/SB workers. This demonstrates that supergenes such as the social chromosome can have complex effects on phenotypes at various stages of development.

Intraspecific variation in social organization by genetic variation, developmental plasticity, social flexibility or entirely extrinsic factors

Previously, it was widely believed that each species has a specific social organization, but we know now that many species show intraspecific variation in their social organization. Four different processes can lead to intraspecific variation in social organization: (i) genetic variation between individuals owing to local adaptation (between populations) or evolutionarily stable strategies within populations; (ii) developmental plasticity evolved in long-term (more than one generation) unpredictable and short-term (one generation) predictable environments, which is mediated by organizational physiological effects during early ontogeny; (iii) social flexibility evolved in highly unpredictable environments, which is mediated by activational physiological effects in adults; (iv) entirely extrinsic factors such as the death of a dominant breeder. Variation in social behaviour occurs between individuals in the case of genetic variation and developmental plasticity, but within individuals in the case of social flexibility. It is important to study intraspecific variation in social organization to understand the social systems of species because it reveals the mechanisms by which species can adapt to changing environments, offers a useful tool to study the ultimate and proximate causes of sociality, and is an interesting phenomenon by itself that needs scientific explanation.

From monogamy to complexity: social organization of arctic foxes (Vulpes lagopus) in contrasting ecosystems

Canids display pronounced intraspecific variation in social organization, ranging from single breeding females to large and complex groups. Despite several hypotheses in this matter, little is understood about the ecological factors underlying this flexibility. We have used the arctic fox ( Vulpes lagopus (L., 1758)) to investigate how contrasting ecosystem conditions concerning resources and predation influence group formation. We predicted that complex groups are more common in resource-rich ecosystems with predators, whereas simple groups occur in more marginal ecosystems without predators. Samples from 54 groups were collected from four populations of arctic foxes with contrasting prey resources and predation and these samples were genotyped in 10 microsatellite loci. We found considerable variation between ecosystems and a significant relationship between resources and formation of complex groups. We conclude that sufficient amounts of food is a prerequisite for forming complex groups, but that defense against predation further increases the benefits of living in larger groups. We present a conceptual model suggesting that a trade-off between the cost of resource depletion and the benefits obtained for guarding against predators explain the differences in social organization. The variable ecology of the arctic foxes makes it is a plausible model species for understanding the connection between ecology and social organization also in other species.

The social organization of sperm whales in the Gulf of California and comparisons with other populations

Intra-specific variation in social organization provides valuable insights into the selective forces driving social evolution. Sperm whales are distributed globally and live far from shore, thus obtaining large sample sizes on social organization in multiple areas is logistically challenging and few comparative studies exist. In order to address how ecological factors influence sociality, we investigated the social organization of sperm whales in the Gulf of California (GoC) using a long-term study (1998–2004) and compare our results to other published studies. Standard photo-identification and behavioural observation techniques were used. Group size was calculated from photographic mark–recaptures using a Petersen estimator. Social organization was investigated using SocProg 2.3. Mean typical group sizes in the GoC were similar to those in the Galápagos Islands, Chile and Seychelles (24.7, 24.8, 30.4 and 18 individuals respectively), but substantially larger than in the Sargasso Sea, Caribbean and northern Gulf of Mexico (12.0, 6.4 and 6.9 individuals respectively). Sperm whale social organization in the GoC best fitted a constant companion/casual acquaintance model, where permanent units sizes were 12.5 individuals and two units usually associated together to form a group. This structure is similar to the situation in the Galápagos Islands and Chile areas. However, groups were more stable in the GoC than in the South Pacific, as groups stayed together for periods of about 80 days versus about ten days in the Galápagos Islands and Chile. It is likely that differences in the social organization between the study areas in the Pacific and Atlantic Oceans were due to differences in predation pressure and/or food resources. We suggest that, site-specific ecological factors are likely to influence fundamental aspects of sperm whale social organization.

Feral asses ( Equus africanus): intraspecific variation in social organization in arid and mesic habitats

Feral asses have been studied in the arid habitats of the southwestern United States [Moehlman, P.D., 1974. Behavior and ecology of feral asses ( Equus asinus). PhD dissertation, University of Wisconsin, Madison, 251 pp.; Moehlman, P.D., 1979. Behavior and ecology of feral asses ( Equus asinus). Nat. Geogr. Soc. Res. Reports 1970, 405–411.; Woodward, S.L., 1979. The social system of feral asses ( Equus asinus). Z. Tierpsychol. 49, 304–316] and in the mesic habitat of Ossabaw Island, Georgia [Moehlman, P.D., 1979, ibid; McCort, W.D., 1980. The feral asses ( Equus asinus) of Ossabaw Island, Georgia. PhD dissertation, Pennsylvania State University, University Park, 219 pp]. The feral ass populations in these two locales exhibited intraspecific variation in polygynous mating systems and social organization which were consistent with the ecological classification of mating systems of Emlen and Oring (1977) [Emlen, S.T., Oring, S.W., 1977. Ecology, sexual selection, and the evolution of mating systems. Science 197 (4300), 215–223]. Feral asses in the arid environment have a `resource defense' polygynous mating system, and those in the mesic habitat exhibit `female (harem) defense' polygyny. The intraspecific variation observed in feral asses encompasses the interspecific variation observed in the family Equidae.

Feral asses ( Equus africanus) of Volcano Alcédo, Galapagos: behavioral ecology, spatial distribution, and social organization

Feral asses were studied on Volcano Alcédo, Galapagos Islands, Ecuador, during the wet season of 1980. On the volcano rim during March/April, two stable groups were observed to have a `female (harem) defense' polygynous mating system [Emlen, S.T., Oring, S.W., 1977. Ecology, sexual selection, and the evolution of mating systems. Science 197 (4300), pp. 215–223] and social behavior patterns and feeding ecology similar to feral asses living in a habitat where forage and climate are similar, e.g., Ossabaw Island, Georgia [Moehlman, P.D., 1979. Behavior and ecology of feral asses ( Equus asinus). Nat. Geogr. Soc. Res. Rep., 1970, pp. 405–411; Moehlman, P.D., 1997. Feral asses ( Equus africanus): intraspecific variation in social organization in arid and mesic habitats. J. Appl. Anim. Behav. Sci., this issue; McCort, W.D., 1980. The feral asses ( Equus asinus) of Ossabaw Island, Georgia., PhD Dissertation, Pennsylvania State University, University Park, 219 pp.].

Water economy of the Somali donkey.

Water economy of the Somali donkey.