Group structure in a social huntsman spider (Delena cancerides) reveals seasonal variation in group complexity

Unlike all other social spiders, the social huntsman spider, Delena cancerides, has been reported to rapidly respond to non-nestmates with lethal aggression, similar to the behavior of some eusocial insects. We tested for the presence of nestmate recognition in D. cancerides under laboratory conditions by introducing 105 unrelated alien conspecifics into foreign colonies and comparing their behavior to 60 control spiders removed and returned to their natal colony. Spiders demonstrated nestmate recognition by investigating alien spiders far more than nestmates and by resting closer to nestmates than to aliens. Serious attacks or deaths occurred in 23% of all trials; however, aggression was not directed significantly more toward aliens than to nestmates. Most notably, aggression was largely mediated by the adult females (resident or alien), who were most likely to attack or kill other subadult or mature individuals. Young individuals (resident or alien) were largely immune from serious aggression. Spiders recently collected from the field tended to be more aggressive than spiders born and raised in the laboratory, possibly due to blurring of recognition cues related to laboratory husbandry. Our findings support the prediction that nestmate recognition should evolve when there is a benefit to discriminating against non-kin, as in this social spider system where foraging individuals may enter a foreign colony and the colony retreat is a limited resource.

Movement among social groups interacts with the costs and benefits of group-living in complex ways. Unlike most other social spiders, the social huntsman spider, Delena cancerides, appears to enter foreign colonies, discriminates kin from non-kin, and has very limited dispersal options because their bark retreats are rare, making this species an interesting model organism with which to examine the role of inter-colony movement on group-living. We examined movement among field colonies of D. cancerides in three ways: (1) by tracking the dispersal and immigration of marked spiders into foreign colonies; (2) by recording resident spiders' behaviour toward introduced immigrants; and (3) by inferring intra-colony relatedness and immigration patterns through allozyme electrophoresis. Of the marked spiders, only young juveniles moved into neighbouring colonies, whereas subadults and adults did not. Introduced juveniles were tolerated in foreign colonies, whereas introduced adult males and subadults were usually attacked by the resident adult female, unless she had similar sized subadult/adult offspring of her own. Allozyme profiles from unmanipulated field colonies showed that 47% of sampled colonies contained at least one immigrant and that average within colony relatedness was below 0.5. These data align with previous research on the costs and benefits of group-living for D. cancerides, suggesting that spiders actively seek and regulate group membership based on interests of both the immigrant and the colony.

Nearly all social spiders spin prey-capture webs, and many of the benefits proposed for sociality in spiders, such as cooperative prey capture and reduced silk costs, appear to depend on a mutually shared web. The social huntsman spider, Delena cancerides (Sparassidae), forms colonies under bark with no capture web, yet these spiders remain in tightly associated, long-lasting groups. To investigate how the absence of the web may or may not constrain social evolution in spiders, we observed D. cancerides colonies in the field and laboratory for possible cooperative defense and foraging benefits. We observed spiders’ responses to three types of potential predators and to prey that were introduced into retreats. We recorded all natural prey capture over 447 h both inside and outside the retreats of field colonies. The colony’s sole adult female was the primary defender of the colony and captured most prey introduced into the retreat. She shared prey with younger juveniles about half the time but never with older subadults. Spiders of all ages individually captured and consumed the vast majority of prey outside the retreat. Young spiders benefited directly from maternal defense and prey sharing in the retreat. However, active cooperation was rare, and older spiders gained no foraging benefit by remaining in their natal colony. D. cancerides does not share many of the benefits of group living described in other web-building social spiders. We discuss other reasons why this species has evolved group living.

Cooperative breeding is a system where helper individuals care for breeding individuals’ offspring. As a result, social environment is likely to play a key role in regulating reproductive success. In primates, cooperative breeding is only found in the family Callitrichidae. Callitrichid males typically provide more infant care than non-breeding females, and in many callitrichid species, the presence of multiple males has been linked to infant survival. Leontopithecus chrysomelas (the golden-headed lion tamarin) is an endangered callitrichid found in the Atlantic Forest of Brazil. We used long-term data for wild L. chrysomelas to assess the influence of social group composition on reproductive success. Our survival model found that infant survival was negatively associated with group size, but this cost was mitigated by the presence of multiple adult males vs a single adult male. We also found that infants raised in groups with multiple adult males exhibited faster growth rates and higher adult weights than infants raised with a single adult male. This study adds novel evidence for the positive influence of adult males on callitrichid reproduction, demonstrating that adult males influence infant growth, as well as survival, in wild populations of cooperatively breeding primates. We suggest that social group composition, particularly the presence of adult males, be considered in future conservation strategies given its importance for reproductive success. In cooperatively breeding species, group members care for breeding individuals’ offspring. Due to this care, group composition may have a strong influence on infant success. In cooperatively breeding primates, males often provide more infant care than females. We investigated the influence of group composition on infant success in a cooperatively breeding primate, the golden-headed lion tamarin. Using long-term field data, we found that infant survival decreased as group size increased. However, this effect was reduced when multiple adult males were present in the group compared to a single male. We also found that infants grew faster and reached larger adult weights in the presence of multiple adult males compared to a single male. Our results demonstrate the importance of group composition for cooperative breeders and provide new evidence for the positive influence of adult males on cooperatively breeding primate infants.

A molecular phylogeny of the Australian huntsman spiders (Sparassidae, Deleninae): Implications for taxonomy and social behaviour

Although the benefits of group foraging are important for evolution of sociality in spiders, the factors that influence group-level benefits of prey sharing in social spiders are still poorly understood. In the unusual transitional social spider Delena cancerides Walckenaer 1837 (Sparassidae), prey sharing almost certainly occurs occasionally among non-kin in the wild, and so we tested the effects of kin relationships and familiarity on the amount of prey consumed in this species. To determine whether the amount of prey sharing increased with relatedness or with familiarity, we fed treatment groups containing spiderlings of varying relatedness and familiarity a single prey item and measured the amount of weight gained by sharing groups. We found no effect of relatedness or familiarity on the amount of prey consumed by prey-sharing groups of D. cancerides. Increased duration of sharing, number and age of the spiders involved, and size of the prey item all increased the amount of prey consumed. The benefits of prey sharing in this species likely overwhelm any possible inclusive fitness benefits derived from kin discrimination in this highly outbred social spider. Hence, we reject the hypothesis that groups of kin consumed proportionately larger amounts of prey biomass than groups of non-kin, as proposed by Schneider and Bilde in 2008 with Stegodyphus lineatus Latreille 1817 (Eresidae).

The influence of siblings on body condition in a social spider: is prey sharing cooperation or competition?

fully solitary lifestyle. Most of the others are characterized by permanent associations between two or more adults and their young. Pair living is relatively rare, being characteristic of only 3% to 4% of primate species. Some of these socially monogamous species (e.g., tamarins and marmosets) have a flexible social organization,inwhich oneadditionaladultmale orfemalemaybetemporarilyassociatedwiththe breeding pair. Stable groups with one adult male and several reproductively active females (i.e., harems) are shown by approximately 35% of primate species. The most common type of social organization in nonhuman primates consists of social groups with multiple adult males and females and their young. These multimale/ multifemale social groups represent the stable form of organization of about 45% primate species, with an additional 15% of species showing fluctuations between such groups and groups with a harem structure. Variation in social organization among primate species has been explained on the basis of variation in ecological variables such as diet, food-related competition and cooperation (e.g., Wrangham, 1980 AQ1 ), and need for protection from predators (e.g., van

Molecular genetic evidence for social group disruption of wild vicuñas Vicugna vicugna captured for wool harvest in Chile

Recent comparative studies on the evolution of mammalian sociality came to opposite conclusions regarding the direction and drivers of evolutionary transitions in social organization, particularly concerning the evolution of pair-living among primates. Because the genus Eulemur is one of the few exceptional primate genera that contain both group- and pair-living species, eulemurs offer a rare opportunity to illuminate the processes driving social diversity among species with very similar life histories and ecologies. The aims of our study were therefore 1) to reconstruct the evolution of social organization in eulemurs with new demographic and phylogenetic data and 2) to evaluate several hypotheses about the evolution of pair-living with data from our long-term study of Eulemur rufifrons and published data on other eulemurs. Our review of published data confirmed that E. mongoz and E. rubriventer are pair-living, whereas the 10 other species of Eulemur live in groups with multiple adult males and females. Furthermore, pair-living evolved at least once among eulemurs from group-living ancestors. The available comparative evidence indicated that this evolutionary transition was probably not driven by male infanticide, social instability, or singular ecological factors. Instead, we propose that female competition has favored small female group size and that this effect may have been exacerbated by interspecific competition with sympatric congeners in the ancestors of the pair-living species. In conclusion, we confirmed that the evolution of pair-living from group-living ancestors among true lemurs is exceptional, but not unique, among mammals, and that commonly invoked hypotheses for the evolution of pair-living do not appear to be supported for this genus.

Beaked whales (Ziphiidae), with 24 currently recognized species, are arguably the least known large animals on the planet, despite being widespread and at times abundant.Mesoplodon, with 16 currently recognized species, is by far the most speciose cetacean genus. Recent technological and taxonomic advances, long-term photographic-identification studies, and historical whaling data have allowed new insights into their social and mating strategies and how these may have driven diversification within the family. In most beaked whales, only adult males have exposed teeth—a single “tusk” erupts from each lower jaw and is used in contests to determine access to breeding females. How forcefully males of different species engage conspecifics varies widely based mainly on differences in tooth size/placement and jaw structure. We compiled data on key dimorphic traits including beak modification, tooth size and location, and prevalence of scarring in adults, for all beaked whales. More detailed information is given for the four best-studied species—northern bottlenose whale and Baird’s, Cuvier’s, and Blainville’s beaked whales. We then compared these traits with what is known about their social organization and reproductive anatomy to make inferences about mating strategies. More aggressive species tend to occur in small groups with only one dominant adult male present and have small testes relative to body size, suggesting that male reproductive success is largely determined by precopulatory contest competition and female defense polygyny. Less aggressive species tend to occur (at least at times) in larger, mixed-sex groups with multiple adult males present, and females may mate with multiple males, which favors postcopulatory sperm competition and polygynandry. We further discuss how conflicting pressures arising from males using their beaks for both feeding and fighting could have sparked an adaptive radiation in tooth development and beak morphology among beaked whales, especially withinMesoplodon, which would have had implications for male-male competition, social structure, sexual strategies, and, perhaps ultimately, evolutionary divergence and speciation within this group.

Social behaviour in spiders is rare: of the 39 000 species of spiders known, only 23 are considered to be cooperatively social.Delena canceridesis a social species of the huntsman spider that is endemic to Australia. This species is virtually unique among social spiders, having evolved social behaviour in the absence of a snare web. It is thought that this form of social behaviour inD. cancerideshas evolved via the sub‐social route, that is, the extension of an ancestrally occurring period of maternal care and the delayed dispersal of juveniles. Most social spiders show no aggression towards non‐kin conspecifics, prompting suggestions that spiders cannot recognize kin; however,D. canceridesindividuals are highly aggressive towards conspecifics introduced from outside their own colony. In order to determine whether selective aggression inD. cancerideshas its basis in kin recognition, tolerance behaviour was assessed in the context of kinship and size. We observed that, in general, juveniles preferred to starve than engage in cannibalism of any conspecifics, related or not. However, where cannibalism did occur, non‐kin were preferentially eaten, indicating that this species is clearly capable of kin recognition. Size thresholds were also established, below which juveniles are tolerated by adults and above which aggressive interactions leading to death occur. We conclude that kin recognition and juvenile dispersal explain the uncharacteristically high levels of genetic polymorphism in this species.

We tested Kanter’s (1977a, 1977b) theory concerning the effects of group proportions (sex ratios) on visibility, polarization and assimilation, using natural groups of women and men in academia. Study 1 compared male-skewed and male-tilted settings and found evidence of greater polarization by minority women than majority men. The only effect of group proportions occurred for perceived dispersion as a measure of assimilation; replicating Brown and Smith (1989), men showed an out-group (OH), and women an in-group (IH), homogeneity effect, and both effects were accentuated in the skewed setting. Study 2 extended the research to include male-skewed, male-tilted, balanced and female-tilted sex ratios. Men’s OH effect declined as relative out-group size increased, and women’s IH effect declined as relative in-group size increased. There was also a linear decrease in relative perceived in-group impact and status as actual relative in-group size declined. We discuss our findings with respect to the validity of Kanter’s theory, gender and group size as moderators of perceived variability, and methodological issues in studying diversity.

Balanced (1 to 1) sex ratios are thought to evolve by a process known as frequency- dependent selection of the minority sex. Five populations of a fish with genetically based variation in temperature-dependent sex determination were maintained for 5 to 6 years in artificial constant-temperature environments that initially caused the sex ratio to be highly skewed. Increases in the proportion of the minority sex occurred in subsequent generations until a balanced sex ratio was established, thus confirming a central premise underlying the theory of sex-ratio evolution.

From 1983 to 2018, in the Alas Purwo National Park (APNP) beach, the olive ridley sea turtle (Lepidochelys olivacea) nested population showed a significantly increasing number compared to the other three species using the same beach. One of the reasons is believed to be the success of conservation efforts (nest relocation and protected egg incubation) to produce a balanced hatchling sex ratio. However, a balanced sex ratio is only the first step in maintaining a good number of individuals in the population. We reviewed several other factors that also influenced the increase in the olive ridley population. The Olive Ridley as the shortest sea turtle species reaches maturity faster, so the population can grow in a shorter time compared to other species. The shortage of males in the olive ridley sea turtle population might not have a dangerous effect because, owing to the ocean current, they can mate with males from other populations. They are also highly polygamous and polyandrous, and have a high degree of genetic variation that helps them to be more adaptable to the changing environment. Lastly, the low illegal hunt rate for this species can also help the population to thrive.