When to diversify,and with whom? Choosing partners among out-group strangers in lowland Bolivia

Evidence from the ethnographic and archaeological records reveals that humans often rely on out-group relationships for access to non-local resources and resource buffering. However, little is known about how actors choose out-group cooperative partners. The existing literature suggests that (in-group) partner choice is based on characteristics associated with greater cooperation (e.g., trustworthiness and productivity). Is out-group partner choice based on the same criteria as in-group? Because out-groups may be unique sources of resource access, we suggest that out-group partner choice should track characteristics of both the candidate partner and the partner's group that are associated with benefits for the actor. To assess partner choice, we employed a non-anonymous, one-shot economic game where participants could allocate money towards in-group and out-group strangers. Participants were from three populations of Bolivian horticulturalists (n = 200) that range in their market integration and their mobility, thus capturing variation in potential benefits to out-group cooperation. We find that individual-level qualities of prospective partners, such as wealth and trustworthiness, affect allocation behavior differently for in-group vs out-group prospective partners. While we find no consistent effects of perceived group qualities on a donor's giving to in-group and out-group members, the relevance of out-group market resource access for Tsimane' donors' allocations suggests that, at least when it comes to dividing a limited resource, qualities associated with a group can affect partner preference. Taken together, results provide insight into patterns of intergroup relationship building that have been crucial in the human lineage.     

An elementary treatment of the genetical theory of kin-selection

Hamilton's (coefficient of relation) rule for the spread of a gene influencing altruistic behavior is derived under a simple model (discrete generations, single locus, infinite population, panmictic mating) using elementary probability theory. The rule holds quite well with slow selection and it makes no difference whether the gene is dominant or recessive. The rule is altered, however, if the pool of relatives (in interaction) is finite, and I derive a correction factor for interaction between sibs in a sexual haploid. In general the ratio of benefit to cost (necessary for the altruist gene to be favored) increases as the family size decreases.     

Kin selection and ethnic group selection

Ethnicity looks something like kinship on a larger scale. The same math can be used to measure genetic similarity within ethnic/racial groups and relatedness within families. For example, members of the same continental race are about as related (r = 0.18–0.26) as half-siblings (r = 0.25). However (contrary to some claims) the theory of kin selection does not apply straightforwardly to ethnicity, because inclusive fitness calculations based on Hamilton's rule break down when there are complicated social interactions within groups, and/or groups are large and long-lasting. A more promising approach is a theory of ethnic group selection, a special case of cultural group selection. An elementary model shows that the genetic assimilation of a socially enforced cultural regime can promote group solidarity and lead to the regulation of recruitment to groups, and to altruism between groups, based on genetic similarity – in short, to ethnic nepotism. Several lines of evidence, from historical population genetics and political psychology, are relevant here.     

Cross-cultural variation in the development of folk ecological reasoning

Two studies examined children's reasoning about biological kinds in populations that vary in formal education and direct experience with the natural world, a Western (urban U.S.) and a Non-Western population (Tanna, Vanuatu). Study 1 examined children's concepts of ecological relatedness between species (N = 97, 5–13-year-olds). U.S. children provided more taxonomic explanations than Ni-Vanuatu children, who provided more ecological, physiological, and utility explanations than U.S. children. Ecological explanations were most common overall and more common among older than younger children across cultures. In Study 2, children (N = 106, 6–11-year-olds) sorted pictures of natural kinds into groups. U.S. children were more likely than Ni-Vanuatu children to categorize a human as an animal and the tendency to group a human with other animals increased with age in the U.S. Despite substantial differences in cultural, educational, and ecological input, children in both populations privileged ecological reasoning. In contrast, taxonomic reasoning was more variable between populations, which may reflect differences in experience with formal education.     

Conflict over male parentage in social insects

Mutual policing is an important mechanism that maintains social harmony in group-living organisms by suppressing the selfish behavior of individuals. In social insects, workers police one another (worker-policing) by preventing individual workers from laying eggs that would otherwise develop into males. Within the framework of Hamilton's rule there are two explanations for worker-policing behavior. First, if worker reproduction is cost-free, worker-policing should occur only where workers are more closely related to queen- than to worker-produced male eggs (relatedness hypothesis). Second, if there are substantial costs to unchecked worker reproduction, worker-policing may occur to counteract these costs and increase colony efficiency (efficiency hypothesis). The first explanation predicts that patterns of the parentage of males (male parentage) are associated with relatedness, whereas the latter does not. We have investigated how male parentage varies with colony kin structure and colony size in 50 species of ants, bees, and wasps in a phylogenetically controlled comparative analysis. Our survey revealed that queens produced the majority of males in most of the species and that workers produced more than half of the males in less than 10% of species. Moreover, we show that male parentage does not vary with relatedness as predicted by the relatedness hypothesis. This indicates that intra- and interspecific variation in male parentage cannot be accounted for by the relatedness hypothesis alone and that increased colony efficiency is an important factor responsible for the evolution of worker-policing. Our study reveals greater harmony and more complex regulation of reproduction in social insect colonies than that expected from simple theoretical expectations based on relatedness only.     

Kin and kinship psychology both influence cooperative coordination in Yasawa,Fiji

Genes shared through common ancestry are among the oldest social bonds. Despite these ancient roots, humans often co-opt the psychology of genetic relatedness and extend it to genetically unrelated others through culturally-acquired kinship systems. We investigate how genealogical relatedness and kinship norms might mutually support or oppose each other within a known kin network in Yasawa, Fiji. Yasawans' reliance on intensive, kin-based cooperation for daily life makes Yasawan kinship an interesting test case to compare the effects of genealogy and kinship norms. Confirming qualitative ethnographic claims, we find that Yasawan kin terms can be described in two dimensions of respect/closeness and joking/authority. Individual players use different strategies for genealogical relatives and non-relatives by making economic game choices that are increasingly beneficial to partners who share a higher percentage of genes through common ancestry. However, pairs of players are most successful in coordinating their game choices despite conflicting self-interests based upon kinship norms relevant to hierarchy. Thus, while genealogical relatedness may boost generosity, the extra behavioral structuring from kinship norms facilitates more productive but difficult coordinated action even when communication is not possible.     

Social evolution in the shadow of asymmetrical relatedness

The persistence of altruism and spite remains an enduring problem of social evolution. It is well known that selection for these actions depends on the structure of the population—that is, on actors'' genetic relationships to recipients and to the ‘neighbourhood’ upon which the effects of their actions redound. Less appreciated, however, is that population structure can cause genetic asymmetries between partners whereby the relatedness (defined relative to the neighbourhood) of an individual i to a partner j will differ from the relatedness of j to i. Here, we introduce a widespread mechanism of kin recognition to a model of dispersal in subdivided populations. In so doing, we uncover three remarkable consequences of asymmetrical relatedness. First, altruism directed at phenotypically similar partners evolves more easily among migrant than native actors. Second, spite directed at dissimilar partners evolves more easily among native than migrant actors. Third, unlike migrants, natives can evolve to pay costs that far outstrip those they spitefully impose on others. We find that the frequency of natives relative to migrants amplifies the asymmetries between them. Taken together, our results reveal differentiated patterns of ‘phenocentrism’ that readily arise from asymmetries of relatedness.     

Synchronous Rhythmic Interaction Enhances Children’s Perceived Similarity and Closeness towards Each Other

Inter-personal synchronization is important for performing many cooperative tasks. Notably, synchrony has also been shown to have considerable positive social influences, possibly mediated by synchrony-induced changes in social attitude such as an increased sense of similarity and affiliation between interacting individuals. This effect has been demonstrated in adults, but it is unknown whether synchrony might have a similar impact on the social attitudes of children. We thus set to directly examine the influence of synchronous rhythmic interaction on perceived similarity and closeness in pairs of 8–9 year old children. We found that children who had participated in a synchronous interaction regarded their interacting partner as more similar and closer to themselves than children who had not interacted at all or who had taken part in an asynchronous interaction. These findings reveal that synchronous interaction can positively alter social attitudes between interacting children, suggesting a potential mechanism by which synchrony may enhance positive social interaction through attitudinal shift.     

Cooperation and Competition Between Twins: Findings from a Prisoner’s Dilemma Game

Attention to factors influencing cooperation and competition during human social interaction has increased within recent years. This study tested the hypothesis that higher levels of cooperation would be associated with increased genetic relatedness between interactants, and explored questions concerning the expression of cooperative behavior over time. A Prisoner’s Dilemma game, in which participants elect to display cooperative, competitive, or exploitative behaviors relative to a partner, was administered to 59 monozygotic and 37 dizygotic twin pairs, between 10.92 and 82.67 years of age. Results from multivariate analysis of variance procedures, cross-lag sequential analyses, and hierarchical linear modeling supported associations between zygosity, and frequency and continuity of cooperation. Mechanisms by which zygosity may affect cooperation were explored by examining relationships between response combinations, and twins’ IQ similarity and social closeness. The findings are considered with reference to an evolutionary perspective on behavior that offers a theoretical basis for considering how the relative genetic relatedness of social partners affects their social-interactional processes and outcomes. This report is the first in a series of studies designed to address mechanisms underlying differences in cooperation among pairs who vary in average genetic commonality.     

Social closeness increases salivary progesterone in humans

We examined whether interpersonal closeness increases salivary progesterone. One hundred and sixty female college students (80 dyads) were randomly assigned to participate in either a closeness task with a partner versus a neutral task with a partner. Those exposed to the closeness induction had higher levels of progesterone relative to those exposed to the neutral task. Across conditions, progesterone increase one week later predicted the willingness to sacrifice for the partner. These results are discussed in terms of the links between social contact, stress, and health.     

Equal temperature–size responses of the sexes are widespread within arthropod species

Sexual size dimorphism (SSD) is often affected by environmental conditions, but the effect of temperature on SSD in ectotherms still requires rigorous investigation. We compared the plastic responses of size-at-maturity to temperature between males and females within 85 diverse arthropod species, in which individuals of both sexes were reared through ontogeny under identical conditions with excess food. We find that the sexes show similar relative (proportional) temperature–body size (T–S) responses on average. The high degree of similarity occurs despite an analysis that includes a wide range of animal body sizes, variation in degree of SSD and differences in the sign of the T–S response. We find no support for Rensch''s rule, which predicts greater variation in male size, or indeed the reverse, greater female size variation. SSD shows no systematic temperature dependence in any of the 17 arthropod orders examined, five of which (Diptera, Orthoptera, Lepidoptera, Coleoptera and Calanoida) include more than six thermal responses. We suggest that the same proportional T–S response may generally have equivalent fitness costs and benefits in both sexes. This contrasts with effects of juvenile density, and food quantity/quality, which commonly result in greater size plasticity in females, suggesting these variables have different adaptive effects on SSD.     

Growth and Body Mass Index of Brazilians Ages 9 Through 17 Years

Obesity during adolescence is considered a strong predictor of adult obesity, and obesity and overweight have been increasing among Brazilian adults. To gauge the relative frequency of overweight among adolescents in Brazil, we compared the distributions of body mass index (kg/m²) and stature in national population based samples of the U.S. and Brazil. U.S. adolescents were on average about 10 cm taller than Brazilians, although growth spurts occurred at the same age for both populations. Brazilian adolescents were leaner than their U.S. counterparts. This difference was reduced among girls in the postpubertal period. At age 17 years, U.S. boys were about 10 kg heavier than Brazilian boys, but the difference among girls was only 2 kg. In families above the poverty level in the more developed South region, body mass index distribution for boys was closer to that of the U.S., and older girls tended to have higher body mass index than U.S. girls. Within Brazil, body mass index varied by ethnicity with Mulattos, but not Blacks, of both sexes having lower body mass index than Whites of the same age. Urban adolescents had higher body mass index than those living in rural areas. In general, the patterns seen among Brazilian adults were found among children. Among girls, in particular, overweight has become an identifiable problem during adolescence.     

Facial resemblance between women's partners and brothers

Research on optimal outbreeding describes the greater reproductive success experienced on average by couples who are neither too closely related, nor too genetically dissimilar. How is optimal outbreeding achieved? Faces that subtly resemble family members could present useful cues to a potential reproductive partner with an optimal level of genetic dissimilarity. Here, we present the first empirical data that heterosexual women select partners who resemble their brothers. Raters ranked the facial similarity between a woman's male partner, and that woman's brother compared to foils. In a multilevel ordinal logistic regression that modeled variability in both the stimuli and the raters, there was clear evidence for perceptual similarity in facial photographs of a woman's partner and her brother. That is, although siblings themselves are sexually aversive, sibling resemblance is not. The affective responses of disgust and attraction may be calibrated to distinguish close kin from individuals with some genetic dissimilarity during partner choice.     

Aspects of the thermal ecology of the rusty crayfish Orconectes rusticus (Girard)

Summary Orconectes rusticus currently is undergoing an explosive range expansion in the midwestern U.S.A., but information on the potentially important effects of water temperature on the species' biology is lacking. The thermal ecology of O. rusticus in southwestern Ohio, U.S.A., was examined by determining 1) the effects of four water temperatures (16, 20, 25, and 29°C) on survival and growth of juveniles, 2) the responses of juveniles and adults to a thermal gradient (7–27° C), and 3) the thermal tolerances (critical thermal maximum, CTMax, and critical thermal minimum, CTMin) of free-living, field-acclimatized juveniles and adults on a biweekly basis throughout the summer. Month-long growth experiments predicted maximum growth rates of juveniles at water temperatures between 26 and 28°C, but greatest survival between 20 and 22° C. Laboratory-acclimated (22° C) adults and field-acclimatized (2.5° C) juveniles both had an acute preferred temperature of 22° C. CTMaxs and CTMins of juveniles were 0.5–2.6° C higher than those of adults throughout the summer, suggesting that juveniles were exposed to water temperatures 1.5–6.8° C warmer than those of adults. Juvenile and adult O. rusticus prefer habitats where water temperatures favor maximum survival, but they usually are not found together in the same habitat; adults apparently displace the juveniles into warmer habitats. Warmer temperatures can decrease survival of juveniles but improve their growth rates, leading to enhanced fecundity and competitive ability. The past and future success of O. rusticus in expanding its range may depend, in part, on the species ability to adjust to new thermal environments occupied by other species of crayfish.     

Tracking from the Tropics Reveals Behaviour of Juvenile Songbirds on Their First Spring Migration

Juvenile songbirds on spring migration travel from tropical wintering sites to temperate breeding destinations thousands of kilometres away with no prior experience to guide them. We provide a first glimpse at the migration timing, routes, and stopover behaviour of juvenile wood thrushes (Hylocichla mustelina) on their inaugural spring migration by using miniaturized archival geolocators to track them from Central America to the U.S. and Canada. We found significant differences between the timing of juvenile migration and that of more experienced adults: juveniles not only departed later from tropical wintering sites relative to adults, they also became progressively later as they moved northward. The increasing delay was driven by more frequent short stops by juveniles along their migration route, particularly in the U.S. as they got closer to breeding sites. Surprisingly, juveniles were just as likely as adults to cross the Gulf of Mexico, an open-water crossing of 800–1000 km, and migration route at the Gulf was not significantly different for juveniles relative to adults. To determine if the later departure of juveniles was related to poor body condition in winter relative to adults, we examined percent lean body mass, fat scores, and pectoral muscle scores of juvenile versus adult birds at a wintering site in Belize. We found no age-related differences in body condition. Later migration timing of juveniles relative to adults could be an adaptive strategy (as opposed to condition-dependent) to avoid the high costs of fast migration and competition for breeding territories with experienced and larger adults. We did find significant differences in wing size between adults and juveniles, which could contribute to lower flight efficiency of juveniles and thus slower overall migration speed. We provide the first step toward understanding the “black box” of juvenile songbird migration by documenting their migration timing and en route performance.     

Relatedness among arbuscular mycorrhizal fungi drives plant growth and intraspecific fungal coexistence

Arbuscular mycorrhizal fungi (AMF) form symbioses with most plant species. They are ecologically important determinants of plant growth and diversity. Considerable genetic variation occurs in AMF populations. Thus, plants are exposed to AMF of varying relatedness to each other. Very little is known about either the effects of coexisting AMF on plant growth or which factors influence intraspecific AMF coexistence within roots. No studies have addressed whether the genetics of coexisting AMF, and more specifically their relatedness, influences plant growth and AMF coexistence. Relatedness is expected to influence coexistence between individuals, and it has been suggested that decreasing ability of symbionts to coexist can have negative effects on the growth of the host. We tested the effect of a gradient of AMF genetic relatedness on the growth of two plant species. Increasing relatedness between AMFs lead to markedly greater plant growth (27% biomass increase with closely related compared to distantly related AMF). In one plant species, closely related AMF coexisted in fairly equal proportions but decreasing relatedness lead to a very strong disequilibrium between AMF in roots, indicating much stronger competition. Given the strength of the effects with such a shallow relatedness gradient and the fact that in the field plants are exposed to a steeper gradient, we consider that AMF relatedness can have a strong role in plant growth and the ability of AMF to coexist. We conclude that AMF relatedness is a driver of plant growth and that relatedness is also a strong driver of intraspecific coexistence of these ecologically important symbionts.     

Sex differences in political leadership in an egalitarian society

We test the contribution of sex differences in physical formidability, education, and cooperation to the acquisition of political leadership in a small-scale society. Among forager-farmers from the Bolivian Amazon, we find that men are more likely to exercise different forms of political leadership, including verbal influence during community meetings, coordination of community projects, and dispute resolution. We show that these differences in leadership are not due to gender per se but are associated with men's greater number of cooperation partners, greater access to schooling, and greater body size and physical strength. Men's advantage in cooperation partner number is tied to their participation in larger groups and to the opportunity costs of women's intrahousehold labor. We argue these results highlight the mutual influence of sexual selection and the sexual division of labor in shaping how women and men acquire leadership.     

Preliminary Observations of Population Genetics and Relatedness of the Broadnose Sevengill Shark,Notorynchus cepedianus,in Two Northeast Pacific Estuaries

The broadnose sevengill shark, Notorynchus cepedianus, a common coastal species in the eastern North Pacific, was sampled during routine capture and tagging operations conducted from 2005–2012. One hundred and thirty three biopsy samples were taken during these research operations in Willapa Bay, Washington and in San Francisco Bay, California. Genotypic data from seven polymorphic microsatellites (derived from the related sixgill shark, Hexanchus griseus) were used to describe N. cepedianus genetic diversity, population structure and relatedness. Diversity within N. cepedianus was found to be low to moderate with an average observed heterozygosity of 0.41, expected heterozygosity of 0.53, and an average of 5.1 alleles per microsatellite locus. There was no evidence of a recent population bottleneck based on genetic data. Analyses of genetic differences between the two sampled estuaries suggest two distinct populations with some genetic mixing of sharks sampled during 2005–2006. Relatedness within sampled populations was high, with percent relatedness among sharks caught in the same area indicating 42.30% first-order relative relationships (full or half siblings). Estuary-specific familial relationships suggest that management of N. cepedianus on the U.S. West Coast should incorporate stock-specific management goals to conserve this ecologically important predator.     

No evidence of inbreeding avoidance in a polygynous ungulate: the reindeer (Rangifer tarandus)

In polygynous species, mate choice is an integrated part of sexual selection. However, whether mate choice occurs independently of the genetic relatedness among mating pairs has received little attention, although inbreeding may have fitness consequences. We studied whether genetic relatedness influenced females' choice of partner in a highly polygynous ungulate--the reindeer (Rangifer tarandus)--in an experimental herd during two consecutive rutting seasons; the herd consisting of 75 females in 1999 and 74 females in 2000 was exposed to three 4.5-year-old adults and three 1.5-year-old young males, respectively. The females' distribution during peak rut was not influenced by their genetic relatedness with the dominant males of the mating groups. Further, genetic relatedness did not influence the actual choice of mating partner. We conclude that inbreeding avoidance through mating group choice as well as choice of mating partner, two interconnected processes of female mate choice operating at two different scales in space and time, in such a highly female-biased reindeer populations with low level of inbreeding may not occur.     

The primacy of trust within romantic relationships: Evidence from conjoint analysis of HEXACO-derived personality profiles

Mate preference research often focuses on traits that indicate a romantic partner's personal worth (e.g., their physical attractiveness, resource potential) rather than their tendency to leverage that worth for mutual vs. zero-sum benefit (i.e., their trustworthiness). No one has assessed the contribution of trustworthiness to perceived mate value relative to other personality dimensions. Here we examined the desirability of a partner's trustworthiness relative to five other personality indicators of mate quality during initial partner selection. Participants (n = 918) ranked multivariate partner profiles constructed from the HEXACO model of personality (i.e., honesty-humility, emotionality, extraversion, agreeableness, conscientiousness, and openness to experience) and provided partner ratings for each trait. Using conjoint analysis, we found that honesty-humility influenced participants' ranking decisions substantially more than each other characteristic (all Cohen's ds > 0.62). This was true for both long- (i.e., committed) and short-term (i.e., purely sexual) partner evaluations, though honesty-humility was relatively more important for long- vs. short-term contexts. There were no sex differences. A different pattern, including sex differences, emerged for partner ratings. Based on these findings, we hypothesize that the challenge of avoiding romantic interpersonal predation may have been a relatively stronger selection pressure during the evolution of human mate preference than has the challenge of identifying other valuable partner traits.