The opportunity cost of walking away in the spatial iterated prisoner's dilemma.

Previous work with the spatial iterated prisoner's dilemma has shown that the ability to respond to a partner's defection by simply "walking away" allows so-called walk away cooperators to outcompete defectors as well as cooperators that do not respond to defection. These findings are important because they suggest a relatively simple route by which cooperation can evolve. But it remains to be seen just how robust the walk away strategy is to ecologically important variables such as population density, strategic error, and offspring dispersal. The results of our simulation experiments show that the evolutionary success of walk away cooperators decreases with decreasing population density and/or with increasing error. This relationship is best explained by the ways in which population density and error jointly affect the opportunity cost of walking away. This opportunity cost also explains why naive cooperators regularly outcompete walk away cooperators in pair-wise competition, something not observed in previous studies. Our results further show that local offspring dispersal can inhibit the evolution of cooperation by negating the protection low population density affords the most vulnerable cooperators. Our research identifies socio-ecological conditions in which forgiveness trumps flight in the spatial iterated prisoner's dilemma.

Cultural Transmission on the Taskscape: Exploring the Effects of Taskscape Visibility on Cultural Diversity.

Culturally transmitted behavior can be structured in its performance both geographically and temporally, in terms of where and when implements are made and used on the landscape (what Ingold calls "the taskscape"). Yet cultural transmission theory has not yet explored the consequences of behaviors transmitted differently due to their enactment at different taskscape locations, what Tostevin calls "taskscape visibility." Here, we use computer simulations to explore how taskscape visibility and forager mobility affect the diversity of two selectively neutral culturally transmitted traits within a single population of social learners. The trait that can be transmitted from residential bases only (lower taskscape visibility) shows greater diversity than the trait that can be transmitted from residential bases and logistical camps (higher taskscape visibility). In addition, increased logistical mobility has a positive effect on the diversity of the trait with the lower taskscape visibility while it generally shows little to no effect on the diversity of the trait with higher taskscape visibility. Without an appreciation for the ways in which taskscape visibility and mobility can structure cultural transmission in space and through time, the difference in the observed equilibrium diversity levels of the two traits might be incorrectly interpreted as resulting from qualitatively different forms of biased cultural transmission. The results of our simulation experiment suggest that researchers may need to take the taskscape visibility into account when inferring cultural transmission from archaeological data.

Modeling the effects of weaning age on length of female reproductive period: implications for the evolution of human life history.

OBJECTIVES: Human life history is unique among primates. Despite our extended lifespan, the length of the female reproductive period is shorter in humans than in our closest living relatives, chimpanzees. Here, we investigate whether this difference could be explained by another unique aspect of human life history-a young weaning age. METHODS: Age-dependent female fertility is modeled with the Brass polynomial. We model female reproductive period length as single locus with multiple alleles. Selection acts on the length of the female reproductive period in an evolutionary agent-based simulation. We quantify the effect of weaning age on the optimal length of the female reproductive period under a range of adult mortality rates. RESULTS: Females sacrifice a smaller proportion of their reproductive potential due to nursing by weaning their offspring at younger ages. As a consequence, the optimal length of the female reproductive period decreases as weaning age decreases, even when adult mortality is low. CONCLUSIONS: Natural selection will favor mutations or strategies that can decrease weaning age without incurring fitness costs. In the presence of younger weaning ages, selection favors a shorter female reproductive period. To the extent that allocare can decrease weaning age without decreasing fitness, its ubiquity in human societies and near absence in other primate societies may explain why women have a shorter reproductive period. Furthermore, allocare may have provided human ancestors with an avenue to decreased weaning age-and, ultimately, a shorter female reproductive period-that was unavailable to their hominoid contemporaries.

Grandmothering and natural selection.

Humans are unique among primates in that women regularly outlive their reproductive period by decades. The grandmother hypothesis proposes that natural selection increased the length of the human post-menopausal period-and, thus, extended longevity-as a result of the inclusive fitness benefits of grandmothering. However, it has yet to be demonstrated that the inclusive fitness benefits associated with grandmothering are large enough to warrant this explanation. Here, we show that the inclusive fitness benefits are too small to affect the evolution of longevity under a wide range of conditions in simulated populations. This is due in large part to the relatively weak selection that applies to women near or beyond the end of their reproductive period. However, we find that grandmothers can facilitate the evolution of a shorter reproductive period when their help decreases the weaning age of their matrilineal grandchildren. Because selection favours a shorter reproductive period in the presence of shorter interbirth intervals, this finding holds true for any form of allocare that helps mothers resume cycling more quickly. We conclude that while grandmothering is unlikely to explain human-like longevity, allocare could have played an important role in shaping other unique aspects of human life history, such as a later age at first birth and a shorter female reproductive period.

Modeling effects of local extinctions on culture change and diversity in the paleolithic.

The persistence of early stone tool technologies has puzzled archaeologists for decades. Cognitively based explanations, which presume either lack of ability to innovate or extreme conformism, do not account for the totality of the empirical patterns. Following recent research, this study explores the effects of demographic factors on rates of culture change and diversification. We investigate whether the appearance of stability in early Paleolithic technologies could result from frequent extinctions of local subpopulations within a persistent metapopulation. A spatially explicit agent-based model was constructed to test the influence of local extinction rate on three general cultural patterns that archaeologists might observe in the material record: total diversity, differentiation among spatially defined groups, and the rate of cumulative change. The model shows that diversity, differentiation, and the rate of cumulative cultural change would be strongly affected by local extinction rates, in some cases mimicking the results of conformist cultural transmission. The results have implications for understanding spatial and temporal patterning in ancient material culture.

Culture, population structure, and low genetic diversity in Pleistocene hominins.

Paleogenomic research has shown that modern humans, Neanderthals, and their most recent common ancestor have displayed less genetic diversity than living great apes. The traditional interpretation that low levels of genetic diversity in modern humans resulted from a relatively recent demographic bottleneck cannot account for similarly low levels of genetic diversity in Middle Pleistocene hominins. A more parsimonious hypothesis proposes that the effective population size of the human lineage has been low for more than 500,000 years, but the mechanism responsible for suppressing genetic diversity in Pleistocene hominin populations without similarly affecting that of their hominoid contemporaries remains unknown. Here we use agent-based simulation to study the effect of culturally mediated migration on neutral genetic diversity in structured populations. We show that, in populations structured by culturally mediated migration, selection can suppress neutral genetic diversity over thousands of generations, even in the absence of bottlenecks or expansions in census population size. In other words, selection could have suppressed the effective population size of Pleistocene hominins for as long as the degree of cultural similarity between regionally differentiated groups played an important role in mediating intraspecific gene flow.

Patterns of subgrouping and spatial affiliation in a community of mantled howling monkeys (Alouatta palliata).

Studies of social affiliation and social spacing offer important insight into the dynamics of subgroup formation and social strategies in living primates. Among the 11 species in the genus Alouatta, mantled howlers (A. palliata) are the only species to consistently form large, stable social groups composed of several adult males and several adult females. In this study, we examine patterns of subgrouping, activity, and partner preferences in a troop of 26-29 wild mantled howling monkeys (including 12-13 marked individuals) inhabiting Isla de Ometepe, Nicaragua. During two study seasons in 2000 and 2001, we simultaneously monitored the size, composition, and activities of individuals in two to three different subgroups. A half-weight association index was used to calculate partner preferences and patterns of spatial association. Results indicate that our howler study troop fragmented into subgroups of 1-20 with subgroups averaging five and six individuals. Subgroup size and membership reflected individual patterns of social affiliation and social tolerance, and in general remained consistent across activities and from year to year. We also found evidence of cliques or social networks of three to four individuals embedded within larger subgroups. A small number of adult males appeared to play an important social role as the nucleus of clique formation. We argue that the persistence of strong male-male and male-female partner preferences in mantled howlers helps to explain the stability of relatively large multimale-multifemale groups.