The ‘social brain hypothesis’ for the evolution of large brains in primates has led to evidence for the coevolution of neocortical size and social group sizes, suggesting that there is a cognitive constraint on group size that depends, in some way, on the volume of neural material available for processing and synthesizing information on social relationships. More recently, work on both human and non–human primates has suggested that social groups are often hierarchically structured. We combine data on human grouping patterns in a comprehensive and systematic study. Using fractal analysis, we identify, with high statistical confidence, a discrete hierarchy of group sizes with a preferred scaling ratio close to three: rather than a single or a continuous spectrum of group sizes, humans spontaneously form groups of preferred sizes organized in a geometrical series approximating 3–5, 9–15, 30–45, etc. Such discrete scale invariance could be related to that identified in signatures of herding behaviour in financial markets and might reflect a hierarchical processing of social nearness by human brains.

Red coloration is a sexually selected, testosterone-dependent signal of male quality in a variety of animals^1,2,3,4,5, and in some non-human species a male's dominance can be experimentally increased by attaching artificial red stimuli⁶. Here we show that a similar effect can influence the outcome of physical contests in humans — across a range of sports, we find that wearing red is consistently associated with a higher probability of winning. These results indicate not only that sexual selection may have influenced the evolution of human response to colours, but also that the colour of sportswear needs to be taken into account to ensure a level playing field in sport.

We argue that grooming is a commodity that female primates can trade, either for itself or in exchange for other services (sensu biological markets theory) and that the decision to do either will depend on the degree of competition within a social group. We test this using data from four chacma baboon troops, living in two populations that differ markedly in the degree of contest competition. As predicted by the predominance of grooming dyads in which females are closely ranked there was, in all four troops, a positive correlation between the time invested by one partner and that by the other. In addition, as predicted, the allocation of time was more closely matched in troops where grooming could not be exchanged for anything else. In troops where resource competition was high, we found in one of two troops a positive relationship between rank distance and the discrepancy in time allocation, with the lower ranking of the partners contributing more grooming.

Although ecologists have long recognized that animal space use is primarily determined by the presence of predators and the distribution of resources, the effects of these two environmental conditions have never been quantified simultaneously in a single spatial model. Here, in a novel approach, predator‐specific landscapes of fear are constructed on the basis of behavioral responses of a prey species (vervet monkey; Cercopithecus aethiops), and we show how these can be combined with data on resource distribution to account for the observed variation in intensity of space use. Results from a mixed regressive–spatial regressive analysis demonstrate that ranging behavior can indeed be largely interpreted as an adaptive response to perceived risk of predation by some (but not all) predators and the spatial availability of resources. The theoretical framework behind the model is furthermore such that it can easily be extended to incorporate the effects of additional factors potentially shaping animal range use and thus may be of great value to the study of animal spatial ecology.