ObjectiveLiving primates vary considerably in tail length‐body size relation, ranging from tailless species to those where the tail is more than twice as long as the body. Because the general pattern and determinants of tail evolution remain incompletely known, we reconstructed evolutionary changes in relative tail length across all primates and sought to explain interspecific variation in this trait.MethodsWe combined data on tail length, head‐body length, intermembral index (IMI), habitat use, locomotion type, and range latitude for 340 species from published sources. We reconstructed the evolution of relative tail length to identify all independent cases of regime shifts on a primate phylogeny, using several methods based on Ornstein‐Uhlenbeck (OU) models. Accounting for phylogeny, we also examined the effects of habitat, locomotion type, distance from the equator and IMI on interspecific variation in tail length‐body size relation.ResultsPrimate tail length is not sexually dimorphic. A phylogenetic reconstruction allowing multiple optima explains the observed regime shifts best. During the evolutionary history of primates, relative tail length changed 50 times under an OU model. Specifically, relative tail length increased 26 and decreased 24 times. Most of these changes occurred among Old World primates. Among the variables tested here, interspecific variation in IMI and the difference between leaping and non‐leaping locomotion explained interspecific variation in relative tail length: Evolutionary decreases in relative tail length are generally associated with an increase in IMI and an absence of leaping behavior.ConclusionsRegime shifts for relative tail length in living primates occurred in concert with fundamental changes in IMI and a change from leaping to non‐leaping locomotion, or vice versa. Exceptions from this general pattern are linked to the presence of a prehensile tail or specialized foraging strategies. Thus, the primate tail appears to have evolved in functional coordination with limb proportions, presumably to assist body balance.

Proactive information donation is crucial for human cumulative cultural evolution. Evidence for proactive information donation in nonhuman animals is limited to a few examples, and its evolutionary origin remains debated. Here, we examine the role of cooperative breeding for the evolution of proactive information donation by comparing cooperatively breeding common marmosets and an independently breeding sister taxon, the squirrel monkeys, in a series of food sharing experiments. Food sharing was virtually absent in squirrel monkeys but highly prevalent in marmosets. Furthermore, experienced adult marmosets shared more food with immatures when food was more difficult than easy-to-access. This increase was mainly driven by the experienced adults, who proactively initiated sharing more often in the difficult condition, consistent with proactive information donation. These notable species differences have significant implications for the evolution of information donation in cooperatively breeding humans and its absence in other great apes who all breed independently.

Human hyper-cooperativity and the emergence of division of labor enables us to not only solve problems effectively within a group but also collectively. Collective problem solving occurs when groups perform better than the additive performance of separate individuals. Currently, it is unknown whether this is unique to humans. To investigate the evolutionary origin of collective problem solving and potential precursors, we propose a continuum of group effects on problem solving, from simple to complex ones, eventually culminating in collective problem solving. We tested captive common marmosets with a series of problem-solving tasks, either alone or in a group. To test whether the performance of a group was more than the sum of its parts, we compared real groups to virtual groups (pooled scores of animals tested alone). Marmosets in real groups were both more likely to solve problems than marmosets within the virtual groups and to do so faster. Although individuals within real groups approached the problem faster, a reduction in neophobia was not sufficient to explain the greater success. Success within real groups arose because animals showed higher perseverance, especially after a fellow group member had found the solution in complex tasks. These results are consistent with the idea that group problem solving evolved alongside a continuum, with performance improving beyond baseline as societies move from social tolerance to opportunities for diffusion of information to active exchange of information. We suggest that increasing interdependence and the adoption of cooperative breeding pushed our ancestors up this scale.

Heintz & Scott-Phillips propose that the partner choice ecology of our ancestors required Gricean cognitive pragmatics for reputation management, which caused a tendency toward showing and expecting prosociality that subsequently scaffolded language evolution. Here, we suggest a cognitively leaner explanation that is more consistent with comparative data and posits that prosociality and eventually language evolved along with cooperative breeding.

Although the spread of innovations through social learning is well documented in animals, resulting animal cultures have remained simple without an increase in complexity over time. Human culture, in contrast, evolves constantly and is unparalleled in terms of complexity and diversity. Why only human culture is cumulative is the subject of ongoing debates, but the most prevalent suggestions are that animals lack high-fidelity transmission and complex innovations. This article examines how the combination of two factors may have helped humans overcome these limitations: first, our having a big brain, inherited from our great-ape-like ancestors; second, our reliance on extensive allomaternal care that evolved convergently with other cooperatively breeding species. We provide support for this suggestion with recent evidence from cooperatively breeding common marmosets ( Callithrix jacchus), showing that motivation for cooperation can trump intelligence when it comes to solving problems and information transmission to the next generation.