Short-term memory is implicated in a range of cognitive abilities and is critical for understanding primate cognitive evolution. To investigate the effects of phylogeny, ecology and sociality on short-term memory, we tested the largest and most diverse primate sample to date (421 non-human primates across 41 species) in an experimental delayed-response task. Our results confirm previous findings that longer delays decrease memory performance across species and taxa. Our analyses demonstrate a considerable contribution of phylogeny over ecological and social factors on the distribution of short-term memory performance in primates; closely related species had more similar short-term memory abilities. Overall, individuals in the branch of Hominoidea performed better compared to Cercopithecoidea, who in turn performed above Platyrrhini and Strepsirrhini. Interdependencies between phylogeny and socioecology of a given species presented an obstacle to disentangling the effects of each of these factors on the evolution of short-term memory capacity. However, this study offers an important step forward in understanding the interspecies and individual variation in short-term memory ability by providing the first phylogenetic reconstruction of this trait’s evolutionary history. The dataset constitutes a unique resource for studying the evolution of primate cognition and the role of short-term memory in other cognitive abilities.

No abstract

I report the first documented case of a wild siamang killed by a clouded leopard. The event, involving a juvenile male (weighing 3.7 kg), occurred at dawn, near a known sleeping tree in a lowland tropical forest in southern Sumatra (Indonesia). Examination of the siamang’s body revealed that the first 2 cervical vertebrae were crushed and the face was partially eaten. This attack highlights the potential importance of predation as an evolutionary pressure for hylobatids, and poses some questions about the antipredator adaptations of this population of siamang.

The link between laterality in humans and other primates is still hotly debated. Hylobatids have been rather neglected in this research area, yet they can provide important insights because: (1) they share with humans a complex vocal repertoire, which in humans is thought to be associated with brain hemispheric specialization and lateralized behaviors; (2) their adaptation to arboreality has produced unique postural constraints; (3) the little that is known about laterality in gibbons is contradictory (captive studies have provided conflicting results, while a field study on siamangs reported a population-level left-hand preference). To clarify this, we investigated hand preference in captive hylobatids [n = 42; 22 siamangs (Symphalangus syndactylus) and 20 gibbons (Hylobates sp., Nomascus leucogenys)] in nine Japanese facilities. We had a large sample size, controlled for possible confounds (posture, enclosure limitations) and used a well-established testing protocol (tube task). Handedness indices calculated from raw frequencies and bouts were highly correlated and showed a significant left-hand skew, which is consistent with data from wild siamangs. Major differences between captive and wild siamangs were a larger number of ambiguously handed individuals, and no significant age-related variation in captivity. The use of the index finger elicited a much more strongly lateralized response than the thumb. These results confirmed a left-hand preference in siamangs, but were equivocal in other hylobatids, and suggest selective pressures that may have acted on the highly arboreal hylobatids to favor handedness. Our study also indicates factors that might explain the discrepancy in the literature between handedness studies on captive and wild primate populations.

Water is one of the most important components of an animal's diet, as it is essential for life. Primates, as do most animals, procure water directly from standing or free-flowing sources such as pools, ponds and rivers, or indirectly by the ingestion of certain plant parts. The latter is frequently described as the main source of water for predominantly arboreal species. However, in addition to these, many species are known to drink water accumulated in tree-holes. This has been commonly observed in several arboreal New World primate species, but rarely reported systematically from Old World primates. Here, we report observations of this behaviour from eight great ape and Old World monkey species, namely chimpanzee, orangutan, siamang, western hoolock gibbon, northern pig-tailed macaque, bonnet macaque, rhesus macaque and the central Himalayan langur. We hypothesise three possible reasons why these primates drink water from tree-holes: (1) coping with seasonal or habitat-specific water shortages, (2) predator/human conflict avoidance, and (3) potential medicinal benefits. We also suggest some alternative hypotheses that should be tested in future studies. This behaviour is likely to be more prevalent than currently thought, and may have significant, previously unknown, influences on primate survival and health, warranting further detailed studies.