Biology subjects

van Dijk, E. S. J.

Publications and source records attributed to van Dijk, E. S. J..

4 recordsLinked to original sources

bioRxiv2026

The social structure of a captive group of Rüppell's vultures (Gyps rueppellii)

Studies on the evolution of intelligence have so far mainly focused on mammals, primates in particular, and only few avian taxa, mainly corvids and parrots. Vultures, however, have relatively large brains too, and those of the Gyps genus are highly social. Their obligate scavenging lifestyle and sociality make them an excellent taxon for investigating the role of social and ecological selective pressures on the evolution of intelligence. However, little is known of Gyps vultures, particularly regarding their social complexity (sensu Social Brain Hypothesis). To investigate the complexity within their social structure, we studied the dominance hierarchy and social bonds of a captive group (n {equiv} 23) of Ruppell's vultures (Gyps rueppellii) over two consecutive years. Our results revealed that dominance ranks and affiliative bonds were stable, although there were some differences between breeding and non-breeding seasons. During the breeding season, pair-bonded individuals were more dominant and less social than unpaired individuals. Pair-bond status, age and season influenced dyadic bond strength. Most notably, pair-bonded dyads had substantially stronger bonds, but maintained multiple other strong bonds, too. Our results highlight that multiple forces shape the dominance hierarchy and affiliative network of captive Ruppell's vultures. Individual characteristics shape social bonds, social relationships are differentiated, and changes between the breeding and non-breeding season can result in social uncertainty, adding to the complexity of this system. The complexity of Ruppell's vultures' social structure seems comparable to that of some social corvid and parrot species, where it has been linked to their sophisticated cognition. Future socio-cognitive work on this and other less social vulture species could further identify the social challenges they face, and reveal whether a complex social environment may have been a selection pressure on the evolution of intelligence, both in general, and in this promising new taxonomic group for cognition studies specifically.

animal behavior and cognition↗

bioRxiv2025

Social status predicts self-directed behaviors, contingent on broader social dynamics

Emotional states like anxiety are adaptive, assisting individuals during social uncertainty and threats. While humans can self-report anxiety, assessing comparable states in animals requires objective measures. Self-directed behaviours (SDBs), including self-scratch, autogroom, and yawn, are indicators of anxiety, particularly in nonhuman primates. Yet it is unclear whether they reflect a common underlying process or distinct responses shaped by demography, social interactions and status, and the broader social dynamics. We studied 13 macaque groups across six species along a despotic-egalitarian social style gradient to investigate the factors associated with variation in SDBs. Our findings revealed partial correlational consistency among the SDBs, where only self-scratch and autogroom positively correlated. Further, we found that males yawned more than females, and individuals receiving less aggression exhibited longer autogroom bouts. Dominance rank showed behaviour-specific effects, with weak evidence for an association with self-scratch, longer autogrooming in subordinates, and a non-linear pattern in yawning, which was lowest at intermediate ranks. Alpha status was linked to more frequent yawning than others. And finally, social style showed no consistent effects on SDBs. Our research demonstrates that SDBs in macaques reflect differential responses related to social uncertainty rather than one anxiety dimension, providing insights into primate emotion regulation.

animal behavior and cognition↗

bioRxiv2025

Despotism promotes cooperation through enhanced interdependencies in non-human primate societies

The evolutionary mechanisms of cooperation are well-studied, yet what motivates individuals to cooperate remains unclear. Although conventionally associated with enhanced social tolerance (self-domestication hypothesis) and prosociality (cooperative-breeding hypothesis), individuals belonging to species that are neither self-domesticated nor cooperatively-breeding also cooperate. An overarching interdependency hypothesis posits that inter-individual dependencies promote cooperation, but comparative evidence is lacking. We experimentally studied cooperation, prosociality, and tolerance in six macaque species along a despotic-egalitarian gradient. Within-group cooperation was higher in despotic than egalitarian societies yet restricted to few partners. Prosociality, kinship, and tolerance positively predicted cooperation success. Agent-based models consistently showed that despotic societies have fewer but more stable social bonds and, thus, higher interdependencies than egalitarian ones. Our results suggest that interdependencies facilitate the emergence and maintenance of cooperation.

animal behavior and cognition↗

bioRxiv2023

Hand preference predicts behavioral responses to threats in Barbary macaques

The structure and functioning of the brain are lateralized - the right hemisphere processes unexpected stimuli and controls spontaneous behavior, while the left deals with familiar stimuli and routine responses. Hemispheric dominance, the predisposition of an individual using one hemisphere over the other, may lead to behavioral differences; particularly, an individual may be programmed to act in a certain way concerning hemispheric dominance. Hand preference is a robust estimator of hemispheric dominance in primates, as each brain hemisphere controls the opposing side of the body. Studies have found links between hand preference and the exhibition of different behaviors in contexts such as exploring and manipulating objects. However, little is known about whether hand preference can predict behavioral variations in other ecologically relevant contexts like, for example, predation. We investigated the relationship between hand preference and the behavioral responses to two types of predator models in captive Barbary macaques (Macaca sylvanus) (n=22). Hand preference was determined by observing unimanual foraging, whereas focus and tension behaviors were quantified during experimental exposure to predator models. We found 91% of the macaques to be lateralized with no group-level bias. In contrast to their right-hand counterparts, individuals with a strong left-hand preference elicited frequent focus and tension behavior. Additionally, the behavioral response varied with predator type. We also found an interaction effect between hand preference and the predator type. Our study suggests that hand preference can reliably predict behavioral variations in the context of potential predation. While these results are consistent with the lateralized brain function, indicating lateralization as a potential neural mechanism of behavioral variation, the interaction effect between hand preference and predator type elucidates the importance of context-specificity when investigating laterality non-invasively. Future research on other non-human primates using the current framework may shed light on the evolution of laterality and underlying behavioral predispositions.

animal behavior and cognition↗