Older animals in a managed setting (e.g., zoos) may require special care. Previous studies have suggested reduced water intake in rhesus macaques may result from cold environment and aging, leading to a decline in their welfare. This study aimed to assess whether providing warm drinking water influences water consumption and preference in aged rhesus macaques. The experiment was conducted on 11 older rhesus macaques at the Kyoto City Zoo, Japan. Observations were conducted for 42 days between November 21, 2024 and April 12, 2025. Containers filled with room-temperature water or warm water were placed in an indoor habitat, and the drinking behavior of the rhesus macaques was recorded for 20 min. No significant difference was found in the drinking frequency between the two water temperatures. However, the time spent drinking in each bout was significantly longer when warm water was provided than when room-temperature water was provided. Furthermore, when room-temperature and warm water were placed simultaneously, the drinking behavior of rhesus macaques was significantly biased toward the warm water container. In other words, rhesus macaques showed a preference for drinking heated water, suggesting that installing warm water may improve the drinking environment for older rhesus macaques.

Appeals to the simplicity of hypotheses about cognitive processes are common in comparative psychology. Much recent work has discussed the role of simplicity in privileging some hypotheses over others. Simpler hypotheses tend to be taken as the default, working hypothesis, so long as there is not any strong evidence against them. Here, I argue that cognitive simplicity also plays a role in hypothesis generation, aiding comparative psychologists to create new hypotheses about behavioural processes. I attempt to justify the role that cognitive simplicity plays here. One approach is to justify that some hypotheses really are simpler than others. Unfortunately, there are several jointly contradictory and individually problematic ways of defining cognitive simplicity that undermine this effort. Instead, I propose that cognitive simplicity is more appropriately interpreted as a family of idealizations about behavioural processes. Idealizations are useful abstractions about phenomena, based on potentially false assumptions, which are justified by serving a purpose for practicing scientists. Idealizations about the properties of behavioural processes help comparative psychologists to creatively generate novel hypotheses about animal behaviour. This is a useful strategy when handling the fact that there are usually several empirically distinct hypotheses that could explain behavioural observations. This view preserves cognitive simplicity as a useful concept for hypothesis generation, while blocking it from involvement in hypothesis selection, in line with previous work.

Humans use proper names as vocal labels to identify and communicate with and about social agents. The comprehension of spoken proper names requires the ability to interpret socially specific verbal signals, or social vocal labels, and use cross-modal perception to identify and discriminate between group members. Individuals that recognize and comprehend familiar proper names can use these labels to identify and discriminate between groupmates, gain third-party knowledge, and guide decision-making. Use of vocal labels for conspecifics is noticeably rare in the animal kingdom, and has only been found in species (dolphins, elephants, and marmosets) that are phylogenetically distant from humans. We therefore investigated the phylogenetic trajectory of this capacity by studying our closest living primate relatives, chimpanzees (Pan troglodytes) and bonobos (Pan paniscus). We implemented a cross-modal non-invasive eye-tracking and playback study with multiple populations of apes (N = 24) living in zoos and sanctuaries, none of whom were specifically language-trained. We tested whether chimpanzees and bonobos spontaneously attend toward an image of a groupmate whose name has been called by a human caretaker. We found limited evidence that apes link the caretaker-given names of their groupmates to images on a screen, and therefore cannot make strong conclusions about apes’ comprehension of these social vocal labels. Our playback and eye-tracking paradigm offers a novel tool for studying cross-modal perception and knowledge of vocal labels. Future work will be critical to identify the sociocognitive foundations underlying socially specific referential communication and the evolution of language.

In two previous experiments, Abramson and colleagues (1996) used a mirage device to demonstrate that honey bees could be influenced by a virtual reality illusion. The current study sought to replicate several key findings of their second experiment by simultaneously presenting honey bees with both a real and illusory target from which to feed. Results for sixteen subjects largely replicated those reported by Abramson and colleagues, but only when individual response patterns were considered. As a group, illusion vs. real target error rates decreased linearly over study trials. However, using organism-centered (i.e., person-centered) analyses, this effect was discovered to be attributable to only two honey bees. Considered as individuals, and consistent with previous findings, the error rates for most of the bees in this sample did not decrease over time. Results therefore support the use of the mirage device in future studies of visual illusions in insects. Implications for analyzing error rates and other important outcomes in such studies are further discussed.

The Geoffroy’s spider monkey (Ateles geoffroyi) has distinctive features, including a vestigial external thumb, elongated fingers and forelimbs, and a prehensile tail. The purpose of the present study was to understand how this derived morphology influences visually mediated reach and grasp movements during fruit picking. Wild spider monkeys, habituated to human observers, were filmed in Sector Santa Rosa (SSR), Área de Conservación Guanacaste in northwestern Costa Rica. We analyzed frame-by-frame video recordings of the monkeys picking 14 fruit species. The most frequent reach strategy was a branch-withdraw (62%; 1,338 of 2,164 fruit items), in which the monkeys hooked their fingers around a branch and pulled it toward themselves, to take the attached fruit by mouth. They sometimes used just their mouth to reach (17% of observations). Reaching with arm and hand extension to pick fruit by hand (21% of observations) was achieved using power grasps that relied on tactile cues. It is likely that picking fruit by hand or mouth involved foveal vision whereas grasping and manipulation branches used peripheral vision. Most fruit-picking sequences featured tail prehension on a branch, which extended the monkeys’ reach horizontally and ventrally into the small distal branches of the canopy. The observed reach strategies and the configuration of visually guided reaching skills of the hands and mouth—contrasted with the absence of visually mediated grasps—indicate that spider monkeys exhibit a reach phenotype. This contrasts with the grasp phenotype of the sympatric capuchin monkey, which uses the thumb and fingers to pick individual fruit items. These findings support the idea that the reach and the grasp have separate evolutionary histories in spider and capuchin monkeys, potentially facilitating fruit harvesting in different microcanopy locations and thereby contributing to niche partitioning.

Birth is one of the most important life history events for eutherian mammals, yet it also remains one of the least understood. Perhaps surprisingly, there is no systematic field of research focused on birth and parturition through a comparative lens. In this paper, we introduce the field of comparative ilithology (named for Ilithyia, the Greek goddess of childbirth and midwifery): the study of birth and parturitional behaviors across species. We outline ten potential new research areas that have historically been overlooked, but which will benefit from comparative study. For each, we provide testable hypotheses and a roadmap for more systematic methodologies and conventions for studying and reporting birth across species. Comparative ilithology has great potential to inform debates around the evolution of human childbirth and inspire novel questions across species surrounding this fundamental life history event.

Changes in the social structure of two neighboring communities of estuarine bottlenose dolphins in Ecuador were assessed over 14 years (2011–2024). During this period, 632 surveys were conducted, covering a cumulative distance of 23,171 km. A total of 133 distinct individuals, representing all age and sex classes, were recorded. The annual abundance of dolphins decreased from 50 in 2014 to 24 in 2024, coinciding with a decline in the proportion of non-resident individuals, from 35% to 0%. Changes in the social structure were analyzed across three periods: 2011–2015, 2016–2019, and 2020–2024. Pairwise cluster analyses revealed two distinct communities (El Morro and Posorja) in the first two periods, which merged into a single community in the third period. Significant social changes were observed during this process as the two communities adapted by sharing spaces and resources, altering spatiotemporal patterns, and increasing their time together to benefit from the fusion. Female mixed groups increased from 0% to 77.8%, the inter-community association rate rose from 0.04 to 0.74, and the average group size grew from 8.07 to 11.48 dolphins/group. Reproductive parameters, including calf survival rates and the fertility of Posorja females, improved following the fusion. These findings reveal a high degree of social flexibility within this population, suggesting that even though inshore bottlenose dolphins typically form discrete social units, these structures can adapt and reorganize under certain demographic or social pressures. Given the historical and emerging threats to this population, understanding the drivers of these social dynamics is critical for the long-term conservation of this endangered population.

Shelter construction has been observed in many fauna but is most common in birds, mammals and insects. Whilst nest construction for birthing and rearing young has been observed in the Eurasian wild boar (Sus scrofa) and captive Visayan warty pigs (Sus cebifrons negrinus), non-nest structures are unheard of in suids until now. The Visayan warty pig is a critically endangered species that is currently restricted to two islands in the West Visayan faunal region of the Philippines. Furthermore, in situ ecological research on this species is severely limited, hampering conservation efforts as it is threatened with habitat loss and population decline. Here, we describe the novel discovery of a warty pig made structure with the potential function of providing thermoregulatory assistance and avoiding heavy rain. This structure appears to be constructed by male individuals in the Visayan warty pig, for assumed weather avoidance and environmental regulation, and whilst there is no conclusive proof of the species architectural ingenuity or structure use, we also highlight identical structures made and used in the same way by the sister species the Mindoro warty pig (Sus oliveri) from a separate corroborating observation. The Visayan warty pig was the first wild pig species to be recorded using tools, it is known for its complex social structures and high levels of intelligence, but now it has elevated its intellectual potential as the first pig species to create artificial structures for shelter.

For group-living species, including humans and nonhuman primates, the ability to navigate social encounters and quickly process threats from others is a critical skill. Rapid detection of threatening stimuli, referred to as an attentional bias toward threat, is adaptive in that fast threat detection can lead to improved survival outcomes. Despite this fitness benefit, the evolutionary roots of attentional bias formation are not well understood, and attentional bias toward social threat is not well studied across the primate phylogeny, particularly across more phylogenetically distant species such as the platyrrhine primates. The present review proposes the use of a comparative perspective to explore the evolutionary origins of this bias, to determine how far back in the primate phylogeny attentional bias toward social threat may have emerged. We discuss the methods that have been used to study attentional bias in humans, and then focus on a popular method for measuring attentional bias in nonhuman primates, the dot probe task. Evidence suggests that humans are not unique in their propensity for showing an attentional bias toward socially threatening stimuli when evaluated with a dot probe task, but there are some nonhuman primate species in need of further study to clarify their susceptibility to this bias. We suggest that the prevalence of attentional bias toward social threat in nonhuman primates can be understood in the context of their respective socioecologies and conclude by discussing future directions that can be taken to explore attentional bias toward social threat in other species.