Conference Paper

Relative leg-to-arm strength proportions in Bornean and Sumatran orangutans

March 2022

Conference: Relative leg-to-arm strength proportions in Bornean and Sumatran orangutans

Authors:

Alexandra Kralick

University of Pennsylvania

Lakehead University Thunder Bay Campus

Differences in habitat continuity and predators have been posited as explanations for higher rates of terrestrial locomotion in Bornean compared to Sumatran orangutans. However, it is unclear whether greater terrestriality in Bornean orangutans is due to recent environmental changes to their habitats. Orangutan skeletons collected a century ago may shed new light on this question as habitat continuity on both islands would have been greater in the past. In this study, cross-sectional geometry ratios were calculated for orangutans from CT scans. Both orangutan taxa show significantly less relative leg-to- arm strength than all gorillas, even those that climb the most (i.e., western lowland gorillas and infant mountain gorillas). Moreover, Bornean orangutans display limb bone strength proportions that are consistent with more terrestriality than their Sumatran counterparts. Thus, recent habitat discontinuity in Borneo does not explain greater terrestriality in Bornean orangutans, as also suggested by camera trap data, and is most likely due to the absence of tigers on Borneo.

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Rationality and cognitive bias in captive gorillas’ and orang-utans’ economic decision-making

December 2022 · PLoS ONE

Human economic decision-making sometimes appears to be irrational. Partly, this is due to cognitive biases that can lead to suboptimal economic choices and context-dependent risk-preferences. A pertinent question is whether such biases are part of our evolutionary heritage or whether they are culturally acquired. To address this, we tested gorillas ( Gorilla gorilla gorilla) and orang-utans ( ... [Show full abstract] Pongo abelii ) with two risk-assessment experiments that differed in how risk was presented. For both experiments, we found that subjects increased their preferences for the risky options as their expected gains increased, showing basic understanding of reward contingencies and rational decision-making. However, we also found consistent differences in risk proneness between the two experiments, as subjects were risk-neutral in one experiment and risk-prone in the other. We concluded that gorillas and orang-utans are economically rational but that their decisions can interact with pre-existing cognitive biases which modulates their risk-preference in context-dependent ways, explaining the variability of their risk-preference in previous literature.

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The curious case of great ape curiosity and how it is shaped by sociality

June 2022 · Ethology

Although curiosity has huge implications for human creativity and learning, its evolutionary roots and function in animals remain poorly understood. Modern humans, who lack natural predators, thrive with curiosity, but our ancestors faced more hazardous environments that would not necessarily favor individual curiosity. Instead, being curious may have undergone selection in interaction with ... [Show full abstract] sociality. Our closest living relatives, the great apes (henceforth apes) have evolved facing conditions more like human ancestors and as such, can help us understand the functions of curiosity and its expression in non‐human species. In this study, we defined curiosity as a combination of behavioral traits like neophilia, exploration diversity, and prolonged interest in exploring novelty and compared it, under similar captive environments across four ape species (N = 101): Pan troglodytes, Pan paniscus, Pongo abelii, and Pongo pygmaeus. Results revealed that curiosity followed a linear gradient across the four species in accordance with their sociality. We propose the social curiosity hypothesis to explain the observed pattern, reflecting those individuals in highly social species, like bonobos and chimpanzees, regularly are accompanied by conspecifics, and thereby accustomed to an abundance of social cues, leading to inhibited curiosity when alone, compared to more solitary orangutans. As such, our study implies that ape curiosity evolved interlinked with sociality. Further, a subset of the sample (N = 46) enabled us to examine if curiosity benefits problem‐solving skills, but our data did not support such link. Social influences on intrinsic curiosity in great apes: The ape species in our sample (chimpanzees, bonobos, Sumatran‐ and Bornean orangutan) differ in curiosity but not problem‐solving skills. The less sociable a species of great ape is in the wild, the more curious the individual when tested alone. Ape curiosity may have evolved interlinked with a species social system.

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Covariation between the cranium and the cervical vertebrae in hominids

January 2022 · Journal of Human Evolution

The analysis of patterns of integration is crucial for the reconstruction and understanding of how morphological changes occur in a taxonomic group throughout evolution. These patterns are relatively constant; however, both patterns and the magnitudes of integration may vary across species. These differences may indicate morphological diversification, in some cases related to functional ... [Show full abstract] adaptations to the biomechanics of organisms. In this study, we analyze patterns of integration between two functional and developmental structures, the cranium and the cervical spine in hominids, and we quantify the amount of divergence of each anatomical element through phylogeny. We applied these methods to three-dimensional data from 168 adult hominid individuals, summing a total of more than 1000 cervical vertebrae. We found the atlas (C1) and axis (C2) display the lowest covariation with the cranium in hominids (Homo sapiens, Pan troglodytes, Pan paniscus, Gorilla gorilla, Gorilla beringei, Pongo pygmaeus). H. sapiens show a relatively different pattern of craniocervical correlation compared with chimpanzees and gorillas, especially in variables implicated in maintaining the balance of the head. Finally, the atlas and axis show lower magnitude of shape change during evolution than the rest of the cervical vertebrae, especially those located in the middle of the subaxial cervical spine. Overall, results suggest that differences in the pattern of craniocervical correlation between humans and gorillas and chimpanzees could reflect the postural differences between these groups. Also, the stronger craniocervical integration and larger magnitude of shape change during evolution shown by the middle cervical vertebrae suggests that they have been selected to play an active role in maintaining head balance.

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Social pressure drives “conversational rules” in great apes

December 2021 · Biological reviews of the Cambridge Philosophical Society

In the last decade, two hypotheses, one on the evolution of animal vocal communication in general and the other on the origins of human language, have gained ground. The first hypothesis argues that the complexity of communication co-evolved with the complexity of sociality. Species forming larger groups with complex social networks have more elaborate vocal repertoires. The second hypothesis ... [Show full abstract] posits that the core of communication is represented not only by what can be expressed by an isolated caller, but also by the way that vocal interactions are structured, language being above all a social act. Primitive forms of conversational rules based on a vocal turn-taking principle are thought to exist in primates. To support and bring together these hypotheses, more comparative studies of socially diverse species at different levels of the primate phylogeny are needed. However, the majority of available studies focus on monkeys, primates that are distant from the human lineage. Great apes represent excellent candidates for such comparative studies because of their phylogenetic proximity to humans and their varied social lives. We propose that studying vocal turn-taking in apes could address several major gaps regarding the social relevance of vocal turn-taking and the evolutionary trajectory of this behaviour among anthropoids. Indeed, how the social structure of a species may influence the vocal interaction patterns observed among group members remains an open question. We gathered data from the literature as well as original unpublished data (where absent in the literature) on four great ape species: chimpanzees Pan troglodytes, bonobos Pan paniscus, western lowland gorillas Gorilla gorilla gorilla and Bornean orang-utans Pongo pygmaeus. We found no clear-cut relationship between classical social complexity metrics (e.g. number of group members, interaction rates) and vocal complexity parameters (e.g. repertoire size, call rates). Nevertheless, the nature of the society (i.e. group composition, diversity and valence of social bonds) and the type of vocal interaction patterns (isolated calling, call overlap, turn-taking-based vocal exchanges) do appear to be related. Isolated calling is the main vocal pattern found in the species with the smallest social networks (orang-utan), while the other species show vocal interactions that are structured according to temporal rules. A high proportion of overlapping vocalisations is found in the most competitive species (chimpanzee), while vocal turn-taking predominates in more tolerant bonobos and gorillas. Also, preferentially interacting individuals and call types used to interact are not randomly distributed. Vocal overlap (‘chorusing’) and vocal exchange (‘conversing’) appear as possible social strategies used to advertise/strengthen social bonds. Our analyses highlight that: (i) vocal turn-taking is also observed in non-human great apes, revealing universal rules for conversing that may be deeply rooted in the primate lineage; (ii) vocal interaction patterns match the species’ social lifestyle; (iii) although limited to four species here, adopting a targeted comparative approach could help to identify the multiple and subtle factors underlying social and vocal complexity. We believe that vocal interaction patterns form the basis of a promising field of investigation that may ultimately improve our understanding of the socially driven evolution of communication.

Last Updated: 02 Feb 2023