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captivity, diet, geometric morphometrics, growth, maxilla, sexual dimorphism, variation
Abstract:
Morphological studies typically avoid using osteological samples that derive from cap-
tive animals because it is assumed that their morphology is not representative of wild
populations. Rearing environments indeed differ between wild and captive individu -
als. For example, mechanical properties of the diets provided to captive animals can
be drastically different from the food present in their natural habitats, which could
impact cranial morphology and dental health. Here, we examine morphological differ-
ences in the maxillae of wild versus captive chimpanzees (Pan troglodytes) given the
prominence of this species in comparative samples used in human evolution research
and the key role of the maxilla in such studies. Size and shape were analysed using
three-dimensional geometric morphometric methods based on computed tomogra-
phy scans of 94 wild and 30 captive specimens. Captive individuals have on average
larger and more asymmetrical maxillae than wild chimpanzees, and significant differ-
ences are present in their maxillary shapes. A large proportion of these shape differ-
ences are attributable to static allometry, but wild and captive specimens still differ
significantly from each other after allometric size adjustment of the shape data. Levels
of shape variation are higher in the captive group, while the degree of size variation is
likely similar in our two samples. Results are discussed in the context of ontogenetic
growth trajectories, changes in dietary texture, an altered social environment, and
generational differences. Additionally, sample simulations show that size and shape
differences between chimpanzees and bonobos (Pan paniscus) are exaggerated when
part of the wild sample is replaced with captive chimpanzees. Overall, this study con-
firms that maxillae of captive chimpanzees should not be included in morphological or
taxonomic analyses when the objective is to characterise the species.
