Measuring facial symmetry in the wild: a case study in Olive Baboons (Papio anubis)
Individuals under stress and those of intrinsically low quality may have insufficient resources to invest in developing equally on both sides. Such individuals may thus display higher levels of fluctuating asymmetry (FA)—small, random departures from perfect bilateral symmetry. These small deviations from symmetry are often less than 1 % of trait size, similar to variation in symmetry generated by measurement error. Studies regarding FA must thoroughly assess measurement error if reliable conclusions regarding FA and fitness parameters are to be drawn. Although many studies regarding facial symmetry in humans have been conducted, few have been carried out on non-human primates. The primary aim of this study was to assess whether facial FA in a non-human primate, the olive baboon (Papio anubis) can be reliably measured from digital photographs. Facial FA was measured in three sets of bilaterally symmetrical landmarks from digital images of 35 olive baboons at Gashaka-Gumti National Park, Nigeria. Measurement error was found to be low indicating a high level of reliability based on comparisons of values from two photos of the same individual (r = 0.85). Measurement error was found to be related to the size of the face. This suggests that orientation error increases in large faces and could potentially influence FA scores. These findings highlight the need for FA studies to measure and control error caused by orientation and position of subjects if FA is to be of utility to behavioural ecology and conservation (e.g. if FA reflects whether individuals in a population are suffering from environmental or genetic stress).
KeywordsDevelopmental stability Fluctuating asymmetry Facial asymmetry Measurement error Papio anubis
Field work benefited from a permit by the Nigerian National Parks Service to the GPP, which receives its core funding from the Chester Zoo Nigeria Biodiversity Programme. NCF/WWF-UK provided logistical support. We thank Volker Sommer for his work as Director of the GPP and for all the help and support he has given us. Thanks also to our field assistant Ibrahim Usman, his help with identification and sample collection was vital, and to Halidi Ilyusu who is a pivotal member of our team carrying out long-term monitoring of the baboons. This paper was improved by constructive comments from two referees and from Elise Huchard; we thank all of them for this. Many thanks also to Balbir Josen, Mary McKenzie and Amanda Morgan for their technical advice and assistance. RB also thanks her family and friends for their advice and support. This is a GPP publication.
Conflict of interest
The authors declare that they have no conflict of interest.
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