Fluctuating asymmetry in gorilla canines: a sensitive indicator of environmental stress.

Populations of the western lowland gorilla (Gorilla gorilla gorilla) have decreased markedly as a result of human encroachment. The species is therefore likely to have been exposed to substantial environmental stress. Fluctuating asymmetry (FA) is one measure of environmental stress. We examined the expression of FA in the canines and second premolars of the upper jaw of western lowland gorillas. The canines had patterns of FA typical of secondary sexual characters, whereas the expression of premolar FA was typical of non-sexually selected traits. We found that FA in male canines was positively correlated with year of collection (YOC) or year of collection and acquisition (YOCA) of the specimen. This suggested a sustained environmental deterioration. However, there was no relation between FA and YOC, or FA and YOCA, in female canines. There was a correlation between FA and YOC in male premolars, but this was relatively weak and disappeared when FA and YOCA were considered. There was no correlation between FA and YOC or YOCA in the premolars of females. We conclude that these patterns of FA support the argument that sexually selected structures are sensitive to environmental stress but non-sexually selected structures are not.