The Evolutionary Morphology of Tree Gouging in Marmosets

Abstract

The marmosets, Callithrix spp. and Cebuella pygmaea, are unique among anthropoids in their habitual biting of trees with their anterior teeth to elicit exudate flow. This tree-gouging behavior is thought to offer certain ecological benefits to marmosets, such as routine access to an under-exploited resource, as well as have specific influences on their behavioral ecology.In order to better understand morphological adaptations for tree gouging in the marmoset masticatory apparatus, we characterized the mechanics of this behavior in a laboratory setting and compared these data to field observations of common marmosets (Callithrix jacchus) in northeast Brazil. Common marmosets generate biting forces up to eight times their body mass when biting simulated tree substrates in the laboratory. When gouging, however, marmosets are not biting as forcefully as they can. Comparisons of the mechanical properties of laboratory substrates with trees gouged in the wild suggest that gouging forces observed in the laboratory are comparable to those that marmosets use during gouging in the wild. Moreover, marmosets use relatively wide jaw gapes during gouging both in the laboratory and in the wild. These wide jaw gapes during gouging approach the maximum structural capacity for jaw opening in common marmosets.Morphological comparisons of masticatory apparatus form between gouging marmosets and nongouging tamarins corroborate these laboratory and field data. Marmosets do not exhibit morphologies that offer increased force generation or load resistance abilities. Furthermore, marmosets exhibit several morphologies of their masticatory apparatus that facilitate increased jaw opening. Previous suggestions that marmoset gouging involves relatively large bite forces likely misrepresent the nature of this behavior. Instead, marmosets need only to score the tree bark to elicit the tree's defense response of exudate flow.