Observations of Capybaras, Hydrochoerus hydrochaeris, in the Venezuelan savannah during the dry season show that the sexes differ in the structure and size of their scent glands and in the way they deploy odours. Males have a greatly enlarged nose gland or morrillo, from which they smear copious secretion on vegetation. They also have detachable, secretion-coated hairs within their large anal pockets. Vegetation marked by the morrillo may also be marked by dragging the anal pockets across it, and by urination. The morrillos of females resemble those of males, but are poorly developed and infrequently used. The anal pockets of females are highly developed, and differ externally from those of males in having a more ampulla-like shape and in lacking detachable hairs (osmetrichia). During the dry season, scent marks of all types made by members of both sexes seem to be deployed throughout the Capybaras home range. At this time, males mark more frequently than females, and larger animals have larger, more active morrillos and anal glands than smaller ones, and probably use them more often. The histology of both glands is described: both are composed of sebaceous tissue. The detachable hairs of males were examined through scanning electron microscopy and found to be coated in regular layers of a largely crystaline deposit. The structure of the hairs may be adapted to retain secretion. The chemistry of the secretion of male morrillos and female anal pockets was investigated. The two secretions are different, although both contain lipids including sterols and/or terpenes, and both contain amines and amino acids. The same compounds were generally represented in the secretion of the same gland from different individuals, but in different quantities and proportions, hence suggesting that individual recognition is possible on the basis of a “chemical fingerprint”. The most volatile substance from the male morrillo was a hydrocarbon C30 H50, whereas the bulk of the secretion is a complex mixture of esters.
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