The sternal integument and scent marking in the brushtail possum, Trichosurus vulpecula : gender and seasonal differences

The marsupial brushtail possum (Trichosurus vulpecula) possess a glandular area of skin over its sternum known as the "sternal gland". This region of the integument is composed of two layers: a superficial layer of holocrine sebaceous tissue and a deeper layer of sudoriferous apocrine tissue. Secretions produced by the glandular tissue are rubbed on objects by the possum as a form of scent mark. The secretions are visible as an orange to brown coloured stain on the fur covering the sternum. The structure is found in both males and females. A review of the literature reveals that olfactory communication in the brushtail possum has been studied in some detail. In a variety of observational and experimental studies using captive and free-ranging animals information has been collected on the location and structure of scent glands, the chemical composition of secretions, the role of hormones, the range of scent marking behaviours, the response of conspecifics to odours and the possible function of odours. Despite the broad range of information already collected there are a number of aspects of olfactory communication in the brushtail possum that have not been explored in any detail, and there are many questions about the function of odours that remain unanswered. This study aims to continue the investigation of olfactory communication in the brushtail possum by focusing on the sternal gland and examining differences between the sexes. Three main areas are explored: • An examination of the histology and gross morphology of the sternal gland. • Development of a method of recording sternal gland scent marking under natural conditions. • Investigation of the use and possible function(s) of the sternal gland in the brushtail possum under natural conditions. The first part the study is a histological examination of the sternal integument carried out using tissue from 119 males and 52 female roadkill possums collected over a period of twelve months. Canonical variate analysis was used to look for differences across gender, maturity, season and reproductive status. The histological parameters were: total glandular tissue depth, depth and percentage of holocrine sebaceous tissue and apocrine sudoriferous tissue, holocrine sebaceous and apocrine sudoriferous nuclear diameter, and apocrine sudoriferous cell height and lumen diameter. Information was also collected on changes in the amount of staining of the sternal fur during the year using roadkill animals and animals trapped in the field. A range of differences in the histology of the sternal gland between and within the sexes, between mature and immature animals, and between groups of possums over different seasons was found. Development of the sternal gland is related to the onset of sexual maturity. The tissue of immature males and females is not significantly different, but mature animals have significantly greater tissue development than immature animals. Among sexually mature animals, males show a higher degree of tissue development, having greater glandular tissue depths and a higher percentage of each tissue type, than females. A number of significant seasonal differences in the histology of the sternal gland exist between the sexes and within each sex. The greatest differences between the sexes are seen during the breeding, post-breeding and dispersion periods and are related to differences in the behaviour and activity of each sex at these times. The differences were only observed in the total depth of the glandular tissue and in the holocrine sebaceous tissue parameters.