|The Sternal Integument and Scent Marking in the Brushtail Possum, Trichosurus vulpecula: Gender and Seasonal Differences|
|Kristen L. Hynes|
|School of Zoology
University of Tasmania
The marsupial brushtail possum (Trichosurus vulpecula) possesses 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:
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.
Among mature males differences in sternal gland histology are closely related to the breeding season. During the breeding season and the time leading up to breeding holocrine sebaceous glandular tissue development and activity are at their greatest. These findings are correlated with a number of physiological and behavioural changes observed in male brushtail possums during the breeding period, including increased scent marking activity, an increase in the number of chemical compounds in the secretion, increased prostate size and an increase in testosterone level. Although most of the differences among males appear to be associated with changes in holocrine sebaceous tissue there is some evidence that sudoriferous apocrine tissue parameters show increased development during the period when young are dispersing.
Among mature females seasonal differences in the size of the holocrine sebaceous nuclei were observed, with the greatest development occurring during the pre-breeding period, when females are carrying pouch young. No clear trends in sudoriferous apocrine tissue were apparent. Mature females were also examined with respect to their reproductive state. Although no significant differences were apparent, some variation in the glandular parameters is evident. Holocrine sebaceous tissue shows its greatest development in anoestrus females and its lowest development in oestrus individuals, and the depth of the sudoriferous apocrine tissue is greatest in oestrus females.
The second part of the investigation is a two-year field study conducted to examine sternal gland scent marking in the brushtail possum. This task was made difficult by the nocturnal, cryptic and partly arboreal behaviour of the species. A range of techniques (including: acoustic biotelemetry, direct observation, radio tracking, and implantable transmitters) were trialed to find a suitable method for collecting information on scent marking under natural conditions. The advantages and disadvantages, the success and limitations of each technique are discussed. The method developed and used in this study involved a combination of spool-and-line tracking and application of fluorescent pigments to the sternal region. The combination of these methods has a number of advantages. The materials required are cheap and easy to construct, and they are easily attached and do not appear to affect the behaviour of the animal. It is possible to collect data without an observer being present at the time of the activity, which has the added advantage that the behaviour of the animal is not influenced by the presence of an observer. The technique allows data to be collected on more than one animal at a time and for information on the location and size of the scent mark to be determined.
Spool-and-line tracking and fluorescent pigments were used to investigate the use and possible functions of the sternal gland scent marking in the brushtail possum. Data on the home range, use of dens, the spatial distribution of scent marks within the home range, the types of objects marked, and the timing of scent marking during the year were collected. Seasonal and gender differences were found.
Males had larger home ranges than females. Although the home ranges of males and females overlapped, within each gender there was very little overlap of home ranges. Spool-and-line tracking revealed that individuals cover large areas of their home range during one night and that most of the area within a home range is used, although some areas are used more frequently than others.
The majority of den sites in this study were located off the ground. Dens were used in at least two different ways by possums. Dens found in hollow logs, close to the ground and in trees were often used during the night, in some cases to shelter from inclement weather. During the day den sites in trees were preferred by resting animals, with sites on the ground being used by sick or injured individuals. Between 8 to 12 den sites were recorded per individual, with dens being spread throughout the home range.
Scent marking by brushtail possums was recorded on a variety of objects including, tree trunks, branches of shrubs, clumps of grasses, fallen logs, fallen sticks, branches and bark on the ground, pieces of wood, rocks and traps. Most marks were made on objects on the ground or close to the ground. There is no evidence of boundary marking of the home range by either sex. Some evidence of marking as a method of resource protection is evident.
Maturity, gender and seasonal differences were found in sternal gland marking. No scent marking was observed in sexually immature individuals of either sex. Among mature individuals males were observed to mark more often with the sternal gland than females. The majority of the marks made by males were deposited on or within two metres of a tree or a trap. Females made most of their marks on objects on the ground and only a third were found on or close to trees or traps. The size of scent marks on trees did not differ between sexes, although marks on all other objects were generally larger in males than females. Males performed most marking in the dispersal, pre-breeding and early part of the breeding season. Once mating had occurred and during the time females had young in the pouch sternal scent marking among males was infrequent. Rates of marking in males correlated with changes in the degree of staining of the sternal fur. Among females sternal scent marking was highest when they had young in the pouch, with a lower level occurring during the dispersal phase and during the pre-breeding and breeding seasons in oestrus females. The level of sternal staining was lower during the period of greatest sternal marking in females.
The results of the study indicate that although both male and female brushtail possums posses sternal glands there is significant sexual dimorphism in the gross morphology and histology of the sternal gland and in the deposition of secretions produced by the gland. Reasons for these differences are discussed by examining the possible functions of sternal gland odours and scent marking in the brushtail possum.
Among males there are three distinct periods of sternal gland development and scent marking behaviour. The first occurs during the pre-breeding and breeding periods and is characterised by a higher level of holocrine sebaceous tissue development and scent marking. At this time an increase in scent marking may function to familiarise females with potential mates and/or to deter rival males. The second period in males occurs during post-breeding when females have young in the pouch and is characterised by a low level of gland development and marking behaviour. If the function of odours is to attract a female or deter a rival it is reasonable to expect that marking and gland development would decrease during a period when neither of these functions is operating. The third period corresponds with the dispersal of young and is characterised by greater development of sudoriferous aporcrine elements and an increase in scent marking. Although there is no evidence that scent marks are concentrated around home range boundaries, odours deposited at this time are most likely related to protection of resources such as den trees and feeding trees.
The pattern of sternal gland development and scent marking in mature female possums is most likely related to the protection of resources. During the winter months when females are carrying pouch young an increase in marking may serve to protect trees required for shelter and food.
In both sexes resource protection appears to be the major role for olfactory communication in the brushtail possum. The mechanisms of odour function, however, are more difficult to ascertain. Possible mechanisms of odour function through the establishment of dominance hierarchies among adjacent and overlapping individuals, through "scent matching" and "competitor assessment" by potential intruders and rivals, and through "confidence boosting" and "reassurance" of a resident individual are discussed.
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