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

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Hynes, Kristen L. (Louise) (1999) The sternal integument and scent marking in the brushtail possum, Trichosurus vulpecula : gender and seasonal differences. PhD thesis, University of Tasmania.

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Abstract

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. 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 was 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 dens 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 postbreeding
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.

Item Type: Thesis (PhD)
Keywords: Opossums, Opossums, Opossums
Copyright Holders: The Author
Copyright Information:

Copyright 1999 the Author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
would be pleased to hear from the copyright owner(s).

Additional Information:

Thesis (Ph.D.)--University of Tasmania, 1999. Includes bibliographical references

Date Deposited: 19 Dec 2014 02:39
Last Modified: 30 Aug 2016 05:36
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