Social organisation and behaviour of the Tasmanian devil, Sarcophilus harrisii
thesisposted on 2023-05-26, 07:10 authored by Pemberton, D
There are two conflicting views regarding the nature of the social organisation of the Tasmanian Devil Sarcophilus harrisii. It has been described either as a polygamous species in which the males hold a dominance hierarchy or a promiscuous species where there is no prolonged association between individuals. It is the aim of this thesis to precisely describe the social organisation and behaviour of the Tasmanian Devil and thereby dispel the confusion. Many studies of the social organisation of highly social animals have been carried out but relatively few on solitary animals such as the Tasmanian Devil. Notwithstanding this, it is known that the density of devil populations can be high in any one area, that they have a notorious aggressive behaviour towards each other and also an elaborate communicatory repertoire. This suggests that the pattern of relationships between individuals of a population could be highly complex. In order to investigate this, a three year field study was carried out on a population at Mt. William, north-east Tasmania. This study investigates and describes the population demography and size, social behaviour and the dispersion pattern of the devil population. A trapping preiod of 500 trap nights conducted over a period of ten days was found to be suitable to effectively sample the population. The largest source of error in calculating the population size was the unequal catchability of animals both within and between trap sessions. For this reason, a number of different methods for calculating population size were used and the results evaluated with respect to each other. The population reached a peak of between 200 and 400 animals from summer through to spring after which it declined to between 80 and 100 animals. This trend did not vary between years. The seasonal pattern was the result of an influx of weaned young into the population in January which remained in the area until August when they dispersed. Females were sexually mature in their second year with 81 % carrying on average 2.3 young, of which 73 % survived to weaning. Fertility decreased with age. There was a highly synchronised birth-pulse taking place over a three-week period with April 10 being the median birth date. No breeding was recorded out of season and over the three year period of the study there was no major variation in the median birth date. The females den their young in August and leave the young in the same den each night while foraging. Young were weaned 240 days after birth. There was a male-biased dispersal pattern of juveniles. Those males that remained philopatric to their natal area suffered less mortality than the philopatric females. There were similar numbers of adult male and female residents throughout the year. Male residents remained in the area for the duration of the study, whilst some females were absent for periods of a year or more. This form of dispersion was termed discontinuous residency. Females that showed a discontinuous residency behaviour had lower fertility rates than the continuous residents. Telemetry studies of the dispersion pattern of the devils showed that they occupied overlapping home ranges of 13.3 km^. Devils were tenacious to their dens and used between 2 and 3 dens. Most of the dens were burrows in relict Quaternary dunes. Three foraging strategies were identified with respect to the rate at which the animals travelled through the night: Linear, Exponential and Stepped. The home ranges were located over the habitat containing the highest concentrations of macropods. Observations of social interactions around food showed that devils used a variety of postures and vocalizations. Communication during agonistic interactions between animals competing for access to food was ritualistic and hence seldom resulted in physical damage. The results of these encounters were related to the length of time that the participants had being feeding and not to any form of hierarchy in the population. The form of these interactions is described with respect to the Resource Holding Potential hypothesis. The pattern of stress resulting from social interactions in the population manifested itself in an annual variation in steroid and androgen levels. Stress, as indicated by elevated corticosteroid levels, was highest during the period of maximum population size. Males had lower stress levels than females. Female stress levels showed two peaks, associated with the early phase of breeding when sexual encounters take place and the latter half of lactation, when the females are caring for denned young. The testosterone levels of the males were depressed throughout the year but unlike in all other dasyurids studied to date, there was no peak associated with the copulation period. The varying stress levels in the devil are explained in terms of varying social conflict and its duration. This study has shown that the Tasmanian devil is a solitary, nocturnal predator occupying an overlapping home range with other members of a fluctuating population. Interaction between the members of this population is intense and manifests itself in the lower reproductive potential of those females who are not continuous residents and the latent stress levels in all animals, particularly juveniles, in relation to high population levels. It is this interaction between individuals of the population which, along with the life history pattern of being a facultatively monoestrous species, which dominates the social organisation of the Tasmanian devil resulting in resident animals having a higher reproductive potential than non-residents. It is hypothesised that the communal feeding habits of the animals and use of communal latrines are highly significant forces. The various facets of this study are drawn together to give a description of the social organisation of the species and some of the factors which exert a control over it. Hypotheses are made on how the social organisation may vary with different food regimes in different habitats.
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