Genetic, morphological and ecological variation among populations of agile antechinus that differ in female teat-number

Antechinus are litter-bearing marsupials that usually start a breeding event with a full complement of pouch-young (i.e. all teats occupied). Females will normally give birth to more young than they have teats. During the first weeks of life, each young must become permanently attached to one teat, so neonates that are surplus to the female’s number of teats die. Thus a female’s teat-number effectively limits the maximum number of young that she can produce in one breeding season. Remarkably, differences in female teat-number occur among populations in a number of species of antechinus in southeast Australia. A geographic pattern in the variation, in which females inland tend to have more numerous teats than those nearer the coast, has long been thought to be associated with variation across the landscape in climate and the abundance of available food resources for rearing young. There has also been conjecture that genetic variation underpins intraspecific teat-number differences in antechinus and about the possibility of hybridisation between teat-number phenotypes. The series of research papers presented in this thesis constitute the first empirical investigations of the possibility ofintraspecific genetic variation among antechinus that differ in teat-number and a possible relationship between habitat variability and the geographic distribution of teat-number phenotypes in the study species, the agile antechinus (Antechinus agilis). Whilst long-term genetic divergence was not indicated by sequence analyses of the cytochrome b mitochondrial gene, the six-teat A. agilis phenotype in the Otway Ranges and areas near Portland in southwest Victoria vii formed a genetically distinct cohort based on nuclear markers. Genetic analysis (nDNA and mtDNA) also indicated that the phenotypically different Wilsons Promontory (six teats), Mount Donna Buang (eight teats) Tumut (ten teats) and Otway Ranges (ten teats) populations were no more differentiated from one another than expected from their current degree of geographic separation, providing no support for a scenario of long-term isolation of these phenotypes. Overall, these results indicated that the southwest six-teat phenotype has been reproductively isolated from the other populations for enough time for genetic differentiation to develop in nDNA markers. Conversely, however, interbreeding between the six- and ten-teat number antechinus at places where they meet and mix was also evident in the Otways, suggesting that currently there is incomplete reproductive isolation between these two phenotypes. Such genotypic divergence between the six- and ten-teat phenotypes in the Otways is consistent with either an allopatric or parapatric mode of divergence. Adaptation of individuals to local habitats within a heterogeneous landscape can create barriers to dispersal, resulting in isolation of populations. Environmental factors that were expected to strongly influence habitat quality for antechinus, such as the prevailing climate, the plant species composition and structure of the vegetation and arthropod prey abundance, were investigated. Clear associations were evident between some climate variables and the distribution of A. agilis teat-number phenotypes, with the six-teat phenotype occurring in milder and less seasonally variable climatic conditions. There was also some evidence of a relationship between plant species composition and viii vertical vegetation structure and A. agilis teat-number distribution in the Otway Ranges, but this appeared to be more of a reflection of differences in climate rather than implying a direct influence of the vegetation on teat-number distribution. The investigation of variation in other morphological traits between the six- and ten-teat females identified some features that appear to be characteristic of each phenotype, but no substantial differences were observed in female body mass. The energy demands on mothers that have similar body mass are expected to be greater for those rearing larger litters. Surveys of macroscopic terrestrial invertebrate assemblages and an assessment ofA. agilis population demographics at multiple sites inhabited by each phenotype were conducted in parallel. No clear differences in invertebrate faunal assemblages or biomass were evident between sites grouped according to A. agilis phenotype; however, population densities of females with six teats were substantially higher than those of females with ten teats. The home ranges (estimated via radio-telemetry) offemales with six teats also overlapped those of the adjacent females more often than was evident among ten-teat females. However, there was a significantly greater number ofA. swainsonii living in sympatry with ten- than with six-teat A. agilis. The effects ofinterspecific competition with A. swainsonii may be particularly detrimental for A. agilis during winter, when food resources are at their most scarce. A greater amount of excursive activity around the home range periphery by ten- than six-teat females was observed during winter, which may also reflect a need to forage over greater distances in response to greater levels ofinterspecific competition with A. swainsonii for tenIX teat A. agilis females. A number of factors that may have contributed to the evolution of teat-number variation and the current distribution of phenotypes are proposed and discussed. Such factors, which may not be mutually exclusive, include (i) ‘bet-hedging’ in higher teat-number phenotypes in response to the greater seasonal variability in climate evident in ten- than six-teat habitats so that in years when conditions are good and prey abundance high, mothers are able to rear the maximum litter-size possible, (ii) the possibility that females with fewer teats are more likely to survive to breed a second time, as indicated by the greater apparent longevity ofsix- than ten-teat females detected in the present study and (iii) that a lower level of competition may occur more often among ten- than six-teat mothers due to differences in adult population densities, giving ten-teat females a greater opportunity to raise large litters during the summer when prey are abundant.