Population density and movement data for predicting mating systems of arboreal marsupials
Introduction
The social organisation and mating systems of animals is a major area of study for ecologists and evolutionary biologists (Verner and Willson, 1966, Emlen and Oring, 1977, Henry and Craig, 1984, Goldingay, 1992, Mulder et al., 1994). Such factors can, in turn, influence long-term population viability, which has become a key area of investigation in ecological modelling and conservation biology (e.g. see Shaffer, 1990, Boyce, 1992, Possingham and Davies, 1995). For example, information on mating systems is among the primary input data required to parameterise models for simulating population dynamics and viability (Haig et al., 1993, Lacy, 1993, McCarthy, 1995, McCarthy, 1997).
In the case of Australian arboreal marsupials, studies of mating systems and social organisation can be important because there are substantial spatial differences in the population densities of arboreal marsupials (reviewed by Gibbons and Lindenmayer, 1996) and several authors have speculated that the mating systems of a given species may vary markedly between populations of the same species including those of the yellow-bellied glider (Petaurus australis Shaw) (Henry and Craig, 1984, Russell, 1984, Goldingay and Kavanagh, 1991), the greater glider (Petauroides volans Kerr) (Norton, 1988), and the mountain brushtail possum (Trichosurus caninus Ogilby) (Lindenmayer et al., 1997). For example, the observed mating system of P. volans changed between monogamy and polygamy in different forested areas in southeastern Australia (Norton, 1988). Similarly, Menkhorst (1995)noted that Victorian populations of the squirrel glider (Petaurus norfolcensis Kerr) typically lived alone or in pairs whereas Quin (1995)found that the groups of P. norfolcensis in northern New South Wales nested in colonies of 2–9 animals and he suggested that the mating system was probably polygynous. Moreover, even within the same population of arboreal marsupials, mating systems may vary between groups of animals (e.g. P. volans, Henry, 1984, Henry, 1985) and change over time (e.g. Leadbeater’s possum [Gymnobelideus leadbeateri McCoy, Lindenmayer and Meggs, 1996]). Spatial and temporal variations in the availability of resources may influence the inter and intrapopulation variations in the social structure and mating systems of different species of arboreal marsupials (Goldingay, 1992, McDonald and Carr, 1989, Lindenmayer and Meggs, 1996) and thus spatial variation in the viability of different populations.
One of the best methods to determine the mating system of a species is to apply molecular genetics techniques such as DNA profiling (e.g. Mulder et al., 1994) because even careful field-based observations of animals can result in a misleading assessment of their breeding patterns. The noisy miner (Manorina melanocephala) provides an example of the contrasting results derived from field-based observations and molecular genetics methods (Dow, 1979, Dow and Whitmore, 1990, Poldmaa et al., 1995). However, molecular genetic techniques can be expensive and time-consuming, so it can be valuable to formulate well constructed hypotheses to guide the application of such approaches. Field studies of birds often allow hypotheses of the mating system to be constructed, and DNA profiling can reveal whether putative fathers have been correctly identified (e.g. Mulder et al., 1994). However, in studies of arboreal marsupials it is not often possible to identify putative fathers because they are cryptic and nocturnal, making it difficult to observe them when active. During the day they shelter in tree hollows, sometimes with groups of adults when mating might take place. On this basis, we outline a simple model for integrating information on the movements, home range and population density of animals to make predictions of the proportion of males expected to mate successfully with more than one female in a polygynous population. In conjunction with data on movement and population density derived from radio tracking and trap-recapture studies, the model may be used to generate hypotheses about the mating system of a species. These hypotheses may be subsequently tested using molecular genetics techniques.
Section snippets
The model
The model predicts the probability distribution of the number of females with which males successfully mate in a breeding period. It is assumed that animals only differ in the location of the area they inhabit, being otherwise identical. The model has two parameters, the number of females encountered by each male, and the number of males encountered by each female. Because of variation in local densities of animals, and variation in home range size, these numbers will not be constant for all
Results
For the general case, assuming that individuals are distributed randomly, the predicted level of polygyny for a species at different population densities and for different sex ratios is shown in Fig. 1. Note that if the densities of males and females are equal (as assumed for the arboreal marsupial data), a maximum of 26% of males and 42% of successfully mated males within a population will be polygynous because λ (, ) will never exceed 1.0. Higher rates of polygyny are only possible when the
Discussion
The model developed in this study integrates data on movement and population density to make predictions about the mating system of animals. It indicates that population density and competition among males for mates can limit the level of polygyny expressed in populations even when there are not behavioural or social constraints contributing to monogamy. The predictions of the model can be tested with molecular genetic methods. On the basis of the distribution of female T. caninus within home
Acknowledgements
DBL would like to thank Drs R Lacy and K Viggers for past collaborative work on T. caninus. MAM was supported by an Australian Research Council Postdoctoral Fellowship.
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