MACROGEOGRAPHIC VARIATION IN LITTER SIZE IN ANTECHINUS (MARSUPIALIA: DASYURIDAE).

Abstract

The best documented cases of geographic variation in brood size are the tendency of avian clutches to increase with latitude (Lack, 1947, 1954; Klomp, 1970), and mammalian litters to increase with both latitude (Lord, 1960), and altitude (Dunmire, 1960; Fleming and Rauscher, 1978; Bronson, 1979). There is no consensus on the cause of this phenomenon (Stearns, 1976; Ricklefs, 1980). Lack (1947) proposed that parents produce broods which correspond with the maximum number of young they can nourish. Numerous modifications to this hypothesis have been suggested (Ashmole, 1963; Cody, 1966; Owen, 1977; Hogstedt, 1980), and a number of theoretical models have been developed that suggest brood size should be reduced below the most productive size (Stearns, 1976). Two classes with wide application can be recognized among the latter models. The first includes those based on the tradeoff between fecundity and adult mortality resulting from increased reproductive effort (Charnov and Krebs, 1974). According to this approach adults should limit the size of their brood in certain environments to extend their opportunities of breeding again (Ricklefs, 1977a). This viewpoint has been criticized because it fails to predict what size smaller broods should be to ensure iteroparous reproduction (Ricklefs, 1977b). The second includes those models which predict tradeoffs between demands for resources on the part of reproduction and other functions, particularly avoidance of predation (Cody, 1966; Skutch, 1967; Perrins, 1977). Ricklefs (1977a) presented evidence that predation is not sufficiently important to account for the magnitude of variation commonly observed along geographic gradients. Several methodological difficulties have hindered investigation of these hypotheses. The genetic basis of variation in brood size has rarely been established (Krohne, 1980, 1981). Further, the influence of predation and the proposed trade-off between reduced clutch size and increased adult longevity have been difficult to quantify in field populations. While it is possible to distinguish between semelparous and iteroparous species, an index of the extent of iteroparity remains elusive and is confounded by difficulties in ageing wild individuals and the production of more than one brood within a season. A study of variation in brood size within semelparous species might be informative, but brood sizes in semelparous invertebrates and lower vertebrates and monocarpic plants are enormous and difficult to quantify. In this study we report macrogeographic variation in the litter size of Antechinus, a genus of mammals in which the frequency of iteroparous versus semelparous reproduction can be precisely quantified, and in which we believe litter size differences are genetically determined. Litter size is manageably low and variable (6-13), and only one litter is produced each year. This combination of characters provides a tractable system for resolution of the separate factors which might influence litter size.