Reproduction in Two Marsupial-Mice, Antechinus bellus and A. bilarni (Dasyuridae), of Tropical Australia

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Abstract
Antechinus bellus and A. bilarni occur in northern Northern Territory, a hot tropical region with a wet summer (November–March) and a very dry winter. The species are sympatric but largely separated by habitat, A. bellus inhabiting woodland and A. bilarni rocky escarpments. Both species are monestrous. The breeding season of A. bilarni is about 6 weeks earlier than that of A. bellus. Males are spermatogenic about 2 months before mating. In A. bellus, testicular development begins during May; males are capable of breeding by mid-June but are no longer so by the end of August. A. bilarni is fully spermatogenic in early May and there is a sharp drop in testicular activity in the latter half of July. In both species testicular regression is indicated externally by a decline in testis size. During the breeding season an active sternal gland develops in males of A. bellus, but not in males of A. bilarni or in females of either species. Female Antechinus do not have pouches. Prior to breeding the pouch area is fully haired and undeveloped. During the breeding period it becomes greatly enlarged, largely depilated, tumescent, and covered with a red-brown pigment, and the teats become conspicuous. The histology of the ovaries and endometrium is described for both species. The life span of the corpora lutea is remarkably short. They are at maximum size about the time of embryogenesis but decline rapidly and disappear immediately following parturition. In A. bellus the ovulation rate averages 15 and the embryo count 14. The corresponding figures for A. bilarni are 7 and 7.5, respectively. A. bellus gives birth from late September into October and A. bilarni from mid-August into September. A. bellus and A. bilarni have 10 and 6 teats, respectively, and these figures are also the maximum litter sizes. Because of the small samples it was not possible to decide whether there is a die-off of males at the end of the mating season as happens in some species of Antechinus, but there is suggestive evidence that male A. bilarni may survive to a second breeding season.

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