Abstract
Male-only obligate semelparity is a well-studied reproductive strategy in some Australian marsupials. This strategy has not been documented in South American species, although semelparity in both sexes occurs in some Neotropical didelphids. The fat-tailed mouse opossum, Thylamys bruchi, is an endemic species of the temperate Monte Desert, in Argentina. Seasonality and predictability of resources are two of the attributes associated with habitats where marsupial semelparity has evolved, and both are characteristic of the Monte Desert. We aimed to characterize the life-history strategy of T. bruchi to explore if it can be considered a semelparous species. We studied a fat-tailed mouse opossum population for 7 years with two different capture techniques (Sherman traps and nest boxes). Thylamys bruchi showed strong seasonality in abundance, with the highest captures during summer and autumn. Reproduction and weaning coincided with the most favorable period of the year with respect to climate and resource availability. Every year we observed a single cohort with little overlap until weaning of young. After breeding, all adults disappeared from the population; however, unlike any other didelphids, males showed delayed mortality and died, along with females, after weaning. We found no evidence of survival to a second breeding season for either sex. We therefore propose T. bruchi as a desert-dwelling marsupial with a semelparous reproductive strategy. Because the severity of winters may be acting as an important constraint on the energetic balance of adults in this population, we propose that challenging climatic conditions, coupled with the seasonality and high predictability of food resources, may have contributed to the evolution of the extreme reproductive strategy in this didelphid marsupial.
