Meri Oakwood The University of New England, Armidale, Australia Males of some small mammal species have an extraordinary reproductive strategy – they all die after mating. For decades, zoologists have speculated about the causes, but now, the discovery of this pattern in a larger mammal throws doubt on previous explanations. Why do some male mammals die after sex? One aspect of life history theory that has fascinated ecolo- gists over many decades is the dichotomy between those species that reproduce many times in a lifetime (iteroparity) and those that only reproduce once and die shortly after- wards (semelparity). Death after reproduction is widespread among plants (annuals and biennials), invertebrates and fish and most of these produce large numbers of offspring. These organisms allocate their stored resources to one massive reproductive effort, rather than investing in future growth and reproduction. Synchronous reproduction across the entire population results in non-overlapping generations. Some of the better-known and more spectacular examples are the mass flowering and subsequent death of bamboo and the spawning of Pacific salmon (Oncorhynchus sp). In some species, all individuals die after reproduction. In other species, there is variation in the degree of semelpar- ity in different populations. For example, the American shad (Alosa sapidissima) is a fish species that is semel- parous in the southern portion of its range but, further north, some spawners survive. We have been aware of semelparity in plants, fish and invertebrates for many cen- turies, but the discovery that some mammals also die after reproduction is a relatively recent finding. The discovery of small mammal die-off In the 1960s, Woolley (1966) studied reproduction in a small (usually 20 – 40 g) Australian carnivorous marsupial Antechinus stuartii (family Dasyuridae). She observed a very precise synchrony in the onset of sexual maturity of both sexes at one year of age, frequent and prolonged cop- ulation during a single 11-day mating period each year (in winter) followed by a dramatic decline in the condition of the males. They lost weight and fur, their testes regressed and sperm production ceased, making them sterile. The startling discovery, though, was that all males in the wild appeared to die soon after mating. This unique finding of semelparity in a mammal generated great interest in this species and, in the following decades, A. stuartii became the most intensively studied carnivorous marsupial in Australia (see Lee and Cockburn, 1985). In the late 1960s, Wood conducted the first detailed field study of this species (Wood, 1970). He examined a population of A. stuartii living in a rainforest on the east coast of Australia and confirmed that males ceased to be caught after mating, with the last individuals in each season found moribund or dead in the traps. Wood explored the possibility Biologist (2004) 51 (1) 5 Title image: Northern quoll looking out of a hollow. Photo credit: Ian Morris Death after sex Death after sex