Death after Sex

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 ecologists 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 afterwards (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 semelparity in different populations. For example, the American shad (Alosa sapidissima) is a fish species that is semelparous 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 centuries , but the discovery that some mammals also die after reproduction is a relatively recent finding.