Body mass changes in a biparental incubator: the Redshank Tringa totanus

Incubation is a period of high energetic costs and accordingly body mass losses are often detected. Why birds lose body mass during incubation is not well understood; suggestions are that it is either a consequence of energetic constraints or adaptations to an optimal mass trajectory. We studied body mass changes through the incubation period in the Common Redshank Tringa totanus, a biparental incubator, on southern Gotland in the Baltic Sea. In contrast to what has been found in other biparental incubators, body mass of both sexes decreased linearly through the incubation period. The estimated mean body mass loss was 6.7 g (SE 1.7), corresponding to ca. 5% of initial body mass at incubation start. Hatching success in males was not related to body mass and size. In contrast, reproductive success, measured as successful production of fledged juveniles, in males was negatively related to body mass during incubation and positively related to body size. This finding supports the theory that body mass loss might follow an optimal mass trajectory, possibly to increase agility through the chick-rearing stage. However, energy constraints causing body mass loss cannot be ruled out; in fact, it is not unlikely that body mass may change due to a combination of both adaptation and stress

Post-mating sexual selection increases lifetime fitness of polyandrous females in the wild

Females often mate with several males before producing offspring(1). Field studies of vertebrates suggest, and laboratory experiments on invertebrates confirm, that even when males provide no material benefits, polyandry can enhance offspring survival(2,3). This enhancement is widely attributed to genetic benefits that arise whenever paternity is biased towards males that sire more viable offspring(1,4,5). Field studies suggest that post-mating sexual selection biases fertilization towards genetically more compatible males(6,7) and one controlled experiment has shown that, when females mate with close kin, polyandry reduces the relative number of inbred offspring(8). Another potential genetic benefit of polyandry is that it increases offspring survival because males with more competitive ejaculates sire more viable offspring(9). Surprisingly, however, there is no unequivocal evidence for this process(10). Here, by experimentally assigning mates to females, we show that polyandry greatly increases offspring survival in the Australian marsupial Antechinus stuartii. DNA profiling shows that males that gain high paternity under sperm competition sire offspring that are more viable. This beneficial effect occurs in both the laboratory and the wild. Crucially, there are no confounding non-genetic maternal effects that could arise if polyandry increases female investment in a particular reproductive event(10) because A. stuartii is effectively semelparous. Our results therefore show that polyandry improves female lifetime fitness in nature. The threefold increase in offspring survival is not negated by a decline in maternal lifespan and is too large to be offset by an equivalent decline in the reproductive performance of surviving offspring