Physiologic polyspermy, the regular and normal occurrence of multiple sperm penetration of the ovum, is a requirement in birds for normal egg fertilization and embryo development. The number of sperm trapped between the by the membranes surrounding the yolk after fertilization is directly correlated with the number of sperm that have penetrated the ovum. This number is positively related to the egg size, but there is significant variation both within and across species with similar sized eggs that remains to be explained. The number of sperm reaching the egg is heavily influenced by post copulatory selection, often reducing several million sperm down to less than a few hundred actually reaching the egg. The strength of selection against sperm is likely to vary based on a number of factors including type of species mating system (e.g. how much completion and extra pair copulations occur?), individual experience, and compatibility and quality of sperm.

To better understand how both environmental and behavioural factors impact sperm production and usage we have investigated the impact of pair compatibility, breeding experience, and temperature on sperm morphology, motility, and usage in Australian Estrilid finches: primarily long-tailed (Poephila acuticauda) and zebra finches (Taeniopygia guttata). We are still processing the multitude of samples form the different facets of these studies, but we are seeing some clear results.

For example, compatible pairs (e.g., colour morph matched long-tails) demonstrate lower stress hormone (i.e., corticosterone) levels and show higher and more consistent numbers of sperm reaching the egg than mismatched or hybrid-backcrossed pairs. We suggest this difference in sperm numbers in the egg is due in part because of observable differences in sperm morphology between the two colour morphs, but is also impacted by the stress on adults in experimentally forced unmatched pairings. This could account for the maintenance of the separate morphs in the wild, as low sperm numbers reduces embryo viability and corticosterone impacts chick development. These findings have implications for understanding the evolution of reproductive isolation and the evolution of genetic polyandry in animals.

Contributed by Laura L. Hurley, PhD candidate at Macquarie University.