Older male house sparrows are better than younger males at “cheating” on their mate
Published in the Journal of Evolutionary Biology, the research collaboration between the University of Sheffield, the Max Planck Institute for Ornithology, Germany, and Imperial College London discovered that older male sparrows deliver almost three times more sperm to the female's egg than young males, while showing no difference in sperm length or shape and ejaculate volume.
Across bird species, research into DNA paternity testing has historically taught us that older male birds father disproportionately more offspring outside their own nest. But work to test behaviours, such as whether older males are better at coercing females into “cheating”, or whether females prefer older males, hasn’t produced any support. The team therefore began to investigate the traits that are important after mating - primarily sperm.
Using both captive and wild house sparrows in Germany and Lundy Island off the Devon coast, over three years, eggs, sperm and other male traits were collected from these two populations to test if older males differ from younger males in sperm length, shape, volume and sperm number at the egg.
Previous research tells us that bird pairs differ in the number of sperm at the egg but it remained unclear until now what caused that variation. This new research shows that male age accounts for 16.4% of that variation, suggesting that older males sire more extra-pair offspring because either they inseminate more sperm or females prefer sperm from older males, or both.
Whilst bringing us one step closer in solving the mystery of why older male birds are better at “cheating”, this finding also highlights a risk as high volumes of sperm can lead to embryonic death, caused when more than one sperm fuses with the female pronucleus. Females need to therefore strike a balance between too many and too few sperm for optimal hatching success.
It is hoped that future research will test these findings further, allowing us to experiment in conditions where the sperm of old and young males can compete with each other for fertilisation.
Read the full paper in the Journal of Evolutionary Biology: