In mammals, if more than one sperm enters the ovum (or egg), the results are lethal. But for bird eggs, penetration by several sperm – a process called polyspermy – is actually quite normal. However, because only one sperm actually fertilizes the egg, researchers have long puzzled over what role the “extra” sperm play. According to new work published in Proceedings of the Royal Society B, multiple sperm are necessary for an embryo to develop normally.
University of Sheffield’s Nicola Hemmings and colleagues wanted to see how female birds respond to sperm limitation and how this influences sperm transport in the oviduct (the tube through which eggs pass from the ovary). In their experiments, 16 female zebra finches (Taeniopygia guttata) produced clutches under both a control treatment (where pairs copulated freely) and a sperm-limited treatment via artificial insemination. The team also artificially inseminated 21 female domestic fowl. The researchers removed eggs from the nests frequently to count the number of sperm in the perivitelline layer (the space between the fertilization membrane and the ovum) as well as the holes made by sperm that went on to penetrate that layer. Later on, they checked for embryo survival.
The number of sperm has little impact on the likelihood of fertilization. But when very few sperm penetrate a bird's egg, the embryo is unlikely to survive beyond the earliest stages of development. Furthermore, when female birds are inseminated with low sperm numbers, more sperm than expected end up reaching and penetrating the ova. This suggests that females can control the number of sperm that make it through the oviduct to the egg – minimizing the risk of infertility when she only has access to a limited number of sperm.
Though the mechanism that regulates polyspermic fertilization is still unknown for now, these findings indicate that the “extra” sperm are required to ensure successful embryo development – an achievement that’s further helped along by the female ability to compensate for low sperm numbers.
As for how “extra” sperm contribute to the early stages of embryo development, that remains to be seen but based on other animals, the researchers have an idea or two. When a single sperm penetrates the mammalian egg, it gives rise to calcium waves that spread across the ovum and support the progression of early embryo cell cycles. Bird eggs are massive compared to mammalian eggs. “So one possibility is that bird eggs need a greater number of sperm to trigger and support these early cell cycles,” Hemmings tells IFLScience. “Alternatively, we know that a number of proteins are released when sperm enter the ovum, so it is possible that these 'extra' sperm are contributing essential proteins to the egg or embryo.”