Polyspermy

Such a situation is often deleterious to the female: in other words, the male–male competition among sperm spills over to create sexual conflict.

When sea urchin sperm encounter an egg with a positive charge, sperm-egg fusion is blocked.

[8] Electrical polyspermy blocks operate in many animal species, including frogs, clams, and marine worms, but not in the several mammals that have been studied (hamster, rabbit, mouse).

In sea urchins, fertilization occurs externally in the ocean, such that hundreds of sperm can encounter the egg within several seconds.

This may be the result of the female genital tract being adapted to minimize the number of sperm reaching the egg.

Some vertebrates that are both amniote or anamniote, including urodele amphibians, cartilaginous fish, birds and reptiles, undergo physiological polyspermy because of the internal fertilization of their yolky eggs.

These calcium ions are responsible for the cortical granule reaction, and are also stored in the egg's endoplasmic reticulum.

[15] Unlike physiological polyspermy, monospermic fertilization (described above) deals with the analysis of the egg calcium waves, as this is the typical reproduction process in all species.

It's also known that the chosen pronucleus also has the biggest aster, though details on how this is achieved (and how precisely it affects choice) remains unknown.

[3] In the journal Proceedings of the Royal Society B,[18] as reported in the New York Times,[19] Dr. Nicola Hemmings, an evolutionary biologist at the University of Sheffield, and one of the study's authors reported that the eggs of zebra finches and chickens require multiple sperm, from 10 to hundreds of sperm, to penetrate the egg to ensure successful fertilization and growth of the bird embryo.

Female defenses select for ever more aggressive male sperm, however, leading to an evolutionary arms race.

On the one hand, polyspermy creates inviable zygotes and lowers female fitness, but on the other, defenses may prevent fertilization altogether.

This leads to a delicate compromise between the two, and has been suggested as one possible cause for the relatively high infertility rates seen in mammalian species.