[1] The zona pellucida is a thick membrane that surrounds the unfertilised eggs of mammals.
In order for an egg to be fertilised, sperm must first bind to, and then penetrate the zona pellucida.
[citation needed] Porcine zona pellucida has been used in wildlife contraception since the late 1980s.
Animals with which PZP has been employed in this context include elephants, wild and/or feral horses,[2] elk and whitetailed deer.
The contraceptive effect last for approximately one year in horses, and can potentially be extended by including a controlled-release PZP component.