Some male first generation hybrids may have viable sperm, whereas very few first generation female hybrids lay fertile eggs.

Professor Huleihel added: "We were able to produce viable sperm that could have been used to create baby mice.

Certainly the proof is there, in his DNA: an inability to produce viable sperm.

In general, individuals with 5-ARD are capable of producing viable sperm.

However, several small studies have failed to find any viable sperm in the fluid.

In N. emarginata, it appears from initial observations that storage of viable sperm is limited to a period of two or three months.

An alternative to egg donation in some couples, especially those in whom the male partner cannot provide viable sperm, is embryo donation, sometimes called "embryo adoption".

A certain species of turtles can store and use viable sperm for at least three years following mating.

Non-reproductive males have smaller testes than their reproductive counterparts and produce little, if any, viable sperm.