When it comes to being a male fish, life can be tough when the lady in your life gets around a bit and mates with lots of other males. It means that to make sure you produce lots of offspring you must compete with all the other guys on the scene, and more specifically your sperm has to be up to the job.
Now a team researchers led by John Fitzpatrick from the University of Western Australia have shown for the first time in the journal PNAS that when male fish have to compete with each other on a daily basis for the chance to mate, their sperm evolves to be bigger and faster.
When polygynous fish mate, a female lay eggs and males then rush in to add their sperm, hoping they will be the first in there.
For a long time now, biologists have suspected that polygynous males must evolve tactical sperm that are faster and therefore more likely to reach the eggs first, but up until now, no solid proof of that idea has been found.
Fitzpatrick and his team looked at species of cichlid fish living in Lake Tanganyika in Africa. Cichlids are famous for having evolved extremely rapidly into lots of different species. Some cichlids are monogamous, and others that are highly promiscuous, sharing many different partners throughout their life.
By zeroing in on these cichlids with all different types of mating behaviour, it allowed researchers to unpick this issue of the evolution of sperm.
The researchers went scuba diving in the lake and caught male fish from 29 different species and dissected them to study their sperm. They measured sperm length under a microscope and used digital video cameras to measure how fast the sperm were swimming. They found out that the polygynous species with lots of mates did indeed have larger, faster sperm than the monogamous species.
This was also the first time that good evidence has been gathered to prove that when sperm are bigger they do indeed swim faster, because their tails or flagella are longer, and can generate more propulsion.
And from earlier studies that have drawn up the family tree of all these cichlid fish, Fitzpatrick and his team were able to work out that the ancient cichlids that first colonised the lake had small, slow moving sperm, and as promiscuity increased over time among certain lineages, so sperm began to evolve to be bigger and faster.
Cichlid fish are the first group of species that have demonstrated the evolution of speedy sperm in polygynous species, but it is likely that similar things are going on in many other species where males have to do whatever they can to make sure they pass on their genes to the next generation.