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Mightier sperm in “cuckolded” species

Jan. 26, 2009
Courtesy Uppsala University
and World Science staff

A­mong some an­i­mal spe­cies with more fre­quent mate in­fi­del­ity, sperm evolve to be faster and larg­er to cope with the great­er com­pe­ti­tion, re­search­ers have found.

“In pro­mis­cu­ous spe­cies we found that males pro­duced larg­er and faster sperm than in closely re­lat­ed spe­cies that were monog­a­mous,” said Si­gal Bal­shine of Mc­Mas­ter Uni­ver­s­ity in Can­a­da, sen­ior au­thor of a new study on the sub­ject.

Artist's image of sperm swim­ming to­ward an egg. A­mong some an­i­mal spe­cies with more fre­quent mate in­fi­del­ity, sperm evolve to be faster and larg­er to cope with the great­er com­pe­ti­tion, re­search­ers have found. (Im­age cour­tesy U.S. Nat'l Inst. of Health)


“This re­search of­fers some of the first ev­i­dence that sperm has evolved to be­come more com­pet­i­tive in re­sponse to fe­males mat­ing with mul­ti­ple males.”

“Com­pe­ti­tion among sperms to fer­ti­lize a fe­male’s eggs is an ex­tremely pow­er­ful ev­o­lu­tion­ary force that in­flu­ences var­i­ous char­ac­ter­is­tics of sperms, such as size and speed,” said Niclas Kolm of Upp­sa­la Uni­ver­s­ity in Swe­den.

To­geth­er with sci­en­tists from sev­er­al oth­er uni­ver­s­i­ties, Kolm has stud­ied the mat­ing sys­tem of 29 spe­cies of Af­ri­can fish known as Tan­gan­yi­ka ci­ch­lids. “For the first time, we can show a strong link be­tween the de­gree of sperm com­pe­ti­tion and the size and speed of the sperms,” he said.

Evolution works by pressuring species to adapt to changing circum­stances. In­di­vi­duals with poor­ly adapted genes die out, while the others sur­vive and spread those genes through the po­pu­la­tion. Gra­dually, cha­ract­er­is­tics of the whole group change through this cease­less pro­cess.

Fe­male promiscu­ity is a prob­lem for males be­cause the sperm from ri­val suit­ors will com­pete in the race to pro­cre­ate, not­ed Bal­shine. While the idea that sperm would evolve to be­come more com­pet­i­tive when males com­pete for fer­til­iz­a­tion seems ob­vi­ous, to date there has been lit­tle ev­i­dence for this, he added.

“We based our study on an un­usu­ally large base, with many fish from many dif­fer­ent spe­cies,” said Kolm. “A spe­cial char­ac­ter­is­tic of this group of fish­es is that there are in­cred­i­ble num­bers of spe­cies.” These dis­play a “w­hole spec­trum of mat­ing sys­tems, from mo­nog­a­mous males to fe­males that mate with many many males.”

The find­ings al­so show that the speed and the size of sperm are closely re­lat­ed: larg­er sperms are faster, he said. These sperm swim faster thanks to the great­er pow­er of a larg­er fla­gel­lum, or tail; but faster sperm al­so need to have a larg­er store of en­er­gy, which in turn re­sults in larg­er sperm.

Thanks to new an­a­lyt­i­cal meth­ods, the re­search­ers said they have al­so man­aged to dem­on­strate the or­der of this de­vel­op­ment. The sperm first be­come faster, then larg­er, fol­low­ing in­creased fe­male promiscu­ity in a spe­cies. “No one has pre­vi­ously been able to show what causes what,” said Niclas, whose stu­dy ap­pears in the re­search jour­nal Pro­ceed­ings of the Na­tion­al Aca­demy of Sci­ences.


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Among some animal species with more frequent mate infidelity, sperm evolve to be faster and larger to cope with the greater competition, researchers have found. “In promiscuous species we found that males produced larger and faster sperm than in closely related species that were monogamous,” said Sigal Balshine of McMaster University in Canada, senior author of a new study on the subject. “This research offers some of the first evidence that sperm has evolved to become more competitive in response to females mating with multiple males.” “Competition among sperms to fertilize a female’s eggs is an extremely powerful evolutionary force that influences various characteristics of sperms, such as size and speed,” said Niclas Kolm of Uppsala University in Sweden. Together with scientists from several other universities, Kolm has studied the mating system of 29 species of African fish known as Tanganyika cichlids. “For the first time, we can show a strong link between the degree of sperm competition and the size and speed of the sperms,” he said. Female promiscuity is a problem for males because the sperm from rival suitors will compete in the race to procreate, noted Balshine. While the idea that sperm would evolve to become more competitive when males compete for fertilization seems obvious, to date there has been little evidence for this, he added. “We based our study on an unusually large base, with many fish from many different species,” said Kolm. “A special characteristic of this group of fishes is that there are incredible numbers of species.” These display a “whole spectrum of mating systems, from monogamous males to females that mate with many many males.” The findings also show that the speed and the size of sperm are closely related: larger sperms are faster, he said. These sperm swim faster thanks to the greater power of a larger flagellum, or tail; but faster sperm also need to have a larger store of energy, which in turn results in larger sperm. Thanks to new analytical methods, the researchers said they have also managed to demonstrate the order of this development. The sperm first become faster, then larger, following increased female promiscuity in a species. “No one has previously been able to show what causes what,” said Niclas.