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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. 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)
“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.
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.