Bird sperm research sheds light on basis of delity

Ongoing research searching for the biological basis of fidelity, including that of humans, has spawned new evidence from an unexpected source - bird sperm.

James Briskie of Oxford University in England recently reported that he has found a positive relationship between sperm length and polygamy in Passerine birds.

"The idea here is that if a male has to compete against the sperm of another male, it would be evolutionarily advantageous to produce a faster swimming sperm to outdistance a rival in the race to reach and fertilize the eggs," Briskie said.

The report, published in the July edition of Evolution, is co-authored with researchers Robert Montgomery and Tim Birkhead. Briskie and his coworkers also discovered that polygamous bird species' females have longer Sperm Storage Tubules.

Each SST - about 60 to 70 micrometers in diameter - accommodates up to several hundred sperm. A typical female possesses anywhere from 500 to 20,000 SSTs, bundled together in an assemblage that connects the vagina and uterus.

SSTs appear to serve as a place where sperm is held until needed to fertilize eggs. Briskie's research suggests that increased sperm length may not only help males' sperm swim faster but also keep other sperm out of competition. Sperm cells that fail to secure a place inside an SST will not be allowed to enter the uterus.

"In our paper we suggested long sperm evolved in some species of birds because females in these species had evolved longer SSTs," Briskie said. "The reason longer sperm would be advantageous in polygamous species is that the longer sperm would allow a male to fill up these SSTs and prevent other sperm from being stored."

The sperm size of human males seems fairly consistent with these studies, Briskie said.

"As for humans, we have pretty average-sized sperm," said Briskie. "The fact that humans don't have particularly long sperm suggests we don't have particularly high levels of promiscuity, at least relative to other species. And the paternity studies of humans tend to support this: the levels of 'mis-matches' of paternity in humans is less than 10 percent of the population. Relative to other species, this is quite low, but it is clear we are not angels either."

University psychology Prof. Barbara Smuts also suggested that human biology could be linked with historical fidelity.

"Aspects of our psychology and biology suggest that throughout our evolutionary history, humans have been mildly promiscuous," Smuts said. "Pair bonding was always the basic pattern but it wasn't followed faithfully."

Researchers interested in learning human beings' innate tendencies towards fidelity have invoked systematics, which uses the behavior of other closely related species as data to speculate on human behavior.

"You run the gamut among the apes," said University anthropology Prof. John Mitani. "The gibbons of Asia live in mated pairs. Gorillas live in single-male groups. Orangutans are solitary in mating, any male can mate with any given female."

A primatologist from the University of California at San Diego recently revealed considerable promiscuity among chimps while working on the Ivory Coast, Mitani said.

"The data suggest over half of all chimps born into a troupe (a multi-family group) have fathers that come from outside of their troupe," Mitani said. "And chimps are our closest ancestors."

10-02-97