Cockerels Dole Out Sperm With Precision, Study Says

John Roach
for National Geographic News
November 5, 2003

When it comes to the primal urge to pass genes on to the next generation, red jungle fowls (Gallus gallus) are a sophisticated lot: They dole out their sperm with economic and strategic precision, according to a new study.

Like most species in the animal kingdom, the birds (a type of wild chicken) are sexually promiscuous. Females may mate with several different males in any given reproductive cycle. Males seem to mate whenever they get a chance.

Researchers have learned, however, that males do not invest their sperm equally with each mate. Rather, they partition it out based on perceived competition from other males and a female's reproductive quality.

Exactly how cockerels increase or decrease their sperm output during mating is not known. But differential sperm investment has been observed in a wide range of species, from insects to mammals. This behavior suggests that it is an evolutionary strategy of males to increase their competitiveness in response to female promiscuity.

"Our study reveals that male ability to invest sperm differentially is more complex than originally realized," said Tommaso Pizzari, a biologist with the Swedish University of Agricultural Sciences in Skara currently at the University of Leeds, England, and lead author of the study described in tomorrow's issue of the science journal Nature.

Pizzari's research revealed that males will devote more sperm to a new mate than to a familiar one and will expend more sperm when mating with females with large combs than those with less impressive ornaments. Males will also allocate more sperm to females that are more promiscuous.

Matthew Gage, a fellow in the School of Biological Science at the University of East Anglia in Norwich, England, said the study demonstrates that males are sexually more sophisticated than previously thought.

"Previously an ejaculate was regarded as a fixed unit of spermatogenic investment that waned with sexual activity," said Gage, who wrote an accompanying commentary on the research in Nature. "Now we know that males play careful games with their sperm to maximize fertilization success."

Investment Strategies

To determine how cockerels increase their chances of passing on their genes to the next generation, Pizzari and his colleagues habituated the birds to humans and placed harnesses on females that allowed sperm to be easily collected and analyzed.

In one experiment, the cockerels were allowed to copulate in the presence of none, one, or three male competitors. Researchers observed different sperm investment strategies depending on how respective males ranked in the pecking order.

In the presence of no competitors, all males minimized their sperm investment, keeping their reserves ready for later, more competitive investments.

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