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.

As the competition increased, dominant males always inseminated more sperm. Weaker (or subdominant) males, however, inseminated more sperm when faced with one competitor, but pulled back in the presence of three.

Since dominant males can monopolize females and control weaker males, they sometimes mate in the absence of sperm competition. They can thus afford to invest more sperm when the potential for competition arises, according to the researchers.

"Subdominant males, on the other hand, cannot prevent other males from inseminating a female and typically copulate with females that have in the past, or will in the future, obtain sperm from other males," said Pizzari. "Therefore subdominant males are certain to face sperm competition."

Gage refers to these two different sperm investment strategies evolved by the dominant and weaker males as the "risk" model and the "intensity" model.

"The risk model deals with varying probabilities that there will be sperm competition, while the intensity model assumes sperm competition is occurring, but at varying levels," he said.

As a result, dominant males can afford to be risk investors, whereas weaker males follow the intensity method. Gage likens the strategies to playing a raffle.

Writing in Nature, he stated: "If there is no competition for the prize, then you buy just one ticket to win; if there is little competition you may maximize your chances by purchasing many tickets; but if competition is very high, it pays to save your money and buy tickets in a future raffle with improved odds."

Sexual Ornaments

In addition to competing for a successful copulation with any female, males also compete for access to the females with the largest combs.

Research shows that females with such ornamentation produce a larger mass of eggs—either in quantity or size, sometimes both—than do females with smaller ornaments. These females also invest more resource-rich yolk in every egg they produce.

"If you invest sperm preferentially in these females, each one of the eggs will produce better offspring. That will have a competitive advantage relative to young that come from eggs of lower quality," said Pizzari.

As a result, both dominant and subdominant males invest more sperm in females with larger ornaments. Pizzari adds that dominant males weight these investments more so than subdominant males.

Study Implications

Since the great majority of living organisms are sexually promiscuous, Pizzari said this study has far reaching implications, including insights into the evolution of male sexual behavior, the role of female ornaments in sexual selection, and the breeding of managed populations.

Since sperm are less costly to produce, in terms of energy and nutrients expended, than eggs, males have long been assumed to follow a pretty straightforward reproductive strategy: inseminate as many females as possible.

"Our study reveals that this assumption is not entirely true, and that cryptic male sexual behavior, in the form of strategic sperm allocation, can be much more sophisticated than originally appreciated," said Pizzari.

The study also demonstrated that female ornamentation, long ignored in the face of more developed male ornamentation, may actually set the scene for male choice of partners and differential sperm investment.

The research may have implications for breeding of managed populations, such as zoo animals or endangered species, as well.

"These populations can suffer fertility problems," said Pizzari. "Our study suggests these may sometimes have a behavioral basis and may be ameliorated or solved by looking at the social environment in which the animals interact and reproduce."