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."
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