When it comes to mating, animals are viewed by scholars as being either lovers or fighters. Some have evolved to emphasize their weaponry, such as antlers or canine teeth, to fight off other males, while other species possess qualities that allow them to fertilize the most eggs — big testes and a high sperm count. A new study finds the traits male animals develop depend on the level of male-to-male competition for the females among them and also how easily each male can dominate a female in order to continue mating. “Understanding the ways animals reproduce is important as it helps us understand how species evolve and can prove important for conservation,” said Syracuse University's Dr. Stefan Lüpold, a co-author of the study, which appears in Nature Communications.

Sexual Competition

Male animals are faced with a decision that can make or break their chances at reproducing and thus surviving: Does it pay to be a lover or a fighter? The answer to this question occurs primarily on a species level and has evolved over time, with males of each species having developed the traits that help them fight off other males or conversely traits that help them spread their sperm. In an effort to understand this more deeply, a team of researchers from the University of Manchester, Syracuse University, and the University of Western Australia analyzed the sexual behaviors of over 300 species of males, including mammals, birds, fish, insects, and flatworms. “We set out to see why some species show trade-offs in sexual traits and others do not,” said John Fitzpatrick, a lecturer in animal evolution at the University of Manchester and senior author of the research.

It is well known, for instance, that some animals appear to invest maximally in “expensive” sexual traits, while others are more frugal. (Sexual traits are deemed expensive because personal, immediate survival often requires fighting off predators so it always pays, so to speak, to put more time into developing armor.) “We know animals try to get females in a couple of ways. When they fight for them they sometimes evolve weaponry — such as antlers or a really big body size or big teeth,” Fitzgerald explained in a press release. Other animals though, don’t seem to bother with any pre-copulation battles; instead, they simply attempt to fertilize the most eggs. What scientists don’t know is why different species choose different paths.

After analyzing their research, the team discovered that the males traded-off an investment in weaponry only when they were sure their females wouldn’t fool around with another male behind their backs. The main driver, then, for how males evolved was the ability to dominate a female for continued mating. Elephant seals, for example, appear to have discovered that fighting for the right to mate brings greater control of the females so they invest more in weapons and less in testes size. Males are almost five times the size of the females, so they easily dominate their partners.

“It is quite costly to invest in everything,” Lüpold explained. “You don’t get something for nothing in evolution, so we wanted to see which species invested in weapons over testes.” Yet some species do invest in both. In those species, fighting is not just the first step in sexual relations; they must also fight after mating. These species also show large testes and a high sperm count. Pheasants, minnows, and bush crickets, for example, all display traits that help them fight as well as love. The researchers remain mystified as to why some species invest in both, nevertheless they are continuing along the trail. "We will now look at whether maximising investment in sexual traits means you pay the price in some other aspect of life,” Lupold stated. Perhaps the human species might be included in that study.

Source: Lupold S, Tomkins JL, Simmons LW, Fitzpatrick JL. Female monopolization mediates the relationship between pre- and postcopulatory sexual traits. Nature Communications. 2014.