Mixed-up world.
A female topi attacks a male to get him to mate with her.

Jakob Bro-Jørgensen

Not Again, Dear. I'm Sperm-Depleted

Who says males are always the persistent sex? Female topi--a type of African antelope--become so intent on mating repeatedly with the most desirable partner that males sometimes have to fend off their aggressive advances to avoid running out of sperm, a researcher reports. The study is the first to suggest that sperm depletion causes such a role reversal in a mammal.

Topi are "lek" breeders. For a month and a half each year, males congregate at a mating arena, or lek, to compete for barren patches of about 30 square meters. The biggest, fittest males, known as lek males, command plots in the center of the arena, and females, which come into heat for just 1 day per year, seek them out. These prized bulls mate as many as 36 times in just 30 minutes. A female copulates with about four males during her visit to the arena, usually mating with each male multiple times. Although they prefer lek males, nearly 75% of females also mate with less hunky males.

The males keep this up for the entire rut, only occasionally nipping off for a bite to eat, says behavioral ecologist Jakob Bro-Jørgensen of the Institute of Zoology in London. Bro-Jørgensen studies topi in Kenya's Masai Mara National Reserve, and the sheer length of the rut interested him in the role that sperm depletion might play. In related species, he says, repeated mating can deplete sperm, meaning that with each additional ejaculation, the male is less likely to fertilize the female.

Bro-Jørgensen reasoned that female topi, particularly if they have already mated with a less fit male, should want to mate as many times as possible with desirable males to maximize their chances of conceiving high-quality offspring. Conversely, males, which encounter many different females during the rut, pay an "opportunity cost" if they mate mainly with one female because they might run low on sperm.

As Bro-Jørgensen reports online 29 November in Current Biology, lek males with two females on their territory at once tended to focus their attention on the female they'd mated with less. The other female often became aggressive toward the mating pair (see movie), attempting to shift the male's attention back to her. However, her aggression sometimes backfired. Seven percent of the time, the male counterattacked. An aggressive rebuff was particularly likely when she'd already mated with him several times. According to Bro-Jørgensen, it's often assumed that males are the pursuers because for them, mating is less costly. However, he says, "In topi, there is a reversal."

According to behavioral ecologist Brian Preston of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, the finding "challenges conventional wisdom that males should mate whenever the opportunity arises." However, says Preston, the work "rests on an untested assumption that males do become sperm depleted. This has been shown only rarely in natural systems."

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