Battle of the Sperm

Once a male has earned a female’s attention and has successfully mated, he doesn’t automatically become a father. His sperm still have to struggle through her reproductive system to find an egg to fertilize. And quite often, his sperm won’t be alone: they will be competing with the sperm of other males that the female has mated with.

It may seem odd that a female would go to great lengths to choose a male and then end up mating with another one. But nothing is simple when it comes to sex. Sometimes a female’s choice gets overridden by big males who grab her and forcibly mate with her. In other cases, a female who has chosen one male may encounter a better specimen and mate with him as well. Hens, for example, prefer to mate with dominant roosters, but a subdominant male can sometimes mount a hen before a dominant rooster can scare him off. The hens don’t treasure these flings. If a subdominant male mates with a hen, she is likely to squirt out his semen. That increases the odds that when the dominant male mates with her later, his sperm will fertilize her eggs, producing stronger chicks.

Promiscuity is rampant in the animal world, even in species that generations of scientists had been convinced were utterly faithful. Around 90 percent of all bird species live monogamously, for instance, with a male and female joining together for a year or even a lifetime, building nests together and working together to raise chicks. Monogamy is a matter of survival: without the help of both parents, the helpless chicks may not live to adulthood. But when ornithologists began to sample the DNA of chicks in the 1980s, they found that in many species, some of the birds did not have the father’s genes. In most species, a few percent of chicks were illegitimate; in some, the percentage was as high as 55 percent.

A monogamous female bird doesn’t cheat on her mate randomly. Female barn swallows, for example, judge the quality of males in part by the length of their tail feathers. Females who pair up with short‑tailed males for the season are much more likely to cheat on their partners than females who pair up with long‑tailed males. Female swallows have only a limited amount of time to choose a male companion for the breeding season, so they can’t wait forever to find the perfect partner. But they can offset the shortcomings of their partner by mating with more desirable males that come visiting. And they can enjoy the help of their partner in raising the chicks, even if he is not their father.

A male thus faces a quandary. Despite all his efforts to court a female, he has no guarantee that his sperm will fertilize her eggs. She may carry the sperm of another male in her, or she may later mate with another male. As a result, males in many species have evolved ways to compete in utero.

One way is to make a lot of sperm. The scramble of sperm inside a female is like a lottery: the more tickets the males buy, the higher their chances of winning. Among primate species, for example, the average size of their testicles is directly proportional to the average number of partners the females mate with. The more intense the competition, the more sperm a male primate produces.

A sneakier way to win a lottery is to destroy the tickets of the other contestants. Male fruit flies have poisonous semen that disables the sperm of previous suitors inside a female. Male black‑winged damsel‑flies have penis‑like organs covered in spines; before they deposit their own sperm inside a female, they use the spines like a scrub brush to clean out the sperm of other males. They can get rid of 90 to 100 percent of another male’s sperm, giving their own a much better chance of fertilizing eggs. The male dragonfly Hadrothemis defecta uses inflatable horns on his penis‑like organ to push the sperm of other males deep into recesses in the female’s body. Only then does he place his own sperm closest to her eggs.

Yet another way for males to win the lottery is to keep other males from buying tickets in the first place. In addition to their sperm poison, male fruit flies have chemicals in their semen that decrease a female’s libido. With less interest in mating, she will be less likely to receive the sperm of other males. Among Sierra dome spiders, the females attract potential mates by scenting their webs with a male‑attracting pheromone. Once a male locates the female, he destroys her web so that other males will have a harder time finding her.

For the males of some species, the best way to raise the odds that their sperm will succeed is to commit suicide. The male Australian redback spider routinely sacrifices himself for sex. He begins his courtship by plucking the strands of the female’s web, transmitting a love song of sorts that may last for hours. If she doesn’t chase him away–or if another male already with her doesn’t do it for her–he approaches. She looms over him, her body weighing 100 times more than his own. To any animal in his position death could come at any moment: her bite is as deadly as that of her relative, the black widow.

The male redback crawls onto her belly. He extends an appendage that sprouts from his head. Known as a palp, it looks like a miniature boxing glove. On its tip is a long coiled tube, which he threads into the female’s body. He then starts to pump sperm through the palp, into her. Suddenly, using his palp as a fulcrum, he swings his body up from the female’s abdomen and flips over onto his back, landing on her fangs. She begins to chew on his abdomen and injects venom into his body, which begins to digest his innards to goo. She dines slowly, as the male continues to inseminate her, and after a few minutes the male pulls away from her. He retreats a few centimeters away and grooms himself for 10 minutes or so. Even as his body is disintegrating from within, he returns for more, inserting a second palp into her body and performing another flip. The female resumes her meal, biting deeper into his body. The mating may take half an hour; by then the male is barely alive, and when he withdraws his second palp, the female weaves a shroud of silk around him. There is no escape for him this time. After a few more minutes of feeding, the female has reduced him to a mummified husk.

Maydianne Andrade, a biologist at the University of Toronto, has studied the suicide of the redback spider to see whether it represents an evolutionary adaptation. Not all male redbacks get devoured, she has found. Only hungry females devour their mates, and as a result a third of males survive their deadly somersault. That discrepancy gave Andrade the opportunity to measure the reproductive success of cannibalism.

Female redbacks seem to be in control of how long copulation will last. In cases where they don’t devour their mate, Andrade has found that sex lasts on average 11 minutes. But when a female chooses to eat a male, copulation can last 25 minutes. As she feasts on her mate’s body, his palps can continue inseminating her. By offering himself as a meal, a male can stretch out the sex. As a result, a cannibalized male delivers more sperm, fertilizing twice as many eggs as a male that survives. And once the male is dead, the female tends to spurn new suitors–perhaps because she is sated with sperm or with food. In either case, the chances of another male injecting her with competing sperm go down, giving the dead male a better chance of having fertilized her eggs.

These sorts of benefits are apparently more valuable to male redback spiders than their own lives. They have few chances to mate for a number of reasons–they have short life spans, and their sperm‑delivering palps snap off during sex, rendering them sterile. So these spiders make their one chance count.