Find out more in our image gallery: “Dangerous liaisons: Animals’ tangled love lives“
IT WAS fatal attraction of the worst kind. A pinnacle of beetle beauty, she drove the local males wild with lust. Although she ignored their mating attempts, her admirers just couldn’t prise themselves from her side, even when attacked by ferocious ants.
Their sacrifice was in vain. The object of their desire was nothing more than a discarded beer bottle the same shade of dark amber as a female jewel beetle. Adding to its lustrous allure, a line of dimples along its bottom looked remarkably similar to the glistening embellishments on the females’ shells, helping to lure the hapless males to their death.
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This sad story was uncovered by . But jewel beetles are far from being the only animals to make unfortunate mate choices. According to a straightforward reading of evolutionary theory, individuals should try to land the fittest mate with the best genes to pass on to their offspring. But over the past few years, a growing number of exceptions to this rule have emerged.
Understanding these puzzling pairings could give us a better grasp of the evolutionary forces driving mate choice. Indeed, the apparent mistakes are often helpful adaptations to difficult circumstances – showing that many more factors can influence partner choice than might be expected. “Sometimes the mistakes tell you more than the times when animals make the right mate choice,” explains David Shuker at the University of St Andrews, UK, who ran a conference at London Zoo last year called: “Why do animals mate with the wrong partner?”
Sometimes, of course, an animal may plump for a poor partner for want of a better choice. “You just make the best of a bad job,” says Shuker.
At other times, the odd pairings can reveal the surprisingly cunning strategies involved in finding and attracting a mate. Such is the case with a fish called the Atlantic molly. Male mollies should opt for the biggest female they can see, because size indicates fertility. But the males are also swayed by popularity, swarming to any female who is basking in another male’s attention. The resulting ambush can harm the first suitor’s chances, so sometimes a male uses a surprising trick to ensure he gets exclusive access.
First he feigns interest in a smaller female to divert attention – nipping at her genitals and perhaps even mating with her. His sacrifice is worth it because he can then make a move on the real object of his affections while everyone else’s backs are turned. His competitors have thus been fooled into mating with the decoy while missing out on the more fertile alternative, says Sabine Nöbel at the University of Siegen, Germany, who studied the phenomenon.
Jealousy also plays a role in the antics of bank voles living in the forests of western Europe. Males whose territories overlap form a hierarchy, according to their strength and dominance. In theory, an amorous female should aim high, but she faces a dilemma – the suitors she rejects may enter her nest and kill any pups they come across.
As a result the female’s best strategy is to have sex with as many of her neighbours as possible – even those of the lowest rank. “They generally mate with any male in their vicinity,” says Ines Klemme at the University of Jyväskylä in Finland. That’s because males are much less likely to kill a brood if there is a chance they might be harming their own young. “It’s a way of avoiding infanticide,” says Klemme.
The female does have one final weapon in her arsenal – she can selectively abort some of the fetuses of the less-dominant males, allowing her to devote more energy to the young of her preferred mate.
On the other hand, some apparent mistakes really are, well, just mistakes. Take the black grouse. For a few days every year, these birds congregate in large groups; the strongest males fight their way to the centre of this “lek” while weaker contenders are relegated to the periphery.
The strutting victors tend to have their pick of the females, but even the losers sometimes strike it lucky. How so? It turns out that they have somehow caught the eye of naive female yearlings going through their first mating season. For grouse, the rules of the dating game seem to be learned from their peers. While studying their habits in Jyväskylä, Finland, Carl Soulsbury at the University of Lincoln, UK, has found that young females tend to make these mistakes before they have honed their skills in successive mating seasons. “Maybe they just haven’t spent enough time looking,” he says.
Even if such pairings aren’t ideal, at least they fit one of the most important criteria of mate choice: that partners are the right species. Yet some creatures ignore even this basic requirement.
Depending on how closely related two species are, they may still be able to produce viable offspring, but such hybrids are often less healthy and fertile than normal. Inter-species matings should therefore be a bad move from an evolutionary point of view. So when hybrids do crop up, it suggests something interesting is going on.
Certain frogs in the western US, for instance, take this radical step as a response to drought. Both parent species live on arid plains, hibernating for most of the year before emerging to mate after heavy storms. The females lay their eggs in the large pools left by the rain, leaving the tadpoles racing against time to grow legs and hop out of the pond before it dries.
If the females of the plains spadefoot frog find themselves in a shallow pool likely to dry up too quickly, they take the drastic action of mating with a different species, the Mexican spadefoot, whose tadpoles mature more quickly. That way their offspring have a better chance of escaping the pond before it vanishes – even if their fertility does suffer. “At least the mother’s going to get some grand-offspring,” says Karin Pfennig at the University of North Carolina at Chapel Hill, who has looked into this hybridisation.
Then again, producing the fittest offspring is not always the primary motive for sex. Take the hump-winged grig, a relative of the cricket. The female chews off her partner’s hind wings during sex and then drinks his blood for an energy boost. Hungrier females become remarkably less choosy about which species of male they allow to mate with them, according to research by Kevin Judge at the Grant MacEwan University, Edmonton, Canada. He thinks they may be particularly tempted to make this decision at the end of their breeding season. “All their own males are tapped out and they just go with what’s available,” says Judge. This ulterior motive for sex might explain why the different grig species haven’t diverged more, he says. “It breaks down the species barrier.”
“The female chews off her partner’s wings and then drinks his blood”
Friends with benefits
If nothing else, the grig’s behaviour makes clear that reproduction need not be the be-all-and-end-all of mating. The many other reasons for sex can lead to lots of apparently maladaptive forms of courtship. Social bonding might explain the active sex lives of bonobos as well as same-sex mating of bottlenose dolphins and many other species.
It is still early days for the study of these unconventional couplings, but what is becoming clear is that there are no hard and fast rules of attraction in the animal kingdom. Mate choice, as with any other decision, can depend on many factors. “There is no mating between ‘wrong’ partners,” says Joan Roughgarden of the Hawaii Institute of Marine Biology, “just wrong interpretations of the function of mating.” Shuker agrees: “We are beginning to realise that there isn’t going to be a best mate that suits everyone. Sometimes it’s best to mate with anything and see how you get on.”
Such was the case with jewel beetles’ dangerous liaison with the beer bottle. The Australian entomologists pointed out that male beetles invest so little in their young that it pays for them to simply sow their wild oats as far and wide as possible. As a result, they have remarkably low standards – anything orangey-brown and shiny will do.
It may be tempting to laugh at their folly – but when it comes to love, who hasn’t chosen unwisely at one time or another? Especially when beer is involved.
