In the animal kingdom, mating calls and pheromones can attract a mate – or a cunning predator
Reproduction is the ultimate goal of life for most animals, but finding a mate is hard work. You must not only find a potential suitor, but hold their attention, identify yourself and make your quality known.
To achieve this, animals use “sexual signals”. These remarkable displays or ornaments help beat the competition in the contest for companions. And some of the most beautiful aspects of the animal world have evolved for this purpose.
Sexual cues are expensive, however, and not just in terms of the energy needed to sing or dance. A seemingly obvious and profound cost is predation. The idea is simple. Just as your private phone call can be heard by curious passers-by, the bright colors and loud calls of sexual displays can also attract the eyes and ears of predators looking for a meal.
From the twinkling wings of butterflies to the sweet songs of birds, we admire these signals daily in other species, even if they are not intended for us. So how often are they intercepted by predators? And is the risk equal for all types of signals?
My colleagues and I sought to answer these questions in a recent publication. We’ve found that the dangers to flaggers are real, although far more varied than we once thought.
Listen to private conversations
Biologists describe the illicit interception of sexual signals as “eavesdropping”, and it has been formally studied since at least Charles Darwin. The Tungara frogs of Central and South America are a classic example; their loud mating calls attract unwanted attention from parasitic flies looking for a blood meal.
To make sense of the wealth of work available on this topic, we scoured the literature for every published study of predatory eavesdropping and found 78 in total. Most were similar in design, in that they placed fake models of animals or their signals in the wild and recorded how often they were attacked by predators.
After statistically combining the results of these studies, we found that, as expected, communication with partners increases the risk of being predatory. Animals carrying sexual signals were about five times more likely to be attacked than those not actively signaling.
Digging deeper, however, we discovered that the risk of being eaten depends on How? ‘Or’ What animals communicate with each other. Those who use calls or pheromones to attract mates are at far greater risk than those who use visual displays, which surprisingly pose no increased risk.
Read more: Here’s how we proved tropical birds are more colorful – and why color helps them survive
Not so risky business
The dangers of loud calls or strong pheromones are quite intuitive, but why don’t bright colors increase the risk of being eaten? We believe there are two related reasons.
The first is that most predators are picky eaters. Even those with a broad diet, such as insectivorous birds and lizards, prefer to eat familiar prey and rarely try new things. As most animals exhibit their sexual displays intermittently, the colorful ornaments may be unfamiliar to predators, who will then avoid them out of caution.
The other possible reason is that many animals use bright colors as warning signals. Consider the striking black and red abdomens of red-backed spiders, which announce that they are dangerous and well-defended. Predators can therefore generally be wary of visible patterns, as animals carrying them often cause more trouble than they are worth.
So what do these results tell us about the evolution of communication? On the one hand, we might expect visual displays to be more conspicuous and elaborate than other types of signals such as calls or pheromones, given that predators pose little threat to extravagance.
And in populations where predation is a persistent threat, we should expect to find that adaptive evolution favors the use of less risky cues, such as color or movement (or the abandonment of signaling altogether). We can see this happening in the Pacific field crickets of Hawaii, where the males have lost the ability to sing in response to intense predation by parasitic flies.
Read more: Tug of war between survival and reproductive capacity: how chameleons grow brighter without predators
Arming the language of sex
Predators aren’t the only ones interested in listening to their prey; humans are too. Pests such as aphids and grasshoppers are not only a nuisance in our gardens, but also wreak havoc on Australian crops to the tune of hundreds of millions of dollars every year.
Enterprising researchers have shown that we can hijack the sexual signals of these parasites to fight them in two ways. One is to use said signals to attract and trap the pests themselves, as in the case of artificial acoustic signals mimicking field crickets and grasshoppers.
Or we can tap into the existing interests of predators to attract them to more pests. This has proven effective in aphid management, for example, where we now commercially synthesize female sex pheromones. This attracts predatory wasps which lay their eggs inside the aphids and eventually kill them.
Of course, our review only offers a small guide to bio-inspired pest control. More generally, it sheds new light on what was once thought to be a fundamental cost of sex, and shows that while attracting friends can be a dangerous game, it entirely depends on how you play.