Breeding is the ultimate goal of life for most animals, but getting a mate is hard work. Not only do you need to find a potential suitor, but you need to stay focused, identify yourself, and advertise your quality.
To achieve this, animals use “sex signals”. These conspicuous displays or ornaments help to outperform the competition in the peer competition. And some of the most beautiful aspects of the animal world have evolved to this end.
Sex signals 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 intrusive passers-by can hear your private phone call, so can the bright colors and audible calls on sex screens catch the eyes and ears of predators looking for food.
From the bright wings of butterflies to the sweet songs of birds, we admire these signs in other species daily, though they are not meant for us. So how often are they intercepted by predators? And is the risk the same in all types of signals?
My colleagues and I have tried to answer these questions in a recent post. We have found that the dangers to signals are real, though much more varied than we thought.
Do animals use colors, calls, and smells to attract mates, but do they also attract predators? Pictured: Sacred Anolis (left), Pseudacris crucifer (right) Ryan Hagerty / USFWS
Listen to private conversations
Biologists describe the illicit interception of sexual cues as “scouting,” and it has been formally studied at least since Charles Darwin. Tungara frogs from Central and South America are a classic example; their mating calls attract unwanted attention from parasitic flies in search of a blood meal.
To make sense of the wealth of work available on this topic, we searched the literature for each published study on predatory scouts and found 78 in total. Most had a similar design, as they placed fake animal models 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, communicating with peers increases the risk of being preyed upon. Animals that carried sex signals were approximately five times more likely to be attacked than those that did not actively signal.
Deepening, however, we discovered that the risk of being eaten depends on how the animals communicate with each other. Those who use calls or pheromones to attract a partner are in much greater danger than those who use visual screens, which surprisingly do not experience any greater risk.
There are some possible reasons why visual displays to attract peers do not increase the risk of predation as much as some other signals. Author provided
Read more: This has shown that tropical birds are more colorful and why color helps them survive
Not so risky business
The dangers of booming calls or strong pheromones are intuitive enough, but why don’t bold colors increase the risk of being eaten? We suspect there are two related reasons.
One is that most predators are demanding eaters. Even those on an extensive diet, such as birds and insect-eating lizards, prefer to eat familiar prey and rarely try new things. Because most animals present their sexual displays intermittently, the colored ornaments may be unknown to predators, who will then avoid them as a precaution.
The other possible reason is that many animals use bright colors as warning signs. Think of the amazing black and red abs of red spiders, which herald the fact that they are dangerous and well defended. Predators can then be wary of visible patterns, as the animals that carry them tend to have more problems 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, as 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 signals, such as color or movement (or total abandonment of signaling). We can see that this is happening among the field crickets in the Pacific of Hawaii, where males have lost the ability to sing in response to the intense predation of parasitic flies.
Read more: Stretching between survival and reproductive fitness: how chameleons become brighter without predators around
Assemble the language of sex
Predators are not the only ones interested in hearing prey; humans are too. Pests such as aphids and locusts are not only a nuisance in our gardens, they also wreak havoc on Australian crops worth hundreds of millions of dollars each year.
Aphids cause hundreds of millions of dollars in damage to Australian crops each year. Shutterstock
Entrepreneurial researchers have shown that we can hijack the sexual signals of these pests to combat them in two ways. One is to use these signals to attract and trap the same pests, as in the case of artificial acoustic signals that mimic field crickets and locusts.
Or we can take advantage of the existing interests of predators to attract them to more pests. This has been shown to be effective in aphid management, for example, where we now commercially synthesize female sex pheromones. This attracts predatory wasps that lay their eggs inside the aphids and eventually kill them.
Of course, our study only offers a brief guide to bio-inspired pest management. More generally, it sheds new light on what was thought to be a fundamental cost of sex and shows that while attracting peers can be a dangerous game, it depends entirely on how you play.