Male stalk-eyed-flies dangle eyes from the ends of long antennae-like stalks. Male widowbirds flaunt tails extending over twice the lengths of their bodies. And male koalas produce deep bellowing calls from highly specialized vocal tracts. There’s no end to the lengths a male animal’s body can be stretched and decorated to win a female’s attention.
But where does the female preference for the elaborate and downright strange come from in the first place?
Mirjam Amcoff, an evolutionary biologist at Uppsala University, suspected that, at least for cichlid fishes, the oddly ornamented males may be tapping into a strong female desire that evolved before it had any connection to choosing a mate.
Females of many of the nearly 2,000 species of cichlids in the Great Lakes of East Africa scoop their eggs into their mouths after mating and don’t release them until the eggs hatch weeks later. Biologists believe this behavior, known as mouthbrooding, evolved to protect eggs from being eaten by other fish.
Meanwhile, in some of these same species, males display yellow, orange, or red markings on their back fins that look an awful lot like eggs. During courtship, the male flashes these markings, known as egg dummies. In response, the female nibbles on the markings, seemingly as though she can’t resist the desire to hide them in her mouth.
This female desire works well for males. The egg dummy distraction delays females from searching for additional mates. Also, some females pick up eggs before males have time to fertilize them. While a female is nibbling on egg dummies, a male is well positioned to release sperm and fertilize the eggs inside her mouth.
For decades, biologists wondered whether egg dummies evolved specifically to take advantage of the female urge to pick up eggs, or whether the female interest in these markings was just a lucky coincidence for males.
Amcoff and her colleagues set out to find an answer. They presented their study in the November issue of the Journal of Evolutionary Biology.
To test whether egg dummies evolved in response to females, the authors needed to show that egg dummies have only evolved in mouthbrooding species. So they created a phylogenetic tree that showed the evolutionary history of over 75% of the 200 cichlid species in Lake Tanganyika, the lake with the longest history of cichlids. This tree uses gene sequence similarities to show which species are most closely related.
This information, along with the researchers’ knowledge of which species are mouthbrooders and which display egg dummies, allowed them to create statistical models that compare whether it’s more likely that egg dummies evolved independently or in response to mouthbrooding.
They found that egg dummy evolution entirely depends on mouthbrooding. In each of the 42 species where males display egg dummies, the females first developed mouthbrooding.
The authors then predicted that if females drove the evolution of egg dummies, then the species where males compete strongly for females would be more likely to evolve the markings. Indeed, the authors found that only in species where the females tend to mate with more than one male have the males developed the alluring egg dummies.
Amcoff says that egg dummy evolution is a clear case of sensory exploitation, where sexual selection favors males “that exploit preexisting biases in the female sensory system.”
Michael Ryan, a biologist at the University of Texas, Austin, who did not participate in the study, agrees: “This hypothesis about egg dummies was suggested some time ago. Some studies had supported the idea, but not others. This study amasses a large amount of data on many species to support this idea.”
While sensory exploitation may benefit showy males, it doesn’t necessarily help females choose mates that make the best father material. Amcoff explains that a female bird might focus on the color red because she needs that search image to find berries. Males that by chance evolve red ornaments might win her attention. However, these males are sometimes not the best at producing healthy offspring. Females that are attracted to these males anyway are stuck in what Amcoff calls an evolutionary trap, “where good foraging ability is associated with suboptimal mating preferences.”
Luckily for cichlid fish, biologists believe that the more egg dummies a male has, the more likely he is to be a strong and healthy mate. So female cichlids can remain happily tricked by the egg dummy hoax.