In animals, female attractiveness is about more than finding a mate
March 29, 2017
Just like us, animals can be show-offs.
The giant plumage of a peacock and the green head of a mallard are evolutionarily designed to get the attention of peahens and female mallards, but flash and ornamentation are not strictly male attributes.
Scientists are discovering more and more species in which the females do the showing off. But new research from Duke University and UNC Chapel Hill suggests that unlike the males, finding the best mate is neither worth the trouble of a little flash, nor the likely motivation for it.
The study’s authors, Courtney Fitzpatrick, a postdoctoral biologist at Duke at the time of the study and Maria Servedio, a biologist at UNC, both study sexual selection. If natural selection is the survival of the fittest, sexual selection is the survival of the prettiest. Animals can develop what are called secondary sex characteristics: pieces of their anatomy apart from reproductive organs that one sex has and the other does not. They usually appear when the animal reaches sexual maturity. Sexual selection occurs when animals choose their mates based on these secondary sex characteristics, so the peacock with the biggest tail feathers or the lion with the fullest mane gets to reproduce and pass its genes on to the next generation.
That is at least how it works when males are the ones growing the ornamental secondary sex characteristic. Peahens will always choose the peacock with the best feathers, so a peacock with a really nice tail can mate with multiple females and pass his genes on to many offspring.
In a less common but ever-growing subset of species where females have the ornaments, the advantage is less clear. Some animals like pipefish, where the males hang onto the fertilized eggs and raise the young, operate the same way, as the females can court as many males as they want with their decorative stripes and then swim away. In others, the females have to rear the young, so there is much lower limit on the amount of offspring they can produce.
The issue with ornaments is that they can be extremely costly in terms of survival. Bright pigments found on the orange claws of female blue crabs or on the yellow chest dots of a rock sparrow take lots of energy to produce; energy that could be used to keep warm or find food. Bright colors, like the red and blue feathers on female eclectus parrots, also make the animals more visible to predators.
Servedio and Fitzpatrick wanted to determine whether ornamented female animals had enough opportunities to create healthy, attractive, fertile offspring to make those costly ornaments worth it. So they created a mathematical model, factoring in the cost of the ornament, the degree to which males would choose that ornament and the strength and fertility of the males. Given how the costs, as well as attractiveness and fertility, are passed down through generations, the researchers can use the model to predict whether male preference for the ornaments is enough to keep them in the gene pool.
The short answer is “no.” Even though the males do the choosing when females have ornaments, they are not the ones that have to carry the children. Since males can mate with multiple females, there are effectively way more males in the mating pool than females. Even if the males prefer the most ornamented females, there will be males that mate with less ornamented females just to pass their genes along so male choice will not ensure that just highly ornamented females reproduce, and the mathematical model bears this out.
The question becomes, then, why do females even bother with the ornaments if males will settle for females without the biggest and brightest ornaments? Fitzpatrick and Servedio hypothesize that another evolutionary process called social selection might be in play.
Imagine the rock sparrow, a songbird with a bright yellow dot on its chest more prominent in females than males. In sexual selection, a female with a bright yellow dot would be chosen by all the best males so she would have the opportunity to reproduce. Social selection is when that same female with the large yellow dot attracts the best males, but they court that special female by bringing her food and other resources or protecting her from predators.
While attracting a marginally better mate may not drastically improve the females’ likelihood of passing its genes to the next generation, having a dedicated group of eligible bachelors working for them could confer a huge advantage. If having the biggest, yellowest dot in the flock guarantees more and better suitors, then the most extravagantly ornamented birds should frequently outlive, and out-reproduce the birds without as much ornamentation.
This research, published in the journal Evolution, reshapes some commonly held assumptions about how ornaments play into mating dynamics. If social selection is the reason animals develop flashy colors, then for the rock sparrow and many other animals, being pretty helps solve problems on a day-to-day basis. Further mathematical modeling and field studies of animals with female ornamentation will be needed to confirm that effect.
Daniel Lane covers science, medicine, engineering and the environment in North Carolina.