Sexual selection, which is a subset of natural selection, is defined as “selection based on mate choice.” It usually, but not always, takes the form of males competing for access to females, and results in the development of either armaments in males that help them compete in the battle for mates (antlers on deer, horns on stag beetles, etc.), or bright plumage, coloration, adornments, calls, or behaviors of males that catch the fancy of females (the bowers of bowerbirds, the plumage, colors, and strange behaviors of the New Guinea birds of paradise, the songs of male frogs, etc.). We understand the competition scenario more than the “female preference” scenario, for it’s hard to figure out why females would prefer one plumage or adornment rather than another.
Nevertheless, the signs of sexual selection of living species are fairly clear (males have the weapons or adornments, females don’t), and we can do tests to see whether sexual selection seems to be a reasonable explanation. In peacocks, for example, you find that males (the ones with the big tails!), who have more “eyespots” in their tails get more mates, sage grouse who display the most vigorously also win the mate race, as do African widowbirds with longer tails.
It’s harder to do these studies in extinct species, but a new paper by Knell et al. in Trends in Ecology & Evolution (reference below), gives some nice examples of possible sexual selection in fossils, as well as a list of things about the fossils that might indicate sexual selection.
First, a few fossils suggestive of sexual selection:
Of course, features like the fan-shaped crest above could have had other functions (e.g., thermoregulation, species recognition, etc.), so we have to use other criteria, delineated below, to see if sexual selection is a likely explanation.
Here’s a weird one—perhaps the strangest trilobite of all, Walliserops trifurcatus from the Devonian of Morocco (Photo: Dr. B.D.E. Chatterton). Check out those horns and the bizarre trident on its head. Richard Fortey showed this one at a talk I saw in Madrid last year, and I believe he gave a functional explanation for it rather than imputing it to sexual selection. (The photo, by the way, is of a real fossil.)
Now, how can we get an idea whether such bizarre features arose by sexual selection as opposed to other mechanisms (direct natural selection not based on mate choice, genetic drift, as nonadaptive pleiotropic byproducts of other features that evolved, and so on)? The authors suggest five observations:
- Sexual dimorphism. This is the most obvious feature: if an exaggerated trait or ornament is found in only one sex but not the other (preferably males), that suggests sexual selection. There are a couple of problems here. First, if you see two forms of fossils, one with a trait and one without, those could be males and females of one species, or they could be two different species, one of which has ornaments in both males and females. Also, there are forms of mutual sexual selection, in which males and females both prefer a trait, that could lead to both sexes having ornaments, and thus no difference to be seen in the fossil record.
But here’s one lovely example from the paper in which sexual selection is likely. It occurs in pterosaurs, flying reptiles of the Jurassic. As the TREE article notes:
Darwinopterus is a small pterosaur from the Middle Jurassic of China, known from numerous specimens. Some individuals are crestless, whereas others possess a bony crest located along the midline of the skull, which was probably associated with soft tissues that enlarged crest size substantially in life . Crested specimens have a proportionally smaller pelvis and ventrally fused pelvic elements, whereas crestless specimens have an unfused, wider pelvis. Furthermore, one crestless specimen has a pterosaurian egg preserved in close association with its pelvis and so is clearly a female.
Here’s the uncrested female Darwinopterus showing her egg (arrow) immediately outside the cloaca (photo by Lü Junchang):
And here’s a reconstruction by Mark Witton that shows the sexual dimorphism of Darwinopterus:
- Change in growth rate during development. Because sexually selected traits are “expensive” in terms of metabolic/reproductive cost, and could make animals susceptible to predation, in living animals they tend to show up only at the end of development, when the animal is an adult and ready to reproduce. Indeed, that’s exactly where we’d expect to see them develop given the modern theory of evolution. And this is indeed the case, as is seen in things like peacock feathers and deer antlers. If one sees this in one sex of fossils (as we do with the bony crests of pterosaurs), it suggests sexual selection.
- “Positive allometry”. This means that as the body of an animals gets bigger during growth, the feature gets proportionally bigger than that. For example, if the linear dimension of body parts increases by a factor of 2 during development, sexual selection might be indicated if the size of the ornament increases by a factor of 3.5. This is really related to the previous point; neither are conclusive, of course, because one can have such positive allometry for other reasons, for example simply because of the constraints of development or biomechanical reasons. To deal with this, the authors suggest testing whether the degree of allometry is one expected under a hypothesis of sexual selection versus hypotheses like thermoregulation or use as a rudder (as in the crests of flying dinosaurs).
- “Morphological disparity”. This means that if there are related species in a group in which one or more species show evidence of sexual selection, that group will show a diversity of different traits that are elaborated. For example, the birds of paradise of New Guinea are sexually selected, with the males having bright ornaments, coloration, feathers, and bizarre courtship behaviors, and yet the traits of males differ drastically among related species. This is probably because female preferences vary erratically among different species (I won’t go into why this might be the case), and so the males also come to differ. Click on this Google image search that shows some of the “morphological disparity” among sexually selected males in those birds.
- Costliness. Sexually selected traits tend to be “costly”: that is, they take a lot of metabolic energy that could be diverted to other traits, or even reduce survival by making the male bearer visible to predators. Male peacocks are not only highly visible to predators because of their coloration, but their long tails make it hard for them to fly. Of course, if such traits are sexually selected, the advantage gained by attracting more females has to outweigh the “cost” of bearing the ornament; and experiments in some species show that this is indeed the case. As the paper explains, if traits arose for another function, say to recognize members of the same species, one wouldn’t expect them to be so costly.
Remember that these features are indications of sexual selection in the fossil record, not absolute proof (which we don’t get in science anyway), and it’s hard to test various alternative hypotheses. The evidence for sexual selection becomes stronger if the traits in a fossil species show a combination of the different properties shown above. I find the example of Darwinopterus (above) pretty convincing.
I’ll leave you with one more example of a fossil species that might have experienced sexual selection: this is a Cambrian trilobite, Parablackweldeeria luensis (from the paper), showing a fossil (a) and reconstruction (b). The animal was unusual in having its eyes on the ends of long eyestalks:
This resembles the long eyestalks shown in diopsid flies, like this speciemen of Teleopsis dalmanni, in which males have eyes on long eyestalks as a result of sexual selection (the males “face off” against each other and butt each other with their heads; long eyestalks give an indication of head size and may help settle contests by forcing the smaller fly to give up sooner.
Knell, R. I., D. Naish, J. J. Tomkins, and D. W. F. Hone. 2012. Sexual selection in prehistorical animals: detection and implications. Trends Ecol. Evol. Early online publication, Sept 7. DOI 10.1016/j.tree.2012.07.015