Killingtonskier

Turns out, they like them a lot. When offered a chance to pay a visit to one of two male fish, female delicate swordtails from two populations in Mexico spent more time with spotted males than spotless ones. And they also preferred to consort with males with big spots over males with little ones.

The Xmrk gene definitely imposes an evolutionary cost on fish. But that cost may be erased by the benefit it gives male fish through sexual selection. By the time a male delicate swordtail dies from an Xmrk tumor, he may have mated with a number of females, which will pass down the gene to their young.”

Allow me to hazard a wild guess as to a possible significant benefit female offspring of a male with large dark spots might receive.

These fish apparently bear their young alive after the female gestates them for 28 days in her oviducts. The dark spots in question look to be situated so as to be likely to cover the area of the oviducts. The Delicate Swordtail is an inhabitant of fast flowing mountain streams in Mexico, and as such would be exposed to a significant amount of UV radiation. It is so tiny (~2in.) that UV could be expected to be able to penetrate to the growing embryos and potentially harm them– unless they just so happened to be protected by something such as a big dark blot of melanin (which functions in many animal species as a protectant against UV).

If this is the case, it is not classical sexual selection, in which the traits selected for are a severe disadvantage to the male, who shows his superior quality and desirability by being able to bear up under them.

In fact, as I mentioned in a previous comment, true sexual selection may be rarer than is commonly supposed. In the case of the peacock for example, I’ve noticed that the spread feathers of the fully extended tail tend to bend forward toward their tips enough, in my opinion, to form a radar dish-like structure which could focus sound right at the ear of the cock, and which would thereby make it much more difficult for any predator (or rival cock) to sneak up on the bird through cover. The eyespots on the feathers might function to enhance the effectiveness of such a system by provoking involuntary startle responses from hidden predators, especially in combination with the peacock’s harsh cry– which could be directed in a beam by the feather dish too, of course. Moreover, I have seen videos in which peacocks seem to be panning around in a steady circle with their feathers fully extended as if they were scanning just as a radar dish would, or as owls do, for that matter.

And peacocks were, as I recall, used as watchbeasts by Persian nobility.