Female fish prefers males with skin cancer

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Getting skin cancer is not necessarily a bad thing for some male Northern swordtails (Xiphophorus cortezi), according to a study published in a recent issue of the Proceedings of the National Academy of Sciences.

Andr Fernandez and Molly Morris studied the mating preferences of three populations of female Northern swordtails and found that two of them preferred males whose tails were painted to resemble the skin cancer spots.

The skin cancer (or melanoma) leaves distinct black spots on the fins and body of the males (caused by the cancer gene being switched on in pigment cells), with the cancer killing the fish in a few months after development.

The authors placed a female swordtail in the middle of a tank with two partitions. A male painted with skin cancer spots was placed on one side, and a male without the pattern on the other.

After releasing the female from an opaque tube into centre of the tank, they observed how much time she spent associating with each male during an eight-minute period.

They found that in two of the swordtail populations, the females spent significantly more time near the painted male, while in the third population, the opposite was true.

The authors surmise that the female preference for more visible males is driving the preference for males with skin cancer spots in the first two populations, whereas in the third population, it is thought that the higher incidence of the skin cancer gene among the females makes them avoid mating with males with skin cancer.

The scientists hypothesize that this is due the significant increase in chances that the offspring will inherit the skin cancer and shorten their reproductive lifespan from such a mating. "Melanoma formation cuts the reproductive life cycle in half," Fernandez said. "It has a huge cost for males."

For more information, see the paper: Fernandez, AA and MR Morris (2008) Mate choice for more melanin as a mechanism to maintain a functional oncogene. Proceedings of the National Academy of Sciences 105, pp. 13503"13507.