Baboon Bottoms

Charles Darwin was the first to write about mate selection leading to the development of non-adaptive traits. At least non-adaptive in the sense that they did not contribute to the survival of the individual. Traits like large antlers and distinct coloration of males or females is assumed to contribute to mating success. The distinctive pink swellings on female baboons have been explained as an example of sexual selection. The assumption is that the swellings make baboon males more likely to attempt to mate with the female.  But a recent study on Tanzanian olive baboons indicates that the offspring from more swollen females were also more likely to survive. Instead of an ornamental signal, the swellings seem to indicate that a female's reproductive value. Why there should be any connection between swelling and viability of offspring is not at all clear. 

• Study Baboon Rears Display Sexual Fitness ABC News
• Big Bottom New Scientist