Something I'd never thought of before, although it's obvious when you think about it - when different species evolve towards a similar color pattern they run the risk of confusing their mates.
A number of butterflies have evolved such similar color patterns, and the process is known as Müllerian mimicry. The hypothesis is that the joint color pattern benefits the butterflies because predators who have previously tasted ANY of them will steer clear of all of them.
In a paper in the latest edition of the Journal of Evolutionary Biology, Estrada and Jiggins conducted experiments in greenhouse conditions to study interspecific attraction between two mimetic butterfly species, Heliconius erato and Heliconius melpomene. Both species spent considerable time approaching and courting females of the co-mimic species. However few males actually tried to mate with the wrong species suggesting the cost was time rather than sperm
Overall, our results showed that there might be a cost of mimetic convergence because of a reduction in the efficiency of species recognition. Such cost may contribute to explain the apparently stable diversity in Müllerian mimetic patterns in many tropical butterfly assemblages.
Here's a link to the paper but I'm reading this at home via the proxy server so in case the link doesn't work for you the paper is:
C. ESTRADA, C. D. JIGGINS (2008) Interspecific sexual attraction because of convergence in warning colouration: is there a conflict between natural and sexual selection in mimetic species?
Journal of Evolutionary Biology 21 (3) , 749–760