Final Thought About Sexual Selection- The Exception that Proves the Rule

When I introduced the topic of sexual selection I mentioned that the critical difference between males and females was the size and function of their gametes and that that difference was the cause for many of the differences between males and females we are familiar with. Because they typically invest more resources in each offspring we expect that females should be choosy about who the mate with and that males should compete for the opportunity to mate with females. Sexual selection has resulted in the production of brightly colored and ornamented males who use coloration and ornamentation to attract females.

Thus, when we see sexual dimorphism in birds we generally conclude that the brightly colored individual is the male while the dull colored individual is the female. At the top of the page is a picture of a male and female Red Phalarope. Based on what we have learned so far it would be easy to conclude that the brightly colored bird was a male. However, if you did so, you would be wrong! How can this be?

It turns out that phalaropes, and othe birds like them, are the exception that proves the rule. Phalaropes have a polyandrous mating system (one female mates with many males). After a female phalarope mates with a male she lays the eggs which are taken care of by the male. While the male is looking after the eggs, the female heads off in search of another male to mate with. Thus, in polyandrous birds, the investment of time by the male actually means that the male invests more in each offspring. Thus, in polyandrous species males are more choosy about who they should mate with than are females and females are brightly colored in an attempt to attract males. Pretty cool!!!!