The Phalaropes are quite the interesting group. First of all, two of the three species are considered pelagic – meaning they spend a great part of their time out at sea, away from visible land. The Wilson’s Phalarope, pictured throughout this post was the most abundant of the shorebirds during our visit to Quivira NWR, and do nest within the refuge. The Phalaropes exhibit reverse sexual dimorphism – both in visual and behavioral characteristics. This means that females are the more brightly colored of the sexes. Similarly, the females display and fight for males. The males will immediately take over the responsibilities of incubating the eggs and raising the chicks, while the polyandrous female will attempt to find and mate with other males. See the photo below of a relatively drab male with his mate.
After reading of this role reversal in a field guide recently, of course I had to find out why. This is not an easy one to answer. Do a quick search and terms like Operational Sex Ratio (ratio of sexually active males and females in a given time and place) and Potential Reproductive Rate (potential offspring production rate if mate availability is unlimited) and Bateman’s Principle (in most species, reproductive success is more variable in males than in females – primarily due to anisogamy) come up often.
So what forces add up to cause this reversal in sexual selection (females competing for males – males choosing females)? Ultimately, this is quite a complex question to answer, and I cannot begin to cover all possible variables that affected this change. The primary changes that had to occur in these species is that OSR of males and females had to be skewed to a situation where females outnumber males in mating opportunities. This produces a scenario where females are forced to compete for males. There are anecdotal accounts of males actually competing for females in this species when the OSR switches to favor males (although males still kept their brood-rearing responsibilities). Another aspect to this model is that PRR must shift so that females can reproduce at rates close to that or greater than males. For instance, nesting sites should not be limited based on other resources. And, similar to traditional sex role systems, the male must be pretty certain that the eggs and brood he will be caring for are his. Observational reports confirm very few extra pair copulations/fertilizations occur in species where this role reversal has developed.
I must admit I only have a hint at the factors that go into creating sexual role reversal in these birds. Obviously there are important endocrinological and physiological changes that must go with the social and environmental changes. Here are a few of the sources that I have used to get these thoughts together. I look forward to learning more about this interesting phenomenon.
- Marcel Eens, Rianne Pinxten. Sex-role reversal in vertebrates: behavioural and endocrinological accounts. Behavioural Processes 51 (2000) 135–147.
- Ingrid Ahnesjö , Charlotta Kvarnemo, and Sami Merilaita. Using potential reproductive rates to predict mating competition among individuals qualified to mate. Behavioral Ecology (2001) 12 (4):397-401.
- Hanna Kokko and Pat Monaghan. Predicting the Direction of Sexual Selection. Ecology Letters (2001) 4: 159-165.
- Mark A. Colwell. Shorebird Ecology, Conservation, and Management. University of California Press, Oct 17, 2010.
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