Sunday, April 10, 2005

More [and more technical] info about Reverse Sexual Dimorphism from Steve Watson to John Blakeman

Steve Watson, the Kestrelcam correspondent, sends in some results from his research into the reverse sexual dimorphism enigma -- that is, why female raptors are bigger than males of the various species. He intended his letter for Blakeman, but since he sent it along, I am posting it for those readers who studied statistics somewhere along the line.

Here's a challenge for Watson: for those who haven't taken a course in statistics, can you explain in ordinary language what a correlation coefficient is? [After all, I just explained in ordinary language what reverse sexual dimorphism is!] At least interpret the numbers in the final paragraph

Hi, Marie,

... here's some more info [about reverse sexual dimorphism in raptors]: .

In my research on kestrels (and specifically, dimorphism in their eggs), I found an article which summarized some average weights for males and females, by species, and their ratios, broken down by foraging type. This partially responded to my question about increasing dimorphism ratio as a function of increasing body mass, but the results appear less than conclusive to me (although, frankly, I'm not a statistician, so I'll defer proper analysis to the experts). Anyway, I've attached a chart I made from the data, just thought it might be of interest to John and perhaps others. The paper is Anderson, J. et al., Prey Size Influences Female Competitive Dominance in Nestling American Kestrels (Falco Sparverius), Ecology 74(2), 1993, pp. 367-376. The table summarizes Cade (1982) and Kemp (1987), all of which with I'm sure Dr. Blakeman is familiar.

As a bit of additional info for John, I did a quick correlation coefficient on these three groups (correlating female mass to sex ratio) and got -.209, .419 and .426 for vertebrate/invertebrate generalists, vertebrate generalists and bird/bat specialists, respectively.