Jake Buehler is a writer.

Males of some hummingbird species may have evolved to be smaller than females due to the fact that they can put on more impressive displays.

Sexual dimorphism occurs when the sexes of animals differ in size and appearance. Males in mammals and birds are usually larger than females. In a group of very small bee hummingbird, the pattern is reversed.

Sean Wilcox and Christopher Clark were interested in the mystery of these tiny males.

Data from other studies on both sexes of hummingbird species were compiled by the researchers. The length of the wings and the sternal keel, a large bone extension from the bird's chest, were among the information gathered. During the characteristic shuttling and diving of courting in 30 bee hummingbird species, the researchers measured the males' wing beat frequencies.

The bees are proportionally shorter and beat faster than females. Their muscles are larger than the females'. The species with the shortest wings have the fastest wings.

The findings show that females may be driving the evolution of males' flight athletics.

Wilcox doesn't think that fights between males are behind their minute proportions. He says that large males tend to do better in fighting in hummingbirds.

It is not known what parts of the displays are most important to females. Wilcox doesn't know if females watch the males for how quickly they fly or if they just pay attention to the speed of the wings.

Wilcox was surprised by the wing beat speeds he and Clark recorded. The upper limit of hummingbird activity was 80 beats per second.

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Wilcox says that they are ramping up their wing beat frequencies. Are they close to a flight limit while doing these displays?

Wilcox wants to know if the bee hummingbird's special muscle type has evolved as part of a push for more impressive flying feats.

There are hummingbird species where the males don't rely on shuttling and diving during courting, but instead gather in groups called "leks", vying for the attention of females. He says it would be interesting to see how the males adapt to being in a hummingbird colony.

Behavioral Ecology is a journal.

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