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Males of some hummingbird species may have evolved to be smaller than their female counterparts because it allows them to put on more impressive courtship displays.

Sexual dimorphism – where the sexes of an animal differ in size and appearance – is common across the tree of life. When dimorphism occurs in mammals and birds, the males are typically the larger sex. But not so in a species group of very small bee hummingbirds, where the pattern is reversed.

Intrigued by the mystery of these tiny males, – now at Moorpark College in California – and at the University of California, Riverside, investigated evolutionary explanations.

The researchers compiled data from other studies on both sexes of 92 hummingbird species, including body mass and how fast the wings beat when hovering. They also gathered information on adaptations for flight, such as the length of the wings and the sternal keel, a large bony extension from the bird’s chest where powerful flight muscles attach. The researchers also measured the males’ wing beat frequency during the characteristic shuttling and diving of courtship in 30 bee hummingbird species.

They found that male bee hummingbirds aren’t just smaller than females, but that their wings are proportionally shorter and beat faster too. Their keels are also longer than those of the females, supporting bigger muscles. Species with especially short wings and long keels also have the fastest wings during courtship flights.

The findings suggest discerning females may be driving the evolution of males’ flight athletics, and their diminutive size along with it.

It is possible that fights between males are behind their minute proportions and nimbleness, but Wilcox says this isn’t likely. “Most of the research on fighting behaviour in hummingbirds has shown that larger males tended to do better,” he says.

If females are preferentially choosing tiny aerial aces, it’s not yet clear what parts of the displays are most important to them. “We really don’t know if females are watching the males for how quickly they fly or if they’re just paying attention to the speed of the wings,” says Wilcox.

Wilcox is particularly surprised by the intense wing beat speeds he and Clark recorded – some males hover at 100 beats per second and hit 132 in courtship dives. Until now, as confirmed in scientific research, 80 beats per second was considered hummingbirds’ upper limit.

“They’re flying side to side, they’re cutting these turns, ramping up their wing beat frequencies,” says Wilcox. “Are they approaching a muscle performance or flight limit while they’re doing these displays?”

Wilcox says that some bee hummingbirds have a specialised muscle type, and he would like to know if these unique muscle fibres have evolved as part of a push for ever more impressive flying feats.

at San Francisco State University in California notes that there are hummingbird species where the males don’t rely on shuttling and diving during courtship, but instead gather in groups called “leks”, vying for the attention of females stopping by. It would be interesting to see how adaptations in these males’ wings compare to those seen in bee hummingbirds, he says.

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