Male bee hummingbirds evolved to be far smaller than females, perhaps because their petite size enables them to perform more complex and rapid courtship flights.
Males of some species of hummingbird may have evolved to be smaller than females so that they may perform more attractive courtship displays.
Sexual dimorphism, in which the sexes of an animal vary in size and appearance, is widespread throughout the tree of life. In mammals and birds with dimorphism, males are often the larger sexe. The pattern is reversed in a species group of extremely little bee hummingbirds.
Sean Wilcox – currently at Moorpark College in California – and Christopher Clark at the University of California, Riverside researched evolutionary theories for these little males.
The researchers collated data from previous studies on both sexes of 92 species of hummingbirds, including body mass and hovering wingbeat rate. In addition, they collected data on adaptations for flight, such as the length of the wings and the sternal keel, a massive bony protrusion of the bird's chest to which powerful flight muscles attach. The researchers also assessed the wing beat frequency of 30 species of bee hummingbird males during courtship's distinctive shuttling and dive.
Male bee hummingbirds are not only smaller than females, but their wings are also proportionally shorter and beat faster. Additionally, their keels are longer than those of females, supporting larger muscles. Species with very short wings and long keels also possess the fastest courtship flight wings.
The results show that discerning females may be responsible for the evolution of males' flight abilities and their tiny stature.
Wilcox argues that it is unlikely that battles between males are responsible for their diminutive size and agility. “The majority of study on the fighting behaviour of hummingbirds has demonstrated that larger males do better,” he explains.
If females prefer little aerial aces, it is not yet known which aspects of the displays are most essential to them. Wilcox states, “We really don't sure if females are observing the speed with which males fly or if they are simply observing the speed of the wings.”
Wilcox is especially astonished by the strong wing beat speeds that he and Clark recorded; some males hover at 100 beats per second and reach 132 beats per second during courtship dives. As established by previous scientific studies, 80 beats per second was previously believed to be the maximum heart rate for hummingbirds.
Wilcox states, “They are flying side to side, cutting these turns, and increasing their wing beat frequencies.” “Are they nearing a flying or muscular performance limit while performing these displays?”
Wilcox claims that some bee hummingbirds have a specialised muscle type, and he would like to know if these specific muscle fibres have developed as a result of a drive to do ever-more-impressive aerial manoeuvres.
Males of certain hummingbird species, according to Derrick Groom of San Francisco State University in California, do not rely on flitting and diving during courtship, but instead congregate in groups called “leks” and compete for the attention of females passing by. It would be fascinating to compare the wing modifications of these guys to those of bee hummingbirds, he says.