Sexual selection function to male tail (but not female tail)


In addition to the naturally selected pursuit-deterrent function of the tail, my research has shown that sexually selected benefits are associated with the male tail, but not the female tail.


Males with longer tail-wires (barbless region of the central tail feathers) have greater pairing success, pair with females that lay larger clutches, and have greater fledgling success. In contrast, female tail length is not related to measures of performance and reproductive success. Additionally, there is no evidence for assortative mating for tail length, further indicating that females do not use their tails as sexual signals.


The tail is sexually dimorphic after controlling for differences in body size and the most dimorphic part of the tail is the wire, which is ten percent longer in males. The shorter female tail length is likely to represent the naturally selected optimum to efficiently signal during predator-prey communication, whereas sexual selection is thought to account for the extension of the male tail beyond this length.


Thus, two selective forces work in concert to maintain the elaborate monomorphic plumage in males, but that natural selection acts alone to maintain female tails.

Turquoise-browed Motmot

Eumomota superciliosa

Both males and females of many avian species maintain elaborate plumage traits, and ‘elaborate monomorphic’ plumage can convey adaptive benefits to one or both sexes as inter- or intraspecific signals.


The selective forces maintaining elaborate monomorphic traits are often assumed to be similar for the sexes; however, in some species, elaborate traits confer different selective benefits to males and females. Male and female turquoise-browed motmots express an elongate racket-shaped tail, and my research has shown that the slightly longer male tail is maintained by a combination of sexual selection and natural selection for predator-prey communication, whereas the female tail is maintained by natural selection alone.

Pursuit-deterrent function to male and female tail


My research indicates that males and females use their tail in a pendulous left-to-right wag-display, which is performed in the presence of predators. Results from predator-presentation experiments indicate that the display is likely to deter ambush by communicating awareness of the presence of a predator (i.e., perception advertisement pursuit-deterrent signal).


When I experimentally presented predators to motmots, and when natural predators were observed, the wag-display was often performed in the absence of conspecifics (i.e., when only the predator is present). Furthermore, the wag-display does not appear to function as an alarm signal oriented to the mate or to general conspecifics because the display is performed by unparied and solitary birds in the same manner as it is performed by paired birds and gregarious birds at nesting colonies.


The wag-display is thus likely to communicate that the motmot is aware of the predator and is prepared to escape.  This form of interspecific pursuit-deterrent signal provides a benefit to both the motmot and the predator; the display prevents the motmot from wasting time and energy fleeing, and the predator avoids a costly pursuit that is unlikely to result in capture.

Significance

This research highlights that selection should not be expected to act on signal characters of males and females in similar ways. Additionally, this research emphasizes that elaborate plumage characters do not always function as sexually selected signals, as is often assumed, but rather that multiple selective forces can work together to favor elaborate monomorphic traits.

Motmot pics

painting by Gabriel Willow

Research

© 2006 Troy Murphy