Exaggerated Traits and Breeding Success in Widowbirds:
A Case of Sexual Selection and Evolution
by
J. Phil Gibson
Department of Zoology and Department of Botany and Microbiology
University of Oklahoma
Figure 1. Long-tailed widowbirds showing breeding and non-breeding plumage. (Long-Tailed Whydah, 1899. Chromolithograph after Frederick William Frohawk, printed by Brumby & Clark Ltd. in Hull and published in Frohawk’s Foreign Finches in Captivity, 1899. Image courtesy of ancestryimages.com.)
Part I—Introduction
Natural selection is the evolutionary force that shapes the traits of a species in response to the various demands of its environment. Any process or event during the life of an organism that influences the number and/or quality of offspring produced by an organism due to a heritable trait(s) of that organism can act as a selective force. Selective forces can be produced by either the biotic or abiotic components of an organism’s environment. Hence, natural selection is manifest as not a singular action, but rather as many different events occurring over the life of an organism. In this case study, the different and potentially conflicting manifestations of natural selection will be explored and studied.
The Study Species
Widowbirds are small, finch-like birds in the genus Euplectes. They are striking members of the bird community in grasslands and shrubby savannas of southern and eastern Africa. This group is noted for the pronounced sexual dimorphism between males and females. During the non-breeding season, both male and female widowbirds have a brownish or buff coloration that blends with the grass and other vegetation. During the breeding season, however, males molt and produce black feathers on most of their body. Males also produce characteristic bright red and/or yellow epaulets and chevrons on their wings. Additionally, males of several widowbird species grow elaborately long tail feathers that can be up to half a meter in length (see Figures 1 and 2).
During the breeding season, males secure and defend a territory from other males where they then build multiple nest frames. Males then perform a flight display that has a “bouncy rowing” appearance with loops and exaggerated wing beats to attract females to their territory. Females choose a male for breeding, line a nest frame in his territory with fine grass, and then incubate the eggs and feed the nestlings in that frame. After the breeding season, males molt to return to their pre-breeding coloration and appearance. Beyond initially building the nest frame, males do not participate further in raising their offspring.
These elaborate behaviors and the striking seasonal sexual dimorphism in male widowbirds have provided researchers with an intriguing system in which to study how natural selection can shape traits of a species.
Figure 2. Male long-tailed widowbird during courtship display flight. (Photograph ©Callie de Wet/Wild Images.)
Sexual Selection
The bright breeding coloration and long tails produced by male widowbirds are thought to be adaptations to attract the attention of female widowbirds. Reproductive systems in which individuals differentiate among potential mates and choose one based upon some characteristic(s) is a form of natural selection called sexual selection. The fundamental idea of sexual selection is that choosing the best mate increases the probability of producing high quality offspring who will pass on the parental genes to the next generation. Intrasexual selection occurs when members of one sex compete with one another for the opportunity to mate with members of the opposite sex or through actively securing and protecting a territory and its resources. Intersexual selection, frequently referred to as female choice, occurs when females choose among males based upon some criterion reflecting resources the male can provide or the quality of his genes.
Concept Check
1. Evolution of reproductive characters and behaviors in long-tailed widowbirds has been shaped by natural selection. Develop hypotheses that could be used to test how intrasexual and intersexual selection could be shaping the evolution of traits in male long-tailed widowbirds.
- Hypotheses of intrasexual selection:
- Hypotheses of intersexual selection:
2. Explain the reasoning behind these hypotheses. How would natural selection be acting in them?
Date Posted: June 11, 2008.
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