Multiple Sexual Signals and Behavioral Reproductive Isolation in a Diverging Population

Abstract

Sexual trait divergence has been shown to play a role in the evolution of reproductive isolation. While variation in multiple sexual signals is common among closely related species, little is known about the role of these different axes of phenotype variation with respect to the evolution of behavioral reproductive isolation. Here we study a unique population of barn swallows (Hirundo rustica transitiva) that can be distinguished phenotypically from its neighboring populations only on the basis of two features of male plumage: exaggerated expression of both long tail streamers and dark ventral coloration. Using phenotype manipulation experiments, we conducted a paternity study to examine whether both traits are sexually selected. Our results show that an exaggerated form of the local male phenotype (with both tail elongation and color darkening) is favored by local females, whereas males whose phenotypes were manipulated to look like males of neighboring subspecies suffered paternity losses from their social mates. These results confirm the multiple signaling role of the unique tail and color combination in our diverging population and suggest a novel possibility according to which multiple sexual signals may also be used to discriminate among males from nearby populations when prezygotic reproductive isolation is adaptive.

Geographic variation in sexual selection and implications for speciation in the Barn Swallow

“…Again, there appears to be variation in sexual selection on tail streamers among these subspecies. Male H. r. transitiva have tail streamers that are nearly as long as those in H. r. rustica, and, accordingly, males with longer streamers breed earlier and have fewer extra pair young in their nests in both correlational (Vortman et al 2011) and experimental (Vortman et al 2013) studies. By contrast, male H. r. gutturalis, which have much shorter streamers than H. r. rustica and H. r. transitiva, do not gain any significant reproductive benefits based on tail length in correlational datasets (Kojima et al 2009;Hasegawa et al 2010).…”

“…Experimental work in this subspecies has shown that both ventral color and tail streamer length predict different aspects of breeding success, with darker males raising more young over the course of a season, and males with longer tail streamers breeding earlier and having fewer extra-pair chicks in their nests (Vortman et al 2011(Vortman et al , 2013.…”

“…There is some evidence that sexually selected traits act as reproductive barriers in Barn Swallows: in an Israeli population of H. r. transitiva, males with color and tail length manipulated to look like the geographically proximate H. r. savignii were not attractive to females, suggesting there may be some selection against heterotypic matings in this populations (Vortman et al 2013). These reproductive barriers are not complete, however: there is evidence for hybridization between H. r. rustica and H. r. tytleri in central Siberia, between H. r. tytleri and H. r. gutturalis in eastern Siberia (Turner 2006), and between H. r. rustica and H. r. transitiva in Israel (Dor et al 2012), despite morphological differences between all these subspecies.…”

“…The North American populations are on average much darker in ventral color, with tail streamers tending to be intermediate in length to other subspecies (Safran et al in review). Populations of the two sedentary subspecies (H. r. savignii in Egypt and H. r. transitiva in the Middle East) have relatively large body size, dark plumage coloration, and long streamers (although H. r. savignii is darker and shorterstreamered than H. r. transitiva, (Vortman et al 2013)). H. r. tytleri, in the Baikal region, is of intermediate body size and exhibits intermediate streamer lengths with dark ventral color, while East Asian H. r. gutturalis has very small body size, pale orange-brown ventral color, and the shortest tail streamers of all subspecies.…”

Beauty alone is insufficient: female mate choice in the barn swallow

“…Thus, when females obtain higher net benefits by preferring specific combinations of male traits, preference for the combination is expected. Vortman et al's study may represent one such case: males with two experimentally enhanced ornaments (tail length and plumage coloration), but not males with either experimentally enhanced one ornament, had increased within-pair paternity, indicating female preference for the combination (Vortman et al 2013). This preference would be adaptive for females to avoid interbreeding with other adjacent subspecies in this EastMediterranean subspecies (where an adjacent subspecies has more colorful plumage with shorter tails and another has less colorful plumage with longer tails than the focal subspecies).…”