Who runs the world? Hybrid zone dynamics in a bird species where females compete for males

Introgression of a female competitive trait across a Jacana hybrid zone.

The only law in biology is that there is always an exception.” This was one of the propositions of my PhD thesis. A few weeks ago, I wrote about how sexual selection is mainly driven by male-male competition and female choice. Well, there are exceptions. In some mating systems, such as polyandry, the female defends a harem of males. Here, competition between females can determine reproductive success. One nice example of such as system are Jacanas, Neotropical shorebirds. A recent study in the journal Evolution explored the consequences of a polyandrous mating system on hybrid zone dynamics.


Northern Jacana (from: http://www.audubon.com/)


Hybrid Zone

Northern Jacana (Jacana spinosa) and Wattled Jacana (J. jacana) interbreed along a narrow hybrid zone in Central America. Previous work indicated that there is some gene flow between these species. To see if the peculiar mating system of these birds has an effect on the patterns of introgression, Sara Lipshutz and her colleagues characterized the hybrid zone with about 13,000 molecular markers (SNPs).

They also measured several morphological characters, such as body mass and the length of wing spurs. Northern Jacanas have larger body mass, longer wing spurs and are more aggressive compared to Wattled Jacanas. The wing spurs are used during fighting and in threat displays.


Wattled Jacana (from: https://landsinlove.com/)



The researchers performed cline analyses on the genetic and morphological data. If you need to refresh your cline theory, you can read this blog post. But I will briefly explain the concept. A cline shows the distribution of character along a geographical transect. Clines can show smooth, continuous gradation in a character, or they may show more abrupt changes in the trait from one geographic region to the next.

The shape and the position of a cline can provide information about the biological processes underlying it. For example, a steep cline suggests a potential role in reproductive isolation or local adaptation because the species have totally different characters.


Big Birds

The cline analyses of the Jacana hybrid zone revealed a striking pattern. The cline for female body mass was shifted to the east relative to the center of the hybrid zone. This suggests asymmetric introgression of this trait from the larger Northern Jacana into Wattled Jacana. Increased competition between females might select for bigger body mass, because larger females can defend larger harems of males. So, Wattled Jacanas that got the “bigger-body-genes” from Northern Jacanas might be more successful.

Similar patterns have been reported for male traits, such as red plumage in Fairy-wrens and yellow plumage in Manakins. This study provides the first example of introgression of a female trait from one species into another.


The cline for female body mass (purple) is shifted to the east relative to the general pattern (grey), suggesting introgression of this female trait (from Lipshutz et al. 2019 Evolution).



Lipshutz, S.E., Meier, J.I., Derryberry, G.E., Miller, M.J., Seehausen, O. & Derryberry, E.P. (2019) Differential introgression of a female competitive trait in a hybrid zone between sex-role reversed species. Evolution.


This paper has been added to the Charadriiformes page.

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