Every blog post I have presented so far has talked about males competing with one another for females, and referred to the choosy sex as the female. While this is predominantly the case, with females providing in most cases greater postfertilisation care, what if this wasn't always the case? Are there certain environmental conditions and community dynamics that would favour sex role-reversal? If this was ever the case and reversal in parental care is strong enough, the direction of sexual selection may become reversed and act upon females more strongly than males (Emlen & Wrege 2004)
Jacanas are exceptional in that seven out of eight species of the family Jacanidae exhibit highly developed sex role-reversal (Butchart 2000, Emlen & Wrege 2004). Males of these species are the sole incubators of eggs and provide the majority of care for the dependent young (Butchart 2000, Emlen & Wrege 2004). Jacanas also display the most extreme reversal of sexual size dimorphism of any other group of birds or mammals, with females weighing up to almost 50% more than males (Butchart 2000). Polyandry has been well documented within the family, with females mating and providing clutches for several males while defending their exclusive territories (Butchart 2000).
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| Female Bronze-winged Jacanas (Metopidius indicus) are generally more than 60.3% heavier and behaviourally dominant to males, which carry out all the incubation and chick care. Photo by Rupal Vaidya https://alchetron.com/Bronze-winged-jacana-1982762-W. Viewed on 10th May 2017. |
The Wattled Jacana (Jacana jacana) is a perfect example of a sex role-reversed bird. Not only do they show all the characteristics mentioned above, females also display greater development of secondary sexual characters such as wing spurs and fleshy facial ornamentation and are more behaviorally dominant than males (Emlen & Wrege 2004). Intrasexual competition between females to accumulate multiple mates is often intense, with larger, more ornamented females shown to hold more territory and have more mates (Emlen & Wrege 2004). Studies have indicated that sexual selection is indeed operating more strongly on female Wattled Jacanas (Jacana jacana) than males (Emlen & Wrege 2004).
So why has the Wattled Jacana (Jacana jacana) developed sex role-reversal? A combination of two main factors has been suggested. Clutch predation of about 50% has selected for females to produce multiple clutches and a tropical climate has allowed for low levels of incubation (Butchart 2000, Osborne 1982). This, coupled with the production of precocial chicks has allowed females to invest less in parental care and more in reproduction (Butchart 2000, Osborne 1982). While this explains how and why polyandry may have developed, it still does not explain exactly why males display greater parental care than females (Butchart 2000). Whatever the reason, it appears to be a successful strategy for the Jacanidae family that has evolved a way in which to tailor their breeding strategy to their wetland environment. I spent a lot of time watching Comb-crested Jacanas (Irediparra gallinacea), in Kakadu National Park - they are known as "Jesus birds" as they appear to walk on water!
References
Butchart, S.H.M. 2000, "Population structure and breeding system of the sex‐role reversed, polyandrous Bronze‐winged Jacana Metopidius indicus", Ibis, vol. 142, no. 1, pp. 93-102.
Emlen, S.T. & Wrege, P.H. 2004, "Size dimorphism, intrasexual competition, and sexual selection in Wattled Jacana (Jacana jacana), a sex-role-reversed shorebird in Panama", The Auk, vol. 121, no. 2, pp. 391-403.
Osborne, D.R. 1982, "Replacement nesting and polyandry in the Wattled Jacana", The Wilson Bulletin, vol. 94, no. 2, pp. 206-208.
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| The Wattled Jacana (Jacana jacana) is a great example of a sex role-reversed bird - females compete for males, have developed greater secondary sexual characters and have abandoned nearly all parental care. Image sourced from http://photography-on-the.net/forum/showthread.php?t=1284873. Viewed on 10th May 2017. |
So why has the Wattled Jacana (Jacana jacana) developed sex role-reversal? A combination of two main factors has been suggested. Clutch predation of about 50% has selected for females to produce multiple clutches and a tropical climate has allowed for low levels of incubation (Butchart 2000, Osborne 1982). This, coupled with the production of precocial chicks has allowed females to invest less in parental care and more in reproduction (Butchart 2000, Osborne 1982). While this explains how and why polyandry may have developed, it still does not explain exactly why males display greater parental care than females (Butchart 2000). Whatever the reason, it appears to be a successful strategy for the Jacanidae family that has evolved a way in which to tailor their breeding strategy to their wetland environment. I spent a lot of time watching Comb-crested Jacanas (Irediparra gallinacea), in Kakadu National Park - they are known as "Jesus birds" as they appear to walk on water!
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| The Comb-crested Jacana (Irediparra gallinacea) is one of the seven out of eight species in the family Jacanidae that exhibits sex role-reversal. It is also known as the "Jesus bird" as it appears to have the ability to walk on water. Photo by David Taylor, http://www.hbw.com/ibc/photo/comb-crested-jacana-irediparra-gallinacea/comb-crested-jacana-struck-lovely-pose-amidst. Viewed on 9th May 2017. |
References
Butchart, S.H.M. 2000, "Population structure and breeding system of the sex‐role reversed, polyandrous Bronze‐winged Jacana Metopidius indicus", Ibis, vol. 142, no. 1, pp. 93-102.
Emlen, S.T. & Wrege, P.H. 2004, "Size dimorphism, intrasexual competition, and sexual selection in Wattled Jacana (Jacana jacana), a sex-role-reversed shorebird in Panama", The Auk, vol. 121, no. 2, pp. 391-403.
Osborne, D.R. 1982, "Replacement nesting and polyandry in the Wattled Jacana", The Wilson Bulletin, vol. 94, no. 2, pp. 206-208.
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