16 October 2013

Bringing home the bacon - Male kea and their unusally big beaks.

This blog post was written by postgraduate student Jenny Dent as part of the course, Research Methods in Ecology (Ecol608). Jenny revisits a Lincoln University research area that looks atthe differences between males and females published way back in 1991.

"Argh, he's attacking the thermos!!"

This particular tourist doesn't seem overly thrilled about his first kea encounter. To be fair, said kea isn't exactly making a stella first impression. Dismantling someone's lunch is never a good icebreaker. But what basis did this tourist have for assuming that the offending kea was in fact a Mr Kea? Why not a curious Mrs Kea? Chances are he had no idea (nor did he look like he cared terribly much) but he's certainly not alone. In fact, many of New Zealand's early taxonomists were in the same boat.

Mr Kea or Mrs Kea? (Photo by Jennifer Dent 2013)
There was a widespread belief amongst New Zealand's early taxonomists that male and female kea could be told apart by their appearance. The problem was that no one was exactly sure HOW they could be told apart. They all had their theories of course, but for the most part these were vague and poorly tested. My personal favourite was the assertion that males could be identified by their "aggressive demeanour".

Surprisingly there was no quantitative assessment of the difference in size between sexes (called sexual dimorphism) in kea until 1991 when Alan Bond, Kerry-Jayne Wilson and Judy Diamond took it upon themselves to solve this conundrum once and for all. Their findings confirmed that kea do express a degree of sexual dimorphism. Males are about 5% larger overall, and, even more impressively, have upper bills which are 12-14% longer than their female counterparts. Now this difference may not seem like much, especially when you consider some of nature's more extreme examples of sexual dimorphism - but for a parrot species this is actually pretty extraordinary. Sexual dimorphism (differences between the sexes) relating to size is only found in seven of the 81 parrot genera. Furthermore, at the time of this study, only one other parrot - the Palm Cockatoo - was known to have bill size dimorphism greater than the overall difference in body size. Its no surprise then, that researchers were pretty keen to see if this uniqueness extended to the only other species in this genus, the kaka.

It would appear that it does. In 1999, Ron Moorehouse and his colleagues were able to demonstrate the presence of bill size dimorphism in the North Island Kaka. The level of dimorphism that they observed was very similar to that observed in kea, with male bills being 14% longer on average. Because of this similarity, and the rarity of the condition overall, there is thought to be a phylogenetic basis for this condition. In other words, it is assumed that kea and kaka have inherited this trait from a common ancestor.                                                                                                                        

With this new discovery in mind, researchers set about trying to explain how bill size differences could have arisen. The little evidence available seems to suggest that it may have arisen as a result of differential niche utilisation (a fancy term for the specialisation of male and females for different roles). In both kea and kaka, the females take on most of the chick-rearing responsibilities and the males assist by provisioning the female and chicks with food. Essentially Mrs Kea becomes a stay-at-home mum and Mr Kea is the breadwinner – think 1950's gender roles.

This all ties in with the bill size dimorphism because having a larger bill is thought to enhance a males provisioning ability. Food availability is strongly correlated with breeding success so natural selection would have favoured the larger billed males over time. Of course, at this stage, a lot of this is based on speculation and assumption. It will be interesting to see how this hypothesis stands up in future studies. However, if you just can't wait for these future studies and want to discover more right now, I suggest you start with these two papers:
Bond, A.B., Wilson, K.J. & Diamond, J. (1991) Sexual dimorphism in kea Nestor notabilis. EMU, 91, 12-19.                                                                                              
Moorehouse, R. J., Sibley, M.J., Lloyd, B.D.  & Greene, T.C. (1999) Sexual dimorphism in the north island kaka Nestor meridionalis septentrionalis: selection for enhanced male provisioning ability? Ibis, 144, 644-651.

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