The Funny Thing About Leks

There is a huge question still in evolutionary biology, which concerns the absurdly high degree of sexual traits (such as the tail of a peacock) of males who form leks. Now, a lek is when a large group of males all get together in a huge group and display for females, who then come and choose who they wish to mate with, and then promptly leave.

Females who visit leks almost always choose the same male to mate with, and that male is generally the one with the largest display, usually of plumage. The larger and more extreme these display traits are however, the lower the chances are of male survival. So the big, baffling question is why would females prefer these large displays, which increase the chances of death to the male, when they receive no other benefit than simply sperm?

A Sage Grouse lek, with displaying males.

In a review article written by Mark Kirkpatrick and Micheal J. Ryan (Kirkpatrick & Ryan 1991)on this very subject, they discussed three of the larger hypothesis about this behaviour. Spoiler Alert: We still don’t know which is the driving force behind extreme sexual displays and lekking! Which is why I personally think it’s pretty cool.

The first hypothesis essentially a direct selection by females on males, where the males with the largest displays get the most copulations wholly for selfish reasons on the female part. This means that the preference of larger displays by females is because, even though those traits increase the chances for the males of dying, choosing those mates will increase the females chances for survival or to reproduce themselves. For example, mating with larger, flashier males may reduce the pressures of searching for and choosing a male for the female. In addition, there is another very important factor called pleiotropy, which is when a gene associated with higher survival of the individual also affects that individual’s preference for mates. This means that a certain gene that may make a female bird of paradise REALLY like males with huge plumage also helps boost its immune system, or something similar. Since that gene is going to be kept, because it’s very helpful, that means the preference stays and the females will select for the males that meet that preference whether it’s good for the males or not.

Another hypothesis that was discussed in the paper was termed the “Runaway Process”. In this case it is thought that a sexual trait is linked to a gene, meaning that quality of environment or amount of food obtained doesn’t make the feathers brighter or longer. Essentially, the sexual trait appears and females select a bit for it and after a few generations almost the entire male population has developed this trait due to selection. The ever-present preference for the largest, most stunning trait will cause that trait to evolve to be bigger, brighter, more magnificent than any other male in order to meet that demand of the females no matter the consequences! Which is, as always, a higher mortality rate for the males. But now that the preference for the largest trait has been established, it will just continue to increase in speed like a runaway train. Smaller populations usually provide results that support this hypothesis, and the affects of genetic drift in those smaller groups most likely play a large role in establishing the immediate increase in the presence of a sexual trait by males.

The final hypothesis is based on a concept that is pretty familiar with most people, in that the brighter and bigger the bird the healthier he his and the stronger his genes for survival are. Also known as the parasite hypothesis, which is confusing until you think that the better a bird is at fighting of parasites that cause disease the healthier they are.  So those birds that look the best are probably free of parasites and have stronger immune systems. So by congregating in large numbers, they are better able to show off that they are brighter than other males, and increase their chance of mating.

Now, there are many other hypothesis about why certain displays have evolved over time, but I personally thought this article did a fabulous job explaining them. It goes on to explain why we are so unsure as to which one is true, and ways they think we should test it. I personally feel as though the runaway hypothesis is fairly week, and would be most likely to back the first one in which pleiotropy is correlating preference to female survival, but then I have never been one to immediately agree with parsimony.

 Original Review Article:  

Kirkpatrick, M & Ryan, MJ 1991, ‘The evolution of mating preferences and the paradox of the lek’, , Published online: 07 March 1991; | doi:10.1038/350033a0, vol. 350, no. 6313, pp. 33–38.

Picture Credit:

‘Eco-Vocab: Lek’ The Olive Tree, accessed February 25, 2014, from <http://oleaeuropea.wordpress.com/2011/01/25/eco-vocab-lek/>.