Darwin’s other big idea (in addition to the theory of evolution by natural selection) was sexual selection.

“We are, however, here concerned only with that kind of selection, which I have called sexual selection. This depends on the advantage which certain individuals have over other individuals of the same sex and species, in exclusive relation to reproduction.” (Darwin, 1859).

In other words, that sexual selection could occur by means of male–male combat and by female choice of the most attractive males. Darwin thought that the peacock’s long tail developed as a result of female choice favouring long-tailed mates because of the beauty of their tail, particularly the iridescent eyespots.

Many of the key aspects of sexual selection are still hotly contested, but most researchers agree that’ Darwin’s contributions are still surprisingly relevant to the modern study of sexual selection’ (Jones and Ratterman, 2009). Furthermore, recent research has shown that the mating success of male peacocks is correlated with the brightness and iridescence of the blue-green region of the eyespot (Loyau et al., 2007); and also that eyespots are a ‘major focus of female attention’ and not just a trait that is correlated with something else that might influence female choice (Dakin and Montgomerie, 2013) . 

But male peacocks (Pavo species) are more than just eyespots; they also have wonderful iridescent plumage – structural colours – as well (see below). Surely these catch the eyes of the female as well? It can’t all be about eyespots? Can it?!

I took these photographs of a Green peafowl (Pavo muticus) in Chiang Mai zoo. I think it got a bit fed up of me chasing it across the road!

This is what the whole organism looks like (below). What a beautiful creature!

I can certainly envisage how female peacocks could be entranced and enraptured by the male’s beauty, and also how they might be ruthlessly choosy of the most showy and delightfully shiny male of them all! But is it all down to spots? In my limited research, I came across a recent paper that suggested that female choice is based on the male (train) size, quality and vigour, and by variation in train display, and not eyespot number (Singh and Jagadeeshan, 2020). In other words, the females may be basing their choice much more on the whole package, including ‘how males use their trains [tails] to establish dominance and to charm females’. So these composite features – behaviour and morphology (the shiny feathers and eyespots together with how they show them off) – could be be the force driving the evolution of the peacock’s elaborate train?

References

Dakin, R., & Montgomerie, R. (2013). Eye for an eyespot: how iridescent plumage ocelli influence peacock mating success. Behavioral Ecology, 24(5), 1048-1057.

Darwin, C. (1859) On the Origin of Species by Means of Natural Selection (Murray, London).

Jones, A. G., & Ratterman, N. L. (2009). Mate choice and sexual selection: what have we learned since Darwin?. Proceedings of the National Academy of Sciences, 106(Supplement 1), 10001-10008.

Loyau, A., Gomez, D., Moureau, B., Théry, M., Hart, N. S., Jalme, M. S., … & Sorci, G. (2007). Iridescent structurally based coloration of eyespots correlates with mating success in the peacock. Behavioral Ecology, 18(6), 1123-1131.

Legagneux, P., Théry, M., Guillemain, M., Gomez, D., & Bretagnolle, V. (2010). Condition dependence of iridescent wing flash-marks in two species of dabbling ducks. Behavioural Processes, 83(3), 324-330.

Singh, R. S., & Jagadeeshan, S. (2020). A long tail of truth and beauty: The developmental basis of complexity, symmetry, and beauty in the evolution of the peacock’s tail. BioRxiv.