The strange case of Mabuya quinquetaeniata.

Fig. 1. Mabuya quinquetaeniata with male above and female below.

As boys my brother and I first encountered these beautiful lizards (Fig. 1), near the Limpopo river in South Africa. Now, as a retired biologist, I look back on those early days and wonder about the evolution of the vivid uniforms of these lizards. The colours are especially noteworthy because both sexes are highly decorated, and in quite different ways. This is not unknown but it is typically the male alone that is decorated. They look so different that we believed them to be different species until one day a mating pair was observed. Even after establishing that the two lizards are of the same species, there is still the difficulty of determining which is male and which female. Many years ago I dissected a blue-tail and found eggs - hence we at last knew that the blue tails are female. This is as far as we got as boys. In those days, in the 1950s, there was no internet and none of  the many identification aids available today, so these simple observations carried a fine sense of discovery.  

What we had unknowingly stumbled onto was an area of evolutionary biology, sexual colour dimorphism, full of active debate ever since the conflict between Darwin and Wallace over 200 years ago (Cronin, 1991) pp123-155, (Roughgarden and Akcay, 2010). I here adopt sexual selection as the selective force responsible for the evolution of mating displays including an adaptive role for colour (Andersson, 1994). In principle, I use natural selection to account for colour in the female and sexual selection for that in the male. But there are caveats (see below). This is in contrast to Darwin's conclusion in The Origin (C. D. Darwin, 1859), but in full agreement with his later revision seen in The Descent (C. Darwin, 1871), quoted by (Cronin, 1991), p148.

To return to my central point to look more closely at the fact that both sexes in Mabuya, rather than just the male are uniquely decorated. Within the world of sexual selection the bright colours of the males can be understood to have evolved under choice by females but the same does no apply to the female. Unlike the male, the tail alone is  bright. The key to understanding how selection is operating here lies in the phenomenon of autotomy, widespread among lizards. This is the ability to shed the tail when attacked. When detached under attack the colourful tail squirms vigorously so it is easy to imagine a predator like a bird, attacking the conspicuous tail while the cryptic female escapes. Although I  have not attempted it, this prediction could readily be tested by experiment. So here is natural selection operating in the female.

But this is not the full story. To better understand these lizards some further points must be made. First, like the female, the male is capable of autotomy and here too the tail is the most conspicuously coloured part of the body. So he also has an anti-predator device, thus after all his colours are not determined solely by sexual selection. Second, the body of the female is not really cryptic but is dark with conspicuous pale longitudinal stripes. Such stripes are seen among many reptiles. For example, in the elegant and common schaapsteker (Fig. 2). Here longitudinal stripes make the barer difficult to catch when it is on the move as we discovered  as boys attempting to catch these snakes by hand. Presumably a predator has the same difficulty. This hypothesis too, is readily tested by experiment. Both this and the experiment to test the adaptive role of autotomy are based on such commonly observed phenomena that they have probably been done many times for many species - but perhaps not.  Have they actually been carried out or merely inferred?

Fig. 2. A striped Schaapsteker on the move.

Thus we can go a long way to understanding the adaptive value of colour in this  lizard. To summarise; for the female, defence against predators is a mixture of a predator confounding pattern and  a highly coloured tail which can behave as a predator lure. Appearance of the female is therefore solely under the control of natural selection.  In the male, by contrast, both sexual attraction (sexual selection), and predator avoidance (natural selection), are the adaptive forces involved. The bright colours also tell us something about the sensory systems of lizards and of predators. We can suggest that female Mabuya prefer orange/green and predators, possibly mostly  birds, bright blue.

Fig. 3. Eclectus roratus parrots with male on the left and female on the right.

Just to emphasise that these lizards are not unique in having both sexes highly coloured, I add a picture of a parrot with both sexes highly coloured (Fig. 3). What are the selective forces operating here? And what about the zebra below (Fig. 4.). Perhaps the greatest mystery of all, with indistinguishable sexes.

Fig. 4. Male and female zebra



My sole work into sexual dimorphism has concerned, not colour dimorphism at all but size differences between the sexes, referred to as sexual size dimorphism (SSD), (McLachlan, 2015; McLachlan, MacLeod, and  Neems, 2016), thus colour dimorphism is something new to me. but what a stimulating departure from the familiar it has been.

 References

Andersson, M. (1994). Sexual Selection. Princeton: Princeton University Press.

Cronin, H. (1991). The Ant and the Peacock (1993 ed.). Cambridge: Cambridge University Press

Darwin, C. (1871). The descent of man and selection in relation to sex. (2 ed.). New York, 1959: The Modern Library, Random House.

Darwin, C. D. (1859). The origin of species by means of natural selection, or the preservation of favoured races in the struggle of life. (Fascimile 1901 ed.). London: John Murray.

McLachlan, A. J. (2015). The midge in the mating system - A sheep in wolf's clothing?, http://www.google.co.uk/atholmclachlan.blogspot.co.uk.

McLachlan, A. J., MacLeod, K. J., and Neems, R. M. (2016). Sexual Size Dimorphism in the chironomid midge: A Sheep in Wolf's Clothing? Journal of Insect Behaviour (submitted).

Roughgarden, J., and Akcay, E. (2010). Do we need a Sexual Selection 2.0? Animal Behaviour, 79, e1- e4.