"Fig. 33. Sitana minor. Male, with the gular pouch expanded (from Gunther's 'Reptiles of India')."
In The Descent of Man and Selection in Relation to Sex (1871), Darwin laid out his ideas about sexual selection. Darwin believed that sexual selection was distinct from natural selection; while natural selection operates through survival and fecundity, sexual selection operates through differences in mating success among individuals.
The puzzle that Darwin set out to solve was the existence of traits like the peacock’s tale or the elk’s antlers. These traits differ between sexes but aren’t related directly to mating. And, these traits appear to be quite costly. Darwin reasoned that these costly traits must benefit the bearer in some way related to mating success. Most likely, these benefits accrued during battle with other members of same sex for mates or during the courtship of the opposite sex. In both cases, having the costly trait benefitted an individual’s mating success beyond the incurred survival or fecundity cost.
Now, how does this discussion relate to anoles? Well, by now you’ve no doubt guessed the reason: many Anolis species have males with dewlaps (1,2,3) that have all the characteristics of a classic, sexually selected trait (but see 4). Dewlaps are large, colorful, conspicuous, and they are used in territorial interactions with other males and courtship interactions with females. It’s no suprise then that Darwin considered dewlaps. Here’s what he had to say about dewlaps in The Descent, in his splendid Victorian English. Enjoy.
Lacertilia.—The males of some, probably of many kinds of lizards fight together from rivalry. Thus the arboreal Anolis cristatellus of S. America is extremely pugnacious: “During the spring and early part of the summer, two adult males rarely meet without a contest. On first seeing one another, they nod their heads up and down three or four times, at the same time expanding the frill or pouch beneath the throat; their eyes glisten with rage, and after waving their tails from side to side for a few seconds, as if to gather energy, they dart at each other furiously, rolling over and over, and holding firmly with their teeth. The conflict generally ends in one of the combatants losing his tail, which is often devoured by the victor.” The male of this species is considerably larger than the female; and this, as far as Dr. Günther has been able to ascertain, is the general rule with lizards of all kinds.
The sexes often differ greatly in various external characters. The male of the above-mentioned Anolis is furnished with a crest, which runs along the back and tail, and can be erected at pleasure; but of this crest the female does not exhibit a trace. In the Indian Cophotis ceylanica, the female possesses a dorsal crest, though much less developed than in the male; and so it is, as Dr. Günther informs me, with the females of many Iguanas, Chamelions and other lizards. In some species, however, the crest is equally developed in both sexes, as in the Iguana tuberculata. In the genus Sitana, the males alone are furnished with a large throat-pouch (fig. 33), which can be folded up like a fan, and is coloured blue, black, and red; but these splendid colours are exhibited only during the pairing-season. The female does not possess even a rudiment of this appendage. In the Anolis cristatellus, according to Mr. Austen, the throat-pouch, which is bright red marbled with yellow, is present, though in a rudimental condition, in the female. Again, in certain other lizards, both sexes are equally well provided with throat-pouches. Here, as in so many previous cases, we see with species belonging to the same group, the same character confined to the males, or more largely developed in the males than in the females, or equally developed in both sexes. The little lizards of the genus Draco, which glide through the air on their rib-supported parachutes, and which in the beauty of their colours baffle description, are furnished with skinny appendages to the throat, “like the wattles of gallinaceous birds.” These become erected when the animal is excited. They occur in both sexes, but are best developed in the male when arrived at maturity, at which age the middle appendage is sometimes twice as long as the head. Most of the species likewise have a low crest running along the neck; and this is much more developed in the full-grown males, than in the females or young males.
(Citation: Darwin, C. R. 1871. The descent of man, and selection in relation to sex. London: John Murray. Volume 2. 1st edition. From Chapter 12: “Secondary sexual characters of fishes, amphibians, and reptiles.”)
"Fig. 34. Ceratophora Stoddartii (sic). Upper figure male; lower figure female."
A couple of notes on this passage:
1) Anolis cristatellus is not from South America. Rather it is from Puerto Rico, although it has established invasive populations in Miami and Costa Rica. That said, the description of the dewlap color was consistent with A. cristatellus, so Darwin seems to be talking about the right lizard but in the wrong place.
2) Tail breakage (autotomy) rates in lizard populations are often taken as a rough measure of the amount predation pressure in a given locality and there are several anole studies on this topic. However, as Darwin points out, tails may break during intraspecific interactions as well. This is an important point that needs to be considered in these sorts of studies. (I like the claim that victorious males eat the autotomized tail of their vanquished rival; however, I doubt its veracity. Although the tail wiggles for a short time after breaking off, could it really resemble a moving insect and get eaten? Have any anolologists seen this happen?)
3) Extensible dewlaps have evolved multiple times in lizards, covered here.
- DNA Sequencing from Formalin-Preserved Specimens - December 16, 2014
- Sans the Claws, Anoles Present Under The Trees? - December 5, 2014
- Out Damned Spot! Or, How They’d Be Loving If They Weren’t Fighting - November 18, 2014
Bill Bateman
Autotomy induced by conspecifics seems to be rare – I have seen competing skinks grasp each other’s tail and never induce autotomy, but I recall mention of Gallotia lizards doing it (and eating the tail), tuatara as well, and I vaguely remember reading a note on a captive skink, which in escaping, shed its tail and then came back, found it, and ate it. Sensible way of recouping the energy lost.