It is said that ‘all is fair in love and war’. That is certainly the case in the animal kingdom where competition for resources is fierce. Animals compete for food, territories, shelter and mates in an effort to survive, and most importantly, to reproduce. The Tenerife lizard (Gallotia galloti) is no exception. Males battle violently for mates and bigger, faster and stronger males are usually the victors. But what are the proximate mechanisms driving the evolution of these traits? A potential mechanism is the effect of testosterone on physiological and morphological characteristics that will ultimately affect the performance of a male during combat. Kathleen Huyghe and her colleagues attempted to answer whether this is the case by determining how certain phenotypic traits are affected by testosterone levels.
The team collected blood samples from Tenerife lizards and determined their initial testosterone levels. They also determined several morphological characteristics such as head length, height and width, as well as two indicators of performance: maximal bite force capacity and maximum sprint speed. Although the authors did not find a correlation between the initial testosterone levels and any of the morphological traits, they did find a positive correlation between initial testosterone levels and bite force capacity. In 2005 Huyghe and her colleagues observed that bite force is a good predictor of the outcome of a battle between Tenerife lizard males. This, together with their findings that testosterone levels affect lizards' bite force, strongly suggests that intrasexual selection is driving the evolution of these traits. On the other hand, sprint speed was not correlated with testosterone level, an interesting observation since sprint speed is not important during male–male interactions.
After obtaining the basal morphological and performance measurements, the team artificially manipulated some of the lizards' testosterone levels by implanting them with crystalline testosterone; this type of implant allows the release of the steroid over the course of several months. About 5 months later they measured the morphological and performance parameters one more time as well as the size of the lizards' penises and of several muscles such as the jaw closer muscles and one of the jaw opener muscles. Testosterone induced a dramatic increase in the jaw closer muscles, which provide the bite force, but not in the jaw opener muscle. However, to the team's surprise the steroid did not affect biting force itself. The authors suggest that this outcome may be explained by a lack of muscle training, since the lizards were captive during the experiments and thus devoid of challenges they would normally experience in the wild such as fights or the necessity to crush tough foods. Lastly, testosterone caused an increase in penis size. Although it is unclear how penis size may increase fertilization success, Huyghe proposes that bigger penises may facilitate copulation despite female resistance, provide an advantage in postcopulatory sperm competition or play a role in female choice.
The results from this study suggest that testosterone is an important mechanism modulating morphological and performance traits fundamental for the reproduction of this lizard; traits which in turn can be favoured by sexual selection. As it turns out, it appears that steroids give male lizards the edge they need to attract a female, and what male wouldn't want that!
