The function of red colouration in the spiny-footed lizard ("Acanthodactylus erythrurus", Schinz 1833)

Abstract

In many animal taxa, conspicuous colouration is a visual signal used for communication purposes among conspecifics or heterospecifics. This kind of colouration may have different functions depending on the body part where it is present and also depending on the age or reproductive status of the individual showing it. Conspicuous colouration is common in lizards, and spiny-footed lizards (Acanthodactylus erythrurus) are particularly good subjects to test different hypotheses explaining the functions of conspicuous colouration and possible changes in these functions during ontogeny, as hatchlings develop conspicuous red colouration on the ventrolateral part of their tails that spreads towards the hind limbs in juveniles. This colouration is lost when males achieve sexual maturity, but is retained, and even enhanced, in adult females while they are sexually receptive, although it is lost when they become gravid. In this PhD thesis we addressed several possible functions of the red colouration in spiny-footed lizards. One hypotheses is the possible function of red colouration as an antipredatory mechanism. We tested whether red colouration in individuals of all age classes was related to their risky behaviours, so we recorded lizard behaviour in the field and calculated several indices of lizard activity, the use of open areas far from refuges and stereotyped movements of limbs and tail. We found that redder (less orange) individuals spent more time further from refuges, a result consistent with the antipredatory function of red colouration. We also tested whether red colouration in the tail of hatchlings was a decoy for avian predators. Using plasticine and plaster lizard models with red or striped dark and light tails, both in the field and with a common lizard predator (Falco tinnunculus) in captivity, we found that red tails increased model conspicuousness in the field, but also that they were an effective decoy for avian predators, diverting attacks from vulnerable body parts towards the tail, thus supporting the antidepredatory function. Another hypothesis about the function of conspicuous colouration in juveniles (the aggression-avoidance hypothesis) states that it reduces aggression from adult conspecifics. Therefore, we tested the influence of juvenile red colouration on the aggression received from adults by conducting videotaped encounters in captivity between adults of both sexes and juveniles that showed either their natural red colouration, or a painted red or white colouration covering their natural red parts. Our results supported the hypothesis, as redder juveniles received less adult aggression. When conspicuous colouration is present in adults, it is commonly associated with sexual selection. Therefore, we tested whether red colouration in adult females, that has already been suggested to have a mating-related function, is used by males in mate selection. We presented adult males with pairs of females differing in their age/size/sexual maturity (adult or juvenile) and/or their colour (red or white) and recorded the time the male spent near or in contact with each female. We found that males spent more time near adult females regardless of their colour, and within adult females, they preferred the red ones, suggesting that female red colouration is a sexual ornament. Red colouration might be providing information about the female reproductive status, and to test this hypothesis we studied the possible link between colouration and female sex steroid hormones that regulate their reproductive cycle, specifically β-estradiol and progesterone. We explored natural variation of colouration and hormone levels along the reproductive cycle, determined the relationships between colouration and hormones and, finally, experimentally manipulated plasma β-estradiol and progesterone concentrations to check the effects on colouration. Our results suggest that red colour in adult females signals the reproductive status, i.e., fertility, as redder females were closer to ovulation. Moreover, high concentrations of either β-estradiol alone or β-estradiol together with progesterone trigger the loss of red colouration immediately following ovulation. In conclusion, red colouration in spiny-footed lizards is a multifunctional signal that is used for different purposes at different age classes. Signals are costly to produce, so using the same signal for different functions or reutilizing the same signal at different age classes should have advantages. In spiny-footed lizards, the retraction of the red colouration to more ventral parts along the ontogenetic process suggests that predation pressures might be modulating the expression of the signal, and points out that individuals of different age classes may be suffering very different selective pressures. This study is also a good example of how signals can change during the evolutionary process, and provides evidences about the conservation of a multifunctional signal during the ontogeny of a species, something poorly studied.