Colour change in New Zealand geckos - a defence against avian predation?
Reptilian colouration is important for a number of functions, including predator evasion, social signalling, and thermoregulation. Colour may assist reptiles to avoid predation in several ways, including camouflage, mimicry, aposematism, and deimatic displays. Camouflage is particularly important for avoiding predators that predominantly use vision to detect prey. Evading visually hunting avian and reptilian predators is thought to have influenced the evolution of New Zealand’s endemic lizards, therefore it is plausible that their colouration has an anti-predator function. Rapid physiological colour change has been observed in at least seven of New Zealand’s 48 endemic gecko species, but the triggers for this phenomenon are poorly understood. I investigated whether rapid colour change in geckos could serve as a defence mechanism against avian predators. Because many geckos are capable of tail autotomy as a defence against predation, and colour has been observed to change differently in the body and tail of two New Zealand gecko species, I also investigated whether colour change in the tail differed from the rest of the body. To achieve this, I (1) measured the colour responses of geckos to predator presence in ngāhere geckos (Mokopirirakau ‘southern North Island’) and Raukawa geckos (Woodworthia maculata), and (2) compared the responses of wild birds to model geckos displaying increased brightness and contrast in their tails. To examine gecko colouration responses to avian predator presence, I presented 15 ngāhere geckos and 20 Raukawa geckos with a randomly ordered series of visual and auditory cues and photographed their skin colour before and after each cue. The four visual cue ‘treatments’ consisted of a model animal flying by the enclosure, with a fifth control treatment for which no object passed overhead. The models were of a (1) morepork (Ninox novaeseelandiae; a nocturnal lizard predator), (2) kingfisher (Todiramphus sanctus; a diurnal lizard predator), (3) fantail (Rhipidura fuliginosa; a bird that does not eat lizards), and a (4) monarch butterfly (Danaus plexippus; an insect that does not eat lizards and is not eaten by geckos). The three auditory treatments were a (1) morepork call, (2) fantail call, and (3) no call. Photographs of geckos were calibrated using an xrite color standard and ImageJ was used to measure skin brightness, contrast, saturation, and hue. A before-after-control-impact (BACI) framework was used for statistical modelling of the repeated measures data taken from each gecko. There was no evidence for differing body and tail colouration in ngāhere geckos, except in the response of saturation to auditory cues. Conversely, there was evidence that brightness and saturation of Raukawa geckos differed between the body and tail. While variation in skin colour of individual geckos was observed across treatments in both species, the BACI analysis did not provide evidence of colour change in response to predator presence. These results suggest that the tail and body of both ngāhere and Raukawa geckos can be differently coloured when measured at the same time. However, uniformity in colour across the whole body appears to be displayed much more frequently in ngāhere geckos. Changes observed in the skin hue, brightness, and saturation of individual ngāhere and Raukawa geckos provide the first experimental evidence that these species are capable of changing aspects of their colour. The apparent lack of a direct response to simulated predator presence in the experiments may have been due to the experimental design providing insufficient time for skin colour to change substantially between treatments, or it may be that there are limitations to the frequency, magnitude, and speed of colour change in geckos following previous colour changes. It may also be that the colour changes observed in the field were a result of another trigger, such as increasing light levels, stress, or temperature.
To test whether the increased levels of brightness and/or contrast in the tail observed in nature resulted in an increased probability of avian attack on a gecko’s tail compared with other body parts, I used trail cameras and model lizards coated in clay. The three treatments were: tails the same colour and brightness as the rest of the body, brighter tails, and more highly contrasted tails. Trail cameras were not effective for capturing avian predator attack behaviour, but I was able to deduce their behaviour from peck marks left in the clay. I calculated area-standardised pecks for the head, body, tail, and limbs of each model type, and investigated which body part was pecked most often within each model. Across the whole gecko model, contrasted-tailed models received more pecks, however more of these pecks were directed at the tail and limbs. There was a weak signal that a similar pattern may be occurring in brighter-tailed models, however this was not statistically significant. The number of area-adjusted pecks to the head and body did not differ significantly between plain-tailed controls and the models with brighter or more contrasted tails. In models with brighter or more contrasted tails, the most-pecked body part was never the head, and most often the tail (at around 1.3 times the rate of the models with plain tails). Rapid colour change observed in geckos could be an effective defence response, but without further data there is insufficient evidence to demonstrate colour change is being utilised in this way. Alternative study designs for predator presentation to geckos may be worth pursuing and colour change may also be triggered by background matching for camouflage, stress, changing light levels, or changing temperature.
Determining the triggers for colour change in geckos is important, as these can affect conservation management decision-making around habitat changes and climate change. As colour affects thermoregulation, if geckos are matching colouration to the surrounding background, habitat colouration could affect thermoregulation efficiency and limitations. The combination of increasing temperatures through climate change and the colour of the habitat could affect management decisions relating to restoration.