Trends in body size and female reproductive frequency with elevation and temperature in a primarily nocturnal, viviparous gecko

Abstract

Bergmann’s rule predicts an increase in body size with increasing elevation and decreasing temperature; however, a published review has found the reverse pattern in the majority of lizards. It is thought that for these ectotherms, small body size may provide thermal benefits, when basking, that would be highly advantageous in cold environments. This suggestion is supported by experimental studies that report faster heat gain in small-sized sun-basking lizards than in larger lizards of the same species. However, this explanation may not apply to species that do not avidly sun-bask, and it is possible that for nocturnal species, a small body size at high elevations is not advantageous. Bergmann’s rule has not been closely studied in a nocturnal lizard, and, furthermore, it is not known which body size (small or large) confers thermal benefits for lizards that do not frequently sun-bask. In this thesis, the Otago-Southland gecko (Woodworthia “Otago-Southland”) was used as a model to examine how body size changes with elevation in a primarily nocturnal lizard. I measured adult and juvenile geckos from seven sites over a range of 54-1039 metres above sea level (m asl). I also examined variation in inferred reproductive frequency of females (which have previously been reported as reproducing biennially at c. 600-700 m asl). Environmental temperature was recorded using data loggers. In a separate laboratory study, I used physical models to investigate whether small body size (which confers thermal benefits for sun-basking lizards) is also advantageous for nocturnal lizards. I found that the Otago-Southland gecko showed a significant increase in size with elevation, the opposite of the dominant pattern in diurnal lizards. Female size at maturity, maximum size of adults and size at birth were larger at higher elevations and at lower temperatures. Inferred reproductive frequency did not vary significantly with elevation or with temperature; however, it was clear that the populations were not all on the same inferred reproductive cycle. Using physical models, I found that large body size might confer thermal benefits for nocturnal lizards that remain under cover within warm retreat sites. The results of my thesis contrast with those of other studies, suggesting that, for this nocturnal lizard, large body size at high elevations is advantageous. I discuss mechanisms that may cause large body size, for example slow growth rates at cold temperatures. I also consider benefits to large size at high elevations, including larger energy stores to manage the challenges of reproduction and prolonged inactivity over winter. The findings of my thesis suggest that Bergmann’s rule should not be dismissed for all lizards, but may in fact be valid for nocturnal species that bask indirectly.