Axolotls (Ambystoma mexicanum) are aquatic salamanders that are widely used in research. Axolotls have been bred in laboratories for nearly 150 years, yet little is known about the basic biology of reproduction in these animals. We investigated the effects of changing day length, time of year, and food availability on levels of circulating estradiol and androgens in adult female and male axolotls, respectively. In addition, we examined the effects of these variables on the mass of ovaries, oviducts, and eggs in females and weight, to calculate a form of gonadosomatic index (GSI). In both sexes, GSI was not correlated with levels of circulating steroids. In female axolotls, estradiol levels were influenced by food availability, changes in day length, and season, even when animals were held at a constant temperature and day length was decorrelated with calendar date. In addition, the mass of ovaries, oviducts, and eggs varied seasonally, peaking in the winter months and declining during the summer months, even though our animals were not breeding and shedding eggs. In males, levels of androgens appeared to vary independently of external conditions, but GSI varied dramatically with changes in day length. These results suggest that reproduction in axolotls may vary seasonally, as it does in many other ambystomid species, although both male and female axolotls are capable of reproducing several times each year. The physiological basis of this ability remains enigmatic, given the indications of seasonality contained in our data. IntroductionThe axolotl (Ambystoma mexicanum) is a paedomorphic salamander: it becomes sexually mature while retaining many morphological characteristics of the larval form.Axolotls were first brought to Europe in 1863, where they generated much interest for their ability to reproduce while resembling aquatic larvae, and then in some cases to metamorphose into an animal resembling a common terrestrial salamander, which was equally capable of reproducing [50]. Since their introduction to the research community, axolotls have become a model organism for biological research, used primarily in studies of embryology and regeneration.The natural history of axolotls is poorly understood. Axolotls are native to two lakes, Xochimilco and Chalco, which have been subsumed by present-day Mexico City. Human alteration of this habitat for agricultural purposes dates back hundreds of years [7]; their environment is now so badly degr on IUCN Red List. Recent studies indicate that few individuals exist in the wild [e.g., 12], and it seems likely that the ecology and natural history of axolotls will never be known.Given that axolotls have been bred in laboratories for nearly 150 years, it is perhaps particularly surprising that their reproductive patterns have not been thoroughly documented. Their courtship behaviors [18] and the anatomy of both male and female reproductive systems have been described [38,56], but the physiology of reproduction in axolotls has not been explored. In this study, we measu...