Growth marks in natterjack toad (Bufo calamita) bones:: histological correlates of hibernation and aestivation periods

Skeletochronological assessment of the growth marks in the phalange bones of two natterjack populations (Bufo calamita) inhabiting northern Spain (Balaguer, Mas de Melons, Catalonia, 41 degrees N) revealed that most individuals produced more than one line of arrested growth (LAG) per year, in contrast to conspecifics in other populations throughout the geographic range. Moreover, up to 19.2% of toads showed faint broad lines of reduced growth (annuli) in a given year. We developed a quantitative method based on the distinction between Summer and winter growth periods in the surface area of periosteal bone produced to identify multiple LAG formation. Statistical evaluation of growth areas demonstrated that a precise distinction was feasible for individuals of a longevity up to five years. We related the number of LAGs per winter (one in 53.3% of 334 observations, two in 32.0%, three in 11.7% or four in 3.0%) to features of the local winter climate. The influence of climate (mainly temperature) accounted for 88.2% of total variation in the number of LAGs per year. Juveniles interrupted hibernation more frequently for winter growth periods than adults, indicating ontogenetic variation in hibernation behaviour. We also investigated the frequency of annuli generally produced during the summer growth period and thus representing histological traces of slowed growth during aestivation. There was no significant correlation between aestivation rates and local climate suggesting that neither ambient temperature nor moisture were key factors in inducing aestivation at the studied localities. In conclusion, the incidence and frequency of multiple growth marks per year represent informative histological correlates of local environmental impacts on life history.