A Latitudinal Variation in Growth Rate Caused by the Differences in the Length of the Period Suitable for Larval Growth in the Japanese Black Salamander, Hynobius nigrescens.

In ectotherms, body size differences between latitudes resulting from shorter activity periods at higher latitudes may disappear due to higher growth rates. Although such latitudinal variations have been examined for various taxa, only a few studies have examined such variations in Japanese vertebrates. Hynobius nigrescens is widely distributed in the northern part of Honshu, Japan, and although their larval period in the wild is shorter at high latitudes, there is no latitudinal variation in the body size of juveniles in the wild. Therefore, larvae may grow faster at high latitudes, and they can grow into juveniles with body sizes similar to those at low latitudes. To examine whether the growth rate of larvae is faster at high latitudes, we reared groups at 15°C and 20°C. We found that the larval period was shorter and the growth rate was greater at high latitudes. Next, we examined whether the body sizes of reared individuals differed between latitudes. We found that the body sizes of metamorphosed juveniles were smaller at high latitudes. Some ectotherms are known to feed more at high latitudes, leading to the disappearance of latitudinal differences in body size between latitudes. In this study, we provided the same amount of food regardless of latitude, which might have caused the differences in body size between latitudes. Our study suggested that the high growth rate at relatively high latitudes can compensate for the short activity period, leading to similar body sizes of juveniles among latitudes.