Gestational age determination, variation of conception date, and external fetal development of sika deer (Cervus nippon yesoensis Heude, 1884) in eastern Hokkaido.

The authors determined gestational age from fetal weight, examined the range in conception dates, and classified the external fetal development process in sika deer of eastern Hokkaido. Gestational age (T) can be estimated from fetal weight (W) with the equation: T = (3 square root of W + 2.730)/0.091. Conception date can then be calculated back from date of kill of pregnant female, using gestational age. Though estimated conception dates ranged from 7 October to 17 January, most of them were concentrated between mid October and early November. In late November and after, nine conceptions were observed and six out of the nine late cases occurred in yearling females. This phenomenon is probably due to the excellent nutritional condition of deer in the study areas, which allows yearling females to mature sexually even in early winter. In fetuses, periods of tactile hair development, appearance of white spots, and development of general hairs overlap very little. In addition, these developmental changes occur at specific fetal weights. Based on these external phases, the gestation period can be divided into four stages. These stages will be useful for simple determination of fetal age in various sika deer populations.