Mechanism of Seasonality in Japanese macaques (Macaca fuscata)

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Seasonality in Japanese macaques (fu) was examined by the comparison with long-tailed macaques (fa) which are close to fu and without clear seasonality. Seasonality is expressed in nutritional and reproductive states. Fat mass was measured with Dexa to elucidate variability profile (sex difference, age change, and seasonality). Statistical analyses proved the utility of such indirect fat indicators as body weight (BW), physique index, and skinfold. Standard weights are 8 and 12 kg in adult fu females and males respectively, and fat mass is about 10% of BW. No immature individual with more than 10% of fat was found in the two species. Adults accumulate fat rapidly in fa, but not in fu. It is considered that the seasonal loss of fat in fu prevents the excessive accumulation. Time-series analysis was applied on the longitudinal BW and endocrinological data. BW seasonal cycle is not greatly influenced by reproduction, but mainly by nutrition. Fat mass contributes to the BW seasonal fluctuation by about 40%, but the increase and decrease of reproductive organs do not. Lean phase in BW cycle is longer in fu female than in male, reflecting the sex difference in feeding behavior or in the rate of assimilation. The nutritional state in autumn appears to determine the rate of conception. Some nutritional signal (e.g., blood glucose) is considered to stimulate the endocrinological system of reproduction. Leptin, although it showed seasonality, was not considered to participate in that function. The convenient method for steroid hormones using fecal samples is expected to apply on wild monkeys. Direct relationships have been reported for some cercopithecoids, that is the amelioration in nutritional state initiates breeding. However, fu has developed the additional safeguard endogenous calendar mechanism, because the survival of winter appears more important than reproduction.