Three phases of lactation in free-ranging Japanese macaques

Abstract

Suckling behaviour was studied in free-ranging Japanese macaques, Macaca fuscata, at Jigokudani Monkey Park, Japan, from April 1984 to December 1990. Nutritive sucking occurred at a constant low local rate (the number of sucks per second); non-nutritive sucking occurred at an increasing and high local rate. The duration of a nutritive sucking bout, which accurately reflects the amount of milk transferred to the infant, correlated linearly with the duration of a lactation period, defined as the interval from the end of a sucking bout to the end of the next bout. An unbiased estimate of the daily rate of milk transfer was thus obtained as the gradient of this linear regression (the ratio of the duration of a nutritive sucking bout to that of a lactation period). The daily rate was stable at first, dropped abruptly at the deceleration of infant growth, and was then constant again. When the next offspring was born in the following year, lactation lasted until parturition ( N=3); when the next offspring was born after 2 years, lactation continued into the early part of the next gestation ( N=2); when the next offspring was born after more than 2 years, lactation ceased before conception ( N=2). Japanese macaques thus appear to have three modes of lactation: lactation in amenorrhoea, with a high level of milk energy output; lactation permitting menses, ovulation and conception, with a low level of milk energy output; and lactation with a high level of milk energy output occurring just before the next parturition if an inter-birth interval lasts a year. The first phase appears to be linked with rapid growth in infancy; the second appears to be related to a long period of slow juvenile growth in catarrhines; and the third appears to be connected with preparation for the next neonate.