Relationship between menarche and fertility in long-tailed macaques (Macaca fascicularis)

Abstract

We attempted to elucidate female reproduction in long-tailed macaques (Macaca fascicularis). These monkeys have a non-seasonal menstruation cycle, which makes them suitable subjects for studies in a variety fields including medical science and regenerative medicine. We analyzed individual breeding data including time of menarche, start of regular menstruation, and first pregnancy. These three events are related to the maturation of female long-tailed macaques. All research subjects were female long-tailed macaques bred at the Tsukuba Primate Research Center. The study comprised 45 females; we included time of menstruation, male-female cohabitation, and first pregnancy in their growth records. We extracted age and weight data relating to menarche, start of regular menstruation, and first pregnancy from these records. In the two years typically required from menarche to first pregnancy, the body weight increased by approximately 500 g (21% of the weight at menarche); it is clear that there is a significant physical change after menarche. Our findings suggest that female monkeys are not necessarily mature enough for pregnancy at menarche. Therefore, the use of the word "maturity" in terms of fecundity may be more accurate after the start of regular menstruation. This is what we term "adolescence" in the developmental process. Therefore, M. fascicularis monkeys are candidates for an animal model of human adolescence.

Keywords: Female fertility; Female reproduction; Long-tailed Macaque; Menarche.

Fig. 1.

The process of sexual maturity in 45 animals. The horizontal line shows the days from the first detection of menses (menarche); the vertical line at 0 indicates the start of regular menstruation. The open circles represent menstruation and the closed circles represent the number of cohabitations subsequent to menstruation. Each animal’s first open circle represents menarche, and the last filled circle represents first pregnancy. Animals 7, 13, 18, 19, and 35 were not observed from the start of regular menstruation because they became pregnant before they began to menstruate regularly.