ovary of the female contained a follicle larger than 4 mm or
immediately post=ovulatory. Secretion of P4 was evident from
ovaries with follicles larger than 5 mm but still pre-ovulatory
and after ovulation rose to a peak (about 14 Ng/ml) before de-
clining prior to menstruation.

Mating behavior showed a similar pattern when tests with
Rhesus females were carried out over several successive cycles (5).

2. PIGTAIL MACAQUES. Swelling of the female's sex skin began
shortly after menstruation, reached a peak and declined prior to
next menstruation. By laparotomy, ovulation was seen to have
occurred during the first day on which the size of the sex skin
began to decline, Taking this day as the beginning of the luteal
phase, cycle length was highly correlated with length of the
follicular (tumescent) phase (r = 0.987), which varied from 12 to
24 days compared to a range of from 14 to 17 days for length of
the luteal phase.

The time required for the male to achieve ejaculation was
inversely related to the size of the sex skin, reaching a minimum
close to the time of maximum swelling. Maximum probability of
mating coincided with peak swelling and less than 10 per cent of
ejaculations occurred in the luteal phase. Behaviors such as
frequency of male mounting or intromissions and "presenting' by
the female or social behaviors, such as grooming, showed no marked
association with any particular stage of the cycle,

Figure 1 shows that the decline in incidence of ejaculation
by the male coincides with the rise of P4 in the blood of the
female. It is not clear how information about the female's
hormonal status is conveyed to the male. Swelling of the sex
skin may well be a visual signal, although other factors, such
as pheromones (6), could be involved. It is known that estrogen
causes swelling of the sex skin and that P4 brings about its
collapse (7,8). This is confirmed by our data on endogenous P4,
the rise in which coincides with the onset of detumescence.
Sampling intervals (48 h) were too long to reliably determine
whether or not a rise in P4 was present in advance of presumptive
ovulation.

(5) R.E. KUEHN and W.C. YOUNG, 1965, Amer. Zoologist 5: 687-688.
(6) R.,P. MICHAEL and E.B. KEVERNE, 1968. Nature 218: 746-749.
(7)5.J 5 CLLEMAN,, 1938;. .S Afr, J. Meds Sci.~3:1:66-71,

(8) J. GILLMAN, 1940. Endocrinology 26: 80-87.