Previous reports of increased sexual activity associated with genital swelling during chimpanzee pregnancy have suggested that the tumescent pregnant female may be regarded as a regularly cycling estrous female. Analysis of additional data from seven pregnant and eight cycling females, however, indicated that tumescent pregnant females differed from cycling females in some social interactions with males. As in earlier reports, pregnant females were more receptive to male-initiated copulatory bouts. In addition, pregnant females initiated grooming bouts with males much less frequently than did cycling females, and males spent less time grooming and less frequently inspected the genitals of pregnant than cycling cagemates. This tendency to decrease social interaction during pregnancy is consistent with field reports that pregnant chimpanzees prefer the company of their offspring and other noncycling females. In addition, the genital swelling patterns of 40 chimpanzee pregnancies were analyzed. Length of gestation averaged 231.5 days. Although a high degree of individual variation existed, females were swollen an average of 41% of the days observed, yet reached maximum swelling only 8.7% of the time. Genital swelling was less frequent during the third trimester, averaging 50.5,47.9, and 22.6% tumescent days in each trimester. Young pregnant females showed maximum swelling more often than did older females. This finding is discussed with respect to field reports of intercommunity transfer of young, tumescent female chimpanzees.

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Daily observations during a 5-year period showed that the mean frequencies of copulation per day during various sexual states of the female chimpanzee were almost identical, except for during maximum tumescence and lactation. The mean frequency of observed copulation peaked during maximum tumescence and was lowest during lactation.

Twin lambs were divided into two groups: an unmothered group consisting of one of each pair of twins separated from its mother at birth, and a mothered group consisting of the other member of each pair reared by its mother. Lambs in both groups were placed individually on a "visual cliff" once every hour. In all instances lambs in the mothered group exhibited "cliff avoidance" behavior before those in the unmothered group. In another experiment, mothered lambs were fitted with translucent goggles for half the time required by unmothered lambs, matched on the basis of sex and birth weight, to acquire depth perception. Again, in all instances the mothered, goggled lambs avoided the cliff before the unmothered lambs.