Suckling and lactational anoestrus in wild gorillas (Gorilla gorilla).

The present observations of wild gorillas demonstrate that lactation has a major influence on birth spacing. The frequency of suckling declined as infants matured, from greater than 1/h during the first year to about 1/2 h by 30-36 months when mothers usually resumed sexual cycling. In contrast, the length of suckling bouts remained relatively constant throughout lactation, averaging between 2.6 and 3.2 min per bout. Within infant age categories, suckling frequencies of less than or equal to 0.5 bouts/h were associated with mothers who had resumed cycling, whereas higher frequencies were associated with mothers who were still anoestrous. These results complement those obtained from studies of breast-feeding in humans, and support the suggestion that the frequency of suckling is a critical factor in the contraceptive effects of lactation.