Unlike females of other great ape species, orangutans do not exhibit sexual swellings to signal fertility. The average age in which a female first gives birth is 15 years and they have a six to nine year interbirth interval, the longest among the great apes. Gestation lasts around nine months and infants weigh at birth. Usually only a single infant is born; twins are a rare occurrence. Unlike many other primates, male orangutans do not seem to practice infanticide. This may be because they cannot ensure they will sire a female's next offspring, because she does not immediately begin ovulating again after her infant dies. There is evidence that females with offspring under six years old generally avoid adult males.