Western lowland gorilla (Gorilla gorilla gorilla) birth patterns and human presence in zoological settings.

There is some evidence to suggest that zoo visitors may have a disruptive impact on zoo-housed animals, especially primates. While some consider western lowland gorillas (Gorilla gorilla gorilla) to be particularly reactive to large crowds, the evidence of these effects is mixed, and is likely highly influenced by exhibit design, and group composition. While the majority of studies have focused on behavioral responses to human presence, there is the potential for physiological effects as well, including the possibility of affecting the timing of parturition. Such effects have been demonstrated in laboratory-housed callitrichids and chimpanzees, but unlike laboratory settings where human presence is lowest during the weekends, human presence might peak during weekends in public zoo settings. However, in a study of zoo-housed chimpanzees, there were no significant differences between the number of chimpanzee births that occurred on weekdays compared to weekends [Wagner and Ross, 2008], and we sought to test these questions with gorillas. We analyzed the timing of 336 live gorilla births and 48 stillbirths at 53 accredited North American zoos from 1985-2014, and similarly to chimpanzees, found no weekend or weekday effect on number of births (live births: G = 0.000, p = 1; stillbirths: G = 0.166, p < 0.684). These data add to our understanding of the potential influence of human presence on primate behavior and physiology, and add to evidence suggesting that the effects of zoo visitors on exhibited species may be less profound than previously assumed.