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

Abstract

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.

Testing the weekend effect hypothesis: Time of day and lunar phase better predict the timing of births in laboratory‐housed primates than day of week

“…Contrary to the predictions of the hypothesis, the three monkey genera we studied showed extremely little variation in their birthing patterns across weekdays and weekends compared. This provides further support for previous studies that have also challenged the weekend effect hypothesis and reported no strong effect of day‐of‐week on parturition rates (Goodlin & Sackett, ; Kurtycz & Ross, ; McGrew & McLuckie, ; Wagner & Ross, ).…”

“…In other words, a more general formulation of the weekend effect hypothesis would state that it is not any specific day of the week that predicts the timing of birth, but rather, changes in human activity levels. An examination of the hypothesis based on chimpanzee births in zoos accredited by the Association of Zoos and Aquariums found that births were no more likely during weekdays than weekends (Wagner & Ross, ), and a similar finding was reported more recently for zoo‐housed gorillas (Kurtycz & Ross, ).…”

“…It remains unclear whether this cross‐facility difference could be due to idiosyncratic characteristics of the first facility or an artifact of sampling. Additional studies with both apes ( Gorilla gorilla gorilla , Kurtycz & Ross, ; P. troglodytes , Wagner & Ross, ) and monkeys ( Macaca nemestrina , Goodlin & Sackett, ; Macaca arctoides , McGrew & McLuckie, ) also reported no evidence for a weekend effect, calling this previously‐identified phenomenon into question.…”

“…Not only have recent studies failed to find definite evidence in support of the weekend hypothesis proposed by McGrew and McLuckie () but also the number of births considered in most early studies was limited (e.g., McGrew & McLuckie, , N = 24 Callitrichidae births and N = 95 M. arctoides births). Interestingly, the two studies of zoo‐housed apes, which did not find much variance across the week for numbers of births per day, included considerably more births in their analyses (Wagner & Ross, , N = 231 chimpanzee births; Kurtycz & Ross, , N = 336 gorilla births). A more robust evaluation of the hypothesis would benefit from an evaluation that includes numerous taxa, numerous births, and a long time frame.…”

Is wounding aggression in zoo-housed chimpanzees and ring-tailed lemurs related to zoo visitor numbers?

“…Interestingly, human presence has also been implicated in altering the timing of births in some laboratory primates [Alford et al, ], but this effect appears not to occur in zoo‐housed chimpanzees [Wagner and Ross, ] or gorillas [Kurtycz and Ross, ]. We can only agree with the latter authors that the effects of zoo visitors on captive animals may be less profound than previous studies suggested.…”