Dither: A unifying model of the effects of visitor numbers on zoo animal behavior

Abstract

Interest in the impact of human presence on the behavior and well-being of zoo and aquarium animals is increasing. Previous work has conceptualized the presence of zoo visitors as having one of three impacts on the behavior of animals in zoos: positive, negative, or neutral. Research suggests the same species may exhibit all three responses under different conditions, calling into question whether the positive/negative/neutral framework is the most useful way of considering visitor impact on animal behavior. Here we present a model of visitor effects that unifies these three predictions. Our model suggests that zoo-goers may provide a "dither effect" for some animals living in zoos. We posit animals may show nonlinear behavioral responses over a range of visitor densities, effectively exhibiting changes in both comfortable and anxiety-like behaviors under different levels of human presence. We tested this model during two COVID-19 related closures at the San Francisco Zoo, studying seven species for evidence of nonlinear relationships between visitor numbers and animal behavior. Our results support the dither effect acting in several species observed.

The Impact of Visitors on Non-Primate Species in Zoos: A Quantitative Review

“…As has been highlighted previously [ 5 , 27 ] and throughout this review, the relationship between zoo animals and zoo visitors is complex. Although there was no association between behavioural responses to visitors and the visitor metrics recorded, it is probably not just the presence of zoo visitors that impacts on animals, rather it is more likely to also relate to visitor activity and behaviour.…”

“…However, although this sounds straightforward, researchers have suggested that the visitor effect may be more complex than was first postulated. Animal behaviour may be affected by differing levels of human presence, with some species principally showing increased responses to both low and high numbers of visitors but reduced responses to intermediate levels of visitors [ 27 ].…”

Do not disturb: Visitors' effect on the behavior and welfare of female and male big cats in zoos

“…Several studies highlighted that the presence of visitors in zoological gardens might lead to changes in animal behavior and physiology (Cooke & Schillaci, 2007; Davey, 2007; Larsen et al, 2014) and the extent of such changes might be due to the physical and behavioral characteristics of the species (e.g., diurnal vs. nocturnal, closed‐habitat species vs. open‐habitat, terrestrial vs. arboreal/scansorial) (Queiroz & Young, 2018). Research on the effect of zoo visitors' presence on animal behavior and welfare has shown that the public can be enriching for animals (Bloomfield et al, 2015; Hosey et al, 2013; Sherwen & Hemsworth, 2019) or it can have no effect (Hosey et al, 2013; Krebs et al, 2022; O'Donovan et al, 1992; Sherwen & Hemsworth, 2019). Other studies have reported that visitors can have a negative impact on animal behavior, promoting the performance of abnormal behaviors or excessive self‐directed behavior (Mallapur et al, 2005; Sherwen & Hemsworth, 2019; Wells, 2005), altering the duration and frequencies of species‐typical normal behaviors (e.g., increasing vigilance as well as hiding behavior) (Carlstead et al, 1993; Clark et al, 2012; Davey, 2007; Hosey et al, 2013; Larsen et al, 2014; Mallapur & Chellam, 2002; Morgan & Tromborg, 2007; Quadros et al, 2014; Sellinger & Ha, 2005; Sherwen & Hemsworth, 2019; Williams et al, 2021) and impairing social interactions (e.g., reducing affiliative behavior and increasing aggression) (Chamove et al, 1988; Hosey et al, 2013; Mallapur & Chellam, 2002; Queiroz & Young, 2018; Sellinger & Ha, 2005; Simpson, 2004).…”