Risk-Taking in Samango Monkeys in Relation to Humans at Two Sites in South Africa

Abstract

A fundamental step in the management and conservation of wild species is advancing our understanding of how animals perceive and use their habitat. Spatial variation in risk either from natural predation or human disturbance generates a “landscape of fear” that can be measured and assessed using experimental patch approaches such as giving-up densities (GUDs). For primates inhabiting a matrix of human habitation, exotic plantations, and indigenous forest, it is possible to explore the “risk-disturbance hypothesis,” which posits that human risk parallels natural predation risk in its effects on animals’ habitat use and decision-making. Here we report on a combination of studies on arboreal samango monkeys (Cercopithecus albogularis spp.) and their risk-sensitive foraging at two sites subject to varying anthropogenic pressures. Using GUDs experiments, we documented pronounced effects of humans on monkeys’ patch use but the impact of humans did not simplistically follow predictions from the risk-disturbance hypothesis. At a site with limited human infrastructure, monkeys exploited food patches at typically high-risk strata (ground level) more intensively in the presence of researchers, whom they likely perceived as shields against terrestrial predators (leopards Panthera pardus, caracals Caracal caracal). Meanwhile, at a predator-poor site where monkeys regularly came into contact with human infrastructure and gardens (adjacent to indigenous forest), the experiments indicated that monkeys preferred to forage in indigenous forest given experimental patches in both forest and gardens, where they showed sensitivity to risk (likely from both humans and domestic dogs Canis lupus familiaris). However, once already in gardens, monkeys depleted patches to extents similar to inside indigenous forest. In this chapter, we also elaborate on monkeys’ risk-sensitive responses to negative, neutral, and rewarding objects associated with humans, and explore the impact of prior experience on risk-taking behavior in proximity to humans and their infrastructure. We conclude that the risk-disturbance hypothesis cannot be used as a blanket term to describe primates’ interactions with humans, as we show here how nuanced and flexible samango monkeys’ risk-taking responses are.

Appendices

Appendix 1: Model 1

Linear mixed-effects model fit by REML

AIC BIC logLik

12701.97 12746.64 −6342.986

Random effects: Formula: ~ExpDay | Tree

Structure: General positive-definite, Log-Cholesky parametrization

StdDev Corr

(Intercept) 2.2702498 (Intr)

ExpDay 0.1373389 −0.652

Residual 5.9674331

Fixed effects: PeanutsLeft ~ BasinHeight + Researchers + BasinHeight:Researchers

	Value	Std. error	DF	t-Value	p-Value
(Intercept)	7.702045	0.4870477	1947	15.813737	0.0000
BasinHeight	−0.637220	0.0599672	1947	−10.626151	0.0000
Researchers	−2.619970	0.4862516	1947	−5.388094	0.0000
BasinHeight:Researchers	0.1028469	1947	2.841911	0.0045	0.292282

Appendix 2: Model 2

glm (Visit ~ Location + Site + Group, family = “binomial”)

Deviance Residuals:

Min 1Q Median 3Q Max

−2.4478 0.3203 0.5323 0.7338 0.9669

Coefficients: (1 not defined because of singularities)

	Estimate	Std. error	z Value	Pr (> |z|)
(Intercept)	1.8827	0.2342	8.038	9.14e−16***
LocationG	−1.3651	0.2862	−4.770	1.84e−06***
SiteLajuma	1.0617	0.4318	2.459	0.0139*
GroupHouse	−1.7698	0.4074	−4.344	1.40e−05***

Signif. codes: 0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1

(Dispersion parameter for binomial family taken to be 1)