Letter to the Editor

Response to Ross, Lonsdorf and Stoinski: Assessing the welfare implications ofvisitors in a zoo setting—A comment on Wells (2005)

I would like to take this opportunity to respond to the letter of Ross and colleagues

regarding the recent paper published on the effect of visitor density on zoo-housed gorillas

(Wells, 2005).

Firstly, Ross et al. raise the possibility of confounding variables influencing the results

of the research. Whilst confounding factors cannot be overlooked in any study, those

mentioned by Ross and associates as potential sources of variation are unlikely to apply to

this particular investigation. The temperature of the gorillas’ housing was kept constant

year around (day-time: 17–18 8C, night-time: 19–20 8C) and the same keepers were

responsible for the group of animals during both periods of testing. As with other groups of

gorillas (e.g. Ogden et al., 1993), the animals in this study spent most of their time on flat

ground in the vicinity of their holding quarters (i.e. indoors) and differences in the use of

enclosure space, even across seasons, were never observed. This group of animals has been

studied extensively over the past 5 years (see, for example, Blaney and Wells, 2004; Wells

et al., in press). The only factor, besides visitor presence, that has been shown to influence

their behaviour is that of diet, which, for the purpose of this investigation, was deliberately

kept the same during the two periods of data collection.

Ross and colleagues secondly question the choice and use of dependent variables in the

study, in particular querying why certain behaviours (notably sitting and standing) were not

influenced by visitor density. There is absolutely no reason why all behaviours within an

animal’s repertoire should be equally influenced by any independent variable, regardless of

how similar they appear on the surface or how they are arbitrarily classified by humans, e.g.

‘inactive’. Numerous studies on animals in captivity have shown significant effects of the

independent variable/s under investigation on some, but not all, of the dependent

behaviours recorded (e.g. De Rouck et al., 2005; Tod et al., 2005), and it is incorrect to

assume that this negates any of the statistically significant effects observed.

Finally, Ross and associates query whether the behavioural measures recorded represent

reliable indicators of stress. Other researchers, including ourselves, have recorded similar

behavioural changes in primates exposed to high numbers of visitors, and have interpreted

these as signs of excitation/stress (e.g. Blaney and Wells, 2004; Chamove et al., 1988;

Glaston et al., 1984; Wells et al., in press). Physiological measurements may indeed

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Applied Animal Behaviour Science 102 (2007) 134–136

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provide a further useful measurement of stress in primates (for example, see Davis et al.,

2005), however, the collection of stress hormones, and indeed other indices of short-term

physiological well-being, can be extremely invasive and temporally meaningless (e.g.

Broom and Johnson, 1993) and hence is not advocated with a group of endangered animals.

Ross indicates that his observations on a group of gorillas in Lincoln Park Zoo do not

support the results of the paper under question. It is impossible to compare the results of

this study to the data collected by Ross since the latter has not as yet been scientifically

reviewed, verified or published, and hence is unavailable for public scrutiny. Great care

must be taken in comparing studies which on the surface appear to investigate the same

phenomenon, but in reality differ greatly in their methodology, sampling procedures and

scientific robustness. Moreover, one must be aware that a lack of significant results does not

necessarily mean an absence of an effect (see Aberson, 2002 for a discussion on

interpreting null results).

The author welcomes Ross and colleagues’ comments on her paper and agrees that

much further work in this field is required before firm conclusions regarding the so called

’visitor effect’ can be drawn. Thus far, only one study has been published on the influence

of the human audience on captive gorillas (Wells, 2005); further published work in this area

is needed before generalisations can be made. Visitors are likely to have different effects

upon different primates (see Hosey, 2005). Factors that may influence the visitor effect

include, amongst others, individual variation, exhibit design, management practices,

visitor characteristics, etc. Thus, for some animals, even within the same group, visitors

may have an enriching effect, for others they may be stressful, and for yet others, they may

have no effect whatsoever. The paper under scrutiny, and work by others, clearly indicates

that visitors have an effect on primate behaviour. The degree to which the welfare of

captive primates is adversely influenced by visitors, however, will only be determined by

further study.

References

Aberson, C., 2002. Interpreting null results: improving presentation and conclusions with confidence intervals.

JASNH 1, 36–42.

Blaney, E.C., Wells, D.L., 2004. The influence of a camouflage net barrier on the behaviour, welfare and public

perceptions of zoo-housed gorillas. Anim. Welfare 13, 111–118.

Broom, D.M., Johnson, K.G., 1993. Stress and Animal Welfare. Chapman & Hall, London.

Chamove, A.S., Hosey, G.R., Schaetzel, P., 1988. Visitors excite primates in zoos. Zoo Biol. 7, 359–369.

Davis, N., Schaffner, C.M., Smith, T.E., 2005. Evidence that zoo visitors influence HPA activity in spider monkeys

(Ateles geoffroyii rufiventris). Appl. Anim. Behav. Sci. 90, 131–141.

De Rouck, M., Kitchener, A.C., Law, G., Nelissen, M., 2005. A comparative study of the influence of

social housing conditions on the behaviour of captive tigers (Panthera tigris). Anim. Welfare 14, 229–

238.

Glaston, A.R., Geilvoet-Soeteman, E., Hora-Pecek, E., Van Hooff, J.A.R.A.M., 1984. The influence of the zoo

environment on social behavior of groups of cotton-topped tamarins, Saguinus oedipus oedipus. Zoo Biol. 3,

241–253.

Hosey, G.R., 2005. How does the zoo environment affect the behaviour of captive primates? Appl. Anim. Behav.

Sci. 90, 107–129.

Ogden, J.J., Lindburg, D.G., Maple, T.L., 1993. Preferences for structural environmental features in captive

lowland gorillas. Zoo Biol. 12, 381–395.

Letter to the Editor / Applied Animal Behaviour Science 102 (2007) 134–136 135

Tod, E., Brander, D., Waran, N., 2005. Efficacy of dog appeasing pheromone in reducing stress and fear related

behaviour in shelter dogs. Appl. Anim. Behav. Sci. 295–308.

Wells, D.L., 2005. A note on the effect of zoo visitors on the behaviour and welfare of captive gorillas. Appl. Anim.

Behav. Sci. 93, 13–17.

Wells, D.L., Coleman, D., Challis, M.G., in press. A note on the effect of auditory stimulation on the behaviour and

welfare of zoo-housed gorillas. Appl. Anim. Behav. Sci.

Deborah Wells*

School of Psychology, Queen’s University Belfast,

Belfast BT7 1NN, Northern Ireland, United Kingdom

*Tel.: +44 28 90 974386;

fax: +44 28 90 664144

E-mail address: d.wells@qub.ac.uk

10 February 2006

Available online 18 April 2006

Letter to the Editor / Applied Animal Behaviour Science 102 (2007) 134–136136