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Impala fGCM Levels Were 57% Higher on Dry-Season Weekends in Four-Species Tourism Study

Male impala grazing in Kruger National Park, South Africa
Research · Physiology

A four-species study at Somabula Nature Reserve in Gauteng, South Africa, found that ecological and reproductive conditions generally explained more variation in stress-related endocrine activity than tourist pressure. The clearest tourism-associated signal occurred in impala (Aepyceros melampus): during the dry season, faecal glucocorticoid metabolite concentrations were about 57% higher over weekends than on weekdays.

The study also included plains zebra (Equus quagga), blue wildebeest (Connochaetes taurinus) and southern giraffe (Giraffa giraffa). The paper used the broader scientific name Giraffa camelopardalis; under the current species treatment, these South African animals are southern giraffe.

fGCMs measure stress-related endocrine activity, not harmful stress directly

Researchers collected 533 fresh faecal samples after observing known animals defecate: 108 from impala, 133 from zebra, 217 from blue wildebeest and 75 from giraffe. Previously validated species-specific enzyme immunoassays were used to measure faecal glucocorticoid metabolites, or fGCMs.

Glucocorticoids help animals mobilise physiologically when ecological, social or other demands increase. Higher fGCM concentrations can therefore reflect normal adaptive responses to challenges such as food shortage, reproduction or disturbance; they are not by themselves evidence of chronic stress or poor welfare.

Seasonal patterns differed strongly among species

Impala fGCM concentrations were about 93% higher in the dry season than in the wet season, and zebra showed the same direction with an increase of about 24%. The authors link this mainly to poorer forage and greater competition around limited resources during the dry season in a fenced reserve.

Blue wildebeest showed the opposite pattern, with mean fGCM concentrations about 31% higher in the wet season. Sampling then overlapped with reproductive and social processes including calving, maternal investment and post-rut restructuring.

Giraffe showed no clear seasonal change. Sex was more important: females averaged about 53% higher fGCM concentrations than males. Two of the five females were visibly pregnant during both sampling seasons and gave birth in March 2023, making pregnancy-related endocrine changes a plausible contributor.

Only impala showed a clear weekend effect

Tourist activity was higher over weekends. Because faecal metabolites reflect hormone activity some time before defecation, samples collected from Tuesday to Friday represented weekday conditions, while samples from Saturday to Monday represented the preceding weekend period.

During the dry season, impala fGCM concentrations were about 57% higher over weekends than on weekdays. No clear weekend effect appeared in impala during the wet season, and blue wildebeest, zebra and giraffe showed no significant time-of-week effect.

The authors interpret this pattern as evidence that tourism-related disturbance may add to ecological pressure when conditions are already demanding. They discuss impala’s smaller body size, vigilance and social behaviour as possible reasons for greater sensitivity, but these mechanisms were not tested directly.

The study measured tourist intensity, not individual encounters

The researchers did not record each tourist’s behaviour, activity or distance from the sampled animals. Hiking, cycling, camping and self-drive traffic therefore could not be separated as individual sources of disturbance, and elevated fGCM values cannot be translated directly into impaired welfare.

The broader result is consequently more nuanced than “tourism raises stress”. Seasonal ecology, reproduction and sex accounted for major species-specific variation, while the clearest tourism-associated signal was an additional dry-season weekend increase in impala. The study therefore supports species- and season-specific management rather than assuming that all ungulates respond to visitors in the same way.

Sources and image credit

  1. Crossey, B., Pienaar, E. F., & Ganswindt, A. (2026). Ecological factors rather than tourism pressure primarily affect the stress-related endocrine response of four ubiquitous mammal species in a small South African reserve. African Zoology, 61(2), 99–109.Primary open-access article under CC BY 4.0. The study analysed 533 faecal samples from impala, plains zebra, blue wildebeest and giraffe at Somabula Nature Reserve, testing season, sex and time of week as a proxy for tourist activity.

Image: Joao Paulo de Vasconcelos — male impala grazing in Kruger National Park, South Africa, 9 April 2016; Wikimedia Commons / Flickr; CC BY 2.0. Representative species image, not a Somabula study animal. · CC BY 2.0 · Source

About this content: This story was produced with AI assistance within an editorial workflow developed by Wildlife Vagabond. Editorial responsibility remains with Wildlife Vagabond.How AI is used

Physiology

SPECIES IN THIS STORY

Species in this story

Impala Aepyceros melampus Explore species Plains Zebra Equus quagga Explore species Blue Wildebeest Connochaetes taurinus Explore species Southern Giraffe Giraffa giraffa Explore species

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