This blog post is provided by Arjun Dheer and tells the #StoryBehindthePaper for the paper ‘Diurnal pastoralism does not reduce juvenile recruitment nor elevate allostatic load in spotted hyenas‘, which was recently published in Journal of Animal Ecology. In his study, he explores the impact of pastoralism on spotted hyena populations in Ngorongoro, discovering that they don’t seem stressed and numbers of recruited cubs didn’t differ between areas with human influence and areas without.
The blog can be found in Swahili here.
Humans affect wildlife, but it’s not always easy to assess the conservation-relevant impacts. In fact, we affect them in so many ways – they might be forced to avoid us and become more active at night, alter where they look for food, or even change their diet. It is often assumed that these effects are bad for the animals, and that wildlife struggles to cope with those behavioral changes. Surely, this is an issue of serious conservation concern. But is it really?
Not necessarily. Many animals can adjust their behavior to changes in their environment without this affecting their ability to survive or reproduce. Changes in behavior thus do not always mean that the persistence of a wildlife population is actually threatened. To figure out whether a population’s persistence is threatened is difficult. And it is especially difficult when studying larger, longer-lived species – such as spotted hyenas (Crocuta crocuta) – which often come into conflict with humans and thus need to be prioritized to promote coexistence. Doing such an investigation requires long-term data, focused particularly on two main elements: fitness and physiology.
Fitness refers to the ability of an organism to survive, reproduce, and contribute offspring to the next generation. Physiology is a broad concept referring to the functions in an organism’s body that allow it to survive and reproduce. Measuring changes in these variables is very conservation-relevant because of how directly they impact a wildlife population’s continued presence. And that is exactly what we investigated.
We used a natural experiment to study hyena clans exposed and unexposed to human activity. For our study, we collected 24 years of data (1996-2019) from the eight spotted hyena social groups (“clans”) resident on the floor of the Ngorongoro Crater, Tanzania. Two of the Crater clans – which we call the Airstrip and Forest clans – were exposed to pastoralism conducted by the local Maasai community from 1996-2016. Pastoralism is a globally widespread human activity and lifestyle whereby people accompany their livestock to grazing pastures, water bodies, and mineral licks. The other six clans were not exposed to pastoralism. This provided us with a natural experiment: we could compare how the exposed clans performed compared to the unexposed ones.
Measuring juvenile recruitment provides key information on the “health” of hyena clans. To measure fitness effects, we compared juvenile recruitment – that is, the number of surviving offspring – in exposed and unexposed clans. As for physiology, we compared the concentration of glucocorticoids in their poop, or “stress”. The level of “stress” indicates the cumulative burden an organism experiences due to life events, be it social interactions with groupmates or repeated human disturbance. To do so, we collected 975 poop samples from 475 different hyenas across our study period and measured the “stress” in each and every one.
Last but not least, in order to disentangle the effect of pastoralism from other environmental conditions, we also collected data on other factors during the course of our study period – the occurrence of disease outbreaks, the risk of encountering lions, and prey availability.
Contrary to what we expected, juvenile recruitment in the exposed clans was slightly higher than in the unexposed clans. Similarly, “stress” was very similar between hyenas from the exposed and unexposed categories. Altogether, it seems that pastoralism did not pose a threat to exposed clans in the Ngorongoro Crater spotted hyena population at all. The exposed clans did just as well as the unexposed clans, if not slightly better!
Two potentially big reasons why there was no negative effect on the exposed clans are that the pastoralism (i) was predictable, as it occurred strictly on dedicated paths into and out of the Crater and (ii) happened only during the day. Because it was predictable, the hyenas may have been able to adjust to the daily rhythm of the pastoralism without much issue. And because it occurred during the day, it meant the pastoralism was not very disruptive to key behaviors in the hyenas – namely hunting and suckling. Suckling of very young cubs does often occur during the daytime, but the mothers may have been able to adjust the suckling times to the night in order to avoid the pastoralism.
On top of all that, the hyena social system might have played a role. Hyena clans have a strict hierarchy or ranking system. Top-ranked hyenas tend to contribute more cubs to the population, and due to several factors, are less likely to be directly exposed to challenges such as pastoralism. It is possible that the lower-ranking hyenas might have absorbed any effects of the pastoralism; most of their cubs die even in the best of times. When exposed to pastoralism, their cubs may just die earlier, leaving the clan’s overall performance unaffected.
Finally, the Ngorongoro Crater’s rich prey abundance relative to the size of the hyena population (i.e., very high “prey per capita”) might have protected the exposed clans from experiencing serious effects of pastoralism. The Crater has one of the highest densities of large mammals on Earth, and that means the hyenas enjoy a banquet of delicious food year-round, at least during most of our study period. This may have allowed mother hyenas to produce plenty of milk to raise healthy cubs, and ultimately may have kept the clans productive.
Our results have strong implications… but should be interpreted with caution. Our findings suggest that human activity may be sustainable and conducive to coexistence with wildlife provided they are not too disruptive to key behaviors. Second, we have highlighted the importance of considering the effects of human activity in light of an animal’s behavioral patterns and social system. And third, by measuring juvenile recruitment at the level of clans, we have offered a new approach to the study of human-wildlife interactions focused on group-living species.
All that being said, our findings apply to a very specific situation. In areas where human activities are more intense and environmental conditions less ideal than in the Crater, pastoralism may have a stronger effect even on behaviourally flexible species such as the spotted hyena. We encourage other scientists to conduct studies that focus on a variety of human activities and animal species with various social systems and behavioral patterns so we can continue developing evidence-based strategies for coexistence.
Arjun Dheer is a doctoral student working with the Ngorongoro Hyena Project, based at the Leibniz Institute for Zoo and Wildlife Research. He studies human-carnivore coexistence in Ngorongoro Conservation Area, Tanzania. Follow him on Twitter @ArjDheer and the Hyena Project at @HyenaProject and on YouTube at www.youtube.com/c/hyenaproject!
Read the paper
Read the full paper here: Dheer, A., Davidian, E., Courtiol, A., Bailey, L. D., Wauters, J., Naman, P., Shayo, V., & Höner, O. P. (2022). Diurnal pastoralism does not reduce juvenile recruitment nor elevate allostatic load in spotted hyenas. Journal of Animal Ecology, 00, 1– 12. https://doi.org/10.1111/1365-2656.13812
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