Large wild mammals – from elephants to antelopes – are already struggling to cope with global warming. Now new research shows that even the small creatures adapted to harsh, arid landscapes may be reaching their limits.
Even though they’re accustomed to living in semi-desert environments, the African striped mice of southern Africa are showing signs that they too suffer from climate-related rising heat and worsening droughts.
As a team of eco-physiologists, we discovered this recently through a study of how the effect of global warming on wild mammals in the desert can be measured by the thickness of their blood. It is generally known from physiology that dehydration leads to higher concentration of substances in the blood, but this is the first study to see whether drought has this effect in wild living mammals.
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To conduct our research, we needed many blood samples drawn from wild mammals during dry and rainy seasons over multiple years. This had rarely been attempted before, but without analysing blood taken over a long time, we would not be able to understand how differences in temperature and food availability over many years affect blood parameters in free-living mammals.
Luckily, we just happened to have the perfect collection of blood samples waiting to be tested. We had collected and frozen more than 8,000 blood samples from a wild mammal living in one of the driest environments in the region – the African striped mouse (Rhabdomys pumilio) – over 12 years of fieldwork in Namaqualand, South Africa.
This tiny rodent lives in the Succulent Karoo semi-desert, a region with cold, moist winters where food is easily available and hot, dry summers with temperatures of between 30°C and 40°C, where resources are scarce. They are no springs or lakes from which they can drink water, and they stay hydrated through the moisture in the succulent (juicy, water-rich) plants that they eat.
We’d initially collected the samples for multiple studies into how hormones influence the social behaviour of the striped mice, but had more than we needed. We froze the extra blood for later work and used it for this study, which was great because we did not have to disturb more mice.
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To make use of this unique resource, we invited researcher Neville Pillay, who’d already led a 25-year study of desert-dwelling rodents in South Africa, to join the team. We also needed someone willing to sort through years of frozen material. Student Aurelie Vinot stepped up, spending weeks combing through –80°C freezers assembling the samples.
Our research found that the blood of striped mice becomes thicker in the dry season in association with temperatures often exceeding 40°C. As we predicted, this increase in blood thickness became more pronounced as water-rich food plants became less available.
We can assume the same would happen in harder-to-study species, like lion or antelope.
This makes the striped mouse a useful model species for understanding how desert mammals respond to drought.
The striped mouse: a drought-adapted survivor
Studies have been done for many years on how birds’ dehydration levels can be measured via blood osmolality.
Osmolality means the concentration of dissolved substances in blood. When animals cannot drink or obtain enough water from food, blood becomes more concentrated. Highly concentrated blood can damage organs, disrupt cellular function, and ultimately cause death.
This means osmolality is a powerful way to assess how heat and drought affect wild species. Simply put: the thicker the blood, the more animals suffer from drought.
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Our research found that during the rainy seasons, the mice did not get dehydrated.
But during the dry seasons, osmolality in striped mice was 2%-3% higher. While this may sound small, blood chemistry is tightly regulated. Even a minor increase indicates substantial physiological strain on the animal. For comparison, a similar rise in ocean salinity would disrupt entire marine ecosystems.
High osmolality, known as hyperosmolality, is not a disease in itself. However, it is a sign of dehydration or reduced kidney function. Severe hyperosmolality can lead to confusion, damaged cells, organ failure and ultimately death. In the wild, even a modest rise signals that an animal’s survival margin is narrowing.
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We also found that the thickness of the mice’s blood depended on how harsh the dry season was. The risk of dehydration shot up as temperatures became hotter. Even more important was the availability of juicy, moisture-rich succulent plants.
We expected the mice’s blood to get thicker in hot weather, because animals lose more water when it’s very hot. But what mattered even more was whether the mice could find succulent plants to eat. When these plants were scarce, their blood became much more concentrated. This shows that staying healthy in dry seasons depends not just on dealing with the heat, but also on having enough food that provides water.
Drought challenges animals. Every extra degree of heat or decrease in succulent plants pushes animals closer to their physical limits.
A new tool for conservation under climate change
Our study is the first to show that serum osmolality is a practical and sensitive measure of environmental harshness in wild mammals, with far-reaching implications.
Conservation biologists can use osmolality to determine whether wild mammals are struggling mainly with water shortage, lack of energy-rich food (by also tracking body mass), or both. This helps guide their decisions about whether to provide these mammals with water, food, or both to improve survival during drought.
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As climate change worsens, tools like this will become more and more important. Osmolality is a way to detect dehydration in wild mammals and step in to help them before their populations go into decline. If scientists have an osmometer, which is not expensive and can easily be set up in a field laboratory, they can measure osmolality in minutes. All wildlife vets should have one.
Blood tells a quiet story – of heat, strain, and survival. Thanks to an osmometer, a freezer full of samples, and a determined student, we can finally hear it.
Lindelani Makuya receives funding from CNRS. She works for the CNRS.
Antoine Stier is affiliated with the CNRS and the University of Turku
Carsten Schradin receives funding from the CNRS, CNRS SE-Life, and the ANR.
This article was originally published on The Conversation. Read the original article.
