Why Harshness Is not Stress – and Why That Matters in a Warming World – Journal of Zoology Blog

Schradin, C., Pillay, N. and Rimbach, R. (2026). Adaptation to harshness is fundamentally different from the adaptive stress response: Results from a 20-year-long case study in African striped mice. Journal of Zoology, vol. 328, pp. 107-120. https://doi.org/10.1111/jzo.70084

It is hot. More than 40°C in the shade. But there is almost no shade.

At least it is a dry heat, where sweating helps – but only if you have enough water to drink. The animals that live here have nothing to drink – and little to eat.

It didn’t rain last week, last month, not for the last three months. It’s February, the middle of summer in Namaqualand, South Africa, and the last rain was in October. Earlier in the year, it was much cooler and wetter. It rained 140mm last winter, a high amount for this region and enough for an amazing wildflower display in September and a good cover of annual succulents in October.

But now most of this is dried out, gone. There are no trees here, too little water. Only succulent shrubs, less than 1 meter high, and deserted sandy spaces in between.

This is a harsh environment, at least in summer. But some animals thrive here. They have evolved mechanisms to cope with this harshness, to survive the long dry and hot summer until the next spring, when they can breed.

For 25 years I have been studying the African striped mouse in this habitat, a small 60-g rodent that is active during the day, occupies an open habitat, and is easily habituated to observers. This allowed us to get accurate behavioural observations from many generations inhabiting this harsh environment, and to trap thousands of individuals over the years, from which we obtained blood samples to analyse hormones, blood parameters related to metabolism, and even their daily energy expenditure. In a current review we summarise results from multiple studies conducted over more than two decades. What we learned is surprising at first, and then eye opening for how animals survive hot droughts.

The striped mice decreased their stress hormone levels during droughts. Corticosterone levels in the dry season were half as high as in the moist season.

There has been an ongoing discussion among scientists about how to define stress. For vertebrates, the best definition is: Stress is what activates the physiological stress response. When stress hormones increase, then it`s stress!

So logically, if stress hormones are not increased, but decreased, then it cannot be stress. Thus, it is wrong to talk about heat and food stress for striped mice during the dry season. Instead, it is harshness they experience. And therefore harshness is not stress! In fact, it is fundamentally different from stress.

How to survive in a harsh environment

Striped mice have evolved multiple ways to deal with the low food availability that persists for months in their hot environment. They warm each other at night in the nest to save energy, then get up to bask in the early morning sun, being largely inactive during the day. They reduce the secretion of several hormones, reducing their basal metabolism. They even shrink their energetically most expensive organ in the dry season, the brain. With this flexibility in behaviour and metabolism they manage to decrease their daily energy expenditure by 30% in the dry season.

Lessons from the striped mouse: Flexibility and energy saving

So striped mice survive the dry season by saving energy. They don’t invest energy to overcome it, unlike animals that overcome a stressor such as a competitor or predator by fight or flight. Instead, striped mice endure harshness.

In fact, endurance is what many animals will need to cope with the consequences of climate change as the planet warms. Droughts will become more common and more severe. To survive this, animals have to be able to either avoid it by migration, or endure it by saving energy and water. And indeed, when an animal reduces its metabolism, it also automatically reduces its use of water significantly.

The main mechanisms, also in other mammals: reduce stress hormones and save energy

There are many other animals living in arid or semi-arid environments that reduce stress hormone levels, and that reduce energy expenditure during the dry season. Desert birds, desert rodents, and zebras do this. Reduced activity, sunbasking, huddling and a reduction in basal metabolism occurs in many of these species.

Climate change will increase harshness

So the question is to what extent species not evolved to survive in harsh environments will be able to reduce their energy expenditure via behavioural and physiological flexibility when harsh periods such as droughts become more common in the future. Thanks to our long-term studies on the striped mouse, describing the many different aspects of their harshness response, it is now possible to study the same traits, and how flexible they are, in other species predicted to be affected by droughts. The most important insight here is that harshness does not induce stress, but that harshness is fundamentally different from stress! This also means that for all species to endure harshness via energy savings, it is important to avoid any stress, which would itself induce increased energy expenditure and therefore increased vulnerability to harshness.

Carsten Schradin

School of Animal, Plant and Environmental Sciences, University of the Witwatersrand, South Africa; Université de Strasbourg, CNRS, IPHC UMR 7178, F-67000 Strasbourg, France