Temperature tunes circadian timing in some desert mammals

When the desert heat rises, dehydrated dromedary camels—the one-humped variety found in North Africa and the Middle East—let their body temperature run wild: Their internal thermometer starts to fluctuate with the ambient temperature to help them retain water, one of many adaptations that equip them for life in an arid environment.

This adaptation is also serving neuroscientists who study circadian clocks, which are set by various external cues, or zeitgebers (literally, “time givers” in German). Light, long considered the most dominant zeitgeber, has received most of the field’s attention since the 1960s. But over the past decade, a growing number of studies have turned the spotlight onto yet another—temperature.

For desert camels and other animals in extreme environments, temperature might actually be a more important zeitgeber than light, says Khalid El Allali, associate professor of anatomy at the Hassan II Agronomy and Veterinary Institute in Morocco.

Mammals—including hydrated camels—use their metabolism to keep their body temperature within a set range that is synched with the natural light/dark cycle. But camels kept in constant darkness or constant light with scheduled temperature fluctuations start to entrain their body temperatures on those thermometer changes instead, El Allali and colleagues reported in a 2013 paper: Their body temperature rose as ambient temperature rose and vice versa, regardless of the lighting. And shifting the timing of the peak ambient temperature shifted their body temperature cycles to match, El Allali found.

Ambient temperature also helps wind two other clock outputs, locomotor activity and melatonin levels, El Allali and fellow researchers reported in the 2013 work and a 2020 follow-up study, which he then replicated in desert goats. And temperature may shape circadian cycles in the Damaraland mole rat as well, according to early results from a group of researchers in South Africa published in March.

These studies are starting to fill in a gap in the chronobiology literature, says Roberto Refinetti, professor of psychology at the University of New Orleans, who was not involved in any of the African mammal work but has done his own temperature studies in mice and ground squirrels.

“The amount of information about light synchronizing circadian rhythms is probably 1,000—if not 10,000, 100,000—times more than temperature,” Refinetti says. But researchers should examine all potential zeitgebers to gain a “full understanding of how animals will behave in nature.”

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hronobiologists have focused on light for several reasons, Refinetti explains: “Light is the most obvious [zeitgeber] for us who have eyes” and it’s simple to study in the lab. “You can buy those little timers to control the lights in a grocery store for $5 and attach your lights to it and you’ll have a light/dark cycle.” Controlling temperature can be more costly, he adds.

Light also has a clear pathway to the brain. The suprachiasmatic nucleus, a region inside the hypothalamus, functions as the circadian clock. Neurons in the retina project directly to the suprachiasmatic nucleus, which “neuroanatomically makes it easy” to study, says Satchidananda Panda, professor of regulatory biology at the Salk Institute for Biological Studies. Panda was not involved in the desert studies but says he has recently begun circadian temperature experiments in mice.

But it’s likely that most mammals can tune in to ambient temperature changes and prioritize that as a zeitgeber when needed, Panda says. “Why not design a system that will sense both temperature and light?” he says. “The system has evolved to use all the sensory mechanisms to sense light and temperature to entrain the clock so that in the absence of one stimulus, maybe the other stimulus will work.”

This kind of flexibility holds true for mice living in less extreme biomes than the desert, for example. In a pair of studies in laboratory mice, Refinetti reported that temperature can entrain the circadian rhythm of locomotor activity, but not as effectively as light.

El Allali’s experiments clearly demonstrate that temperature can set circadian clocks in camels and goats but do not determine which signal is a stronger zeitgeber, says Shin Yamazaki, professor of neuroscience at the University of Texas Southwestern Medical Center. To find out, Yamazaki adds, the researchers could mismatch the light and temperature cues—low temperatures during the day and high temperatures at night—and see which wins out.

El Allali conducted a version of this experiment in the 2013 camel work. Under mismatched light and temperature cues, the ambient temperature entrained the body temperature of all seven camels tested but entrained the melatonin rhythm of only three. More dramatic temperature changes might entrain melatonin in more animals, El Allali says.

Moving forward, he wants to explore whether circadian adaptations are present in smaller desert mammals, such as two species of Egyptian jerboa. He is curious, he says, if the lifestyles of people who live in the desert—sleeping more during the day and staying active late into the night—are partly due to heat-driven clock changes.

“I mean, I’m a veterinarian. I could not do such studies in a human,” El Allali says, but it’s an open question for other researchers to explore.