The desert can be, primarily, described in two words: hot and dry. And while the desert often experiences high highs, the temperature at night can vary. The Sahara Desert is one of the main regions where fennec foxes can be found. On average, the daytime temperature of the Sahara is about 100º F, but during the summer it can skyrocket to 125º F. During the winter nights, it can get as cold as -36º F or even -52º F (ThinkMorocco)!
The thermal neutral zone (TNZ) is the range of temperatures where a body, human or animal, can regulate and maintain its core body temperature through regulation. It is when a body falls out of its thermal neutral zone that heat production or heat loss (i.e. sweating) occurs in order to stabilise the core temperature (Kingma et al.). For a fennec fox, their TNZ is between 68 - 89.6ºF (Geffen, Girard pg. 223). Because the fennec fox lives in the desert, they have adaptations in place to help them live and survive the extreme heat.
Due to these extreme ranges of heat, the fennec fox has honed their thermoregulation in order to survive both the heat, but loss of water as well. The fennec fox is a nocturnal mammal, and it is partly by choice. By avoiding direct exposure to the heat during the day, a rough average of about 10 hours of light, fennec foxes are able to conserve water loss (Geffen, Girard pg. 223). This is partially due to the fact that the temperature the fennec foxes experience in their burrows are well within their TNZ, meaning that they can continue to regulate their body temperature (Geffen, Girard pg. 226). To do so, they spend the majority of their day burrowed deep in the sands trying to avoid the heat. It is not until near dusk that they begin their day, and the hunt for food [OpenLearn, 2020]. In order to conserve their water balance, the fennec fox has adapted a strategy. Because water is hard to come by in the desert, fennec foxes have adapted to be able to go without water indefinitely. Almost all of their water intake comes from the food that they do eat. Because they have extremely limited access to water, they conserve the water in their body by producing concentrated urine to conserve their water balance (Geffen, Girard pg. 223). Producing concentrated urine is a key survival mechanism for a few different reasons: not only does it not disrupt their water balance, but it allows them to keep their blood plasma at normal levels (Sands, 2007). Essentially, concentrated urine means that more solutes, such as sodium, are being excreted from the body in relatively little water. By doing so, they are able to lose the amount of water that they just drank.
When looking at the fennec fox, perhaps one of the most notable things about them is their big ears, and while it helps them hear better when looking for food, it also plays a role in regulating their body temperature. During high temperature exposures, researchers noted that the ears of the fennec fox had lower temperatures, indicating that their ears may be key in helping them regulate their body temperature and minimize water loss. This same study, by Maloiy et al. found that at the higher end of the TNZ, the blood vessels in their ears and legs became dilated. Vasodilation allows for there to be more blood flow to the skin, allowing heat to be carried out by the blood. As the heat and blood reach the skin and exit allowing the body to cool (BBC).
It should be noted that the fennec fox also has a low metabolic rate, or rather they burn calories slower when producing energy (Diamond, Mueller). Their basal metabolic rate is only 61% of what it should be for their body size. This trait is important because it also helps them to conserve their water balance, less water is wasted trying to produce energy, and is instead saved for cooling their bodies (Geffen, Girard pg. 226). It has also been noted that fennec foxes pant when temperatures are equal or higher to 104ºF and it is one of the main ways that they are able to cool their bodies down as they exhale hot air. It has also been noted that the feet of the fennec fox are covered in ears, this not also allows them to walk or run through the sandy desert, but it also protects them from feeling the heat on their paws (Gauthier-Pilters pg. 714)
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