Not too hot and not too cold: mammals (endotherms) in the Kalahari. (Part 6)

The Kalahari Desert can get extremely hot in summer & very cold in winter. Large & small mammals in the desert maintain their optimal temperature despite these large changes in ambient temperature. As we have previously discussed in the reptiles behavioral mechanisms also play an important role in mammals. But their own endogenous heat production allows a more successful adaptation to cold conditions.

Mammals of the Kalahari usually maintain their core temperature in a narrow range ((37-38oC) i.e. they are Homeotherms. The temperature of their skin & extremities varies much more widely with changes in the ambient temperature. Their basic metabolic rate is higher than that seen in reptiles. This endogenous heat input together with physiological adjustments in heat output allows them to maintain constant body temperatures in a range of ambient temperatures as the seasons change in the Kalahari.

Thermal Neutral Zone:- At moderate environmental temperatures  heat input from both exogenous & endogenous sources  balances heat output in mammals. In this “Thermal Neutral Zone” the animal does not need to expend much energy to maintain its body temperature. These “small” adjustments include:

a) Regulating skin blood flow will influence heat loss or gain by radiation, convection, & conduction

b) Postural changes to alter body shape or orientation towards the sun will alter heat loss or gain by radiation, convection, & conduction

c) Using pilomuscles to raise or lower fur to change amount of insulating air that is trapped i.e. lower or raise heat loss respectively. Interesting to note that “goose bumps” in humans are the vestige of this mechanism.

Evaporative heat loss:-  Heat loss through evaporation will occur in all animals through the lungs. Evaporation through the skin will vary in animals with different skin thickness & fur. Many mammals sweat but it seems that only man & horses have sufficient sweat glands to effectively cool down. So many mammals use panting to increase evaporative heat loss from the respiratory tract e.g. dogs & cats such as lions. Note that cooling via evaporation will result in water loss & dehydration can occur unless water uptake is increased to maintain the balance in water input & output.

Radiant, conductive, & convective heat loss:- As long as ambient temperature is lower than the body surface temperature heat loss will occur. The core heat will be transferred to the skin by the peripheral circulation & as discussed in reptiles this can be actively regulated i.e. vasoconstriction when it is cold & vasodilation when it is hot.

The amount of heat lost or gained via these routes will depend on the animal’s surface area & this is relatively greater in small mammals. They have less tissue volume to distribute & store the heat in. Thus they can rapidly overheat or cool down.

Extremely hot:-  The Cape ground squirrel (Xerus inauris) can spread out its tail into a wide parasol with the white ventral surface facing upwards to shade itself from radiant energy. But if it is still too hot it will disappear into its underground burrow to cool off e.g. conductive loss into the sand. When the surface temperature in the Kalahari reaches 43oC the burrows deeper than 60cm have a temperature around 30oC. Small mammals do not sweat but they use grooming with saliva to cool down. They spread saliva over limbs & body parts. The method is less effective than sweating (reference 1).

The larger mammals have to adopt different behavioral strategies to decrease exogenous heat input & increase heat loss. They will avoid radiant heat by seeking shade e.g. wildebeest, & bathe if pools e.g. elephant, gemsbok, zebra, are available to cool down by conduction & convection in the water & evaporation when they emerge (Figure 1).

Figure 1. Large mammals also use behavioral strategies to keep cool. As well as bathing, standing in the shade & wallowing in the mud are commonly seen. (click to enlarge)

Warthogs love to wallow in the mud.

Mammals also open or shut “heat windows” to regulate heat loss by altering the peripheral circulation to areas with little or no hair e.g. the large ears of hares & elephants, or the ventral body of the gemsbok. There is an interest theory that the stripes of the zebra may aid in cooling. Differential temperatures may build up over the black & white stripes inducing the flow of small cooling convection currents.

Another approach to coping with heat in the desert is heterothermy i.e. allowing the temperature to fluctuate (see part 3 of this topic in the blog index). As discussed above large endotherms have relatively low surface to mass ratios & large heat capacities. This topic has been extensively studies in camels but a similar mechanism is present in gemsbok & eland. The large mass acts as a heat buffer & the heat that the slowly gain in the day is slowly lost in the cooler nights. This advantage is further increased because they can tolerate higher core temperatures in the day & lower levels at night i.e. heterothermy.  Thus a low start point & high end point each day significantly increases the amount of heat they can absorb without affecting heat sensitive physiological functions.

Carnivores expend a tremendous amount of energy when hunting & their endogenous heat input can become excessive. This is one reason lions & leopards prefer to hunt when the sun is down. Interestingly, cheetahs do hunt at midday. They do this because there is less chance they will lose their prey to the larger cats, which are then resting. They are however limited by the dramatic rise in temperature, & must catch their prey e.g. springbok within a few hundred meters of starting the chase to prevent overheating.

Extremely cold:- Analogous to the situation in preterm human baby, it is the small mammalian species that have trouble coping with the cold of the Kalahari. They will try to capture as much exogenous heat as possible from the weak winter sun e.g. sitting on top of rocks or termite mounds. They will also need to need to increase their endogenous heat production at a time when food is scarce & attempt to decrease heat loss. They lose a lot of heat by radiation, convection, & conduction via their relatively large surface areas. Some e.g. hottentot golden mole limit their exposure by restricting activity to shorter more intense periods & also grow a thicker fur. Others limit losses by nest building, huddling or layering (lying on top of each other).

Heterothermy: is also practiced by small African mammals. The largest in South Africa is the hedgehog. Other examples are the golden mole, rock elephant shrew, dormouse, rock mouse, & mole rat. By decreasing their body temperature below normal summer values (38oC)& temporarily defending much lower winter set points (e.g. 25oC), these small mammals decrease their energy needs to a fraction of what they would otherwise require. The figure 2 shows a) homeothermic temperature balance in summer with endogenous heat input matching the exogenous output. However in winter the figure b) illustrates that heat output increases dramatically without compensation by endogenous input & the core temperature is allowed to fall to a lower level.

Figure 2: shows how small mammals adapt to extreme cold by converting from homeothermy to heterothermy. During the winter they allow their core temperatures to fall from the normal 38°C to around 25°C. They thus spare their sparse energy reserves. (click to enlarge)

See excellent review on African heterotherms by Andrew McKechnie & Nomakwezi Mzilikazi (3) & general discussion in Merrit’s book on small mammals (1) & Ekherts Animal Physiology (2).

References:

1. Merrit JE. The biology of small mammals. John Hopkins Univ Press 2010. pp. 147-207.

2. Randall D et al. Eckhert Animal Physiology; Mechanisms & Adaptations. WH Freeman & Co 2002.

  1. 3.  McKechnie AE, Mzilikazi N. Heterothermy in Afrotropical Mammals and Birds: A Review. Integ Comp Biol 2011;51:349.

 

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