Not too hot and not too cold: reptiles (ectotherms) in the Kalahari.(Part 5)

The Kalahari desert can get extremely hot in summer & very cold in winter. Snakes & lizards in the desert have to maintain their optimal temperature despite these large changes in ambient temperature.

Behavioral mechanisms play an essential role & when it is cold e.g. early morning, reptiles will expose the maximal body surface to the sun’s rays (radiant heat) or warm rocks (conductive heat) to increase exogenous input. (Figure 1)

Figure 1:Ground Agama basking in the sun to warm up. (click to enlarge

Furthermore some reptiles e.g. the iguana increase their heart rate & divert the flow of core blood to the skin circulation to more rapidly increase their heat uptake.

These mechanisms are successful & reptiles are widespread & dominant species in the tropics. The higher ambient temperatures allow them to be nocturnal. They do not need metabolic energy sources to keep warm & this energy is diverted to other survival strategies e.g. reproduction.

The ectothermic physiology is however not helpful in cold environments & reptiles are much less widespread or even absent in colder areas.

Excess Heat:- There is a critical thermal maximum & above this physiological functions will fail. For example oxygen binding to hemoglobin falls, & as discussed before enzyme function decreases despite the protection by heat shock proteins.

Thus ectotherms must respond to rising temperatures to avoid reaching critical levels. In principle this must be done by decreasing input or increasing output of heat. The main protective mechanism is behavioral. They will seek the shade under plants & rocks, or enter deep cool burrows. Some will increase conductive & convective heat loss by submerging themselves in water or digging deep into the cooler layers of sand.

Figure 2: When it is cold reptiles bask in the sun to increase exogenous heat uptake. When it gets too hot they will seek shade or burrow underground. Behavior is the major mechanism for heat regulation in ectotherms. (click to enlarge)

A marvelous example of how they adapt to the extreme heat of the desert is revealed by the shovel snouted lizard (Meroles anchietae) in Namibia. The sand is so hot this lizard continually “dances’ balancing on its tail & lifting two of its feet every 10 seconds to cool them. If it gets too hot it dives down into the loose desert sand swimming to a cooler level. The streamlined shape of its nose helps it to penetrate the sand. It can survive for 24 hours buried down under using air trapped in the sand. It is interesting to note that it also has a special water bladder, a blind tube branching off from its intestine. There it stores a large reserve of water, obtained by drinking when fog settles on cold nights along the coast of Namibia. Do watch the wonderful BBC wildlife video to see the dance recorded with a thermal imaging camera.

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

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