Welcome to part two of the keeping cool in the warm summer heat series where Reptiles will be the focus. These somewhat feared and misunderstood creatures can be wonderful to observe and admire whilst out on a game drive. After a period of reduced activity through winter, caused by a combination of cooler temperatures and lack of water, the cold-blooded creatures that we are used to seeing in summer have now re-emerged. Sightings of African Rock pythons in trees, Leopard tortoises crossing the roads and lizards sunning themselves on the rocks are great examples of this.
With the change of seasons, many species of birds and mammals have the ability to migrate or thermoregulate in response to tough conditions whereas Reptiles tend to “go with the flow” and sit tight and wait for better times to get active. This can be for an extended period of time. Referred to as “cold-blooded”, reptiles do in fact produce metabolic heat, which can be misleading. The problem is they can’t produce enough of it to maintain their own body temperature. On average, birds and mammals have metabolic rates that are about ten times that of a reptile of the same body mass.
Metabolic heat is heat generated through biological processes occurring in the body where energy is burnt.
Remote thermography of a trio of lions, picture taken from the work by Craig Packer
Also known as ectotherms, Reptiles do not rely entirely on metabolic heat to maintain their body temperature but take on the temperature of the environment around them. To do this, they have developed certain adaptations that help them maintain a healthy body temperature regardless of the climate.
An adaptation is a physical or behavioural feature of an animal that helps that animal to survive better in its surrounding environment. These features having been passed down from generation to generation by natural selection.
We are going to look at the following animals and discuss the adaptations they are able to use in order to cool down in warm summer weather.
Crocodiles
Regarded as the most famous reptile in the animal kingdom, the crocodile is an incredibly successful species that has been unchanged through evolution for millennia. This success is largely due to its ability to conserve energy and use the surrounding environment to the best of its ability. Driving across the Sand River recently I noticed that a large Nile Crocodile was basking on the bank but with its mouth wide open. A crocodile will regulate its core body temperature through where it positions itself during the day. While in the water, the crocodile will remain cool but by sliding itself onto the banks of the water it allows the sun to warm it.
Once the body temperature has reached an optimum level the crocodile is able to make small adjustments by opening its mouth. This exposes a dense network of blood capillaries situated in the mouth either to the shade if facing away from the sun and allowing the breeze to cool it down through evapotranspiration or by facing the sun and allowing the direct sunshine to warm the mouth.
Another obvious way for a crocodile to cool down is too move back into the cool water, however, this will require more energy which is a precious resource for an animal with a very slow metabolism.
Tortoises
Depending on the extent and duration of the heat, a tortoise will escape this by moving into a shady spot, burying down or by using a burrow. Londolozi has a vast array of habitats where open crests give way to bush willow thickets and deep dry river beds which are lined with tall beautiful trees. There is ample shade available for a hot tortoise.
Tortoises are often seen just before the heavens open over the reserve and the reason for this is that they are highly dependent on water. They utilise an organ known as a bursa sac for storing excess water to help survive during dry conditions. During the summer months, when water is abundant, tortoises are often seen drinking from puddles in the road, assisting with cooling down the body.
Chameleons:
Probably one of the more fascinating animals found at Londolozi, chameleons are mostly active during the day, and therefore exposed to the heat while moving around. In response to this chameleons have the unique ability to change the colour of their skin as a direct correlation to the ambient temperature and to portray their mood as well.
During hotter conditions and being mostly arboreal, they will climb to shadier parts of the tree and change their skin colour to a lighter complexion which reflects the heat from the sun.
They are able to change the colour of their skin because the outermost layer of the chameleon’s skin is transparent. Beneath this are several more layers of skin that contain specialized cells called chromatophores. The chromatophores at each level are filled with sacs of different kinds of pigment. As the chameleon warms up, its nervous system tells specific chromatophores to expand or contract. This changes the colour of the cell to counteract the heat.
The texture and detail of the skin of a chameleon are remarkable.
Why is it important for a reptile to cool down?
‘Thermoregulation’ and ‘Cold Blooded’ are often terms people associate with the need for a reptile to gain heat from its surrounding environment. While this is true, it is probably even more important for a reptile to lose heat through thermoregulation. The majority of reptiles gain heat quicker than they lose it so if a reptile cannot cool down, its body temperature can rise to dangerous levels.
A young crocodile crawls its way out of the shaded pool of water along the Manyelethi River to take advantage of the last bit of heat for the day before nightfall.
A cold reptile moves slower than a warm one and this may put them at a distinct disadvantage when being hunted by birds of prey or mammals.
Another very important process affected by thermoregulation in reptiles is their ability to fight off disease. If a reptile cannot maintain a stable body temperature, it could be more prone to illness.
I hope you enjoyed the second instalment of the Keeping Cool in the Delightfully Summer heat Series. Look out for Part Three!
Nick Tennick
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