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Thursday, Aug. 16, 2001
WHY REPTILES ARE COOL
Slow and steady wins the dispersal race
By MARK BRAZIL
Second of two parts
Humans have an anthropocentric tendency to look down on "cold-blooded" reptiles. We even use the term "cold-blooded" in a derogatory way to criticize people who seem somehow less than human.
Nevertheless, reptiles do have a few advantages over mammals, including us, and these come the fore when we consider how they have succeeded in reaching remote islands around the world. Just how do they do it?
Their natural dominance over mammals on remote islands is thanks to no special aptitude for island life. Even though reptiles in such locations often fill the ecological niches that mammals would occupy elsewhere, once introduced to islands by people mammals generally do very well. No, the unique abilities of reptiles come to the fore in reaching islands first and becoming established there.
Reptiles are very tolerant of changes in temperature. This may come as a suprise, as, unlike mammals, they are ectothermic, meaning they are unable to regulate their body temperature internally. This, however, makes reptiles more tolerant of variations in their body temperature, which they regulate it by behavioral means using external sources of warmth and coolness.
Each night, a reptile's body temperature slowly falls in tune with its surroundings, so each morning it must regain enough heat to become active. It may soak up energy directly, orienting itself toward the sun so as to expose as much of its body as possible, or it may do so indirectly by draping itself over a warm surface such as a rock.
As the day heats up, the reptile alters its angle to the sun, exposing less of its body. Then, during the hottest part of the day, it seeks out shade, stays inactive, or chooses a spot where a breeze may blow over it. Thus the reptile keeps its body temperature within safe limits.
Being ectothermic has its good points. We endothermic mammals have been slow to catch on to solar energy (many of our global environmental problems are related to this tardiness). Because reptiles obtain so much energy externally from the sun, they are more energy-efficient than mammals of similar size, requiring comparatively less food and water to survive. Their normal diet provides them with much of the water they require, and because of their low energy needs they are able to go for long periods without food.
Finally, reptiles are able to tolerate salt in both their diet and environment. In addition to their kidneys, most have salt-excreting glands, and their skins are tough and relatively impermeable.
So imagine that you are to be swept into a river and out to sea on a fallen tree, and then washed back and forth across an ocean. Which would you prefer to be? A hot-blooded mammal, panting in the heat, needing food and water almost daily, with a low tolerance for salt and mediocre swimming skills? Or a cool, salt-resistant, ectothermic reptile capable of fasting for days, weeks or even months? No surprise that remote islands have so many reptiles and so few mammals!
Being able to survive for long periods without food or water made giant tortoises great survivors, but it also made them a valuable source of fresh meat for boats visiting the Galapagos Islands and Aldabra in previous centuries. In the days before refrigeration, food that could be kept fresh (ie. alive) on board for up to a year without feeding or watering was extremely valuable. Tortoises were upturned into the hold and kept there alive for later consumption.
While giant tortoises were prized as a food supply, early visitors to the Galapagos seem to have been unimpressed by the iguanas. In 1798, Captain James Colnett wrote: "The guanas [sic] are small, and of a sooty black, which, if possible, heightens their native ugliness. Indeed, so disgusting is their appearance, that no one on board could be prevailed on to take them as food."
Even Darwin was repulsed by them, calling the marine iguana "a hideous-looking creature, of a dirty black colour, stupid, and sluggish in its movements.
"The black Lava rocks on the beach are frequented by large (2-3 ft) [60-90 cm] most disgusting clumsy Lizards," Darwin noted. "They are as black as the porous rocks over which they crawl & seek their prey from the Sea. Somebody calls them 'imps of darkness'. They assuredly well become the land they inhabit."
Looks aside, iguanas deserve more credit than these early observers gave them. Iguanas in general are good swimmers, but the marine iguana has physical and physiological characteristics that make it exceptional. Their tails are flattened and they swim by lateral undulation of their bodies, with their limbs held to their sides. Their claws are longer and sharper than those of the land iguana, enabling them to cling to rocks along the shore and resist the pull of heavy waves and the tide.
They are herbivores, feeding on seaweed in the intertidal zone, and they are able to dive down to 5 meters or so for food. Meeting a marine iguana underwater while snorkeling ranks as one of my most memorable wildlife encounters.
Marine iguanas ingest large amounts of salt water while feeding, but they are able to excrete the salt via the salt glands located (as with seabirds) between their eyes and nostrils. They sneeze the concentrated salt out of their nostrils, expelling the waste and also using it as a warning display.
They must still leave the cold ocean regularly to warm up. They prostrate themselves across dark rocks, where their black skin color helps them absorb heat. At night they keep warm by huddling together.
Reptiles' superior ability to survive with little or no water or food for prolonged periods gives them a distinct edge over us mammals in extreme environments.
Deep-space exploration might be easier for reptiles than mammals; for one thing they would need to carry a far smaller load of food and water. With all the gene-splicing going on, future astronauts might benefit from a few reptilian genes! Perhaps calling someone cold-blooded will one day cease to have such negative connotations.
E-mail Mark Brazil at mabrazil@raku Web site: no.ac.jp