Essay Assignment One

WLF 314

September 10, 2010

While the concepts, advantages, and disadvantages endothermy and ectothermy are well understood by researchers, the scientific community is still unsure about the evolutionary history and origins of endothermy. Some folks believe endothermy occurred through convergent evolution by animals increasing aerobic scope andentering new ecological niches. Other folks believe that endothermy evolved alongside higher aerobic scopes so animals could give better parental care. To help advance the scientific community’s knowledge about the issue, crocodilians are studied today to help support or refute claims about endothermy’s origins. Today, some biologists claim that this group of animals were once endothermic and re-evolved ectothermy. This goes against the traditional claim that today’s alligators and crocodiles descended from an ancestor that was ectothermic.

Watanabe’s 2005 article first started out discussing a little bit of the history of reptilian research. Through the 1940s, 1950s, and 1960s many experiments were preformed on alligators, lizards, and snakes. Research included discovering heat tolerance, observing the impacts of insulation to reptiles, and monitoring how certain snakes curled around their eggs and shivered to generate heat. In 1979, researchers then turned to looking at basal metabolic rates (minimum amount of energy needed to keep organs functioning) of reptiles versus the maximum metabolic rates of those same reptiles on treadmills. Ectotherms normally have a maximum to resting metabolic ratio (aerobic scope) of 5-10, but when the alligators were ran on the treadmill their aerobic scope was 40 (Watanabe 2005). This is a number that you would see in endotherms, however alligators are ectotherms. This puzzled researchers as they looked to explain this phenomenon.

So why would alligators and crocodiles need such a high aerobic scope? Some researchers believe they need the maximum oxygen intake and maximum metabolic rate to be so high so these animals can take care of their young. Often defending, feeding, and ensuring a safe environment for young takes up quite a bit of energy. Having a high oxygen intake rate allows these animals to endure more of these stresses at a higher rate for longer periods of time. And when an animal takes care of its offspring, it allows the offspring to grow quicker than if they had to defend themselves on their own. It also increases the odds of those offspring surviving as well. Thus it has been proposed that endothermy evolved from parental care.

However, some researchers believe that endothermy evolved for niche expansion. Researchers are studying groups of fish that have regional endothermy to better understand this hypothesis. Endothermy in various fish might have evolved separately for separate species when the oceans began to cool as a survival strategy. Additionally, endothermy allows these fish the ability to go into cooler waters to hunt prey because they can generate and conserve their own heat (through insulated tissue, countercurrent heat exchanges, increased myoglobin, etc.). Thus some researchers believe this hypothesis of niche expansion can be extracted to explain endothermy in birds and mammals.

After reading the article, I tend to agree with new research that states that crocodilians at one point had an endothermic ancestor. Their four chambered hearts, large body size, high aerobic capacity, parental care, and other characteristics highly resemble those of endothermic animals. I believe they evolved back to ectothermy to fill in a niche in semi-aquatic areas. I’m not entirely sold on the parential care hypothesis because there are many animals that do watch their young but are ectotherms. However, I do agree with the concluding consensus that more genetic and fossil studies need to be conducted to better understand the origins of endothermy.