Wlf 314
Dr. Rachlow
10 September 2010
Assignment One: Evolution of Endothermy
Were crocodilians once endothermic? In the November/December 2004 issue of
Physiological and Biochemical Zoology, Roger Seymour, of the University of Adelaide argued
against John Ruben, of Oregon State University, and Wilhelm Hillenius, of the College of
Charleston, South Carolina, that crocodilians are actually descended from endotherms and that
they later re-evolved back to being ectotherms (Watanabe 2005). In the paper by Myrna
Watanabe, titled, “Generating Heat: New Twists in the Evolution of Endothermy”, the argument
is discussed about whether or not crocodilians are indeed descendents of endotherms or if their
ancestors have always been ectotherms. The problem is that the ancestors that first possessed
endothermy are now extinct, and only speculation and observations of living organisms can be
assessed to support one side or the other (Watanabe 2005). Experiments involving metabolic
rates and parental behavior have been conducted, as well as observations of other ectothermic
organisms such as pythons and fish that give support both for and against the above stated
hypothesis.
In one experiment in the 1950’s by Raymond Cowles, a zoologist at the University of
California, Los Angeles, fur coats were put on monitor lizards to insulate them, but it turned out
they had no effect on the temperature of the animal (Watanabe 2005). Rather, the extra layer
would inhibit the ability to absorb heat by basking; later, it was deemed that in order
to evolve endothermy, possession of insulation would be a must, nixing the idea of lizards ever
being endothermic (Watanabe 2005). Contrary to this experiment was one carried out in which
the aerobic scopes of alligators were measured, and in conclusion, it was found that their aerobic
scopes were measured too high to be considered ectotherms, yet too low to be endotherms
(Watanabe 2005). Finally, Seymour proved that from their four-chambered heart (that they have
in common with other endotherms), crocodile embryos could separate venous and arterial blood
supplies to create high systolic blood pressure (Watanabe 2005). Along with high metabolic
rates, this gave him reason to believe crocodiles were once endothermic (Watanabe 2005).
I cannot say that I know enough to make a positive decision about whether crocodilians
really were once endotherms and then re-evolved into ectotherms, but based on Seymour’s data,
I can confidently say that it is a great possibility that crocodilians went back to being ectothermic
after being endothermic. It can be compared to the evolution of aquatic mammals: giving a short
and condensed version, it is known that quadrupeds (and later mammalian quadrupeds) evolved
on land from aquatic organisms, and over time, became mammals. Eventually, these terrestrial
organisms found their way back to the aquatic life, only this time, they were more complex and
could produce live young and nurse them underwater. To me, this is a very interesting concept.
What pressures caused these terrestrial organisms to go back to where they first came from after
slowly adapting to terrestrial life, and why was it advantageous? The same kinds of questions
arise in regards to crocodilians being endotherms. Perhaps at one point endothermy was
advantageous, only to later become detrimental, causing a shift to ectothermy. Like Seymour
mentions, the only way in which this issue can be solved is by genetic and paleontological
research and discovery. To me though, it is hard to believe the hypothesis that crocodilians have
only been, and always will be, ectothermic.