Geoscience
Research Institute
Origins 9(2):98-100 (1982).
Amphibians and reptiles have often been considered to be primitive,
and to illustrate steps in an evolutionary pathway to higher vertebrates, the birds and
mammals. However, in a notable recent review Pough (1980) points out that the anatomy and
physiology of amphibians and reptiles are as complex as in birds and mammals, but fit a
different mode of life. They have a system based on low energy flow rather than the high
energy flow of birds and mammals.
Birds and mammals maintain a constant high temperature by a high
metabolic rate; they are called "endotherms" to specify that the source of heat
is internal. On the other hand, amphibians and reptiles (and many other animals), choose a
warm environment at the time of activity, and this supplies the necessary heat; they are
called "ectotherms,'' indicating that the heat source is external to the animal. One
consequence is that an active amphibian or reptile may use less than a tenth as much
metabolic energy as an endotherm! Even when at rest the metabolic rate is only 10-20% of
that of birds and mammals of similar size. Further, in nonactive periods of the day (or
year), the body temperature can also drop, further reducing overall metabolic energy
usage.
Part of what makes this low energy flow system possible is that most of
the energy used for muscular activity is limited to anaerobic metabolism, rather than
aerobic as in endotherms. Anaerobic energy stores (glycogen) are immediately available
within the muscles and hence facilitate bursts of activity. But in many cases the animals
would be completely exhausted by 3 to 5 minutes of maximum activity and could require
several hours to completely regenerate their energy stores. At this point one might ask,
"How then do they manage?" The answer is that bouts of activity are brief,
interspersed with "sitting and waiting" (follow the next frog or lizard you
see). In this way these animals may normally avoid the oxygen debt ensuing from continuous
activity.
This might seem a high price to pay. On the other hand, consider the
benefits of low energy flow (see Table 1). Small endotherms, with their large surface/mass
ratio, lose heat so rapidly that they require more food per unit weight. This explains the
incessant food gathering required by small animals such as shrews. In fact, the metabolic
rate rises so fast with decreasing body size that an endotherm smaller than 5 grams would
have an energy demand impossible to meet. But amphibians and reptiles, with a
weight-specific daily energy requirement of less than a tenth that of birds and mammals,
may have body weights of much less than 5 grams. Over 300 of 5000 species surveyed (Pough
1980) weigh less than 1 gram (!) (calculated from his Table 2). Thus amphibians and
reptiles can occupy a whole size range (i.e., <5 grams) unavailable to birds and
mammals.
TABLE 1. Costs and benefits of ectothermy and endothermy. The items listed are not mutually exclusive: some follow from others in the list. Based on Bennett and Ruben 1979 and Pough 1980.
Ectothermy Endothermy Disadvantages
- Rapid exhaustion
- Activity restricted to brief bouts
- Activity restricted to certain hours or habitats
- Require much food
- Require continuous supply of food, water, and O2
Advantages
- Avoids cost of high basal metabolic rate
- Endure shortages of food, water, or O2
- Can be elongate in shape or tiny in size
- Can live in very sandy desert
- Work capacity many times that of ectotherms
- Capable of sustained high activity
- Can be active at a variety of times of day and in a wide range of habitats
Further, an elongate form is possible (long salamanders, lizards,
snakes). To endotherms, this form would result in prohibitive loss of heat across the body
surface. Again we see the variety possible in terms of thermal physiology.
Or suppose there is a food shortage. Because of their low energy
approach, many ectotherms can go for months without food.
Many species of lizards and snakes can survive the extremes of the
desert even in an area of shifting sand, by burrowing under. A mammal could not get enough
oxygen at the depth required, and a tunnel system in this instance, which might provide
oxygen elsewhere, would collapse.
What benefits do endotherms gain from their costly high energy flow
system? The resting levels of oxygen consumption for endotherms equal the maximum
levels for ectotherms, and the maximum levels for endotherms are 5 to 10 times
the resting levels. Hence the capacity of endotherms for supporting work is many times
that of ectotherms.
Birds and mammals are capable of much higher sustained speeds than
ectotherms and can have a much broader behavioral repertoire than ectotherms because of
the greater range of possible speeds and activities. Furthermore, there is greater
independence in timing daily activity, because of constant maintenance of the high
temperature that provides for maximal oxygen consumption.
In summary, here is yet another example of what has been seen before:
when a phenomenon is studied with enough depth and in enough animals, it may show great
design or value in its own right, rather than primitiveness or progressive evolution. It
also may show more diversity in the underlying design than previously suspected — a
little surviving reminder that Eden must have been a more intriguing place, even
physiologically, than we had imagined.
LITERATURE CITED
[As specific examples of current research see (a) Hulbert, A. and P. Else, 1981, Comparison of the "mammal machine" and the "reptile machine'': energy use and thyroid-activity, American Journal of Physiology 241:R350-R356; and (b) Schall, J., A. Bennett, and R. Putnam, 1982, Lizards infected with malaria: physiological and behavioral consequences, Science 217:1057-1059].
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