E N E R G Y I N ANIMAL ECOLOGY
71
Within a single species there may be seasonal variations (Table I).
Animals about t o initiate a fast are generally high in calorific value.
Analysis of calorific value in pre-pupal and newly emerged adult sarcophagid flies clearly demonstrate that on emergence the flies are again
a t the low modal value for calorific content. The pre-pupal larvae of
Sarcophaga bulhta had 5.914, 11 day pupae 5.399 and newly emerged
adults 5.079 kcal/ash-free g.
Certain species have relatively high values. The three highest values
in our initial survey were those for a Tenebrio larva about t o pupate,
Artemia nauplii that had just hatched and were still laden with yolk,
and overfed laboratory -reared Dugesia tigrina fed on Artemia nauplii.
The specimens t o be burned were initially chosen at random. A tentative explanation was developed after the data from the first seventeen
species (cf. Slobodkin and Richman, 1961).
We might have expected any of three possible distributions a priori :
1. A normal distribution, implying that energetic content of organisms is determined by the overall biochemical similarities known to
exist between almost all species.
2. Broad differences between taxonomic groups might have been
expected since taxonomic groups differ in so many ways that it would
not be too surprising if they differed in energetic content. This is compatible with the biochemical similarities when it is considered that a
biochemical potentiality does not necessarily imply the realization of
that potentiality.
3. A distribution skewed in one direction is the only remaining
possibility.
The observed skewed distribution with a low modal value is explained on the assumption that in general, energy is limiting t o almost
all populations almost all of the time and that this limitation has been
the case throughout evolutionary history. This explanation is supported by evolutionary considerations since selective advantage is
defined by Fisher and others in terms of the intrinsic rate of increase ( m
in Fisher, 1958, r in most ecological literature). That is, there is a clear
selective advantage to reproduction but no clear advantage t o adiposity.
Excess calories will, therefore, be converted t o offspring throughout
evolutionary history.
If energy were not limiting, a normal distribution of calorific value
in organisms or taxonomic differences in energy content might be
expected. The limitation of energy would imply that animals usually
maintain the lowest possible set of biochemical components consonant
with survival. The possible argument that the narrow range of calorific
values is due to biochemical inability to maintain other higher values
is refuted by the occurrence on occasion of energy-rich organisms.
71
Within a single species there may be seasonal variations (Table I).
Animals about t o initiate a fast are generally high in calorific value.
Analysis of calorific value in pre-pupal and newly emerged adult sarcophagid flies clearly demonstrate that on emergence the flies are again
a t the low modal value for calorific content. The pre-pupal larvae of
Sarcophaga bulhta had 5.914, 11 day pupae 5.399 and newly emerged
adults 5.079 kcal/ash-free g.
Certain species have relatively high values. The three highest values
in our initial survey were those for a Tenebrio larva about t o pupate,
Artemia nauplii that had just hatched and were still laden with yolk,
and overfed laboratory -reared Dugesia tigrina fed on Artemia nauplii.
The specimens t o be burned were initially chosen at random. A tentative explanation was developed after the data from the first seventeen
species (cf. Slobodkin and Richman, 1961).
We might have expected any of three possible distributions a priori :
1. A normal distribution, implying that energetic content of organisms is determined by the overall biochemical similarities known to
exist between almost all species.
2. Broad differences between taxonomic groups might have been
expected since taxonomic groups differ in so many ways that it would
not be too surprising if they differed in energetic content. This is compatible with the biochemical similarities when it is considered that a
biochemical potentiality does not necessarily imply the realization of
that potentiality.
3. A distribution skewed in one direction is the only remaining
possibility.
The observed skewed distribution with a low modal value is explained on the assumption that in general, energy is limiting t o almost
all populations almost all of the time and that this limitation has been
the case throughout evolutionary history. This explanation is supported by evolutionary considerations since selective advantage is
defined by Fisher and others in terms of the intrinsic rate of increase ( m
in Fisher, 1958, r in most ecological literature). That is, there is a clear
selective advantage to reproduction but no clear advantage t o adiposity.
Excess calories will, therefore, be converted t o offspring throughout
evolutionary history.
If energy were not limiting, a normal distribution of calorific value
in organisms or taxonomic differences in energy content might be
expected. The limitation of energy would imply that animals usually
maintain the lowest possible set of biochemical components consonant
with survival. The possible argument that the narrow range of calorific
values is due to biochemical inability to maintain other higher values
is refuted by the occurrence on occasion of energy-rich organisms.