ENERQY I N ANIMAL ECOLOGY
91
caught. Ivlev indicates (1961). however, that when fish that show sizediscrimination in their feeding are presented with prey of various sizes,
they choose the largest individuals. It might again be argued that this
is adaptation t o acqitiriiig the greatest food energy per unit energy expenditure in limiting and this could not be denied. Critical field evidence is lacking.
Examined from the standpoint of the prey, increasing the relative
population efficiency of those classes of individuals that are normally
taken by predators obviously is of selective advantage t o the prey.
This can be done most feasibly by altering the age distribution of
reproductive activity so as to increase the reproductive value (as
defined by Fisher, 1958) of those animals not taken by predators and
minimize the reproductive value of those normally taken. The persistence in the population of old animals of low reproductive value
will itself presumably be selected against t o save the cost of their
maintenance should predation fnil t o remove them. This argument
may be of significance in connection with Medawar’s suggestions (cf.
Comfort, 1961) on the selective significance of age at physiological
death. I would not be surprised if a rigorous theoretical translation
between reproductive value and the inverse of population efficiency
could be stated but I cannot do this at present.
There is no clear reason why population efficiency should be expected t o have any particular constancy from one species or situation
t o the next except for a general increase with age and even this may
not be monotonic.
Population efficiency was also evaluated on a set of sixteen Hydra
oligactis populations fed on nauplii of Artemia sp. While the relation
between P and P, in Hydra did not neatly conform to Eq. (13) it is
equally impossible t o claim that the general applicability of the equation is denied by the Hydra (Fig. 5 ) .
The population efficiency and maintenance cost of the Hydra populations is also indicated in Table 111. Energy income for the Hydra
was assumed t o be the energy content of the Artemia nauplii that were
provided as food. The similarity between the population efficiency of
Hydra yield and Daphnia egg yield is probably fortuitous, despite the
tempting hypothesis that the relatively simple anatomy of a Hydra
is more similar to an egg of a Metazoan than to anything else. There
is a great paucity of data on the metabolic cost of complexity. Unpublished data collected by Miss Jill Claridge in my laboratory indicate
that the total energy expenditure of a frog embryo between the time
of fertilization and the time of initial heartbeat (c.145 h) is less than
5% of the energy present in the newly-fertilized egg. Energy loss is
considerably more rapid after the heart starts beating. If this result
91
caught. Ivlev indicates (1961). however, that when fish that show sizediscrimination in their feeding are presented with prey of various sizes,
they choose the largest individuals. It might again be argued that this
is adaptation t o acqitiriiig the greatest food energy per unit energy expenditure in limiting and this could not be denied. Critical field evidence is lacking.
Examined from the standpoint of the prey, increasing the relative
population efficiency of those classes of individuals that are normally
taken by predators obviously is of selective advantage t o the prey.
This can be done most feasibly by altering the age distribution of
reproductive activity so as to increase the reproductive value (as
defined by Fisher, 1958) of those animals not taken by predators and
minimize the reproductive value of those normally taken. The persistence in the population of old animals of low reproductive value
will itself presumably be selected against t o save the cost of their
maintenance should predation fnil t o remove them. This argument
may be of significance in connection with Medawar’s suggestions (cf.
Comfort, 1961) on the selective significance of age at physiological
death. I would not be surprised if a rigorous theoretical translation
between reproductive value and the inverse of population efficiency
could be stated but I cannot do this at present.
There is no clear reason why population efficiency should be expected t o have any particular constancy from one species or situation
t o the next except for a general increase with age and even this may
not be monotonic.
Population efficiency was also evaluated on a set of sixteen Hydra
oligactis populations fed on nauplii of Artemia sp. While the relation
between P and P, in Hydra did not neatly conform to Eq. (13) it is
equally impossible t o claim that the general applicability of the equation is denied by the Hydra (Fig. 5 ) .
The population efficiency and maintenance cost of the Hydra populations is also indicated in Table 111. Energy income for the Hydra
was assumed t o be the energy content of the Artemia nauplii that were
provided as food. The similarity between the population efficiency of
Hydra yield and Daphnia egg yield is probably fortuitous, despite the
tempting hypothesis that the relatively simple anatomy of a Hydra
is more similar to an egg of a Metazoan than to anything else. There
is a great paucity of data on the metabolic cost of complexity. Unpublished data collected by Miss Jill Claridge in my laboratory indicate
that the total energy expenditure of a frog embryo between the time
of fertilization and the time of initial heartbeat (c.145 h) is less than
5% of the energy present in the newly-fertilized egg. Energy loss is
considerably more rapid after the heart starts beating. If this result