Environmental Engineering Reference
In-Depth Information
tively smaller weights over much longer periods of time.
But even for large ungulates the total cost of gravidity is
proportional to maternal M in the same ratio as the mean
for other mammals (M
0
:
7
). The maintenance needs of
the gravid uterus are the dominant gestation costs in all
but the smallest rodents and insectivores, whose very
short gestations and large litters make the growth more
demanding. Among larger mammals at least 60%, among
ungulates some 80%, of energy metabolized by the gravid
uterus is for maintenance (Robbins 1983).
Mammalian milks have a wide range of dry matter
levels, from 8.5% in asses to 64.4% in northern elephant
seals (Oftedal 1984). Ungulates and primates have the
most dilute milks, pinnipeds the most concentrated
(to counteract conductive heat loss of newborn in cold
waters and to deposit rapidly the insulating layer of sub-
cutaneous fat). Most of the solids in dense milks are fats,
whereas in thinner milks sugars dominate. Energy den-
sities are up to 21 kJ/g for seal milk, as little as 1.45
kJ/g in ungulates. Cow's milk rates 2.95 kJ/g, human
milk 2.87 kJ/g, a close match with obvious evolutionary
consequences for domestication of cattle. Larger animals
must have faster growth rates in order to attain their ma-
ture body mass. But as organisms grow, the number of
cells that have to be supplied with energy scales faster
than the capacity of the networks needed to supply them,
and this results in generally valid S-shaped growth curves
(West and Brown 2005). When growth data of all multi-
cellular organisms are rescaled in dimensionless terms,
they follow very closely a universal asymptotic growth
curve (fig. 4.9).
Most of the gain comes obviously during a fairly
linear stage, when the maximum daily gains can be well
approximated as power
4.9
Universal asymptotic growth curve. From West and
Brown (2005).
0.0326M
0
:
75
(in g/day). Notable departures from the
trend are the fast-growing pinniped carnivores and
the slow-growing primates. The energy content of these
gains ranges widely as a function of different concentra-
tions of body constituents. Mammalian neonates average
about 12% of protein and 2% of fat, but gray seals have
9%, guinea pigs 10%, and humans 16% of fat and hence
an extraordinarily high energy density of 8.75 kJ/g at
birth, compared to typical mammalian values of 2.9-3.6
kJ/g. The conversion efficiency of these gains is remark-
ably similar at the earliest growth stage, averaging about
35%.
4.5 Heterotrophs in Ecosystems
Mass-related energy needs of heterotrophs must be re-
flected in the densities with which they are encountered
functions of adult M. The
general
relation for placental mammals
scales
as
