Environmental Engineering Reference
In-Depth Information
BMR, and normal activities raise the multiple to 1.6-2.1.
The energy cost of growth is 15-35 kJ/g of lean tissue
(about 20 kJ/g in young children). Pregnancy costs
about 27 kJ/g of new tissue. Lactation is energized by
fat deposited during pregnancy but, as with pregnancy,
its cost among some Asian and African populations may
be minimal. Superior metabolic efficiency in nutritionally
limited circumstances is the only plausible explanation.
Energy expenditure for walking is a U-shaped function
of speed, with minima at 5-6 km/h, but the cost of run-
ning remains nearly identical for a wide range of speeds.
This impressive feat is the result of human bipedalism
and efficient heat dissipation. Core temperature is main-
tained by dilation of peripheral vessels and shifting of
blood to feet and hands and, above all, by copious active
sweating. Human sweating rates surpass those of other
efficient perspirers (camels and horses) and can sustain
prolonged exertions of up to 600 W without any rise in
core temperature. Acclimatized individuals can remove
over 1.3 kW of body heat by sweating. In contrast, ther-
moregulation in cold environment relies on increased
BMR and pronounced vasoconstriction in extremities.
The limits of human physical performance are set by hy-
drolysis of high-energy compounds.
Anaerobic processes support very high but necessarily
brief exertions, rating up to 8-12 kW for trained individ-
uals. All long-term efforts are energized overwhelmingly
by aerobic recharge, and the limits of human power are
thus largely a function of maximum oxygen intakes.
Most people can support rates of 600-900 W, and best
performers can go up to 2 kW, or about 25 times the
basal metabolic rate. Among mammals this metabolic
scope is surpassed only by canids. Efficiency of converting
digested food to work is 10%-13% in anaerobic processes
and 15%-20% in aerobic exertions. This means that dur-
ing an 8-h spell the maximum sustained power output of
300-350 W translates into 1.5-2 MJ of useful work at
50-70 W.
Humans have spent more than 99% of their existence
as foragers or shifting crop cultivators. The earliest forag-
ers were most likely only opportunistic scavengers, and
most gatherers and hunters experienced repeated food
shortages. Only those groups that exploited highly pro-
ductive environments spent a fraction of the time needed
by workers of industrial society in order to secure an ade-
quate diet. Prehistoric foragers favored gathering seeds
and nuts (energy density of up to 25 kJ/g) and often
prolific, though less nutritious, roots and tubers. This
was energetically a highly rewarding strategy (net returns
5-15, up to 40). But in most ecosystems seasonal fluctu-
ations of phytomass availability and a variety of natural
hazards made the foraging a deanding experience.
The great diversity of habitats and subsistence patterns
precludes sweeping generalizations, but several energetic
imperatives are obvious. All foraging societies were om-
nivorous, but except for maritime groups, plant foods
were dominant. Much of the plant gathering fits the op-
timal foraging pattern, but other considerations (the need
to secure water, vitamins, and minerals as well as the
presence of large carnivores and competing foragers)
complicated the process. Gathering in forests, where most
of the phytomass is in inedible tissues, required more fre-
quent residential moves. In contrast, the seasonal surfeit
of grains, nuts, and roots in grasslands and shrublands
allowed for fewer, but longer, camp relocations.
Hunting was largely limited to herbivores, and its net
energy returns were often very low, especially in tropical
forests with their small folivorous fauna. Larger grass-
lands herbivores were also preferred because of their
higher fat content (lean wild meat has only 6 kJ/g).
