Environmental Engineering Reference
In-Depth Information
4.8 (a) Basic food-searching strategies (O'Brien, Bowman,
and Evans 1990). (b) Scaling of the daily movement distance
of mammals (Garland 1983).
height, is the objective of kangaroo jumping. Large
Achilles and tail tendons store elastic energy for highly
efficient locomotion. Whereas the energy cost of penta-
pedal gait (four limbs plus tail) rises rapidly with speed,
once kangaroos start hopping (@6 km/h), the cost levels
off, even declines, and at about 17 km/h it becomes
lower than the expenditure of equally massive running
quadrupeds (T. J. Dawson 1977). Most of the energy
invested in animal locomotion is in the search for food.
The two extreme searching strategies are cruise (or
widely ranging) and ambush (sit and wait) (fig. 4.8).
Many animals fit neatly into these categories, hawks
and tuna in the first one, rattlesnakes and herons in
the second, but most foragers exhibit saltatory patterns
of movement, alternating pauses and moves and fall-
ing somewhere between true cruisers and ambushers
(O'Brien, Bowman, and Evans 1990). As they graze or
hunt, many animals seem to be trying to maximize their
total energy gain per unit of foraging time. In times of
plenty, grazers select feed with higher energy density,
and predators reduce pursuit of elusive prey.
Widely influential optimal foraging theory initially per-
ceived such behavior as one of the great natural laws;
its proponents believed that optimal foraging maxi-
mizes animal fitness and plays a critical role in natural se-
lection (Stephens and Krebs 1987). Critics have pointed
out that foraging behavior is shaped by other factors than
just searching for and consuming food; the necessity for
constant antipredator vigilance is perhaps the most criti-
cal concern for most grazers. The life of heterotrophs
cannot be simply partitioned into independent activities.
Optimal foraging is not one of the universal energetic
