Body Size Matters

How animals interact with their environment is strongly influenced by their body size: the relative importance of

Table 1 Examples of the range of body size seen in various taxa

Taxa

Smallest

Largest

Orders of magnitude

Mammal

Suncus etruscus (pygmy

-1.8 g

Balaenoptera musculus (blue whale)

-1801

8 (mass)

shrew)

Bird

Mellisuga helenae (Bee

6.2cm,

Struthio camelus (North African

-2.75 m,

2 (length)

hummingbird)

-1.8g

ostrich)

-156 kg

Tree

Salix herbacea (dwarf willow)

1-6 cm tall

Sequoiadendron giganteum (giant

83.8 m

3 (length)

sequoia)

tall

Fish

Paedocypris progenetica

-7.9 mm

Rhincodon typus (whale shark)

12.6 m long

4 (length)

long

Frog

Eleutherodactylus iberia

-9.8 mm

Conraua goliath (Goliath frog)

-32 cm,

-1-2 (length)

3.3kg

Spider

Patu marplesi (Samoan moss

0.3mm

Theraphosa blondi (Goliath

28 cm

4 (length)

spider)

bird-eating spider)

170 g

Insect

Nanosella fungi (Feather-

0.25 mm

Goliathus goliatus (Goliath beetle)

>110mm

3 (length)

winged beetle)

For roughly cylindrical organisms, mass scales as the cube of length, so a difference of one order of magnitude in length equates to a three-order difference in mass.

For roughly cylindrical organisms, mass scales as the cube of length, so a difference of one order of magnitude in length equates to a three-order difference in mass.

forces like the surface tension of water, and the influence of gravity and temperature differ greatly depending on the absolute size of organisms. A cat cannot walk on water or up a wall, but some insects or lizards can. Likewise, both tree trunks and the limbs of terrestrial animals must be strong enough to provide support against the force of gravity but not so large that they are crushed under their own weight, or interfere with efficient functioning (transport of water and nutrients in the case of a tree, locomotion in the case of the animal). For a quadrupedal animal, ^140 t is the estimated maximum mass before the width of the limbs would have to be so large as to support the weight of the animal without interfering the movement; this value is close to that estimated for the sauropod Argentinosaursus, the largest known terrestrial animal 1001). Aquatic organisms face different constraints. Although the influence of gravity is ameliorated, allowing the evolution of much larger size (^200 t in the case of the blue whale), water has ^24 times the heat conductance of air (0.58 vs. 0.024 Wm-1 K-1). In practical terms, this means that endotherms (animals maintaining a constant body temperature) must expend considerably more energy maintaining homeostasis. This likely limits the minimum body size; the smallest truly aquatic endotherms are ^100 kg, a mass that reflects selection on the ability of neonates to successfully thermoregulate. Because neonate mass is tightly correlated with maternal mass, an adult mass less than 100 kg results in offspring too small to successfully thermoregulate in water.

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