E5 Thermoregulation and Physical Homeostasis

Topic

A mammal's interaction with its physical environment—whether air, water, or a granular medium like sand or loose soil—generates resistive and propulsive forces in addition to the gravitational forces acting on the body. Each of these three media imposes its own specific type of resistance to the animal's movement.

In air, the aerodynamic resistance the medium exerts against the animal's body increases with the square of its speed; consequently, doubling the speed quadruples this resistance. For mammals that reach the highest speeds—such as cheetahs or greyhounds—aerodynamic resistance can ultimately become the limiting factor on how much faster they can run.

In water, two distinct forms of resistance limit an animal's speed: viscous resistance, which the water exerts against the body sliding through it, and wave resistance, which arises when the body generates waves on the water's surface while moving. At the same time, the water exerts an upward force on the animal's body equal to the weight of the displaced water; this buoyancy reduces the effort the animal must expend against gravity while swimming.

In granular media like sand or loose soil, burrowing mammals and those walking on soft ground encounter resistive forces that increase with both the depth of the foot's penetration into the substrate and the speed of its movement within it; thus, a foot that sinks deeper or moves faster through the substrate meets greater resistance.

The viscous resistance of water—mentioned earlier in the context of swimming—does not affect all aquatic mammals in the same way; a dimensionless number comparing the significance of inertial forces in moving water against the viscous forces that retard it distinguishes between two different regimes. Small aquatic mammals operate primarily in the viscous regime, where viscous resistance outweighs inertial resistance; conversely, large swimming aquatic mammals operate in the turbulent regime, where the inertia of the moving water dominates over viscosity.

Within this same context of swimming, another dimensionless number—relating the frequency and amplitude of the animal's propulsive movement to its forward speed—describes the efficiency of that propulsion: there is a specific combination of frequency, amplitude, and speed that maximizes propulsive efficiency, and aquatic mammals that deviate significantly from this combination swim at a comparatively higher energetic cost.

In small semi-aquatic mammals, such as otters or beavers, an additional mechanism compounds these water-related resistances: friction between the animal's fur and the surrounding water plays a significant role in the total resistance encountered while swimming—a role that is comparatively less important for large aquatic mammals, which have less fur relative to their body size. This completes the picture of resistive and propulsive forces—beyond gravity—that introduced this description of the mammal's interaction with its physical environment.

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