Four Key Physical Properties
- Water and air have very different physical properties.
- These differences profoundly affect how animals move, maintain body temperature, and expend energy in aquatic versus terrestrial or aerial environments.
Buoyancy
Buoyancy
Buoyancy is the upward force exerted by a fluid (liquid or gas) on an object immersed in it.
- In water:
- Water is dense, providing significant buoyant force.
- Organisms with similar density to water experience strong upward support.
- Less energy needed to stay afloat and counteract gravity.
- In air:
- Air is far less dense than water.
- Provides negligible buoyant force.
- Organisms must actively generate lift (e.g., flying) or support their weight on land.
In water, organisms don’t need as much energy for structural support (e.g., bones) as terrestrial organisms, thanks to buoyant forces.
Viscosity
Viscosity
A measure of a fluid's resistance to flow, determined by the internal friction between its molecules. Higher viscosity indicates a thicker, slower-flowing fluid (e.g., honey), while lower viscosity indicates a thinner, faster-flowing fluid (e.g., water).
- In water:
- Water has viscosity about 50 times greater than air.
- Aquatic organisms experience high drag forces while moving.
- Must overcome significant resistance to swim.
- In air:
- Air has low viscosity, creating minimal resistance.
- Allows relatively frictionless movement.
- Imagine walking through a swimming pool versus walking through an empty hallway.
- The water “pushes back” against your movements, slowing you down.
- This is due to its higher viscosity compared to air.
Thermal Conductivity
Thermal conductivity
Thermal conductivity is the rate at which heat transfers through a substance.
- In water:
- Water has high thermal conductivity: conducts heat away from the body efficiently.
- Warm-blooded animals lose heat rapidly in water.
- In air:
- Air has low thermal conductivity: poor conductor of heat.
- Animals retain body heat more easily.
The high thermal conductivity of water makes aquatic environments more thermally demanding for warm-blooded animals, requiring specialized adaptations like blubber or fur.
