How large should whales be?

Area of Science:

• Macroevolutionary biology
• Thermodynamics
• Mammalian physiology

Background:

• Terrestrial mammal body size is governed by a tradeoff between selective advantages and extinction risks.
• This tradeoff operates above a minimum size threshold (2 g) for thermoregulation in air.
• Aquatic mammal body size evolution has not been previously analyzed using this tradeoff model.

Purpose of the Study:

• To investigate if the macroevolutionary tradeoff model explains body size distribution in fully aquatic mammals (cetaceans).
• To determine if a universal body size evolution model exists for all mammals, irrespective of habitat.

Main Methods:

• Adapted the terrestrial mammal tradeoff model, a constrained convection-reaction-diffusion system, for aquatic environments.
• Replaced the terrestrial minimum size threshold with a pelagic minimum (approx. 7000 g).
• Validated the model against observed body masses of extant cetacean species.

Main Results:

• The adapted model accurately predicts cetacean body masses without tunable parameters.
• The model accounts for species ranging from small cetaceans to the Blue Whale (175,000,000 g).
• Increased convective heat loss in water shifts the body size distribution upward for aquatic mammals.

Conclusions:

• A universal macroevolutionary tradeoff governs mammalian body size evolution across terrestrial and aquatic habitats.
• The large body sizes of cetaceans are primarily driven by the need to minimize convective heat loss in water.
• The model predicts the maximum expected species size occurs at an equilibrium between niche colonization and extinction rates.