The ability to navigate both air and water using powered locomotion is rare. These two fluid environments—air, which is thin and offers low resistance, and water, which is dense and provides high drag—demand vastly different physical adaptations. Mastering flight requires structures optimized for lift, while efficient swimming favors density and a streamlined form for propulsion. Consequently, the number of animal species that have evolved to excel at both flying and swimming is exceedingly small. This rarity highlights the limits of biomechanical compromise in the animal kingdom.
Avian Specialists: The Mechanics of Underwater Flight
The most widely recognized masters of dual locomotion are marine birds known as wing-propelled divers, including the Atlantic Puffin, Common Murre, and various Auks and Diving Petrels. They actively use their wings for propulsion in both air and water. In the air, wings function as airfoils generating lift and thrust. Once submerged, the same appendages transition to hydrofoils, optimized for generating thrust in water.
A bird’s underwater wingbeat cycle is an adaptation of its aerial flight stroke, though the motion is stiffer and shorter. The high density of water allows the bird to generate substantial thrust with a smaller surface area and reduced movement amplitude. This underwater “flight” is effective, allowing birds like the Common Murre to reach depths exceeding 100 meters. Their feathers are shorter and denser than those of purely aerial birds, helping to trap an insulating layer of air while minimizing buoyancy that resists deep dives.
The distinction between a true dual mover and a foot-propelled diver lies in the method of submerged movement. Foot-propelled birds, like cormorants and loons, use their large, webbed feet as paddles, keeping their wings passive during the dive. In contrast, wing-propelled divers utilize the powerful pectoral muscles used for flying to drive their wings for swimming thrust. This reliance on the wing for submerged movement defines these avian specialists.
The Evolutionary Dilemma: Trade-offs in Dual Locomotion
The contrasting physical properties of air and water force a fundamental trade-off on any animal attempting dual locomotion. Aerial flight requires a large wing surface area and a lightweight skeleton with pneumatized, or hollow, bones to reduce density. Conversely, efficient diving demands a small, stiff wing to minimize drag and a higher body density to counteract buoyancy. Birds that both fly and swim must compromise between these opposing forces, limiting excellence in either domain.
Wing-propelled divers possess shorter wings than birds specialized only for flight, which increases the energy cost of flying. To reduce buoyancy and make diving easier, these birds often have less pneumatized bones than their purely flying relatives. This morphological compromise means that dual-locomotion animals are generally not the fastest or most maneuverable species in either air or water, but they achieve a functional balance between the two.
Beyond Feathered Flyers: Insects and Other Dual Movers
Dual locomotion is not exclusive to birds; several insect groups also fly and swim. Aquatic insects like the Predaceous Diving Beetle (Dytiscidae family) are highly adapted for this lifestyle. They possess a smooth, streamlined body shape to reduce drag underwater and feature specialized oar-like hind legs covered in fine swimming hairs for propulsion. These beetles use their membranous hindwings for aerial travel, often relocating when their current habitat dries up.
Similarly, the Water Boatman, a common aquatic bug, is a proficient flyer and swimmer. It utilizes its elongated hind legs as paddles, sometimes carrying a bubble of air trapped against its body to breathe while submerged. When conditions change or food is depleted, these insects take flight to find a better location. A unique mammalian example is the Greater Bulldog Bat, which flies low over the water, using its large, hooked claws to rake the surface for fish. Although it does not perform powered underwater swimming for hunting, this bat is capable of paddling to shore with its wings if it accidentally falls into the water.