The Animal Kingdom contains a diversity of life, but two classes, Aves (birds) and Mammalia (mammals), stand apart from most other vertebrates. The last common ancestor of a bird and a mammal lived over 300 million years ago. Despite this vast evolutionary separation, these two groups share complex physiological and behavioral traits not widely seen in fish, amphibians, or reptiles. These similarities arose largely through convergent evolution, where distinct lineages independently developed comparable solutions to similar environmental and metabolic pressures, resulting in a shared, highly active lifestyle.
Internal Temperature Regulation
The foundation of the bird and mammal lifestyle is a shared system of internal temperature control, known as endothermy and homeothermy. Endothermy is the ability to internally generate heat through metabolic processes, contrasting with ectotherms which rely on external sources. Homeothermy is the ability to maintain a stable body temperature within a narrow range, regardless of external conditions. Mammals typically maintain temperatures between 36 and 38°C (97–101°F), while birds operate at a higher range, often between 40 and 42°C (104–108°F).
This constant, high internal temperature allows enzymes and biochemical reactions to operate at peak efficiency, enabling sustained, vigorous activity. The trade-off is a significantly higher resting metabolic rate, often five to ten times that of a similarly sized reptile. To fuel this high-energy lifestyle, birds and mammals must constantly seek and consume large quantities of food. The necessity of generating heat metabolically dictates other shared traits, including efficient oxygen delivery and the need for insulation.
Highly Efficient Circulation
The demands of endothermy require a powerful and highly efficient system for delivering oxygen to tissues. Both birds and mammals evolved a four-chambered heart that completely separates oxygenated and deoxygenated blood, a structure that arose independently in the two classes. This separation ensures that only blood saturated with oxygen is pumped to the body’s systemic circuit.
The four-chambered design creates two distinct circulatory loops: the pulmonary circuit, which moves deoxygenated blood to the lungs, and the systemic circuit, which moves oxygenated blood to the rest of the body. This arrangement allows for high blood pressure in the systemic circulation, facilitating the rapid delivery of oxygen necessary to maintain the high metabolic rate. Without this complete separation and high-pressure system, neither birds nor mammals could sustain the energy expenditure required for stable, high internal temperature.
Specialized Structures for Insulation
To avoid losing the metabolic heat they generate, both birds and mammals evolved specialized outer coverings for insulation. In mammals, this covering is hair or fur, and in birds, it is feathers. Although these structures are fundamentally different in development and form, they share the same physiological function: to trap a layer of still air close to the body, acting as an effective thermal barrier.
Mammalian fur consists of a dense underfur responsible for most insulation, protected by longer guard hairs. Birds rely on downy, plumulaceous feathers for insulation, which are fluffy and lack the interlocking barbs of flight feathers. Both hair and feathers are composed primarily of the protein keratin. Animals can adjust the position of their covering, such as when a bird “puffs up” its feathers, to increase the trapped air layer. This insulative layer reduces the rate of heat loss, minimizing the energy cost of homeothermy.
Complex Parental Care
Beyond their physiological similarities, birds and mammals exhibit a complex and energy-intensive behavioral trait known as parental care. This shared strategy involves a high investment in a relatively small number of offspring, a pattern comparatively rare among other vertebrate groups. Parental care includes activities like nesting, feeding, teaching, and protection from predators.
In birds, over 90 percent of species involve both parents in raising the young, a practice known as biparental care. This energy-sharing is necessary because parents must feed themselves and provision their fast-growing young to maintain their high metabolism. Mammalian care is universally provided by the female due to the necessity of lactation. However, in about five percent of species, the male is also closely involved in feeding and protecting the young. This prolonged care period requires sustained energy output from the parents to ensure the survival and successful development of their offspring.