Mammals and birds are two of the most widespread and well-known classes of vertebrates. While mammals and birds share some similarities as vertebrates, including being multicellular organisms with complex organ systems, there are a number of key differences between these two classes of animals.
Body Temperature Regulation
One major difference between mammals and birds lies in how they regulate their body temperature. Mammals are endothermic, meaning they are able to generate their own body heat and maintain a relatively constant internal body temperature regardless of external temperatures. Birds are also endothermic, but they maintain higher average body temperatures compared to mammals. The average body temperature for birds is around 105°F, while for mammals it is around 100°F.
Birds and mammals also rely on different physiological and anatomical adaptations to regulate their high body temperatures. Mammals utilize insulation from fur or blubber, as well as shivering and non-shivering thermogenesis in brown adipose tissue to generate heat when needed. Birds maintain feather coverage and can alter blood flow to their legs and feet to regulate heat loss. Their small body size and relatively large surface area also allows for more effective heat dissipation.
Reproduction
Mammals and birds exhibit some clear differences when it comes to reproduction. Nearly all mammals give live birth to young after a relatively long gestation period. The exceptions are monotremes like platypuses and echidnas, which lay eggs. Birds uniformly rely on egg-laying for reproduction. Avian egg shells and adaptation like the hard-shelled eggs and brooding behavior provide protection and insulation for the developing chicks.
Another difference is that most mammal mothers nourish their young through lactation after birth. Baby birds are fed by their parents with regurgitated food after hatching, but do not consume milk. Birds also show much greater diversity of parental care strategies, ranging from precocial chicks able to feed themselves to altricial nestlings requiring extensive feeding and brooding.
Limb Differences
The forelimbs of mammals and birds have evolved for very different uses, which is reflected in clear anatomical differences. Mammals posses forelimbs designed for grasping, digging, flight in the case of bats, or a variety of other functions. Bird forelimbs are highly adapted for flight as wings, with feathers, stiff lightweight bones, and powerful flight muscles.
The hind limbs of mammals and birds are more similar in basic design, but still show adaptations for their respective modes of locomotion. Mammals use hind limbs specialized for running, jumping, climbing, swimming, or other motions. Avian hind limbs are adapted for bipedal walking, perching, swimming, and other functions, with scales instead of hair covering the legs in most species.
| Difference | Mammals | Birds |
|---|---|---|
| Body temperature regulation | Endothermic; maintain around 100°F body temperature | Endothermic; maintain around 105°F body temperature |
| Reproduction | Give live birth after long gestation; nourish young with milk | Hard-shelled eggs; no lactation |
| Forelimbs | Adapted for grasping, digging, flight in bats, etc. | Modified into wings for flight |
| Hind limbs | Adapted for running, jumping, climbing, etc. | Adapted for bipedal walking, perching, swimming, etc. |
Skeletal Differences
There are also clear differences between mammals and birds when it comes to skeletal anatomy. Mammals have bones derived from forelimb bones that birds lack, including the collar bone, shoulder blade, radius, ulna, carpals, metacarpals, and phalanges of the forelimb adapted as a grasping organ. Birds of course have a number of bones in the forelimbs modified into the skeleton of a wing.
In the hind limb, a major difference is that birds have a tibiotarsus derived from the fusion of what are separate bones in mammals, the tibia and fibula. Birds also lack an external tail composed of caudal vertebrae, instead having a fused pygostyle as the anchoring point for flight feathers.
Brain Anatomy
There are some clear anatomical differences between the brains of mammals and birds reflecting their different evolutionary histories and functional requirements. Both mammals and birds have highly developed brains compared to other vertebrates, but with distinct structures in several regions.
Mammals possess a neocortex, a brain region associated with higher cognition that is absent in birds. Birds instead have a pallium with different nuclei that support complex behaviors and learning. The cerebellums of mammals and birds also differ in the microscopic structure of folds in the gray matter cortex.
Digestive Systems
The digestive systems of mammals and birds show distinct specializations as well. In mammals, digestion begins in the mouth with saliva and chewing. Food passes through the esophagus to the stomach, where gastric juices break it down further. Birds lack teeth and chewing, swallowing food whole or in large pieces. Birds possess a multi-compartment stomach specialized for grinding food, with a muscular gizzard containing swallowed stones or gravel.
Another difference is that in birds the small intestine leads to the cloaca, a common cavity into which the digestive, reproductive and urinary tracts empty. Mammalian systems have separate openings for the rectum containing the intestines and the urogenital system.
Respiratory Systems
Mammals and birds also utilize different respiratory systems. Mammals breathe using lungs, which are ventilated by negative pressure as the diaphragm and intercostal muscles move the ribs to inhale. Birds lack a diaphragm and instead have air sacs and unidirectional flow of air through the lungs. Inspiration is driven by positive pressure as air flows through the lungs on both the inhalation and exhalation.
Another difference is that birds have tubular, perforated lungs. Mammalian lungs have a spongy texture and are divided into isolated lobules. The efficiency of the avian respiratory system enables the elevated metabolic rates required for flight.
Circulatory Systems
There are several distinct differences between the circulatory systems of birds and mammals.
Birds have nucleated red blood cells, while mammalian erythrocytes lack nuclei. Birds also have thrombocytes that are similar to white blood cells, while mammals possess platelets as their clotting cells.
A key difference is that birds have a four-chambered heart with complete separation of oxygenated and deoxygenated blood, similiar to mammals. However, the avian system has two aortas emerging separately from the left ventricle. Mammals possess a single systemic aorta.
Another major difference is that birds have additional pulmonary capillary beds where gas exchange occurs within the air sacs. Mammals do not have this feature, with gas exchange limited to the surfaces of the alveoli.
Sensory Differences
Mammals and birds differ in certain aspects of their sensory systems as well.
Most mammals have an excellent sense of smell, detected by olfactory nerves and bulbs in the nasal cavity. Most birds have a relatively poor sense of smell, with small olfactory bulbs. However, there are some exceptions, as tubenosed seabirds can detect very dilute scents.
Birds possess excellent color vision compared to most mammals. Avian retinas have four types of color-sensitive cone cells, enabling tetrachromatic vision. Most mammals are dichromats, with some primates as trichromats.
On the other hand, birds generally do not hear well at low frequencies and are less sensitive to sound texture and timbre compared to many mammals. However, birds do have superior detection of high frequency sounds.
Conclusion
In conclusion, mammals and birds exhibit a number of substantial differences across diverse aspects of their anatomy, physiology, reproduction, and sensory systems reflecting 300 million years of divergent evolution. While they share characteristics of endothermy, complex brains, and advanced social behaviors, the specialized adaptations of birds for flight and other avian traits clearly separate them from their mammalian counterparts.
