November 2008

In the next three monthly essays, I will compare birds and mammals .

First, birds and mammals are recently evolved and are closely related. They share many genes and have the same basic anatomy.

 

Second, birds are very similar in size, habitat, and feeding whereas mammals are extremely diverse. The main reason is that flying involves many special adaptations that limit how birds look.

 

Third, even though we humans are mammals, we bird-watch but do not mammal-watch. We are like birds in that we also are active during the day and use color vision and hearing as our main senses.

Part 1. BIRDS AND MAMMALS ARE CLOSELY RELATED BUT HAVE UNIQUE DIFFERENCES.

All life forms are related, since all life started about 3.8 billion years ago with bacteria-like organisms. Birds and mammals share about 5% of the same genes for cell structure and basic housekeeping functions with these earliest forms of life. As terminal twigs on the same branch of the evolutionary tree of life, birds and mammals share about 60 percent of their genes. And within their own classes, birds share 80 percent of their genes with other birds and mammals share about 80 percent of their genes with other mammals. As human mammals we share 98 percent of our genes with our closest living relative, chimpanzees.

Here is a Linnaean classification of one Refuge bird and one mammal. This hierarchal classification reflects the evolutionary history of organisms. It is based on shared and derived characters.

 
 
From the classification above it is clear that birds and mammals share many features. Especially clear is the basic skeletal anatomy of skull, vertebral column, forelimbs with pectoral girdle, and hind limbs with pelvic girdle. Early embryos of birds and mammals have bones of the limbs that are hard to tell apart. The future front appendages of mammals and wings of birds have the same humerus, radius and ulna, carpals, metacarpals, finger digits, and nails. And the future back legs and feet of mammals and birds have the same femur, tibia and tarsus, tarsals, metatarsals, toe digits, and nails.

The only unique characteristics of birds (Class Aves) are feathers and a hard-shelled egg. Other characteristics that are always found in birds are also found in other organisms. Alligators lay their eggs in a nest. A high body temperature maintained by internal heat production (called endothermy) is also found in mammals (Class Mammalia). Wings and powered flight occur in bats (Class Mammalia, order Chiroptera) and in many orders of insects. Even beaks are also found in turtles, squids, and octopuses. And both whales and humans have distinct calls and songs.

The four unique features of mammals are three ear bones, teeth specialized for different functions, hair, and milk producing glands used to suckle young. Other characteristics that are always found in mammals are also found in other organisms. All mammals except the Monotremes (Echidna and Platypus) give birth to live young, but so do some groups of almost all kinds of vertebrate and invertebrate animals. Striking examples are sharks and rays among fish and aphids among insects. Although other classes of vertebrates, like reptiles and fish, have teeth, mammals are the only class that has slightly different ‘baby’ or ‘milk’ teeth and teeth specialized for different functions. It is true that mammals mainly communicate and find their way around by touch and smell but this is also true of many night-active animals in many invertebrate phyla. And mammals are not the only animals that are very intelligent. Consider the octopus. And parrots and ravens even use tools. The implication of the term "bird-brained" is wrong!

A BIRD IS A BIRD IS A BIRD, BUT MAMMALS ARE VERY DIFFERENT. WHY?

Because they have specializations for powered flight, all birds are very similar in their appearance. Even flightless birds like ostriches and kiwis have obvious wings that they flap. An alien would have no trouble recognizing most different species as birds, such as the smallest hummingbird (.0015 kg) and the largest condor (15 kg).

 

In contrast mammals differ greatly in appearance. In size they vary from from tiny to huge (.002 – 136,000 kg). In habitat and mode of locomotion mammals are aquatic swimmers, amphibious jumpers, terrestrial runners, aerial fliers, and subterranean burrowers. In feeding habits mammals are carnivores, insectivores (including ant-eating specialists), omnivores, frugivores (fruit-eating), sanguivores (blood - lapping vampires), and herbivores (browsers and grazers). In appearance they are incredibly different. Contrast blue whales, manatees, hippopotami, elephants, giraffes, cheetahs, antelopes, horses, kangaroos, platypuses, bats, pandas, and panthers.

 

The small diversity of mammals found at the Refuge are extremely different in size, habitat and locomotion, feeding habits, and appearance. Even those that have the same feeding habits look different. Compare the marsh rabbit and vole, both herbivorous. Contrast the armadillo and raccoon, both omnivorous. Consider our smallest and one of our largest mammals, a burrowing shrew (.002 kg) and an aquatic otter (20 kg), both carnivorous. Douglas Florian describes the two:

REVIEW QUESTIONS
[Note:  Internet Explorer users may need to click the warning at the top of the page to permit ActiveX controls]     

a. One unique feature of birds is powered flight with wings.
b. We say a bird is a bird is a bird because evolution of extreme adaptations for flight has resulted in very similar appearances of all birds.
c. Mammals and birds share about 5% of the exactly same genes with the earliest life that appeared 3.8 billion years ago.
d. Front legs of mammals and wings of birds are so different that they could not have a common evolutionary origin.