Exploring Our World

Similarities and Differences: Reptiles and Mammals

Key Changes Needed:

In order for reptiles to transitionally change into mammals, the following changes would be needed:

1. The development of hair.

2. The development of a glandular system.

3. The development of the ability to regulate body heat.

4. A drastic change in their metabolic rate.

5. A drastic change in their growth rate.

6. The re-location of their limbs.

Reptiles and mammals are similar in that they both have vertebrates; they both have a sophisticated nervous system; and they both have spinal cords.

However, to get a better understanding of the unique natures of both reptiles and mammals, let’s consider how they differ from one another.

1. Reproduction

Reptiles are mostly egg-laying creatures, while mammals, except for the monotremes, give birth to a live young.

2. Body Coverings

Reptiles have tough scales and usually cool and dry skin.   Mammals have fur or hair.

3. Regulating Body Heat

Almost all reptiles are cold-blooded and have no system to regulate body heat.   The only known exception is the Argentinian black and white tegu lizard, which can raise its body temperature ten degrees during mating season.   Mammals can produce body heat.

4. Nursing Young

Female reptiles lack mammary glands, and most species abandon their offspring soon after they hatch.  All female mammals have mammary glands that produce milk, allowing them to nurse their young.

5. Teeth

Reptile teeth are uniform in shape, though they might vary in size.  Reptiles' teeth grow and are replaced continually throughout their lives.

Mammals grow only two sets of teeth, with complex cheek teeth, and they do not continue to grow throughout their lives; the only exception being rodents, which have single pair of continuously growing incisors in each of the upper and lower jaws. Mammals have specialized teeth, such as canines for tearing through meat and molars for grinding food. Reptiles do not.

6. Metabolic Rate

Reptiles have a very low metabolic rate.  Mammals have a very high metabolic rate.

7. Hearts

Reptiles have three-chambered hearts with two ventricles and only one atrium.  Mammals have a heart that consists of four chambers, two ventricles and two atria.

8. Growth Rate

According to most sources, reptiles continue to grow throughout their life, although at a slower pace during adulthood.  Mammals reach a growth limit and stop growing.

9. Glandular System

Reptiles do not have sweat glands.  Mammals have sweat glands.

10. Skulls

Reptiles have a skull with a small brain case.  Mammals have a skull with an expanded brain case.

11. Ears

Reptiles have a single middle ear bone.  Mammals have three middle ear bones.

12. Jaws

The lower jaw of reptiles is made up of multiple bones.  The lower jaw of mammals consists of one bone that is firmly attached to the skull.

13. Pelvis

Reptiles’ pelvic bones are separate.  Mammals’ pelvic bones are fused.

14. Limb Location

Reptiles have a sprawling gait with their limbs emerging horizontally from their body. Mammals have an upright stance with their limbs directly beneath their body.

Two Worldview Interpretations:

Undirected Process Formation 

Life forms share common traits and body parts with other life forms because they had a common ancestor.

Directed Purpose Formation

Life forms share common traits and body parts with other life forms because of similar purpose and design.