**Mammals** are species that can maintain a constant body temperature by generating their own heat when they are in a cooler environment, and by cooling themselves when they are in a hotter environment. They are four-chambered hearts, a closed circulatory system, and paired lungs for efficient respiration.

There are two distinct characteristics separate mammals from other vertebrates, and they are:

A) Presence of mammary gland.

B) Presence of hair on their body for insulation.

Based on fossil records, **mammals** are descendants of reptile species. The ancestors of modern mammals emerged and diverged from the ancient reptile species during the **Carboniferous Period**. However, the initial species of true mammals appeared during the late Triassic period, about more than 200 million years ago. These mammals are characterized and identified as very small.

Mammals have the ability to maintain homeostasis by regulating their body temperature. They have various ways and behaviors which can remove excess heat if the weather is too hot, or they can also produce body heat to insulate themselves during a cold weather. Another way to maintain homeostasis is to filter urea from blood, to excrete excess water, or to retain water through the use of the kidneys.

The **endocrine system** is composed of various organs including the pituitary gland and thyroid gland which function to create hormones that can be used in the maintenance of homeostasis, growth, reproduction, and development of an individual.

The endocrine system works closely with other organ systems because it produces hormones that help other organs to grow, develop, and function very well. Moreover, through these hormones, various feedbacks are created to regulate some physiological functions.

The **circulatory system** is the system that functions in the transport of nutrients including oxygen in the body as well as the pick-up of waste materials for excretion. On the other hand, the **respiratory system** is made up of various organs (including the lungs) that function in the exchange of gases between the body and the environment. The respiratory system takes in oxygen and releases carbon dioxide.

The respiratory system and circulatory systems work together as the respiratory system takes in oxygen for the body, and the circulatory system transports this oxygen all throughout the body, The circulatory system, on the other hand, picks up waste materials like carbon dioxide which the respiratory system releases to the environment.