In female mammals, their mammary glands secrete milk which nourishes the young mammals.

C. mammary gland

About 220 million years ago in the late Triassic Period, the first true mammals have emerged.

C. Triassic Period

Bats are the only mammals which are capable of flying. They are members of the order Chiroptera.

A. bats

Diaphragm is a muscle that separates the chest and the abdominal cavities. It helps improve the respiration of mammals.

A. diaphragm

Another way for mammals to maintain homeostasis is to filter urea from blood, to excrete excess water, or to retain water through the use of the kidneys.

B. kidneys

Similar to reptiles, monotremes have a cloaca that serves as an outlet into which the digestive, reproductive, and urinary systems open.

C. cloaca

Marsupium is the protective pouch found in female marsupials wherein their newborn marsupials are placed and carried in order to nourish and to continue its growth and development.

A. marsupium

Marsupials lack placenta. Instead, they have marsupium to nourish their newborn young. Marsupium is the protective pouch found in female marsupials in which their newborn marsupials are placed and carried in order to nourish and to continue its growth and development.

B. marsupials

The prosimians and anthropoids are the two groups of primates that evolved from a common ancestor.

D. prosimians

The only hominid species that exist in Earth today is the Homo sapiens.

a. one

Three ways in which mammals preserve body heat are:

1. Generating internal body heat by having a high metabolic rate
2. The presence of the integumentary system, protects the body from the environment and helps in trapping heat through the presence of hair and coats.
3. The presence of a subcutaneous layer of fat which is beneath the skin also traps through insulating the body.

The endocrine system is in charge of regulating body activities by secreting hormones. Hormones are carried into the tissues and organs of the body by the bloodstream.

The **immune system** is composed of various organs, cells, and molecules that work together in order to protect the body against an antigen.
The immune system recognizes these antigens in the form of bacteria, viruses, toxins, etc., and fights to keep our body in balance.

The immune system is like a castle, the outer gates are like the *nonspecific immunity* which acts as the barrier and the first line of defense against the enemy (antigens). The inner gates and doors will be the *specific immunity* containing the best soldiers and fighters (compared to T cells, antibodies, and B cells) that specifically target and kills the enemies to protect the body which is the main castle

In the course of evolution, mammals have evolved various feeding mechanisms in order for them to adapt and survive the changing environment. The form and functions of their teeth and jaws are connected to their feeding adaptation. **Carnivorous** mammals have sharper and pointed canines and incisors in order for them to grip, slice, and chew the meat of their prey. On the other hand, **herbivorous** mammals have flat-edged teeth that can easily grasp and tear grasses or other vegetation.

When a mammal inhales air, the chest cavity expands. As the air goes inside the lungs, the diaphragm pulls the chest cavity downward to increase the surface volume. Then, as the air goes out of the lungs, the diaphragm relaxes and the volume of the chest cavity starts to decrease.

The **kidney** has a role in maintaining homeostasis through the removal of wastes and excess water from the body and by the regulation and control of the pH of the blood. Nephrons are the functional filtering units of the kidney.

Mammals have the most well-developed cerebrum of vertebrates and the largest brain-to-body size ratio. This enables mammals to think and learn and respond to stimuli in a more sophisticated manner

**Mammals** have evolved and adapted a wide range of mechanisms for movement. Mammals have well-defined backbones that can flex vertically and laterally which enables them to move with a bouncing stride. They have strong pelvic girdles that permit their limbs to move in many different ways.

**Monotremes** are a group of highly unique egg-laying mammals. The young ones of the monotremes get their nourishment as they lick the milk from their mother’s surface abdomen.

**Marsupials** are groups of mammals that have pouches for the nourishment of their young ones. Marsupials do not have a placenta and the young are born prematurely, the rest of the nutrition comes from the mother’s teats and pouch

**Placental mammals** have babies nourished by the *placenta* as they develop in the uterus of the parent, In placental mammals, the embryo develops much longer inside the body of the mother, and as it passed birth it will be nourished through nursing.

The placenta enables the embryo a consistent source of nutrition while the embryo develops fully in an enclosed shock absorbent environment. This increases the chances of the embryo surviving.

Convergent evolution has resulted in striking similarities in different mammals that are living in a similar environment or ecological niche. One example is the evolution between a flying squirrel (a placental mammal) and a sugar glider (a marsupial). Their ancestors were known to be separated millions of years ago when the continents drifted. Flying squirrels have occupied niches in North America. On the other hand, sugar gliders occupied niches in Australia and its surrounding islands. Through convergent evolution, these animals have both acquired wing-like structures that enable them to glide on the treetops, which are their habitat.

Mammals have a highly developed sense to detect and respond to given stimuli. Many primates, for example, have a flat face that caters to both eyes facing forward creating overlapping fields of view. This facial structure enables the primates to have good binocular vision and in-depth perception.

**Primates** are animal species that have good binocular vision, a well developed-brain, and long fingers and toes used for the efficient movement from one place to another.

Primates have flexible fingers and toes that can curl and grasp around objects, enabling them to run along trees, and swing from branch to branch swiftly. Their arms are also well adapted for climbing, and their thumb and big toe are well adapted to hold objects securely using their hands and feet.

Hominids have a larger brain inside a skull that is attached to the S-shaped spine. They have a bowl-shaped pelvis and thigh bones that are faced inward. They have arms that are shorter than the legs and hands that evolved into an opposable thumb that allowed them to grasp objects. In addition, their legs are used for walking on the ground, which improved their locomotion and enabled them to become bipedal.

Examples of hominids are Ardipithecus, Australopithecus, Paranthropus, Kenyanthropus, and Homo. Under the genus Homo is the species called Homo sapiens, which is known as the modern human.

Mary Leaky led the discovery of the first upright-walking hominids by the evidence she found in fossil footprints from a volcanic ash in Laetoli, Tanzania. This fossil evidence is called Laetoli footprints. This discovery shed light on the early hominids that walked upright using their two legs.

With the help of fossils, it was found that there was a species closely resembling humans. Due to their similarity with humans, they Kept this species in the genus Homo. They called this species *Homo habilis*. Homo habilis lived about 2.33 to 1.4 million years ago in the Gelasian Pleistocene era. It was believed that this species first arise in Africa.

Based on the fossil records, researchers called the earliest known species of the genus Homo Homo habilis. which means “handyman.” They called it a handyman because fossils of Homo habilis were found with tools that were made of stone and bone.

Differences between the two flowcharts:

Flowchart 1 tells us that P. robustus, Homo habilis, Homo erectus, and Homo sapiens all shared a common ancestor which is the A. africanus, which evolved from Australopithecus afarensis.

On the other hand, flowchart 2 tells us that Australopithecus afarensis diverged into two species – A. africanus and Homo habilis. From these two species, different hominids emerged. These are P. robustus, Homo erectus, and Homo sapiens. Only P. robustus descended from A. africanus, while H. erectus and H. sapiens came from Homo habilis.

Newborn Placental Mammal:

The embryo develops inside the mother with the help of the placenta for nourishment. After birth, the mother cares for the young by nursing.

Newborn Marsupial:

Marsupials lack a placenta. Instead, they have marsupium to nourish their newborn young after birth. Marsupium is the protective pouch found in female marsupials in which their newborn marsupials are placed and carried in order to nourish and to continue its growth and development.

Dogs have a well developed sense of hearing that allows it detect sounds of different frequencies and sound from far away. Dogs use this information in order to respond to its environment and survive.

Monotremes lay eggs like reptiles but nurses the young from mammary glands like mammals. The monotremes urinary, digestive and reproductive system all opens out to a cloaca, just like the reptile’s cloaca. That is why scientist say montremes are the first mammals evolved from reptiles.

Here is a sample definition of mammals and birds which can help determine why bats are classified as mammals and not birds:

Mammals are animals characterized by the presence of several features such as hair, mammary glands, limbs and digits, set of complex teeth, and jaws connected to the skull.

Birds are animals that have feathers, beaks, legs covered in scales, and front limbs that are modified into wings.

Mammals are animals characterized by the presence of several features such as hair, mammary glands, limbs and digits, set of complex teeth, and jaws connected to the skull.

Birds are animals that have feathers, beaks, legs covered in scales, and front limbs that are modified into wings.

**Mammal A:** The animal can be under the **group of chiropteran**, the characteristics described in the question are almost similar to the characteristics of the members of this group– all animals in this group can fly, and feed on mostly insects, and fruit nectar (liquid diet).
Examples: bats

**Mammal B**: The animal can be under the **group of lagomorphs**, the characteristics described in the question are almost similar to the characteristics of the members of this group– having two pairs of incisors in the upper jaw and adapted hind legs for jumping and leaping.

**Mammal C**: The animal can be under the **group of cetaceans**, the characteristics described in the question are almost similar to the characteristics of the members of this group– being able to live underwater, yet must come to the surface every now and then to breathe.

Knowing the age and structure of hominid fossils can help determine the evolutionary history of hominids. Scientists can create a timeline using the fossil records and the pieces of evidences that they have gathered to better understand how species transformed over time. Analyzing their structures can help us point out how species adapted to their ecosystem and their geographical distribution. These discoveries might also help us uncover a bigger picture about all the changes and events which happened on Earth and its inhabitants over time.

The parental behavior of mammals contributed to the overall fitness of their offspring. Since the new generation has a higher rate of fitness due to the nurturing of the parent, they have better chances of survival and reproduction.

In delivering hormones to the different parts of the body, the circulatory system and the endocrine system are both involved. Once the endocrine system secretes the hormones, these substances are carried into the tissues and organs of the body by the bloodstream.

The absence of dinosaurs has helped the mammals to evolve and multiply quickly. Since there is no competition for food with dinosaurs, mammals were able to dominate many different habitats, including land, water, and air. When vegetation started to regrow and evolve, it provided a great food source for mammals. These conditions have allowed mammals to flourish.