Chordata—Mammalia

Melanie Tabroff

Kingdom: Animalia

Phylum: Chordata

Class: Mammalia
What are mammals and where did they come from?

Mammals are a group of vertebrates that have evolved from reptiles around 220 million years ago, during the Triassic period. Its ancestors are among the Therapsids, part of the synapsid branch of reptilian phylogeny. During the period of dinosaur’s existence, the therapsids disappeared but left Mesozoic mammalian descendants. As the Cretaceous period ended and the Cenozoic era began, the fluctuations in the environment contributed to mammalian adaptations, which are reflected in the diversification of mammals today.

There are three major groups of mammals that exist today:

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  Monotremes: egg-laying
  
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  Marsupials: have pouches
  
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  Eutherian mammals: placental
  

There are about 4,500 different species of mammals on Earth today. Yet, all of these species share the same diagnostic characteristics. The principle mammalian characteristic is having mammary glands, which are organs that produce milk. Mothers nourish their young with the milk from the mammary gland, which is rich in vitamins, minerals, fats, sugars, and proteins. In addition, most mammals are born rather than hatched. Another distinctively mammalian characteristic is having hair, which is made of keratin. The hair, along with a layer of fat, is beneficial for species of mammals because it helps their bodies retain metabolic heat. Mammals also have differentiation of teeth, which enables them to chew foods that vary in size and texture. Along with having a diversified range of teeth, mammals have jaws, which were modified during the evolution of mammals from reptiles. In general, most mammals are proficient learners and have larger brains than other vertebrates of equivalent size. Mammals invest a long time in parental care so that their offspring can learn from their parent’s behavior and be able to eventually survive on its own.

The process of physical and chemical digestion begins in the mouth. Mammals have differentiation of teeth, which helps them chew different kinds of foods to increases its surface area to make it easy to swallow. The presence of food in the oral cavity triggers the deliverance of saliva from the salivary glands. The saliva contains salivary amylase, an enzyme that helps break down carbohydrates. When the tongue senses the food, it pushes the ball of food (or bolus) to the back of the oral cavity into the pharynx. As the mammal swallows, the food is able to travel from the pharynx down the esophagus by peristalsis, waves of involuntary muscle contractions, and then into the stomach. At this stage, the stomach releases an acid fluid called gastric juice, which mixes with the food to further break it down into its chemical components. Then the food travels to the small intestine, where it is broken down even further and its nutrients are absorbed into the bloodstream. Next, the food moves into the large intestine, where water is extracted, and the leftover is released through the anus.

The mammalian digestive system functions with the help of accessory glands that secrete digestive juices into the alimentary canal, the pathway that the food travels, through ducts. The accessory glands include the pancreas, the liver, the gallbladder, and three pairs of salivary glands, which store digestive juices.
Sensing the environment

Mammals can sense different stimuli in the environment with the help of sensory receptors. They have pain receptors, thermoreceptors (sense temperature), chemoreceptors (sense odor), mechanoreceptors (sense pressure, touch, motion, and sound), and electromagnetic receptors (sense visible light and electricity). In general, mammals use the five main senses, which include sight, hearing, touch, smell, and taste, to identify the environment. The ability of mammals to sense and understand their environment is due to the inner workings of the brain.

Mammals have various modes of locomotion. Mammals exist in diversified environments, such as land, water, and air, and so mammals have adapted different modes of movement based on their habitat. Some adaptations include gliding, jumping, walking, or running. Bats are the only mammals that have adapted active flight as a mode of locomotion.

Mammals have lungs, located in the thoracic (chest) cavity, where cellular respiration occurs. The lungs have a spongy texture and are honeycombed with a moist respiratory surface. Air travels to lungs through a series of branching ducts. Air first enters the nostrils, where it is filtered by hairs, and then travels through the nasal cavity to the pharynx and then to the larynx. From the larynx, air passes into the trachea (windpipe), which forks into two bronchi, leading to each lung. The bronchi branch into smaller and smaller tubes called bronchioles, which eventually leads to the alveoli, the cluster of air sacs where gas exchange occurs. Mammals ventilate their lungs by negative pressure breathing, changing the air pressure within its lungs relative to the pressure of the outside atmosphere. During inhalation, the rib muscles and diaphragm contact (moves down). This process expands the rib cage and increases the lung’s volume. The pressure gradient that results allows air to enter through the nostrils to the alveoli. During exhalation, the rib muscles and diaphragm relax which restores the lungs to its normal volume and increases air pressure within the alveoli. The increased air pressure inside the alveoli forces air to rush out via the nostrils.

Mammals, like all animals, remove wastes by excretion. The kidneys of mammals filter out nitrogenous wastes, in the form of urea, from the bloodstream and excrete it during urination. Urea is produced in the liver and carried to the kidneys by the circulatory system. It is not as toxic as ammonia and it can be concentrated to help mammals conserve water during excretion.

Mammals have a four-chambered heart, an efficient respiratory and circulatory system to support their endothermic way of life (ability to maintain their own heat without the help of the environment). The four-chambered heart consists of the right and left atrium, which are chambers the receive blood returning to the heart, and the right and left ventricle, which are chambers that pump blood out of the heart. Arteries function to carry blood away from the heart to organs throughout the body, while veins return blood to the heart. As the right ventricle pumps blood to the lungs via the pulmonary arteries, oxygen and carbon dioxide are exchanged with the air in the lungs. The oxygen-rich blood from the pulmonary veins flows from the lungs to the left atrium of the heart. From there, then the oxygen-rich blood flows into the left ventricle. Blood leaves the left ventricle from the aorta, the major artery that is responsible for supplying blood to the body. This continuous cycle ensures that mammals have a constant flow of oxygenated blood to essential organs.

Mammals have different mechanisms to protect themselves. Their immune system is an involuntary mechanism of self protection. All mammals have complex immune systems that control and eliminate pathogens, like bacteria or viruses. A voluntary method of protection is passive or active defense against predators. Mammals have various ways to defend themselves by using their teeth, claws, fangs, or poisons. In addition, mammals use their senses that help locate and indentify predators or keep out of trouble. For example, mammals use pain receptors to determine when they are in danger.

The mammalian kidney functions to conserve water. The nephron, the functional unit of the kidney, consists of a ball of capillaries, called the glomerulus, and a long tubule that ends with a cup-shape swelling called the Bowman’s capsule. One of the regions of the nephron, the proximal tubule, functions primarily to reabsorb salt and water from the filtrate. As salt diffuses into the cells, the cellular membrane actively transports the sodium ions into the interstitial fluid (the fluid between the cells). Simultaneously, the chloride ions passively transport out of the tubule. As the salt moves into the fluid, water moves out of the filtrate and into the interstitial fluid due to osmosis. Next, the salt and water diffuse from the interstitial fluid into the capillaries. More reabsorption of water takes place as the filtrate moves into the descending limb of the loop of Henle, another region of the nephron. The filtrate continues to lose water do to the increasing osmolarity as it travels from the cortex of the nephron to the medulla. Next, the collecting duct carries the filtrate through the medulla to the renal pelvis, where more water is reabsorbed into the cells due the high osmolarity.