Environmental Science · Science

An Introduction to Mammals


Mammals are the most diverse species on Earth. It is vital to understanding the origin of mammals by using fossil records as evidence. Fossil records that are recently discovered hold a significant information regarding the origin of mammals. Mammals have different sizes, forms and shapes. These varieties help mammals to demonstrate adaptive behavior in order to survive the environmental and habitat changes. All mammals interact with each other. Human, as a species of mammal, domesticates other species. The domestication may impact the intelligence strategy of some mammals. The domestication requires studying the behavior of species related to the physical structure, such as migratory behavior, communication behavior, and sexual behavior. The reproductive system in mammal differs from other species. Mammals have several reproductive strategies that are aligned with the changes in environment and habitat.

The Origin of Mammals

Mammals are evolved from early amniotes as the fossil records show that the therapsids have similar features of both reptiles and mammals. The early amniotes had divided into two main lines; synapsids, and sauropsids. Synapsids are said to be the origin of all mammals including extinct mammals. Therapsid – mammal-like reptiles, belong to synapsids. Sauropsids– that includes anapsids and diapsids, are different than synapsids in the structure of the skull and the number of temporal fenestrae behind eyes. The first mammals arose before the extinction of synapsids in the Jurassic period, 195 Ma (Habib, 2014).  I can argue that mammals are evolved from amphibians. Both amphibians and mammals have two occipital condyles in their skull. Mammals have left aortic arch to resemble amphibians. This fact was assumed by H.T Huxley (1880) (Dhami and Dhami, 1987). However, I do not agree with him on this point. In amphibians, the exoccipital is evolved into occipital condyles, while basioccipital is evolved in mammals into occipital condyles.


Dhami, P. and Dhami, J. (1987). Chordate Zoology. New Delhi: R. Chand & Co.

There are insufficient fossil records that explain the origin of mammals. The fossil evidence that is discovered in Spain, Spinolestes enarthrosis, is very useful. It shows the skeletal morphology changes through the geological time. This is an excellent fossil that explains the ancestor of mammals. It shows some skeletal events in the evolutionary history of mammals, for example, the mandible that has a single dentary bone. It also indicates that the dentition is evolved and specialized. The braincase is large and the body covered by fur, that means it was endothermic. The Cynodont that existed about 260 MA is a transitional fossil (BioLogos, 2012). It shows that the early mammals had two jaw hinges. I would maintain that this fossil is a vital one as it shows the slow development of the ears. Paleontologists have found a pair of bones in the cynodont fossils that developed slowly to ears. Personally, I prefer referring to the Spinolestes xenarthrosus fossil. It was the primary fossil that I based my discussion with my colleagues.

The evolution of synapsid achieved in several steps. Those steps are called “pulses.” There are seven pulses of synapsid evolution (Palaeos.com, 2016):

The first pulse – Permian, the pelycosaurs were occupying the tropical areas. Its body was ectothermic, and the body form was medium to large size. They had a sprawling posture. The early pelycosaurs were meat-eating synapsids. The second pulse existed in early to middle Triassic. Therapsids appeared that have evolved from pelycosaurs. They had a higher metabolic rate than pelycosaurs. They occupied a wider range of habitat than pelycosaurs; the jaw musculature has become bigger, and the posture has improved. In the Late Triassic, the third pulse existed. The cynodont therapsids evolved. The body structure is modified, the first appearance of the diaphragm, secondary palate, specialized dentition and some evidence of endothermic characters. The fourth Pulse – the Late Triassic to Mid-Early Cretaceous, the first appearance of early mammals. They covered wider geographic areas, but with little diversity. The fifth pulse, first thorium mammals, Late-Early Cretaceous to Late Cretaceous. It is associated with the diversity of angiosperm plant. The sixth Pulse, abundant in Early Paleocene to Middle Eocene. The appearance of archaic therians and didelphoid marsupials.  The species were abundant in tropical forests worldwide. The seventh Pulse occurred in Late Eocene to Recent. By Middle Eocene, the modern therions marsupials are limited to the South America, Asia, and Australia. Mammals inhabit a broad range of area with the diversity of different groups such as bats, whales, and hominoids. The terrestrial mammal size is ranged up to 5000 kg while the aquatic mammals size is up to 10000 kg.

Mammalian Adaptability

Mammals have different forms, sizes, and shapes. This variety because it occupied a broad range of area on earth.  All mammals have mammary glands, a group of cells that secrete fluids (Biggs et al., 2004). Mammal also has a unique form of skin and hair. The skin is composed of two layers – outer nonvascular epidermis layer and an inner layer of corium. The surface of the mammalian skin is covered with lipids, and organic salts. This coat has the antifungal and antibacterial properties. It is an imperative part of the mammal form. I believe, its importance is it moderate body temperature and the blood pressure. Terrestrial mammals have hair that covers their bodies. Whale adults are hairless. However, they have hair in utero. Some aquatic species have fur, such as in beavers. Some terrestrial species have also fur that covers their body. The importance of hair is it works as an insulator. Hair also can function as a sensory organ, as in cats, dogs, and whales. The teeth distinguish all mammals. Teeth in mammals are well adapted to their different types of food. Some hoofed mammals have cud chewing that helps mammals to break the cellulose plant wall into nutrients that they can absorb quickly. The aquatic mammals have excellent strategies of adaptation. The body shape of them is unique. It has fish-like shape, head is elongated, and streamline body. It is a vital body shape as it minimizes the resistance of water during locomotion (Chandran, Rani, and Sekar, 2007). Aquatic mammals have large sizes and weights. It reduces the loss of heat and skin friction.

Domestication and Intelligent Strategies

Domestication is a process that has taken an extended period to have a unique economic importance. Domesticated animals initially are used as a source of food, such as sheep and goats. Some of the mammal species, dogs, for example, were used in hung games and to warn breeders of dangers. I would like to maintain that this strategy is a good early strategy of domestication as some of the villagers still used dogs to protect their yards. Here, in Kuwait, and near to my workplace, some of local Camel breeders still use dogs to protect their camels. Later, domestication provided an economic importance. Breeders used mammalian milk for trading. Horses played a significant role in the transportation and the wars as well. There are several examples of successful domestication for economic importance. Llamas Lama glama, wool is used commercially, Water buffalo, Bubalus bubalis, are used to plow rice paddies and pull carts (Ryan, Vaughan and Czaplewski, 2015). Personally, I like the idea of the economic importance of the domestication. The interaction between human and animals is very useful for the human. However, sometimes domestication has some cons.

One of the remarkable drawbacks that might affect the domesticated animal is the reduction of its intelligent strategies. Some studies have noted a systematic decrease in the size of domesticated species brain compared with their wild species (Zeder, 2012). The studies discovered that the mammals with a larger brain have high degrees of brain reduction while the smaller brain size species has a little effect of brain reduction. The decline in the brain size is a clear sign of the reduction of the species intelligent strategies. Some of the species that have a significant decrease in their brain size; pigs, Sus scrofa, Silver fox, Urocyon cinereoarenteuc, and Sheep, Ovis aries. Despite the fact that these studies have very strong evidence of the reduction of intelligent skills of domesticated animals, I can argue that it might miss some of the methods of domestication. In the circus, you can see that the animal trainer trains his animals to do some games. Animals can jump and play with their trainers. However, if we look into the wild, intelligent skills, such as hunting skills, it might be true that the domestication might affect these skills.

Mammalian Behavior, Diversity and Communication Strategies

The mammalian behavior depends on its physical structure and its lifestyle. Mammals are endothermic, so they need more energy than ectothermic of the same size. The thermoregulation plays a vital role in the way of mammal behaves. Sensory organs affect the mammalian behavior. As well, for example, the auditory functions play a major role in some mammal species. Bats and some marine mammals have very developed and specialized echolocation capabilities. They emit a sound signal and interpret the received reflected sound waves to determine the objects’ positions. Also, the vision sensor in mammals is developed in many species. The spectral range of the mammals differs from species to another. Not all species have full-color viewing (Hogan and Ervin, 2016). Nocturnal mammals have large well-developed eyes. Mammals use vision in hunting, navigation, locomotion and foraging. They use vision also in communication. Some species release odors as a defense against predators.  The structure of mammal’s teeth can determine the hunting behavior of mammals. Lions, for example, can hunt by using its limbs and strong teeth to control its prey. Some mammal species that live in territories will defend their areas by pushing away other individuals. Many species use pheromones to identify their territory boundaries. Aggressive behavior involves actual or potential harm to another individual (Huntingford, 2016). A magnificent example of aggressive behavior is found in a male bighorn sheep. When another male threatens the territory of the male bighorn sheep, they fight to the death to protect his territory.

Communication in mammals varies from a species to another. The marine mammals have different means of communication than terrestrial mammals. Some mammals use sounds, sights, touches or smell to signal each other.  In marine mammals, for example, Dolphins use sound to communicate. Some species sing to attract females, such as humpback whales.  Whales are a good example of using low-frequency sound to communicate. These sounds can be used in both social and sexual signals during communication. I would like to argue that the communication in marine ecosystem is tough. Water can transfer few amounts of waves, so marine mammals use water current, some frequencies of sound and sometimes odor to communicate. However, the terrestrial mammals use a wide range of communication skills. Human use languages and writing skills to communicate.

Mammal species occupy wide ranges of Earth surface. They existed in all continents and all marine ecosystems.  There are around 5000 species of living mammals, classified into 125 families and 29 orders (Wilson, 2016). There are three main classes of mammals; Placental mammals, Pouched Mammals, and Monotremes Mammals.  The most diverse living mammals are rodents. The high diversity of mammals is a result of two major events; plate tectonics, as the continents, have moved throughout the Earth geological history and mammal’s migration. Migration is a movement of an animal from its habitat to another. There are many types of migration. The marine mammals and large herbivorous mammals demonstrate seasonal migration strategy. Mammals migrate to the most favorable conditions of food and weather. Bats, for example, in the cold and temperate regions move to warm areas in the winter. Some mammals, especially herbivorous of cold regions, have summer and winter ranges, such as caribou.   In temperate areas, some mammals such as African antelopes, they use the seasonal migration strategy to avoid the draught (Feldhamer et al., 2015).  The migration can be a change of altitude, or latitude or both. Most mammals migrate horizontally; other mammals migrate vertically. Some of the marine mammals move from the deep water to the shallow levels of water.  I believe that these different strategies of the mammal migration helped to provide a larger diversity of mammals. This, in my opinion, gives the mammal a unique advantage to occupy vast areas of Earth.

Mammalian Reproduction

All species of mammals reproduce sexually that means the male’s sperm fertilizes the female’s egg. Mammals viviparous that produce offspring alive. However, there are some egg-laying animals such as the duck-billed platypus. Mammals offspring are not born immediately after fertilization. Offspring need a period of development in the uterus or womb. This period is called gestation; that is called in human as pregnancy (The Open Door Web Site, 2016). Both male and female have specialized reproductive systems. The external structure of the male reproductive tract consists of the penis and the scrotum. The scrotum contains testes that exist outside the body. The urethra is a tube that carries the sperm to the female vagina (Brooker, 2011). The female reproductive structure consists of several organs. Ovary, the site of oogenesis, is responsible for hormone production. Oviducts, uterine tubes, provides a place for fertilization. Cervix and uterus exist at the end of the vagina. The male sperms move through the cervix into the uterus. The vagina is a tube that lies posterior to the uterus.

Brooker, R. (2011). Biology. New York: McGraw-Hill

Reproductive systems in different mammalian species (Prior, 2014):

Monotremata Marsupials Placenta
Method Sexually: They produce a hard-shelled egg Sexually Sexually
Feeding baby No nipples Nipples existed Nipples Existed
Female reproductive Structure –          No vagina but one opening called cloaca.

–          Uterus has two chambers.

–          Paired of ovaries similar to those in birds and reptiles.

–          Mammary glands


–          There is posterior urogenital sinus. Two vaginae and the urethra.

–          A pair of uteri and a cervix extends from the top of each vagina.

–          Mammary glands


–          Two ovaries.

–          Two uterine tubes.

–          A single uterus.

–          Single Vagina

–          Mammary glands are not part of the reproductive system

Male Reproductive Tract Same as in the Placental Mammals Same as Eutherian mammals but the bifurcate penis is posterior to the scrotum. –          Two Testes.

–          Scrotum Sac around testes.

–          Penis

–          Urethra

–          Prostate.

–          Ejaculation ducts at the end of urethra

Examples Platypus Kangaroo Human

Reproductive Strategies

Many mammals used to breed, and to mate seasonally when the environmental conditions are suitable for reproduction and breeding. Females of some species store male sperm until the condition of the environment become suitable for fertilization. This is a vital strategy in order to adapt to the environmental changes. This approach called Delayed Fertilization. Another strategy that some mammals practice is Delayed Implantation. Some females delay eggs. I believe this strategy is not that much effective than the delayed fertilizations. Not all mammalian females can delay producing eggs. However, the third approach is Delayed Development that means the female can delay or slow down the development after implantation (Wund, and Myers, 2016).  This is the most efficient strategy. I can argue that this is more efficient adaptive strategy than the previous two strategies. However, the delayed implantation in many diverse groups of mammals is more diverse than the delayed fertilization and delayed development. The delayed development strategy is rarely understood. I cannot understand this strategy and in which cases the animal may use.  The long-tailed weasel, Mustela frenara, and the least weasel, Mustela nivales, both occur in the same habitat. However, the least weasel demonstrates delayed development; the long-tailed does not (Feldhamer et al., 2015), so there is no particular method for the reproductive strategy in some mammals.


It is evident that mammals are descended from early amniotes. The fossil records show how mammals are related to the early amniotes.  These evolutionary stages have done in several steps that called evolutionary pulses. Domestication has used as a source of food and animal games, but now it has an economical importance. Mammals occupy a large diversity on earth; that is because the successful migratory behavior. They have physical structures that help them to communicate and reproduce.


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