Monotreme in the context of Hatchlings

The beak or bill is an external rostrum structure found mostly in birds. A beak is used for pecking, grasping, and holding (in probing for food, eating, manipulating and carrying objects, killing prey, or fighting), preening, courtship, and feeding young. The terms beak and rostrum are also used to refer to a similar mouth part in some ornithischians, pterosaurs, cetaceans, dicynodonts, rhynchosaurs, anuran tadpoles, monotremes (i.e. echidnas and platypuses, which have a bill-like structure), sirens, pufferfish, billfishes, and cephalopods.

Although beaks vary significantly in size, shape, color and texture, they share a similar underlying structure. Two bony projections–the upper and lower mandibles–are covered with a thin keratinized layer of epidermis known as the rhamphotheca. In most species, two holes called nares lead to the respiratory system.

An egg is an organic vessel grown by an animal to carry a possibly fertilized egg cell – a zygote. Within the vessel, an embryo is incubated until it has become an animal fetus that can survive on its own, at which point the animal hatches. Reproductive structures similar to the egg in other kingdoms are termed "spores", or in spermatophytes "seeds", or in gametophytes "egg cells".

Most arthropods, vertebrates (excluding live-bearing mammals), and mollusks lay eggs, although some, such as scorpions, do not. Reptile eggs, bird eggs, and monotreme eggs are laid out of water and are surrounded by a protective shell, either flexible or inflexible. Eggs laid on land or in nests are usually kept within a warm and favorable temperature range while the embryo grows. When the embryo is adequately developed it hatches; i.e., breaks out of the egg's shell. Some embryos have a temporary egg tooth they use to crack, pip, or break the eggshell or covering.

A cloaca (/kloʊˈeɪkə/ kloh-AY-kə), pl.: cloacae (/kloʊˈeɪsi/ kloh-AY-see or /kloʊˈeɪki/ kloh-AY-kee), or vent, is the rear orifice that serves as the only opening for the digestive (rectum), reproductive, and urinary tracts (if present) of many vertebrate animals. All amphibians, reptiles, birds, cartilaginous fish and a few mammals (monotremes, afrosoricids, and marsupial moles) have this orifice, from which they excrete both urine and feces; this is in contrast to most placental mammals, which have separate orifices for evacuation and reproduction. Excretory openings with analogous purpose in some invertebrates are also sometimes called cloacae. Mating through the cloaca is called cloacal copulation and cloacal kissing.

The cloacal region is also often associated with a secretory organ, the cloacal gland, which has been implicated in the scent-marking behavior of some reptiles, marsupials, amphibians, and monotremes.

The platypus (Ornithorhynchus anatinus), sometimes referred to as the duck-billed platypus, is a semiaquatic, egg-laying mammal endemic to eastern Australia, including Tasmania. The platypus is the sole living representative of its family Ornithorhynchidae and genus Ornithorhynchus, though a number of related species appear in the fossil record. Together with the four species of echidna, it is one of the five extant species of monotremes, mammals that lay eggs instead of giving birth to live young. Like other monotremes, the platypus has a sense of electrolocation, which it uses to detect prey in water while its eyes, ears and nostrils are closed. It is one of the few species of venomous mammals, as the male platypus has a spur on each hind foot that delivers an extremely painful venom.

The unusual appearance of this egg-laying, duck-billed, beaver-tailed mammal at first baffled European naturalists. In 1799, the first scientists to examine a preserved platypus body judged it a fake made of several animals sewn together. The unique features of the platypus make it important in the study of evolutionary biology, and a recognisable and iconic symbol of Australia. It is culturally significant to several Aboriginal peoples, who also used to hunt it for food, and has appeared on stamps and currency.

Whiskers, also known as vibrissae (/vəˈbrɪsi/; sg. vibrissa; /vəˈbrɪsə/), are specialized sensory hairs that help most species of mammals sense their environment. Whiskers function as highly sensitive feelers that help mammals navigate the world in many different ways, including the detection of air currents and physical objects to aid in navigation, hunting, and spatial awareness. Whiskers can trigger reflexes to protect the eyes. Whiskers also play a significant role in the communication between individual animals in different species of mammals. They convey an animal's emotional state, essentially sending a message to other members of their species about what emotion they are feeling at the time. Whisker movement can be thought of as a form of language that animals use to silently communicate with the other animals around them.

Whiskers grow in clusters at various places around the body, including above the eyes, on the chin, on the forelegs, near the ears, and above the upper lip. Most mammals have whiskers, including all non-human primates, marsupials, and especially nocturnal mammals. Monotremes, however, lack them.

Eutheria (from Ancient Greek εὐ- (eú-), meaning "true, well", and θηρίον (thēríon), meaning "beast", and thus, "true beasts"), also called Pan-Placentalia, is the clade consisting of placental mammals and all therian mammals that are more closely related to placentals than to marsupials.

Eutherians are distinguished from non-eutherians by various phenotypic traits of the feet, ankles, jaws and teeth. All extant eutherians lack epipubic bones, which are present in all other living mammals (marsupials and monotremes). This allows for expansion of the abdomen during pregnancy, though epipubic bones are present in a number of primitive eutherians. Eutheria was named in 1872 by Theodore Gill; in 1880, Thomas Henry Huxley defined it to encompass a more broadly defined group than Placentalia.

A mammary gland is an exocrine gland that produces milk in humans and other mammals. Mammals get their name from the Latin word mamma, "breast". The mammary glands are arranged in organs such as the breasts in primates (for example, humans and chimpanzees), the udder in ruminants (for example, cows, goats, sheep, and deer), and the dugs of other animals (for example, dogs and cats) to feed young offspring. Lactorrhea, the occasional production of milk by the glands, can occur in any mammal, but in most mammals, lactation, the production of enough milk for nursing, occurs only in phenotypic females who have gestated in recent months or years. It is directed by hormonal guidance from sex steroids. In a few mammalian species, male lactation can occur. With humans, male lactation can occur only under specific circumstances.

Mammals are divided into 3 groups: monotremes, metatherians, and eutherians. In the case of monotremes, their mammary glands are modified sebaceous glands and without nipples. Concerning most metatherians and eutherians, only females have functional mammary glands, with the exception of some bat species. Their mammary glands can be termed as breasts or udders. In the case of breasts, each mammary gland has its own nipple (e.g., human mammary glands). In the case of udders, pairs of mammary glands comprise a single mass, with more than one nipple (or teat) hanging from it. For instance, cows and buffalo udders have two pairs of mammary glands and four teats, whereas sheep and goat udders have one pair of mammary glands with two teats protruding from the udder. Each mammary gland produces milk for a single teat and is evolutionarily derived from modified sweat glands.

A hedgehog is a spiny mammal of the subfamily Erinaceinae, in the eulipotyphlan family Erinaceidae. There are 17 species of hedgehog in five genera found throughout parts of Europe, Asia, and Africa, and in New Zealand by introduction. There are no hedgehogs native to Australia and no living species native to the Americas. However, the extinct genus Amphechinus was once present in North America.

Hedgehogs share distant ancestry with shrews (family Soricidae), with gymnures possibly being the intermediate link, and they have changed little over the last 15 million years. Like many of the first mammals, they have adapted to a nocturnal way of life. Their spiny protection resembles that of porcupines, which are rodents, and echidnas, a type of monotreme.

The anterior commissure (also known as the precommissure) is a white matter tract (a bundle of axons) connecting the two temporal lobes of the cerebral hemispheres across the midline, and placed in front of the columns of the fornix. In all but five species of mammal the great majority of fibers connecting the two hemispheres travel through the corpus callosum, which in humans and all non-monotremes is more than 10 times larger than the anterior commissure. Other routes of communication pass through the hippocampal commissure or, indirectly, via subcortical connections. Nevertheless, the anterior commissure is a significant pathway that can be clearly distinguished in the brains of all mammals.

The anterior commissure plays a key role in pain sensation, more specifically sharp, acute pain. It also contains decussating fibers from the olfactory tracts, vital for the sense of smell and chemoreception. The anterior commissure works with the posterior commissure to link the two cerebral hemispheres of the brain and also interconnects the amygdalae and temporal lobes, contributing to the role of memory, emotion, speech and hearing. It also is involved in olfaction, instinct, and sexual behavior.

Louse (pl.: lice) is the common name for any member of the infraorder Phthiraptera, which contains nearly 5,000 species of wingless parasitic insects. Phthiraptera was previously recognized as an order, until a 2021 genetic study determined that they are a highly modified lineage of the order Psocodea, whose members are commonly known as booklice, barklice or barkflies.

Lice are obligate parasites, living externally on warm-blooded hosts, which include every species of bird and mammal, except for monotremes, pangolins, and bats. Chewing lice live among the hairs or feathers of their host and feed on skin and debris, whereas sucking lice pierce the host's skin and feed on blood and other secretions. They usually spend their whole life on a single host, cementing their eggs, called nits, to hairs or feathers. The eggs hatch into nymphs, which moult three times before becoming fully grown, a process that takes about four weeks.

Echidnas (/ɪˈkɪdnəz/), sometimes known as spiny anteaters, are quill-covered monotremes (egg-laying mammals) belonging to the family Tachyglossidae /tækiˈɡlɒsɪdiː/, living in Australia and New Guinea. The four extant species of echidnas and the platypus are the only living mammals that lay eggs and the only surviving members of the order Monotremata. The diet of some species consists of ants and termites, but they are not closely related to the American true anteaters or to hedgehogs. Their young are called puggles.

Echidnas evolved between 20 and 50 million years ago and descend from an amphibious, platypus-like monotreme.

The evolution of mammals has passed through many stages since the first appearance of their synapsid ancestors in the Pennsylvanian sub-period of the late Carboniferous period. By the mid-Triassic, there were many synapsid species that looked like mammals. The lineage leading to today's mammals split up in the Jurassic; synapsids from this period include Dryolestes, more closely related to extant placentals and marsupials than to monotremes, as well as Ambondro, more closely related to monotremes. Later on, the eutherian and metatherian lineages separated; the metatherians are the animals more closely related to the marsupials, while the eutherians are those more closely related to the placentals. Since Juramaia, the earliest known eutherian, lived 160 million years ago in the Jurassic, this divergence must have occurred in the same period.

After the Cretaceous–Paleogene extinction event wiped out the non-avian dinosaurs (birds being the only surviving dinosaurs) and several mammalian groups, placental and marsupial mammals diversified into many new forms and ecological niches throughout the Paleogene and Neogene, by the end of which all modern orders had appeared.

The pseudoautosomal regions or PARs are homologous sequences of nucleotides found within the sex chromosomes of species with an XY or ZW mechanism of sex determination.

The pseudoautosomal regions get their name because any genes within them (so far at least 29 have been found for humans) are inherited just like any autosomal genes. In humans, these regions are referred to as PAR1 and PAR2. PAR1 comprises 2.6 Mbp of the short-arm tips of both X and Y chromosomes in humans and great apes (X and Y are 154 Mbp and 62 Mbp in total). PAR2 is at the tips of the long arms, spanning 320 kbp. The monotremes, including the platypus and echidna, have a multiple sex chromosome system, and consequently have 8 pseudoautosomal regions.

A mammary gland is an exocrine gland that produces milk in humans and other mammals. Mammals get their name from the Latin word mamma, "breast". The mammary glands are arranged in organs such as the breasts in primates (for example, humans and chimpanzees), the udder in ruminants (for example, cows, goats, sheep, and deer), and the dugs of other animals (for example, dogs, cats, rabbits and pigs) to feed young offspring. Lactorrhea, the occasional production of milk by the glands, can occur in any mammal, but in most mammals, lactation, the production of enough milk for nursing, occurs only in phenotypic females who have gestated in recent months or years. It is directed by hormonal guidance from sex steroids. In a few mammalian species, male lactation can occur. With humans, male lactation can occur only under specific circumstances.

Mammals are divided into 3 groups: monotremes, metatherians, and eutherians. In the case of monotremes, their mammary glands are modified sebaceous glands and without nipples. Concerning most metatherians and eutherians, only females have functional mammary glands, with the exception of some bat species. Their mammary glands can be termed as breasts or udders. In the case of breasts, each mammary gland has its own nipple (e.g., human mammary glands). In the case of udders, pairs of mammary glands comprise a single mass, with more than one nipple (or teat) hanging from it. For instance, cows and buffalo udders have two pairs of mammary glands and four teats, whereas sheep and goat udders have one pair of mammary glands with two teats protruding from the udder. Each mammary gland produces milk for a single teat and is evolutionarily derived from modified sweat glands.