The other day, when my 9-year-old son, Zack, and I were walking the dog, he turned to me and said, “You know, I’m really glad I’m a mammal. Aren’t you?”
I said, “Sure.” My younger son, Cody, slowed down on his scooter and glanced back at us. Zack then proceeded to list all the reasons why being a mammal is so awesome. We have hair and we’re warm-blooded and we have babies. And even though reptiles outsmarted the Ice Age and have scales and other cool stuff, they just can’t compare to mammals.
It’s hard to argue with that kind of logic, so I said, “Absolutely. I see your point.” Cody finally stopped the scooter, turned around, put his hands in the air, and said, “Really? Mammals? Really? That’s what we’re talking about?” like Zack and I had lost our minds.
Well, I could see that point, too, but not Zack. “Cody, if you weren’t a mammal,” he told his little brother, “you wouldn’t be here with us, because most moms leave the offspring after the eggs crack.” Cody rolled his eyes and got back on his scooter, but I thought about this pack trait. It seems to be the biggest mammal perk.
This year hasn’t been easy for many. Friends and family members have lost jobs or taken huge pay cuts. Neighbors have become ill, couples have separated, and parents have died. One close friend has two boys: one with leukemia, the other with a brain injury. It is too much to bear without a pack.
Human mammals, I believe, extend their pack beyond their offspring. While I watch those I love struggle and suffer, I also see many others drop off meals, turn holiday parties into toy drives, pick up other people’s children, and pour wine when someone needs to talk. Almost every day, I hear a person I know say to someone else, “What can I do for you?” and mean it. No one is leaving after someone else’s eggs have cracked.
That’s what keeps me a believer in all things good, large and small. I don’t know why things happen the way they do. I don’t know why some people are forced to suffer more than others, why they are given more than anyone can reasonably bear. I don’t know why some prayers are answered and others seemingly ignored. But I do know there is a line of people who will bring over a chicken casserole and take the kids to football practice.
I believe those acts—those people in our packs who help us pick up the pieces—are the answered prayer, the miracle. And because of that, I believe my son Zack is right: it’s cool to be a mammal.
Laurie Uttich lives in Oviedo, Florida, a small town outside of Orlando, where she is surrounded by a pack of people who demonstrate the power of good deeds nearly every day. She lives with eight mammals—a husband, three boys, two dogs, and two cats—and teaches creative writing at the University of Central Florida. She recently completed a collection of essays titled, “What the Daughter I Will Never Have Knows.”
Independently produced by Dan Gediman for This I Believe, Inc.
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This article is about the animal class. For other uses, see Mammal (disambiguation).
"Mammalian" redirects here. For the 2010 documentary film, see Mammalian (film).
Mammals are the vertebrates within the classMammalia ( from Latinmamma "breast"), a clade of endothermicamniotes distinguished from reptiles (including birds) by the possession of a neocortex (a region of the brain), hair, three middle ear bones, and mammary glands. Females of all mammal species nurse their young with milk, secreted from the mammary glands.
Mammals include the largest animal on the planet, the blue whale. The basic body type is a terrestrial quadruped, but some mammals are adapted for life at sea, in the air, in trees, underground or on two legs. The largest group of mammals, the placentals, have a placenta, which enables the feeding of the fetus during gestation. Mammals range in size from the 30–40 mm (1.2–1.6 in) bumblebee bat to the 30-meter (98 ft) blue whale. With the exception of the five species of monotreme (egg-laying mammals), all modern mammals give birth to live young. Most mammals, including the six most species-rich orders, belong to the placental group. The largest orders are the rodents, bats and Soricomorpha (shrews and allies). The next three biggest orders, depending on the biological classification scheme used, are the Primates (apes and monkeys), the Cetartiodactyla (whales and even-toed ungulates), and the Carnivora (cats, dogs, seals, and allies).
Living mammals are divided into the Yinotheria (platypus and echidnas) and Theriiformes (all other mammals). There are around 5450 species of mammal, depending on which authority is cited. In some classifications, extant mammals are divided into two subclasses: the Prototheria, that is, the order Monotremata; and the Theria, or the infraclasses Metatheria and Eutheria. The marsupials constitute the crown group of the Metatheria, and include all living metatherians as well as many extinct ones; the placentals are the crown group of the Eutheria. While mammal classification at the family level has been relatively stable, several contending classifications regarding the higher levels—subclass, infraclass and order, especially of the marsupials—appear in contemporaneous literature. Much of the changes reflect the advances of cladistic analysis and molecular genetics. Findings from molecular genetics, for example, have prompted adopting new groups, such as the Afrotheria, and abandoning traditional groups, such as the Insectivora.
The mammals represent the only living Synapsida, which together with the Sauropsida form the Amniota clade. The early synapsid mammalian ancestors were sphenacodontpelycosaurs, a group that produced the non-mammalian Dimetrodon. At the end of the Carboniferous period, this group diverged from the sauropsid line that led to today's reptiles and birds. The line following the stem group Sphenacodontia split-off several diverse groups of non-mammalian synapsids—sometimes referred to as mammal-like reptiles—before giving rise to the proto-mammals (Therapsida) in the early Mesozoic era. The modern mammalian orders arose in the Paleogene and Neogene periods of the Cenozoic era, after the extinction of non-avian dinosaurs, and have been among the dominant terrestrial animal groups from 66 million years ago to the present.
Some mammals are intelligent, with some possessing large brains, self-awareness and tool use. Mammals can communicate and vocalize in several different ways, including the production of ultrasound, scent-marking, alarm signals, singing, and echolocation. Mammals can organize themselves into fission-fusion societies, harems, and hierarchies, but can also be solitary and territorial. Most mammals are polygynous, but some can be monogamous or polyandrous.
In human culture, domesticated mammals played a major role in the Neolithic revolution, causing farming to replace hunting and gathering, and leading to a major restructuring of human societies with the first civilizations. They provided, and continue to provide, power for transport and agriculture, as well as various commodities such as meat, dairy products, wool, and leather. Mammals are hunted or raced for sport, and are used as model organisms in science. Mammals have been depicted in art since Palaeolithic times, and appear in literature, film, mythology, and religion. Defaunation of mammals is primarily driven by anthropogenic factors, such as poaching and habitat destruction, though there are efforts to combat this.
Main article: Mammal classification
See also: List of placental mammals, List of monotremes and marsupials, and List of mammal genera
Mammal classification has been through several iterations since Carl Linnaeus initially defined the class. No classification system is universally accepted; McKenna & Bell (1997) and Wilson & Reader (2005) provide useful recent compendiums.George Gaylord Simpson's "Principles of Classification and a Classification of Mammals" (AMNH Bulletin v. 85, 1945) provides systematics of mammal origins and relationships that were universally taught until the end of the 20th century. Since Simpson's classification, the paleontological record has been recalibrated, and the intervening years have seen much debate and progress concerning the theoretical underpinnings of systematization itself, partly through the new concept of cladistics. Though field work gradually made Simpson's classification outdated, it remains the closest thing to an official classification of mammals.
Most mammals, including the six most species-rich orders, belong to the placental group. The three largest orders in numbers of species are Rodentia: mice, rats, porcupines, beavers, capybaras and other gnawing mammals; Chiroptera: bats; and Soricomorpha: shrews, moles and solenodons. The next three biggest orders, depending on the biological classification scheme used, are the Primates including the apes, monkeys and lemurs; the Cetartiodactyla including whales and even-toed ungulates; and the Carnivora which includes cats, dogs, weasels, bears, seals and allies. According to Mammal Species of the World, 5,416 species were identified in 2006. These were grouped into 1,229 genera, 153 families and 29 orders. In 2008, the International Union for Conservation of Nature (IUCN) completed a five-year Global Mammal Assessment for its IUCN Red List, which counted 5,488 species. According to a research published in the Journal of Mammalogy in 2018, the number of recognized mammal species is 6,495 species included 96 recently extinct.
The word "mammal" is modern, from the scientific name Mammalia coined by Carl Linnaeus in 1758, derived from the Latinmamma ("teat, pap"). In an influential 1988 paper, Timothy Rowe defined Mammalia phylogenetically as the crown group of mammals, the clade consisting of the most recent common ancestor of living monotremes (echidnas and platypuses) and therian mammals (marsupials and placentals) and all descendants of that ancestor. Since this ancestor lived in the Jurassic period, Rowe's definition excludes all animals from the earlier Triassic, despite the fact that Triassic fossils in the Haramiyida have been referred to the Mammalia since the mid-19th century. If Mammalia is considered as the crown group, its origin can be roughly dated as the first known appearance of animals more closely related to some extant mammals than to others. Ambondro is more closely related to monotremes than to therian mammals while Amphilestes and Amphitherium are more closely related to the therians; as fossils of all three genera are dated about 167 million years ago in the Middle Jurassic, this is a reasonable estimate for the appearance of the crown group.
T. S. Kemp has provided a more traditional definition: "synapsids that possess a dentary–squamosal jaw articulation and occlusion between upper and lower molars with a transverse component to the movement" or, equivalently in Kemp's view, the clade originating with the last common ancestor of Sinoconodon and living mammals. The earliest known synapsid satisfying Kemp's definitions is Tikitherium, dated 225 Ma, so the appearance of mammals in this broader sense can be given this Late Triassic date.
In 1997, the mammals were comprehensively revised by Malcolm C. McKenna and Susan K. Bell, which has resulted in the McKenna/Bell classification. Their 1997 book, Classification of Mammals above the Species Level, is a comprehensive work on the systematics, relationships and occurrences of all mammal taxa, living and extinct, down through the rank of genus, though molecular genetic data challenge several of the higher level groupings. The authors worked together as paleontologists at the American Museum of Natural History, New York. McKenna inherited the project from Simpson and, with Bell, constructed a completely updated hierarchical system, covering living and extinct taxa that reflects the historical genealogy of Mammalia.
Extinct groups are represented by a dagger (†).
- Subclass Prototheria: monotremes: echidnas and the platypus
- Subclass Theriiformes: live-bearing mammals and their prehistoric relatives
- Infraclass †Allotheria: multituberculates
- Infraclass †Eutriconodonta: eutriconodonts
- Infraclass Holotheria: modern live-bearing mammals and their prehistoric relatives
- Superlegion †Kuehneotheria
- Supercohort Theria: live-bearing mammals
- Cohort Marsupialia: marsupials
- Cohort Placentalia: placentals
Molecular classification of placentals
Molecular studies based on DNA analysis have suggested new relationships among mammal families over the last few years. Most of these findings have been independently validated by retrotransposonpresence/absence data. Classification systems based on molecular studies reveal three major groups or lineages of placental mammals—Afrotheria, Xenarthra and Boreoeutheria—which diverged in the Cretaceous. The relationships between these three lineages is contentious, and all three possible different hypotheses have been proposed with respect to which group is basal. These hypotheses are Atlantogenata (basal Boreoeutheria), Epitheria (basal Xenarthra) and Exafroplacentalia (basal Afrotheria). Boreoeutheria in turn contains two major lineages—Euarchontoglires and Laurasiatheria.
Estimates for the divergence times between these three placental groups range from 105 to 120 million years ago, depending on the type of DNA used (such as nuclear or mitochondrial) and varying interpretations of paleogeographic data.
The cladogram above is based on Tarver et al. (2016)
Group I: Superorder Afrotheria
Group II: Superorder Xenarthra
- Order Pilosa: sloths and anteaters (neotropical)
- Order Cingulata: armadillos and extinct relatives (Americas)
Group III: Magnaorder Boreoeutheria
- Superorder: Euarchontoglires (Supraprimates)
- Superorder: Laurasiatheria
- Order Eulipotyphla: shrews, hedgehogs, moles, solenodons
- Order Chiroptera: bats (cosmopolitan)
- Order Pholidota: pangolins or scaly anteaters (Africa, South Asia)
- Order Carnivora: carnivores (cosmopolitan), including cats and dogs
Main article: Evolution of mammals
Synapsida, a clade that contains mammals and their extinct relatives, originated during the Pennsylvanian subperiod (~323 million to ~300 million years ago), when they split from reptilian and avian lineages. Crown group mammals evolved from earlier mammaliaforms during the Early Jurassic. The cladogram takes Mammalia to be the crown group.
Evolution from amniotes
The first fully terrestrial vertebrates were amniotes. Like their amphibious tetrapod predecessors, they had lungs and limbs. Amniotic eggs, however, have internal membranes that allow the developing embryo to breathe but keep water in. Hence, amniotes can lay eggs on dry land, while amphibians generally need to lay their eggs in water.
The first amniotes apparently arose in the Pennsylvanian subperiod of the Carboniferous. They descended from earlier reptiliomorph amphibious tetrapods, which lived on land that was already inhabited by insects and other invertebrates as well as ferns, mosses and other plants. Within a few million years, two important amniote lineages became distinct: the synapsids, which would later include the common ancestor of the mammals; and the sauropsids, which now include turtles, lizards, snakes, crocodilians, dinosaurs and birds. Synapsids have a single hole (temporal fenestra) low on each side of the skull. One synapsid group, the pelycosaurs, included the largest and fiercest animals of the early Permian. Nonmammalian synapsids are sometimes called "mammal-like reptiles".
Therapsids, a group of synapsids, descended from pelycosaurs in the Middle Permian, about 265 million years ago, and became the dominant land vertebrates. They differ from basal eupelycosaurs in several features of the skull and jaws, including: larger skulls and incisors which are equal in size in therapsids, but not for eupelycosaurs. The therapsid lineage leading to mammals went through a series of stages, beginning with animals that were very similar to their pelycosaur ancestors and ending with probainognathiancynodonts, some of which could easily be mistaken for mammals. Those stages were characterized by:
- The gradual development of a bony secondary palate.
- Progression towards an erect limb posture, which would increase the animals' stamina by avoiding Carrier's constraint. But this process was slow and erratic: for example, all herbivorous nonmammaliaform therapsids retained sprawling limbs (some late forms may have had semierect hind limbs); Permian carnivorous therapsids had sprawling forelimbs, and some late Permian ones also had semisprawling hindlimbs. In fact, modern monotremes still have semisprawling limbs.
- The dentary gradually became the main bone of the lower jaw which, by the Triassic, progressed towards the fully mammalian jaw (the lower consisting only of the dentary) and middle ear (which is constructed by the bones that were previously used to construct the jaws of reptiles).
The Permian–Triassic extinction event about 252 million years ago, which was a prolonged event due to the accumulation of several extinction pulses, ended the dominance of carnivorous therapsids.