Category: Mammals

[Skip to list of Topics for this Category →]

Mammals are probably the best-known animal group, maybe partly so because we belong to them. Although they occur on all landmasses and in a wide range of habitats, mammals do not play a major role in terms of numbers - they are the smallest vertebrate group with approximately 5,400 species. However, most mammalian genera are extinct and it has been estimated that by including those, the number of species might rise to more than 20,000. Mammals are subdivided into three major groups, the egg-laying monotremes (subclass Prototheria) and the live-bearing marsupials and placentals (subclass Theria), which are distinguished by their reproductive biology as well as skeletal characteristics.

The mammals' name-giving feature is the mammary glands of the females, which produce milk to nourish the young. Further unique characteristics include sweat glands, hair (which can serve various functions, e.g. insulation, camouflage and display), the neocortex, a brain region involved in higher functions such as cognition, and skeletal features including three middle ear bones (which may have evolved several times independently) and a dentary-squamosal jaw joint. Mammalian dentition is often highly complex, with different specialisations in different subgroups, some of which are convergent on dental arrangements in certain reptiles (e.g. teiid lizards). Like birds, mammals are endothermic ("warm-blooded") and possess a four-chambered heart with a double circulation.

Mammals evolved from a lineage of tetrapods. In the late Palaeozoic, one tetrapod line led to sauropsids ("reptiles" and birds), another to synapsids. The early synapsids dominated the land in the middle to late Permian period and diverged into herbivorous and carnivorous forms, pelycosaurs and therapsids. The latter are traditionally referred to as "mammal-like reptiles". However, strictly speaking, mammals did not evolve from reptiles and Reptilia is no longer considered a valid taxon. Within the therapsid lineage, the first true mammals probably arose in the Jurassic period, but a fragmentary early fossil record sparked a discussion of whether mammals are monophyletic or polyphyletic (and thus convergent). This question persists, although the idea of a monophyletic origin of mammals has received more support. In the Cenozoic era, mammals underwent an explosive adaptive radiation, facilitated by the extinction of the non-avian dinosaurs and the separation of the large continental landmass, and by the end of the Tertiary all modern orders had appeared. Mammalian taxonomy is currently being examined by a simultaneous analysis of extensive molecular and phenotypic datasets. While there is still considerable disagreement regarding the interrelationships of placental orders, four placental superorders are now widely accepted - Xenarthra, Afrotheria, Laurasiatheria and Euarchontoglires.

The striking parallels between placentals and marsupials, which have evolved to fill similar ecological niches in different parts of the world, are one of the best-known examples of evolutionary convergence. Numerous placental species have a marsupial counterpart - there are carnivorous, grazing, ant-eating, gliding and burrowing forms. Burrowing mammals generally show extensive convergent adaptations for subterranean life, including anatomy (e.g. reduction or loss of eyes and powerful forelimbs with big claws for digging), physiology, behaviour and even aspects of genetics.

Some groups of placental mammals are particularly interesting in terms of convergence, such as elephants, which parallel other animals in terms of sensory perception, intelligence, tool use and societal organisation, or dolphins, where convergences include body shape and locomotion, cognitive abilities, communication, echolocation and society structure. Striking convergences can also be found within the Carnivora, for example in the viverrids, a highly successful group that includes civets, linsangs, genets and the binturong. And while the cheetah is one of the most popular African animals, it is probably less well known that it is remarkably convergent on the extinct American cheetah, which evolved from a puma-like form. But one of the most spectacular examples of convergence is provided by the sabre-tooth cats. Two distinct ecomorphs are recognised ("dirk-tooth" with elongate sabres and "scimitar-tooth" with somewhat shorter canines), which evidently represent different hunting strategies, and both evolved repeatedly in different felid groups, as well as showing striking parallels in the thylacosmilid marsupials of South America.

When it comes to mammalian reproduction, convergence is ubiquitous. The erectile penis of male mammals, for example, is strikingly similar to that of turtles. Viviparity is not at all a mammalian invention, but has actually evolved more than 120 times in vertebrates alone (mainly in reptiles, but also in some amphibians and fish) as well as in invertebrates (molluscs and insects). Some viviparous insects even secrete "milk" and skinks have remarkably evolved a complex placenta for foetal nutrition. Elaborate parental care is evidently convergent between mammals and birds (and might be linked with endothermy), while sexual mimicry has evolved independently in several mammalian groups, such as hyenas, primates and moles.

The mammalian visual system provides rich insights into evolutionary convergence, for example with respect to trichromatic vision, underwater vision, the detection of UV light and colour-blindness. Apes and humans show a reduction in olfactory capacities, as do whales. More unusually, echolocation has evolved multiple times within the mammals (as well as in some birds), monotremes have independently evolved an electric sense very similar to that of fish and some amphibians, and manatees even possess an equivalent to the lateral line system.

Numerous other fascinating convergences can be found among the mammals, including the production of toxic venom in several species (e.g. platypus, shrews and slow loris), which use poison compounds and venom delivery mechanisms that are similar to those of reptiles, mammalian woodpecker analogues, such as the remarkable aye-aye and the striped opossum, the evolution of eusociality, a social system much better known from the insects, in mole rats and, of course, intelligence and cognition, where primates and dolphins represent prime examples.

Go to the top of the page

This table lists all the Topics which are part of the Category "Mammals"
Topic title Teaser text Availability
Snail eating: an asymmetric diet Snails may not be everyone's first choice on the menu but several distinct colubrid snakes have evolved expert techniques for gorging on these nutritious gastropods. Available
Defensive enrolment Curling up into a ball has evolved many times as an excellent anti-predator defence. Unavailable
Vibrational communication in mammals Kangaroo rats drum their foot on the ground upon encountering a snake. Why? Read on for this and many other fascinating examples of vibrational communication in mammals… Available
Echolocation in bats How can bats navigate in total darkness amongst trees and branches, but still locate a tiny, fluttering insect with extraordinary acuity? All made possible through echolocation, an astonishing sensory mechanism… Available
Foregut fermentation in mammals Foregut fermentation is best known from the ruminants, such as cattle, deer and giraffes, that regurgitate and rechew their food to aid microbial digestion. However, they are not the only mammals to have evolved this digestive strategy... Available
Monochromacy in mammals Underwater environments are dominated by blue light. Ironically, whales and seals cannot see blue, because they have independently lost their short-wavelength opsins. Available
Co-operative breeding n/a Unavailable
Pheromone use in animals, fungi and plants n/a Unavailable
Daily torpor in birds and mammals n/a Unavailable
Endothermy ("warm-bloodedness") n/a Unavailable
Prehensile caudal tails in reptiles and mammals n/a Unavailable
Brain development and complexity in reptiles and mammals n/a Unavailable
Secondary palates in crocodiles and tritylodonts n/a Unavailable
Marsupials with aye-aye-like digits n/a Unavailable
Afrotherian and eulipotyphlan mammal insectivores n/a Unavailable
Teaching in humans, meerkats, birds and ants n/a Unavailable
Placental and marsupial mammal ecology n/a Unavailable
Fin collagen in tuna, dolphins and sharks n/a Unavailable
Ant-eating (myrmecophagy) n/a Unavailable
Kiwi and kokako: mammal-like birds of New Zealand n/a Unavailable
Ultraviolet (UV) vision in insects and vertebrates n/a Unavailable
Agriculture in dugongs When you think of grazing mammals, you might envisage large herds of antelopes roaming African savannahs. Did you know that there is an equivalent in the ocean, feeding on seagrass? Available
Vibrational communication in animals What on earth could an elephant or treehoppers have in common with a seismometer? Available
Pressure sensitivity and the tactile sense (excluding the lateral line) The star-nosed mole is famous for, well, its nose, but do you have any idea what these peculiar 'tentacles' are for? The answer is rather touching and, of course, convergent... Available
Lateral line system in fish and other animals Some cavefish are completely blind, so how do they manage to navigate through their environment with astonishing ease? Available
Anointing in mammals The strategy of anointing the body with the scent of a more dangerous animal has evolved independently several times, including in rodents, hegdehogs and tenrecs. Available
Collagen in animals and bacteria n/a Available
Suction feeding in fish, amphibians, reptiles and aquatic mammals Probably everyone is familiar with the walrus, but did you know that it generates a vacuum in its mouth to suck clams out of their shells? And this is just one example of suction feeding, the feeding mode typically used by bony fish… Available
Crustacean-trapping teeth in mesosaurs and crabeater seals The multi-lobed post-canines of Lobodon carcinophagus are a functional analogue to the long, thin cage-like teeth of Mesosaurus, as both cage and prevent the escape of small crustacean prey. Available
Teeth in aquatic reptiles Aquatic reptiles tend to display one of three dentition types, well adapted to either seize and slice large vertebrate prey, pierce and gouge slippery fish, or entrap small prey such as crustaceans. Available
Dental batteries in ceratopsians, hadrosaurs and elephants The dental batteries or 'pavements' of ceratopsians and hadrosaurs evolved independently, and yet the dentition of several more distantly related animals also converges on their highly adapted tooth form. Available
Teiid lizard dentition: convergence with other reptiles, mammals and fish Teiids are skink-like lizards whose members show a stunning diversity of tooth types, providing rich evidence of convergence within the teiids themselves, in distantly related reptile groups and even in certain mammals and fish. Available
Complex tooth occlusion in notosuchid crocodiles and tritylodonts (proto-mammals) Two unusual Early Cretaceous crocodiles provide a shining example of convergence, as their dentition parallels that observed in a group of advanced proto-mammals called tritylodonts. Available
Reptile dentition: convergence on complex occlusion Some reptiles have transverse chisel-like teeth for slicing, and others have teeth bearing projections ('cusps') that interlock and slice or grind tough food. In each case evolutionary parallels are clear both within and outside the reptiles. Available
Venom and venom fangs in snakes, lizards and synapsids Although the evolution of snake fangs itself provides us with a window on convergence, the presence of fang-like teeth in lizards, therapsids and mammals provides an even broader and more remarkable perspective. Available
Feeding in snakes and lizards The Turtle-headed sea snake feeds on small eggs and its feeding shows intriguing similarities to the way lizards forage, and herbivorous mammals graze and browse. Available
Sociality in mole-rats and meerkats n/a Unavailable
Bats: Insights into convergence Bats show a fascinating array of convergences, from echolocation to flight to nectar feeding. Vampire bats can even detect infrared radiation, while others might be able to see into the ultraviolet end of the spectrum. Available
Echolocation in toothed whales and ground-dwelling mammals Given the extraordinary powers of echolocation in bats, it is not surprising that this group has received the most attention. However, they are not the only mammals to have evolved echolocation. Who invented sonar millions of years before the Navy? Available
Gliding in feathered reptiles A number of reptile species have been discovered in the Mesozoic fossil record, bearing feathers that were apparently used to support gliding locomotion, rather than true, powered flight as we see in present day birds. Available
Gliding mammals Gliding mammals rely primarily on extensive skin membranes or ‘patagia’ that stretch between fore- and hind-limbs, creating a wing-like structure. Available
Gliding reptiles In the reptiles, different forms of skin membrane (called ‘patagia’) and in some extinct species, primitive feathers, have evolved convergently as adaptations for gliding. Available
Mammal-like placentation in skinks (and fish) “Only two types of vertebrates [have] evolved a reproductive pattern in which the chorioallantoic placenta provides the nutrients for fetal development. One is [...] the eutherian mammals […], and the other, a few lineages of the family Scincidae.” A.F. Flemming (2003) J Exp Zool 299A 33-47 Available
Viviparity in lizards, snakes and mammals “In over 100 lineages of […] squamates, the oviduct has been recruited for viviparous gestation of the embryos, representing a degree of evolutionary convergence that is unparalleled in vertebrate history.” D. G. Blackburn (1998) Journal of Experimental Zoology, vol.282, p.560 Available
Trichromatic vision in mammals Who has not enjoyed the splash of colour in a market: gorgeous red peppers, the green of basil and what on earth are these purple vegetables over there? All thanks to trichromatic vision, another story of convergence. Available
Mammalian adaptations to underwater vision One of the more obvious eye adaptations in whales is the extraordinary capacity of the pupil to change the size of its opening, from very small when in surface sunlit water to very large when exploring the abyssal gloom. Unavailable
Nocturnal colour vision in moths, geckos and aye-ayes Nocturnal colour vision is clearly convergent, and found in groups as disparate as the hawkmoths (insects), geckos (reptiles) and aye-aye (mammals). Unavailable
Olfaction: insights into convergence Although olfaction is very widespread, there is abundant evidence for repeated convergence of key features, strongly suggesting that there really is an optimal solution to detecting smells. Available
Venom in mammals (and other synapsids) Beware the venomous shrew! Yes, venomous. And convergent on some formidable lizards... Available
Infrared detection in animals Some snakes are famous for 'seeing' infrared, but did you know that their heat-sensing abilities are rivalled by some beetles that can detect forest fires over considerable distances? Available
Taste in arthropods and mammals The ability to taste is obviously an essential component in the life of any animal, both to assess the potential quality of food, its nutrient capacities and also to detect toxins or other dangers. Available
Loss of olfactory capacity in primates and cetaceans It is widely thought that reduced olfactory capacity in apes is linked to the development of acute vision, especially trichromacy. Available
Defensive spines in animals Sea-urchins, porcupines (and porcupine fish), lizards and many other animals bristle with defensive spines. Unavailable
Lipocalins for milk and pheromone transport Lipocalins are proteins that bind to and transport small hydrophobic molecules such as lipids and steroids, and have been associated with biological processes such as milk production, pheromone transport and immune responses. Available
Adhesive pads: from geckos to spiders In terms of adhesive pads we find they have a remarkably wide distribution evolving in at least four distinct groups, including members of the reptiles, amphibians, arthropods and mammals, with tentative parallels in sea urchins. Available
Lysozyme Lysozymes are common antibacterial enzymes that protect our eyes and nose from infection, but some animals have recruited them for a rather different purpose... Available
Filter feeding in whales, birds and reptiles Filter feeding is most familiar in the baleen whales , but closely analogous arrangements have appeared at least twice in the birds, first the flamingos and second the sub-antarctic broad-billed prions. Unavailable
Gut fermentation in herbivorous animals Ever tried eating a newspaper? Don't. Plant cell walls contain cellulose, which is notoriously difficult to digest. Considering that all vertebrates lack the enzymes to attack this polysaccharide, how do so many of them manage to survive on a plant diet? Available
Baculum (penile bone) in mammals Ouch!! Gentlemen, fancy a bone in your penis? Seems a bit risky, given it could fracture during copulation. Even our near ancestors had such a bone. It has probably evolved several times, but what is its function? Available
Sabre-toothed cats and marsupials Marsupials with giant fangs? Yes, not all of the extinct sabre-toothed cats were actually cats… Available
Cheetahs of Africa and America The American cheetah (Miracinoyx) is an extinct Pleistocene form, and is strikingly similar to the cheetah (Acinoyx) that formerly had a wide range in the Old World but is today effectively restricted to Africa. Unavailable
Viverrid ecomorphs: from linsangs to Binturongs The viverrids or civets are a highly successful group of carnivores, fairly closely related to the cats and showing several striking examples of convergence. Unavailable
Cavitation: bubble formation in plants, reptiles and shrimps The formation of bubbles in a fluid is known as cavitation. Typically this occurs at low pressures, and is perhaps best known in the xylem of plants where embolisms can be destructive to the surrounding tissues. Available
Dolphin communication, cognition and sociality Dolphins are one of the most intriguing sources of evolutionary convergence, having cognitive abilities that seem to find many parallels in the great apes, and rather remarkably even extend to tool use. Available
Sleep in animals Suffering from insomnia? Fruit flies do as well... Available
Eusociality in arthropods and mammals Eusociality is most familiar in the insects, where it has evolved several times, notably in bees, wasps, ants and termites, as well as in thrips and aphids. Unavailable
Agriculture: from ants to dugongs Human farmers tending their fields are a familiar sight. But don't forget about those fungus-farming termites or the fish with a garden of algae… Available
Woodpeckers and woodpecker-like birds and mammals You think woodpeckers are unique? Consider the ovenbirds. Or even the curious aye-aye. Available
Insecticide production: from plants to primates Application of insecticides, such as against mosquitoes, has been documented in several primates and birds. Unavailable
Developmental genetic pathways to convergence At first sight there is a fairly simple dichotomy between convergent features that have effectively the same genetic basis, and those where the same feature emerges but the underlying genetics are different. The former, however, is somewhat more complicated... Available
Fission-fusion societies Fission-fusion societies has evolved repeatedly and are found, for example, in the elephants, dolphins, chimpanzees, as well as some New World Monkeys. Unavailable
Tool use in elephants Elephants are well known for their accuracy in throwing objects, and in addition use sticks for a number of purposes. Also of considerable interest is their use of fly-whisks to keep blood-sucking flies at bay. Unavailable
Elephant response to death Elephants are extremely unusual in their reaction to the dying and death of their compatriots, which includes attempts at resuscitation and grieving. Unavailable
Elephants: senses, intelligence and social structure There is evidence that elephants are sensitive to seismic communication, with the large pads of the feet and the trunk tip capable of picking up vibrations transmitted through the ground. Unavailable
Parental care in vertebrates, echinoids, molluscs and brachiopods The independent evolution of parental care is far more widespread than birds and mammals, extending as far as molluscs and echinoderms! Unavailable
Alcoholism in mammals and flies Identification of alcohol tolerance (or lack thereof) in different animal groups is important because alcoholism in humans may have some genetic basis. Unavailable
Play in birds, mammals and octopus Social play is the hall-mark of the most intelligent of this planet’s species, and there is a particularly striking convergence between birds and mammals. Unavailable
"Colour vision" in Firefly squid The Japanese firefly squid (Watasenia scintillans), which inhabits the deep ocean, has three visual pigments located in different parts of the retina that are likely to allow colour discrimination as they each have distinct spectral sensitivities. Available
Penis form in mammals, turtles, birds and octopus The specific case of a penis with a hydrostatic structure, as well as an array of collagen fibres that allows both expansion and guards against aneurysms, has evolved in a strikingly convergent fashion in mammals and turtles. Available
Personality in vertebrates and cephalopods Personality in the vertebrates might, therefore, be deeply embedded in their phylogeny, although this does not rule out the convergent appearance of more complex personality traits in more advanced vertebrates, notably birds and mammals. Unavailable
Ultrasound communication in mammals and amphibians Amphibians are adept at vocalization, and remarkably ultrasonic communication has also evolved in the Concave-eared torrent frog, from China. Unavailable
Viviparity (live birth) in animals Viviparity is rampantly convergent, with famous examples in the reptiles, notably the lizards and snakes. Unavailable
Lekking in birds, fish, mammals and cephalopods Complex interactions between males and females prior to mating have evolved independently many times. Amongst the most familiar examples are leks. Unavailable
Electroreception in fish, amphibians and monotremes From an evolutionary point of view, electroreception is particularly intriguing as a sense modality that has been repeatedly lost and reinvented again. Available
Burrowing: from worms to vertebrates Quite a few adaptations are useful for burrowing into the soil. So it is not exactly surprising that they have evolved several times... Available
Asymmetric eye use in octopus, dolphins and birds In a number of cases one eye is used in preference to another. This convergent phenomenon is found in octopus (cephalopods), dolphins, birds, and other animals. Unavailable
Sexual mimicry in mammals and cephalopods Sexual mimicry is widespread, and the most famous example is probably the male-like genitalia in the females of the hyaena. Unavailable
Learning and memory in vertebrates and cephalopods Convergence in learning (and by implication memory) is important not only because it will give us clues as to the nature of consciousness, but it will also have a bearing on the reality (or otherwise) of pain and suffering in “primitive” animals. Unavailable
Ultraviolet (UV) absorption in vertebrates and cephalopods In some vertebrates (fish, mammals) and cephalopods we find an interesting convergence whereby some of the incoming ultraviolet is screened out. Unavailable