Mammal | Definition, Characteristics, Classification, Examples, & Facts

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id="content" class="md-content"> <div class="md-article-container template-desktop infinite-pagination"> <div class="infinite-scroll-container article last"> <article class="article-content container-lg qa-content px-0 pt-0 pb-40 py-lg-20 content md-expanded" data-topic-id="360838"> <div class="grid gx-0"> <div class="col-auto"> <div class="topic-left-rail md-article-drawer position-relative d-flex border-right-sm border-left-sm open"> <div class="drawer d-flex flex-column open"> <div class="left-rail-section-content"> <div class="topic-left-rail-header text-truncate bg-gray-50 position-relative text-right d-flex align-items-center"> <div class="tlr-title px-20 py-15 text-left"> <em class="material-icons text-gray-400 d-lg-none" data-icon="toc"></em> <a class="font-serif font-weight-bold text-black link-blue" href="https://www.britannica.com/animal/mammal">mammal</a> </div> <button aria-label="Close" class="js-sections-close-button btn-link btn-sm btn d-lg-none position-absolute top-0 p-10 right-0" > <em class="material-icons font-26" data-icon="close"></em> </button> </div> <div class="section-content pl-10 pr-20 pl-sm-50 pr-sm-60 pl-lg-5 pr-lg-10 pt-10 pt-lg-0 bg-gray-50 clear-catfish-ad"> <div class="toc mb-20"> <div class="font-serif font-14 font-weight-bold mx-15 mb-15 mt-20"> Table of Contents </div> <ul class="list-unstyled my-0" data-level="h1"><li data-target="#ref1"><div class="pl-25"><a class="link-gray-900 w-100" href="/animal/mammal">Introduction & Top Questions</a></div><div class="ml-40 toc-drawer sub-toc-drawer"></div></li><li data-target="#ref51696"><div class="d-flex align-items-center"><button class="h1-link-drawer-button btn btn-xs btn-circle d-flex rounded" type="button" aria-label="Toggle Heading"><em class="material-icons font-18" data-icon="keyboard_arrow_right"></em></button><a class="w-100 link-gray-900" href="/animal/mammal#ref51696">General features</a></div><div class="ml-40 toc-drawer sub-toc-drawer"><ul class="list-unstyled" data-level="h2"><li data-target="#ref235397"><a class="w-100 link-gray-900" href="/animal/mammal#ref235397">Diversity</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref51697"><a class="w-100 link-gray-900" href="/animal/mammal/Importance-to-humans">Importance to humans</a></li></ul></div></li><li data-target="#ref51698"><div class="d-flex align-items-center"><button class="h1-link-drawer-button btn btn-xs btn-circle d-flex rounded" type="button" aria-label="Toggle Heading"><em class="material-icons font-18" data-icon="keyboard_arrow_right"></em></button><a class="w-100 link-gray-900" href="/animal/mammal/Importance-to-humans#ref51698">Natural history</a></div><div class="ml-40 toc-drawer sub-toc-drawer"><ul class="list-unstyled" data-level="h2"><li data-target="#ref51699"><a class="w-100 link-gray-900" href="/animal/mammal/Reproduction">Reproduction</a><ul class="list-unstyled" data-level="h3"><li data-target="#ref235398"><a class="w-100 link-gray-900" href="/animal/mammal/Reproduction#ref235398">Estrus and other cycles</a></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref235399"><a class="w-100 link-gray-900" href="/animal/mammal/Reproduction#ref235399">Implantation, gestation, and birth</a></li></ul></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref51700"><a class="w-100 link-gray-900" href="/animal/mammal/Reproduction#ref51700">Behaviour</a><ul class="list-unstyled" data-level="h3"><li data-target="#ref51701"><a class="w-100 link-gray-900" href="/animal/mammal/Reproduction#ref51701">Social behaviour</a></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref51702"><a class="w-100 link-gray-900" href="/animal/mammal/Territoriality">Territoriality</a></li></ul></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref51703"><a class="w-100 link-gray-900" href="/animal/mammal/Territoriality#ref51703">Ecology</a><ul class="list-unstyled" data-level="h3"><li data-target="#ref51704"><a class="w-100 link-gray-900" href="/animal/mammal/Territoriality#ref51704">Response to environmental cycles</a></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref51705"><a class="w-100 link-gray-900" href="/animal/mammal/Territoriality#ref51705">Populations</a></li></ul></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref51706"><a class="w-100 link-gray-900" href="/animal/mammal/Locomotion">Locomotion</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref235400"><a class="w-100 link-gray-900" href="/animal/mammal/Locomotion#ref235400">Food habits</a></li></ul></div></li><li data-target="#ref235401"><div class="d-flex align-items-center"><button class="h1-link-drawer-button btn btn-xs btn-circle d-flex rounded" type="button" aria-label="Toggle Heading"><em class="material-icons font-18" data-icon="keyboard_arrow_right"></em></button><a class="w-100 link-gray-900" href="/animal/mammal/Form-and-function">Form and function</a></div><div class="ml-40 toc-drawer sub-toc-drawer"><ul class="list-unstyled" data-level="h2"><li data-target="#ref51709"><a class="w-100 link-gray-900" href="/animal/mammal/Form-and-function#ref51709">Skin and hair</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref51710"><a class="w-100 link-gray-900" href="/animal/mammal/Form-and-function#ref51710">Teeth</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref51711"><a class="w-100 link-gray-900" href="/animal/mammal/Skeleton">Skeleton</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref235402"><a class="w-100 link-gray-900" href="/animal/mammal/Skeleton#ref235402">Muscles</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref51714"><a class="w-100 link-gray-900" href="/animal/mammal/Digestive-system">Digestive system</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref51715"><a class="w-100 link-gray-900" href="/animal/mammal/Digestive-system#ref51715">Excretory system</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref51716"><a class="w-100 link-gray-900" href="/animal/mammal/Digestive-system#ref51716">Reproductive system</a><ul class="list-unstyled" data-level="h3"><li data-target="#ref235403"><a class="w-100 link-gray-900" href="/animal/mammal/Digestive-system#ref235403">The male tract</a></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref235404"><a class="w-100 link-gray-900" href="/animal/mammal/Digestive-system#ref235404">The female tract</a></li></ul></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref51717"><a class="w-100 link-gray-900" href="/animal/mammal/Circulatory-system">Circulatory system</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref51718"><a class="w-100 link-gray-900" href="/animal/mammal/Circulatory-system#ref51718">Respiratory system</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref235405"><a class="w-100 link-gray-900" href="/animal/mammal/Circulatory-system#ref235405">Nervous and endocrine systems</a></li></ul></div></li><li data-target="#ref51720"><div class="d-flex align-items-center"><button class="h1-link-drawer-button btn btn-xs btn-circle d-flex rounded" type="button" aria-label="Toggle Heading"><em class="material-icons font-18" data-icon="keyboard_arrow_right"></em></button><a class="w-100 link-gray-900" href="/animal/mammal/Evolution-and-classification">Evolution and classification</a></div><div class="ml-40 toc-drawer sub-toc-drawer"><ul class="list-unstyled" data-level="h2"><li data-target="#ref51721"><a class="w-100 link-gray-900" href="/animal/mammal/Evolution-and-classification#ref51721">The evolution of the mammalian condition</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref51722"><a class="w-100 link-gray-900" href="/animal/mammal/Classification">Classification</a></li></ul></div></li></ul> <a class="toc-extra-link link-gray-900" href="https://www.britannica.com/animal/mammal/additional-info">References & Edit History</a> <a class="toc-extra-link link-gray-900" href="/facts/mammal">Quick Facts & Related Topics</a> </div> <div class="tlr-media-slider pb-10 mb-30"> <a class="section-header link-gray-900 font-serif font-14 font-weight-bold mb-10 mx-10" href="https://www.britannica.com/animal/mammal/images-videos">Images & Videos</a> <div class="slider js-slider position-relative d-inline-flex align-items-center mw-100 "> <div class="slider-container js-slider-container overflow-hidden d-flex overflow-hidden text-nowrap ml-15"> <a href="https://cdn.britannica.com/96/94596-050-D327EF86/polar-bear-cubs.jpg" data-href="/media/1/360838/106976" class="media-overlay-link d-inline-block mr-5"> <img loading="lazy" src="https://cdn.britannica.com/96/94596-004-EC1E5668/polar-bear-cubs.jpg" alt="polar bear and cubs" height="50" /> </a> <a href="https://cdn.britannica.com/69/146369-050-6B1B677E/Okapi.jpg" data-href="/media/1/360838/155742" class="media-overlay-link d-inline-block mr-5"> <img loading="lazy" src="https://cdn.britannica.com/69/146369-004-DF571746/Okapi.jpg" alt="okapi" height="50" /> </a> <a href="https://cdn.britannica.com/63/6563-050-3F5BC4B3/realms-regions-world.jpg" data-href="/media/1/360838/27" class="media-overlay-link d-inline-block mr-5"> <img loading="lazy" src="https://cdn.britannica.com/63/6563-004-C35DF016/realms-regions-world.jpg" alt="faunal realms" height="50" /> </a> <a href="https://cdn.britannica.com/53/5953-050-91075E1D/goats-Silk-Road-China-Takla-Makan-Desert.jpg" data-href="/media/1/360838/570" class="media-overlay-link d-inline-block mr-5"> <img loading="lazy" src="https://cdn.britannica.com/53/5953-004-94200FC6/goats-Silk-Road-China-Takla-Makan-Desert.jpg" alt="goat herding" height="50" /> </a> <a href="https://cdn.britannica.com/94/67394-050-C0CD9347/Steller-sea-cow-shore-kelp.jpg" data-href="/media/1/360838/57537" class="media-overlay-link d-inline-block mr-5"> <img loading="lazy" src="https://cdn.britannica.com/94/67394-004-903736D1/Steller-sea-cow-shore-kelp.jpg" alt="Steller's sea cow" height="50" /> </a> <a href="https://cdn.britannica.com/81/136581-050-EE222F5A/bottlenose-dolphin-calf-swimming-mother-amusement-park-2009.jpg" data-href="/media/1/360838/139730" class="media-overlay-link d-inline-block mr-5"> <img loading="lazy" src="https://cdn.britannica.com/81/136581-004-3B44E2DC/bottlenose-dolphin-calf-swimming-mother-amusement-park-2009.jpg" alt="bottlenose dolphins" height="50" /> </a> <a href="https://cdn.britannica.com/30/94930-050-42F45F28/egg-journey-woman-uterus-mammals-eggs-ovaries.jpg" data-href="/media/1/360838/228268" class="media-overlay-link d-inline-block mr-5"> <img loading="lazy" src="https://cdn.britannica.com/30/94930-004-A1CD7B94/egg-journey-woman-uterus-mammals-eggs-ovaries.jpg" alt="journey of a fertilized egg" height="50" /> </a> <a href="https://cdn.britannica.com/32/117432-050-8C3BE02A/state-development-human-embryo-pregnancy.jpg" data-href="/media/1/360838/228271" class="media-overlay-link d-inline-block mr-5"> <img loading="lazy" src="https://cdn.britannica.com/32/117432-004-8F53EBF7/state-development-human-embryo-pregnancy.jpg" alt="embryonic development" height="50" /> </a> <a href="https://cdn.britannica.com/41/146341-050-B49C48EE/Leopard-cub-Kenya-Masai-Mara-National-Reserve.jpg" data-href="/media/1/360838/155728" class="media-overlay-link d-inline-block mr-5"> <img loading="lazy" src="https://cdn.britannica.com/41/146341-004-CC4C5FAE/Leopard-cub-Kenya-Masai-Mara-National-Reserve.jpg" alt="leopard" height="50" /> </a> <a href="https://cdn.britannica.com/33/160333-050-07E44E66/mammals-pack-Yellowstone-National-Park-limits-members.jpg" 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Armstrong</div> <div class="editor-description font-12 font-serif mt-5 clamp-description text-black">Professor of Biology, University of Colorado, Boulder.</div> </a> <div data-popper-arrow></div> </div> <span class="btn btn-link editor-link p-0 qa-byline-link gtm-byline font-12 byline-contributor text-decoration-underline"> David M. Armstrong</span>, <div class="editor-popover popover p-0"> <a class="d-block p-20 gtm-byline font-12 byline-contributor" href="/contributor/Don-E-Wilson/4519" > <div class="editor-title font-16 font-weight-bold">Don E. Wilson</div> <div class="editor-description font-12 font-serif mt-5 clamp-description text-black">Curator Emeritus of Mammals, National Museum of Natural History, Washington, D.C. Coauthor of <em>Mammals of North America</em> and many others; coeditor of <em>Mammal Species of the World</em> and many...</div> </a> <div data-popper-arrow></div> </div> <span class="btn btn-link editor-link p-0 qa-byline-link gtm-byline font-12 byline-contributor text-decoration-underline"> Don E. Wilson</span><span class="text-gray-700 mx-5">•</span><a class="see-all border-gray-700 gtm-byline" rel="nofollow" href="https://www.britannica.com/animal/mammal/additional-info#contributors">All</a> </div> <div class="font-serif font-12 text-gray-700"> <span class="qa-fact-checked-by">Fact-checked by</span> <div class="editor-popover popover p-0"> <a class="d-block p-20 font-12" href="/editor/The-Editors-of-Encyclopaedia-Britannica/4419" > <div class="editor-title font-16 font-weight-bold">The Editors of Encyclopaedia Britannica</div> <div class="editor-description font-12 font-serif mt-5 text-black">Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree. They write new content and verify and edit content received from contributors.</div> </a> <div data-popper-arrow></div> </div> <span class="btn btn-link editor-link p-0 qa-byline-link font-12 "> The Editors of Encyclopaedia Britannica</span></div> <div class="last-updated font-12 font-serif"> <span class="text-gray-700"> Last Updated: <time datetime="2024-10-31T00:00:00CDT" >Oct 31, 2024</time> •</span> <a class="byline-edit-history" href="https://www.britannica.com/animal/mammal/additional-info#history" rel="nofollow">Article History</a> </div></div> </div> <button class="d-flex d-lg-none btn btn-outline-blue border rounded-sm shadow-sm mobile-toc-button gtm-mobile-toc-inline-button d-none d-sm-block js-sections-inline-button module-spacing btn d-lg-none"> <em class="material-icons mr-5 ml-n10 my-n5 md-icon" data-icon="toc"></em> Table of Contents </button> <div class="d-flex d-sm-none flex-row"> <button class="d-flex d-lg-none btn btn-outline-blue border rounded-sm shadow-sm mobile-toc-button gtm-mobile-toc-inline-button js-sections-inline-button module-spacing"> <em class="material-icons mr-5 ml-n10 my-n5 md-icon" data-icon="toc"></em> Table of Contents </button> <button class="ai-ask-button btn border-2 ai-ask-button btn border-2 module-spacing btn-sm js-inline-ai-ask-button btn-outline-red-400 border-red-400 p-10 ml-5"> Ask the Chatbot a Question </button> </div> <div class="js-qf-module qf-module px-40 px-sm-20 py-15 mx-auto module-spacing font-14 bg-gray-50 rounded"> <div class="facts-list mt-10"> <div class=""> <div class="js-fact mb-10 line-clamp clamp-3"> <dl> <dt>Key People: </dt> <dd><a href="/biography/Steen-Willadsen" topicid="2130858">Steen Willadsen</a></dd> <dd><a href="/biography/Karl-Ernst-Ritter-von-Baer-Edler-von-Huthorn" topicid="48635">Karl Ernst von Baer</a></dd> <dd><a href="/biography/George-Gaylord-Simpson" topicid="545434">George Gaylord Simpson</a></dd> <dd><a href="/biography/Jan-Evangelista-Purkinje" topicid="484087">Jan Evangelista Purkinje</a></dd> <dd><a href="/biography/Carl-von-Voit" topicid="631930">Carl von Voit</a></dd> </dl> <button class="js-more-btn d-none btn btn-unstyled font-12 bg-gray-50" aria-label="Toggle more/less fact data"> <em class="js-content link-blue">(Show more)</em> </button> </div> </div> <div class=""> <div class="js-fact mb-10 line-clamp clamp-3"> <dl> <dt>Related Topics: </dt> <dd><a href="/animal/primate-mammal" topicid="476264">primate</a></dd> <dd><a href="/animal/artiodactyl" topicid="37203">artiodactyl</a></dd> <dd><a href="/animal/perissodactyl" topicid="452044">perissodactyl</a></dd> <dd><a href="/animal/cetacean" topicid="103892">cetacean</a></dd> <dd><a href="/animal/bat-mammal" topicid="55655">bat</a></dd> </dl> <button class="js-more-btn d-none btn btn-unstyled font-12 bg-gray-50" aria-label="Toggle more/less fact data"> <em class="js-content link-blue">(Show more)</em> </button> </div> <div class="text-center"> <a class="btn btn-sm btn-link p-0" href="/facts/mammal"> See all related content </a> </div> </div> </div> </div><script type="application/ld+json"> { "@context": "https://schema.org", "@type": "FAQPage", "mainEntity": [ { "@type" : "Question", "name" : "<div>How are mammals distinct from other animals?<\/div><em><\/em>", "acceptedAnswer" : { "@type" : "Answer", "text" : "An animal is considered a mammal if it can produce <a href=\"https:\/\/www.britannica.com\/topic\/milk\">milk<\/a>. Other features unique to mammals include hair or fur (chemically different from hairlike structures on non-mammals); the malleus, incus, and stapes in the ear; and a diaphragm separating the heart and lungs from the abdomen. Also, mammals lack nuclei in mature red blood cells." } } , { "@type" : "Question", "name" : "<div>How many species of mammals are there?<\/div><em><\/em>", "acceptedAnswer" : { "@type" : "Answer", "text" : "It is estimated that there are more than 5,500 living mammal species. Mammals are incredibly diverse and can be found in every major habitat." } } , { "@type" : "Question", "name" : "<div>What is the biggest mammal?<\/div><em><\/em>", "acceptedAnswer" : { "@type" : "Answer", "text" : "The biggest living mammal\u2014indeed, the largest animal ever\u2014is the <a href=\"https:\/\/www.britannica.com\/animal\/blue-whale\">blue whale<\/a>. It can be as heavy as 180 metric tons (200 short tons) and reach a length of more than 30 metres (98 feet)." } } , { "@type" : "Question", "name" : "<div>Did mammals and dinosaurs exist at the same time?<\/div><em><\/em>", "acceptedAnswer" : { "@type" : "Answer", "text" : "According to fossil records, mammals have existed since the <a href=\"https:\/\/www.britannica.com\/science\/Triassic-Period\">Triassic Period<\/a>, alongside early dinosaurs such as <em>Riojasaurus<\/em> and <em>Saltopus<\/em>. True mammals, such as the shrewlike <em>Juramaia sinensis<\/em>, emerged 160 million years ago, during the <a href=\"https:\/\/www.britannica.com\/science\/Jurassic-Period\">Jurassic Period<\/a>, where they would have lived alongside dinosaurs such as the <a href=\"https:\/\/www.britannica.com\/animal\/brachiosaur\">brachiosaur<\/a> and <em><a href=\"https:\/\/www.britannica.com\/animal\/Stegosaurus\">Stegosaurus<\/a><\/em>." } } , { "@type" : "Question", "name" : "<div>What did mammals evolve from?<\/div><em><\/em>", "acceptedAnswer" : { "@type" : "Answer", "text" : "Mammals evolved from a group of reptiles called <a href=\"https:\/\/www.britannica.com\/animal\/therapsid\">therapsids<\/a>, which lived from 299 million to 200 million years ago. Therapsids were quadrupedal and had such mammalian features as specialized tooth structures and an opening in the temporal region of the skull. They were most likely warm-blooded." } } , { "@type" : "Question", "name" : "<div>Why is the platypus a mammal?<\/div><em><\/em>", "acceptedAnswer" : { "@type" : "Answer", "text" : "The <a href=\"https:\/\/www.britannica.com\/animal\/platypus\">platypus<\/a> has many features not found in other mammals. It is part of a group of mammals known as <a href=\"https:\/\/www.britannica.com\/animal\/monotreme\">monotremes<\/a>, which lay eggs and have specialized mouthparts. They branched off earlier in the evolutionary tree than marsupials and placental mammals, and they retained more reptilian features, such as a lower body temperature." } } ] } </script> <div class="top-questions qa-accordion d-flex flex-column module-spacing"><div class="font-weight-bold font-14 mb-5"> Top Questions </div><div id="intent-accordion" class="md-intent-accordion"><div class="top-question bg-gray-50 rounded" data-value="1"> <h3 class="accordion--question font-14 font-weight-normal cursor-pointer rounded"><div class="pe-none d-flex justify-content-between align-items-center"><div>How are mammals distinct from other animals?</div><em class="material-icons" data-icon="expand_more"></em></div></h3> <div class="accordion--answer hidden p-15 pt-5 font-16 mb-5"><div class="accordion--answer-copy"><p>An animal is considered a mammal if it can produce <a href="https://www.britannica.com/topic/milk" class="md-crosslink " data-show-preview="true">milk</a>. Other features unique to mammals include hair or fur (chemically different from hairlike structures on non-mammals); the malleus, incus, and stapes in the ear; and a diaphragm separating the heart and lungs from the abdomen. Also, mammals lack nuclei in mature red blood cells.</p></div></div> </div><div class="top-question bg-gray-50 rounded" data-value="2"> <h3 class="accordion--question font-14 font-weight-normal cursor-pointer rounded"><div class="pe-none d-flex justify-content-between align-items-center"><div>How many species of mammals are there?</div><em class="material-icons" data-icon="expand_more"></em></div></h3> <div class="accordion--answer hidden p-15 pt-5 font-16 mb-5"><div class="accordion--answer-copy"><p>It is estimated that there are more than 5,500 living mammal species. Mammals are incredibly diverse and can be found in every major habitat.</p></div></div> </div><div class="top-question bg-gray-50 rounded" data-value="3"> <h3 class="accordion--question font-14 font-weight-normal cursor-pointer rounded"><div class="pe-none d-flex justify-content-between align-items-center"><div>What is the biggest mammal?</div><em class="material-icons" data-icon="expand_more"></em></div></h3> <div class="accordion--answer hidden p-15 pt-5 font-16 mb-5"><div class="accordion--answer-copy"><p>The biggest living mammal—indeed, the largest animal ever—is the <a href="https://www.britannica.com/animal/blue-whale" class="md-crosslink " data-show-preview="true">blue whale</a>. It can be as heavy as 180 metric tons (200 short tons) and reach a length of more than 30 metres (98 feet).</p></div></div> </div><div class="top-question bg-gray-50 rounded" data-value="4"> <h3 class="accordion--question font-14 font-weight-normal cursor-pointer rounded"><div class="pe-none d-flex justify-content-between align-items-center"><div>Did mammals and dinosaurs exist at the same time?</div><em class="material-icons" data-icon="expand_more"></em></div></h3> <div class="accordion--answer hidden p-15 pt-5 font-16 mb-5"><div class="accordion--answer-copy"><p>According to fossil records, mammals have existed since the <a href="https://www.britannica.com/science/Triassic-Period" class="md-crosslink " data-show-preview="true">Triassic Period</a>, alongside early dinosaurs such as <em>Riojasaurus</em> and <em>Saltopus</em>. True mammals, such as the shrewlike <em>Juramaia sinensis</em>, emerged 160 million years ago, during the <a href="https://www.britannica.com/science/Jurassic-Period" class="md-crosslink " data-show-preview="true">Jurassic Period</a>, where they would have lived alongside dinosaurs such as the <a href="https://www.britannica.com/animal/brachiosaur" class="md-crosslink " data-show-preview="true">brachiosaur</a> and <em><a href="https://www.britannica.com/animal/Stegosaurus" class="md-crosslink " data-show-preview="true">Stegosaurus</a></em>.</p></div></div> </div><div class="top-question bg-gray-50 rounded" data-value="5"> <h3 class="accordion--question font-14 font-weight-normal cursor-pointer rounded"><div class="pe-none d-flex justify-content-between align-items-center"><div>What did mammals evolve from?</div><em class="material-icons" data-icon="expand_more"></em></div></h3> <div class="accordion--answer hidden p-15 pt-5 font-16 mb-5"><div class="accordion--answer-copy"><p>Mammals evolved from a group of reptiles called <a href="https://www.britannica.com/animal/therapsid" class="md-crosslink " data-show-preview="true">therapsids</a>, which lived from 299 million to 200 million years ago. Therapsids were quadrupedal and had such mammalian features as specialized tooth structures and an opening in the temporal region of the skull. They were most likely warm-blooded.</p></div></div> </div><div class="top-question bg-gray-50 rounded" data-value="6"> <h3 class="accordion--question font-14 font-weight-normal cursor-pointer rounded"><div class="pe-none d-flex justify-content-between align-items-center"><div>Why is the platypus a mammal?</div><em class="material-icons" data-icon="expand_more"></em></div></h3> <div class="accordion--answer hidden p-15 pt-5 font-16 mb-5"><div class="accordion--answer-copy"><p>The <a href="https://www.britannica.com/animal/platypus" class="md-crosslink " data-show-preview="true">platypus</a> has many features not found in other mammals. It is part of a group of mammals known as <a href="https://www.britannica.com/animal/monotreme" class="md-crosslink " data-show-preview="true">monotremes</a>, which lay eggs and have specialized mouthparts. They branched off earlier in the evolutionary tree than marsupials and placental mammals, and they retained more reptilian features, such as a lower body temperature.</p></div></div> </div><button class="show-more-button js-toggle-top-questions btn btn-unstyled font-14 d-flex pr-10 rounded-sm"><em class="material-icons" data-icon="expand_more"></em></button></div></div><div class="bg-gray-50 p-15 rounded module-spacing recent-news d-flex flex-column float-false"> <div> <h2 class="font-weight-bold font-14 m-0 d-inline"> News <span class="text-gray-600">•</span> </h2> <div class="recent-news-item first-recent-news-item d-inline"> <a class="font-14 gtm-ap-news-link" href="https://www.bbc.com/news/articles/cwyg4213597o" rel="nofollow">Fox cub photo at Leeds Castle wins first prize in European contest</a> <span class="font-14 text-gray-600"> <span>•</span> Oct. 30, 2024, 2:13 AM ET (BBC) </span> </div> </div> <div class="rest-of-recent-news-items"> <button class="js-toggle-recent-news d-flex btn btn-unstyled font-14 pr-10 rounded-sm mt-10" aria-label="Toggle additional news items"> Show less <em class="material-icons" data-icon="expand_less"></em> </button> </div> </div><span class="marker before-article"></span><section data-level="1" id="ref1"><span class="marker PREMOD1 mod-inline"></span><p class="topic-paragraph"><strong><span id="ref193712"></span>mammal</strong>, (class Mammalia), any member of the group of <a href="https://www.britannica.com/animal/vertebrate" class="md-crosslink " data-show-preview="true">vertebrate</a> <a href="https://www.britannica.com/animal/animal" class="md-crosslink " data-show-preview="true">animals</a> in which the young are nourished with <a href="https://www.britannica.com/topic/milk" class="md-crosslink " data-show-preview="true">milk</a> from special <a href="https://www.britannica.com/science/mammary-gland" class="md-crosslink " data-show-preview="true">mammary glands</a> of the mother. In addition to these <a class="md-dictionary-link md-dictionary-tt-off eb" data-term="characteristic" href="https://www.britannica.com/dictionary/characteristic" data-type="EB">characteristic</a> milk glands, mammals are distinguished by several other unique features. <a href="https://www.britannica.com/science/hair-anatomy" class="md-crosslink " data-show-preview="true">Hair</a> is a typical mammalian feature, although in many <a href="https://www.britannica.com/animal/whale" class="md-crosslink " data-show-preview="true">whales</a> it has disappeared except in the fetal stage. The mammalian lower <a href="https://www.britannica.com/science/jaw" class="md-crosslink " data-show-preview="true">jaw</a> is hinged directly to the <a href="https://www.britannica.com/science/skull" class="md-crosslink " data-show-preview="true">skull</a>, instead of through a separate <a href="https://www.britannica.com/science/bone-anatomy" class="md-crosslink " data-show-preview="true">bone</a> (the quadrate) as in all other vertebrates. A chain of three tiny bones transmits sound waves across the middle <a href="https://www.britannica.com/science/ear" class="md-crosslink " data-show-preview="true">ear</a>. A muscular <a href="https://www.britannica.com/science/diaphragm-anatomy" class="md-crosslink " data-show-preview="true">diaphragm</a> separates the <a href="https://www.britannica.com/science/heart" class="md-crosslink " data-show-preview="true">heart</a> and the <a href="https://www.britannica.com/science/lung" class="md-crosslink " data-show-preview="true">lungs</a> from the <a href="https://www.britannica.com/science/abdominal-cavity" class="md-crosslink autoxref " data-show-preview="true">abdominal cavity</a>. Only the left aortic arch persists. (In <a href="https://www.britannica.com/animal/bird-animal" class="md-crosslink " data-show-preview="true">birds</a> the right aortic arch persists; in <a href="https://www.britannica.com/animal/reptile" class="md-crosslink " data-show-preview="true">reptiles</a>, <a href="https://www.britannica.com/animal/amphibian" class="md-crosslink " data-show-preview="true">amphibians</a>, and <a href="https://www.britannica.com/animal/fish" class="md-crosslink " data-show-preview="true">fishes</a> both arches are retained.) Mature red blood cells (<a href="https://www.britannica.com/science/red-blood-cell" class="md-crosslink " data-show-preview="true">erythrocytes</a>) in all mammals lack a <a href="https://www.britannica.com/science/nucleus-biology" class="md-crosslink " data-show-preview="true">nucleus</a>; all other vertebrates have nucleated red blood cells.</p><span class="marker MOD1 mod-inline"></span><span class="marker PREMOD2 mod-inline"></span><p class="topic-paragraph">Except for the <span id="ref743242"></span><a href="https://www.britannica.com/animal/monotreme" class="md-crosslink " data-show-preview="true">monotremes</a> (an <a href="https://www.britannica.com/science/egg-biology" class="md-crosslink " data-show-preview="true">egg</a>-laying order of mammals <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="comprising" href="https://www.merriam-webster.com/dictionary/comprising" data-type="MW">comprising</a> echidnas and the duck-billed <a href="https://www.britannica.com/animal/platypus" class="md-crosslink " data-show-preview="true">platypus</a>), all mammals are <span id="ref743333"></span><a href="https://www.britannica.com/science/viviparity" class="md-crosslink " data-show-preview="true">viviparous</a>—they <a href="https://www.britannica.com/animal/bear" class="md-crosslink autoxref " data-show-preview="true">bear</a> live young. In the <a href="https://www.britannica.com/animal/placental-mammal" class="md-crosslink " data-show-preview="true">placental mammals</a> (which have a <a href="https://www.britannica.com/science/placenta-human-and-animal" class="md-crosslink " data-show-preview="true">placenta</a> to <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="facilitate" href="https://www.merriam-webster.com/dictionary/facilitate" data-type="MW">facilitate</a> <a href="https://www.britannica.com/science/nutrient" class="md-crosslink " data-show-preview="true">nutrient</a> and waste exchange between the mother and the developing fetus), the young are carried within the mother’s womb, reaching a relatively advanced stage of development before birth. In the <a href="https://www.britannica.com/animal/marsupial" class="md-crosslink " data-show-preview="true">marsupials</a> (e.g., <a href="https://www.britannica.com/animal/kangaroo" class="md-crosslink " data-show-preview="true">kangaroos</a>, <a href="https://www.britannica.com/animal/opossum-marsupial-group" class="md-crosslink " data-show-preview="true">opossums</a>, and <a href="https://www.britannica.com/animal/wallaby" class="md-crosslink " data-show-preview="true">wallabies</a>), the newborns are incompletely developed at birth and continue to develop outside the womb, attaching themselves to the female’s body in the area of her mammary glands. Some marsupials have a pouchlike structure or fold, the <a href="https://www.britannica.com/science/marsupium" class="md-crosslink " data-show-preview="true">marsupium</a>, that shelters the suckling young.</p><span class="marker MOD2 mod-inline"></span><span class="marker PREMOD3 mod-inline"></span><p class="topic-paragraph">The class Mammalia is worldwide in distribution. It has been said that mammals have a wider distribution and are more adaptable than any other single class of animals, with the exception of certain less-complex forms such as <a href="https://www.britannica.com/animal/arachnid" class="md-crosslink " data-show-preview="true">arachnids</a> and <a href="https://www.britannica.com/animal/insect" class="md-crosslink " data-show-preview="true">insects</a>. This versatility in exploiting <a href="https://www.britannica.com/place/Earth" class="md-crosslink " data-show-preview="true">Earth</a> is attributed in large part to the ability of mammals to regulate their body temperatures and internal <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="environment" href="https://www.merriam-webster.com/dictionary/environment" data-type="MW">environment</a> both in excessive <a href="https://www.britannica.com/science/heat" class="md-crosslink " data-show-preview="true">heat</a> and aridity and in severe cold.</p><span class="marker MOD3 mod-inline"></span></section> <span class="marker h2"></span><section data-level="1" id="ref51696"> <h2 class="h1">General features</h2> <section data-level="2" id="ref235397"><h2 class="h2">Diversity</h2> <span class="marker PREMOD4 mod-inline"></span><div class="assemblies"><div class="w-100"><figure class="md-assembly m-0 mb-md-0 card card-borderless print-false" data-assembly-id="155742" data-asm-type="image"><div class="md-assembly-wrapper card-media " data-type="image"><a href="https://cdn.britannica.com/69/146369-050-6B1B677E/Okapi.jpg" class="gtm-assembly-link position-relative d-flex align-items-center justify-content-center media-overlay-link card-media" data-href="/media/1/360838/155742"><picture><source media="(min-width: 680px)" srcset="https://cdn.britannica.com/69/146369-050-6B1B677E/Okapi.jpg?w=300"><img src="https://cdn.britannica.com/69/146369-050-6B1B677E/Okapi.jpg?w=300" alt="okapi" data-width="742" data-height="600" loading="eager"></picture><button class="magnifying-glass btn btn-circle position-absolute shadow btn-white top-10 right-10" aria-label="Zoom in"><em class="material-icons link-blue" data-icon="zoom_in"></em></button></a></div><figcaption class="card-body"><div class="md-assembly-caption text-muted font-14 font-serif line-clamp"><span><a class="gtm-assembly-link md-assembly-title font-weight-bold d-inline font-sans-serif mr-5 media-overlay-link" href="https://cdn.britannica.com/69/146369-050-6B1B677E/Okapi.jpg" data-href="/media/1/360838/155742">okapi</a><span>Okapi (<em>Okapia johnstoni</em>).</span><button class="js-more-btn d-none btn btn-unstyled font-12 bg-white js-content" aria-label="Toggle more/less fact data"><span class="link-blue">(more)</span></button></span></div></figcaption></figure></div></div><p class="topic-paragraph">The <a href="https://www.britannica.com/science/evolution-scientific-theory" class="md-crosslink " data-show-preview="true">evolution</a> of the class Mammalia has produced tremendous <a href="https://www.britannica.com/science/biodiversity" class="md-crosslink " data-show-preview="true">diversity</a> in form and habit. Living kinds range in size from a <a href="https://www.britannica.com/animal/bat-mammal" class="md-crosslink " data-show-preview="true">bat</a> weighing less than a gram and tiny <span id="ref827679"></span><a href="https://www.britannica.com/animal/shrew" class="md-crosslink " data-show-preview="true">shrews</a> weighing but a few grams to the largest <a href="https://www.britannica.com/animal/animal" class="md-crosslink autoxref " data-show-preview="true">animal</a> that has ever lived, the <span id="ref193714"></span><a href="https://www.britannica.com/animal/blue-whale" class="md-crosslink " data-show-preview="true">blue whale</a>, which reaches a length of more than 30 metres (100 feet) and a weight of 180 metric tons (nearly 200 short [U.S.] tons). Every major <a href="https://www.britannica.com/science/habitat-biology" class="md-crosslink " data-show-preview="true">habitat</a> has been exploited by mammals that swim, fly, run, burrow, glide, or climb.</p><span class="marker MOD4 mod-inline"></span> <span class="marker PREMOD5 mod-inline"></span><p class="topic-paragraph">There are more than 5,500 <a href="https://www.britannica.com/science/species-taxon" class="md-crosslink " data-show-preview="true">species</a> of living mammals, arranged in about 125 families and as many as 27–29 orders (familial and ordinal groupings sometimes vary among authorities). The <span id="ref193715"></span><a href="https://www.britannica.com/animal/rodent" class="md-crosslink " data-show-preview="true">rodents</a> (order Rodentia) are the most numerous of existing mammals, in both number of species and number of individuals, and are one of the most <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="diverse" href="https://www.merriam-webster.com/dictionary/diverse" data-type="MW">diverse</a> of living lineages. In contrast, the order Tubulidentata is represented by a single living species, the <span id="ref743220"></span><a href="https://www.britannica.com/animal/aardvark" class="md-crosslink " data-show-preview="true">aardvark</a>. The Uranotheria (<a href="https://www.britannica.com/animal/elephant-mammal" class="md-crosslink " data-show-preview="true">elephants</a> and their kin) and <span id="ref743230"></span><a href="https://www.britannica.com/animal/perissodactyl" class="md-crosslink " data-show-preview="true">Perissodactyla</a> (<a href="https://www.britannica.com/animal/horse" class="md-crosslink " data-show-preview="true">horses</a>, <a href="https://www.britannica.com/animal/rhinoceros-mammal" class="md-crosslink " data-show-preview="true">rhinoceroses</a>, and their kin) are examples of orders in which far greater <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="diversity" href="https://www.merriam-webster.com/dictionary/diversity" data-type="MW">diversity</a> occurred in the late <a href="https://www.britannica.com/science/Paleogene-Period" class="md-crosslink " data-show-preview="true">Paleogene</a> and <a href="https://www.britannica.com/science/Neogene-Period" class="md-crosslink " data-show-preview="true">Neogene periods</a> (about 30 million to about 3 million years ago) than today.</p><a class="link-module shadow-sm d-block qa-quiz-module" href="/quiz/deadliest-animals-quiz" data-link-module-iframe-link=""> <img loading="lazy" src="https://cdn.britannica.com/54/92754-131-661B61DB/lion-Panthera-leo.jpg" alt="Lion (panthera leo)" class="rounded-sm mr-15" width="70" /> <div class="line-clamp clamp-5"> <div class="module-title bg-green">Britannica Quiz</div> <div class="font-weight-semi-bold mt-5">Deadliest Animals Quiz</div> </div> </a><span class="marker MOD5 mod-inline"></span> <span class="marker PREMOD6 mod-inline"></span><div class="assemblies"><div class="w-100"><figure class="md-assembly m-0 mb-md-0 card card-borderless print-false" data-assembly-id="27" data-asm-type="image"><div class="md-assembly-wrapper card-media " data-type="image"><a href="https://cdn.britannica.com/63/6563-050-3F5BC4B3/realms-regions-world.jpg" class="gtm-assembly-link position-relative d-flex align-items-center justify-content-center media-overlay-link card-media" data-href="/media/1/360838/27"><picture><source media="(min-width: 680px)" srcset="https://cdn.britannica.com/63/6563-050-3F5BC4B3/realms-regions-world.jpg"><img src="https://cdn.britannica.com/63/6563-050-3F5BC4B3/realms-regions-world.jpg?w=300" alt="faunal realms" data-width="2000" data-height="1463" loading="eager"></picture><button class="magnifying-glass btn btn-circle position-absolute shadow btn-white top-10 right-10" aria-label="Zoom in"><em class="material-icons link-blue" data-icon="zoom_in"></em></button></a></div><figcaption class="card-body"><div class="md-assembly-caption text-muted font-14 font-serif line-clamp"><span><a class="gtm-assembly-link md-assembly-title font-weight-bold d-inline font-sans-serif mr-5 media-overlay-link" href="https://cdn.britannica.com/63/6563-050-3F5BC4B3/realms-regions-world.jpg" data-href="/media/1/360838/27">faunal realms</a><span>Faunal realms and major regions of the world.</span><button class="js-more-btn d-none btn btn-unstyled font-12 bg-white js-content" aria-label="Toggle more/less fact data"><span class="link-blue">(more)</span></button></span></div></figcaption></figure></div></div><p class="topic-paragraph">The greatest present-day diversity is seen in continental tropical regions, although members of the class Mammalia live on (or in seas <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="adjacent" href="https://www.merriam-webster.com/dictionary/adjacent" data-type="MW">adjacent</a> to) all major landmasses. Mammals can also be found on many oceanic <a href="https://www.britannica.com/science/island" class="md-crosslink " data-show-preview="true">islands</a>, which are principally, but by no means exclusively, inhabited by <a href="https://www.britannica.com/animal/bat-mammal" class="md-crosslink " data-show-preview="true">bats</a>. Major regional faunas can be identified; these resulted in large part from <a href="https://www.britannica.com/science/evolution-scientific-theory" class="md-crosslink " data-show-preview="true">evolution</a> in comparative isolation of stocks of early mammals that reached these areas. <a href="https://www.britannica.com/place/South-America" class="md-crosslink " data-show-preview="true">South America</a> (the Neotropics), for example, was separated from <a href="https://www.britannica.com/place/North-America" class="md-crosslink " data-show-preview="true">North America</a> (the Nearctic) from about 65 million to 2.5 million years ago. Mammalian groups that had reached South America before the break between the continents, or some that “island-hopped” after the break, evolved independently from relatives that remained in North America. Some of the latter became <a href="https://www.britannica.com/science/extinction-biology" class="md-crosslink " data-show-preview="true">extinct</a> as the result of competition with more advanced groups, whereas those in South America <a class="md-dictionary-link md-dictionary-tt-off eb" data-term="flourished" href="https://www.britannica.com/dictionary/flourished" data-type="EB">flourished</a>, some radiating to the extent that they have successfully competed with invaders since the rejoining of the two continents. <a href="https://www.britannica.com/place/Australia" class="md-crosslink " data-show-preview="true">Australia</a> provides a parallel case of early isolation and <span id="ref743341"></span><a href="https://www.britannica.com/science/adaptive-radiation" class="md-crosslink " data-show-preview="true">adaptive radiation</a> of mammals (specifically the <a href="https://www.britannica.com/animal/monotreme" class="md-crosslink " data-show-preview="true">monotremes</a> and <span id="ref743204"></span><a href="https://www.britannica.com/animal/marsupial" class="md-crosslink " data-show-preview="true">marsupials</a>), although it differs in that Australia was not later connected to any other landmass. The <a href="https://www.britannica.com/animal/placental-mammal" class="md-crosslink " data-show-preview="true">placental mammals</a> that reached Australia (rodents and bats) evidently did so by island-hopping long after the adaptive radiation of the mammals isolated early on.</p><div class="one-good-fact-module"> </div><span class="marker MOD6 mod-inline"></span> <span class="marker PREMOD7 mod-inline"></span><p class="topic-paragraph">In contrast, North America and Eurasia (the Palearctic) are separate landmasses but have closely related faunas as the result of having been connected several times during the <a href="https://www.britannica.com/science/Pleistocene-Epoch" class="md-crosslink " data-show-preview="true">Pleistocene Epoch</a> (2.6 million to 11,700 years ago) and earlier across the <a href="https://www.britannica.com/place/Bering-Strait" class="md-crosslink " data-show-preview="true">Bering Strait</a>. Their faunas frequently are thought of as representing not two distinct units but one, related to such a degree that a single name, Holarctic, is applied to it.</p><div class="module-spacing"> <DIV class="marketing-INLINE_SUBSCRIPTION marketing-content" data-marketing-id="INLINE_SUBSCRIPTION"><style> .student-promo-banner-wrapper { container-type: inline-size; margin-bottom: 15px; } @container (min-width: 475px) { .student-promo-banner { flex-direction: row; } .student-promo-banner-img-wrapper { margin-bottom: 0; margin-right: 10px; justify-content: flex-start; } .student-promo-banner-text-wrapper { text-align: left; margin-bottom: 0px; margin-left: 10px; } .student-promo-banner-button-wrapper { margin-right: 0; } }</style> <div class="student-promo-banner-wrapper"> <div class="student-promo-banner d-flex flex-column align-items-center bg-blue rounded p-20"> <div class="student-promo-banner-img-wrapper mb-20 mr-0 d-flex justify-content-center"> <img class="rounded" style="max-width: 100px; min-width: 80px" src="https://cdn.britannica.com/marketing/BlueThistle.webp" /> </div> <div class="student-promo-banner-text-wrapper ml-0 mb-10 text-center text-white"> <div class="h2 mb-10">Get Unlimited Access</div> <div class="h4 font-weight-semi-bold">Try Britannica Premium for free and discover more.</div> </div> <div class="student-promo-banner-button-wrapper d-flex justify-content-center align-items-center ml-auto mr-auto"> <a class="btn btn-m btn-orange" href="https://premium.britannica.com/premium-membership/?utm_source=premium&utm_medium=inline-cta&utm_campaign=black-friday-2024">Subscribe</a> </div> </div> </div> </DIV></div><span class="marker MOD7 mod-inline"></span></section> </section><span class="marker end-of-content"></span><span class="marker after-article"></span></div> <div id="chatbot-root"></div> </div> </div> </div> <div class="ai-dialog-placeholder"></div> </div> </div> <aside class="col-md-da-320"></aside> </div> </div> </div> </div> </article></div> </div></div> </div> </main> <div id="md-footer"></div> <noscript><iframe src="//www.googletagmanager.com/ns.html?id=GTM-5W6NC8" height="0" width="0" style="display:none;visibility:hidden"></iframe></noscript> <script type="text/javascript" id="_informizely_script_tag"> var IzWidget = IzWidget || {}; (function (d) { var scriptElement = d.createElement('script'); scriptElement.type = 'text/javascript'; scriptElement.async = true; scriptElement.src = "https://insitez.blob.core.windows.net/site/f780f33e-a610-4ac2-af81-3eb184037547.js"; var node = d.getElementById('_informizely_script_tag'); node.parentNode.insertBefore(scriptElement, node); } )(document); </script>  <script> window.ap3c = window.ap3c || {}; var ap3c = window.ap3c; ap3c.cmd = ap3c.cmd || []; ap3c.cmd.push(function() { ap3c.init('ZO4siT4cLwnykPnzZWJtd3Byb2Q', 'https://engage.email.britannica.com/'); ap3c.track({v: 0}); }); ap3c.activity = function(act) { ap3c.act = (ap3c.act || []); ap3c.act.push(act); }; var s, t; s = document.createElement('script'); s.type = 'text/javascript'; s.src = "https://engage.email.britannica.com/app.js"; t = document.getElementsByTagName('script')[0]; t.parentNode.insertBefore(s, t); </script> <script class="marketing-page-info" type="application/json"> {"pageType":"Topic","templateName":"DESKTOP","pageNumber":1,"pagesTotal":11,"pageId":360838,"pageLength":851,"initialLoad":true,"lastPageOfScroll":false} </script> <script class="marketing-content-info" type="application/json"> [] </script> <script src="https://cdn.britannica.com/mendel-resources/3-130/js/libs/jquery-3.5.0.min.js?v=3.130.14"></script> <script type="text/javascript" data-type="Init Mendel Code Splitting"> (function() { $.ajax({ dataType: 'script', cache: true, url: 'https://cdn.britannica.com/mendel-resources/3-130/dist/topic-page.js?v=3.130.14' }); })(); </script> <script class="analytics-metadata" type="application/json"> {"leg":"D","adLeg":"D","userType":"ANONYMOUS","pageType":"Topic","pageSubtype":null,"articleTemplateType":"PAGINATED","gisted":false,"pageNumber":1,"hasSummarizeButton":false,"hasAskButton":false} </script> <script type="text/javascript"> EBStat={accountId:-1,hostnameOverride:'webstats.eb.com',domain:'www.britannica.com', json:''}; </script> <script type="text/javascript"> ( function() { $.ajax( { dataType: 'script', cache: true, url: '//www.britannica.com/webstats/mendelstats.js?v=1' } ) .done( function() { try {writeStat(null,EBStat);} catch(err){} } ); })(); </script> <div id="bc-fixed-dialogue"></div> </body> </html>

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Mammal | Definition, Characteristics, Classification, Examples, & Facts | Britannica