Name:_________________________________________Class:__________Date:____________Mammals
One of these is not a mammal. Which one? Mammalsareaclassofendothermicvertebrates.Theyhavefourlimbsandproduceamnioticeggs.Examplesofmammalsincludebats,whales,mice,andhumans.Clearly,mammalsareaverydiversegroup.Nonetheless,theysharemanytraitsthatsetthemapartfromothervertebrates. Characteristics of Mammals Mammalsmakeupaclassofvertebrateanimalsthatcontainsmorethan4,500livingspecies.Examplesofmammalsincludebats,whales,elephants,anteaters,dogs,andhumans.Astheseexamplessuggest,mammalsmakeupadiversegroupofanimals.Nonetheless,theyshareanumberofcharacteristicsthatservetosetthemapartfromothervertebrates.
Characteristicsthataremostoftenusedtodefinethemammalianclassarethepresenceofmammaryglandsandbodyhair.Allfemalemammalspossessmammaryglands.Theirfunctionistoproducemilkafterthebirthofanoffspring.Milkisanutritiousfluidthatcontainsdisease-fightingmolecules,otherimportantchemicals,andthenutrientsneededbybabymammals.Theproductionofmilktofeedtheyoungiscalledlactation.Allmammalshavehair(orfur)duringsomeoralloftheirlife.Haircanserveseveralfunctions.Themostimportantfunctionusuallyisinsulatingthebodytohelpconservebodyheat(seetheMammals:Endothermy(Advanced)concept).Haircanalsobeusedforsensoryperception.Forexample,catsusetheirwhiskerstosensetheirsurroundings.Inaddition,haircanbeusedtocommunicatethreatstoapotentialrivalorpredator.Manymammals,likethecattotheleftcanraisetheirfurtomakethemselveslooklargerandmorethreatening.
Other Traits of Mammals Mostmammalsshareseveralothertraits.Thetraitsinthefollowinglistaretypicalof,butnotnecessarilyuniqueto,mammals.Forexample,likemammals,birdsarewarm-blooded,andsomefishandreptilesgivebirthtoliveyoung.
• Mammalsarewarm-blooded,meaningtheycanmaintainastable,relativelyhighbodytemperature.Thisiscalledendothermy.
• Theskinofmanymammalshassweatglands.• Mammalianlungshavemillionsoftinyairsacscalledalveoli.• Themammalianheartisdividedintofourchambers,orcompartments.• Thebrainsofmammalsarerelativelylargeandhaveacoveringcalledtheneocortex.• Mostmammalsgivebirthtoliveyoung.Thisiscalledvivipary.
Traits of Fossil Mammals Thepresenceofmanyofthemammaliantraitsisrarelyobservedinthefossilrecord.Thetraitsinvolvesofttissuesthatareunlikelytobepreservedinfossils.Therefore,extinctmammalsusuallyareidentifiedonthebasisofothertraits.Thesegenerallyincludetraitsofthebonesandteeth,whicharemorelikelytobepreserved.Fossilmammalsmaybeidentifiedbasedonthepresenceofoneormoreofthefollowingtraits:
Byraisingitsfur,thiscatlooksbiggerandmorethreatening.Thismightdiscourageapredatorfromattacking.Thisisjustoneofseveralfunctionsofhairorfurinmammals.
Thisdrawingshowsthethreebonesofthemammalianmiddleear,usingthehumanear
asanexample.Thebonesarecalledthehammer,anvil,andstirrup.Onlymammalsusethesebonesforhearing.Inreptiles,the
threebonesarepartofthejawjoint.
• Threetinybones(calledthestirrup,hammer,andanvil)inthemiddleear;thesegivemammalsexceptionallygoodhearing.Theycarrysoundvibrationsfromtheeardrumtotheinnerear(seetheNervousandEndocrineSystemsconcepts).Inothervertebrates,thesebonesarepartofthejawjointandarenotinvolvedinhearing.
• Twobonyknobs(calledoccipitalcondyles)atthebaseoftheskull;theknobsfitintothetopmostbonesoftheneck.Inmostothervertebrates,thereisonlyonebonyknobatthebaseoftheskull.Thetwooccipitalcondylesmayhelpmammalsholduptheirrelativelyheavy,big-brainedhead.
• Teethwithdifferentshapesandfunctions;mammalsgenerallyhavefourdifferenttypesofteeth.Theyhavesharpincisorsthatcutfood,strongpointedcaninesthattearfood,broadmolarswithseveralcusps(bumpsonthechewingsurface)thatgrindfood,andpremolarswithtwocuspsthatfunctionsimilarlytocaninesormolars.
Summary
• Characteristicsthataremostoftenusedtodefinethemammalianclassarethepresenceofmammaryglandsandbodyhair.
• Themostimportantfunctionofhairisinsulatingthebodytohelpconservebodyheat.• Threetinybones(calledthestirrup,hammer,andanvil)inthemiddleeargivemammalsexceptionally
goodhearing.Review
1. Whatcharacteristicsaremostcommonlyusedtoidentifymammals,especiallylivingones?
2. Whatfunctionsdoeshair(orfur)serveformammals?3. Whyisthepresenceofmostmammaliantraitsnotobservedinfossils?4. Whataresomedefiningcharacteristicsofmammalianskeletons?
