Mammalian trends Endothermy Sensory specializations Heterodont dentition Skeletal simplification Dentary/squamosal jaw joint Lactation and increased parental care See Table 2.2 for a longer list of diagnostic characteristics.
Mammalian Skin Adapted from Romer, A. S., and Parsons, S. T. The Vertebrate Body. Saunders, 1977.
Mammary Glands Adapted from Hildebrand, M. Analysis of Vertebrate Structure. John Wiley & Sons, 1974.
Lactation and suckling promote social bonds Milk composition varies – Seal milk has 12 times the fat and 5 times the protein of cow milk Glandular ducts Teat, nipple, or hair tufts Under endocrine control Mammary Glands
Hair Dead epidermal cells with keratin Outer layer of cuticular scales Deeper cortex and medulla layers Color determined by pigments (melanin) Pelagecoat of hair – Provides insulation – May undergo seasonal molting
FIGURE 03: Structure of a guard hair and cuticular scale patterns of the guard hairs of some mammals Adapted from Teerink, B. J. Hair of West-European Mammals: Atlas and Identification Key. Cambridge University Press, 1991.
Pelage coloration Countershading Camouflage Disruptive coloration – Zebra stripes Warning coloration – Skunks Intraspecific communication FIGURE 04: The pattern of postjuvenile molt in the golden mouse (Ochrotomys nuttalli) Adapted from Linzey, D. W. and Linzey, A. V., J., Mammalogy. 48 (1967): 326-241.
Fat and Energy Adipose tissue – Energy storage – Source of heat and water – Thermal insulation Desert or temperate zone mammals – Store fat in tail or abdomen Boreal or arctic mammals – Subcutaneous layer of blubber
Circulatory System Endothermy requires highly efficient circulation – Systemic and pulmonary circuits – 4-chambered heart – Biconcave red blood cells Heart rate varies with: – body size – activity state (e.g. hibernation)
Circulatory System FIGURE T03: Heart Rates of Selected Mammals Data are from Altman and Dittmer (1964: 235); names updated.
Respiratory System Tracheabronchibronchiolesaveolar ductsalveoli Human lungs contain ~300 million alveoli or 70 m 2 of respiratory surface area Muscular diaphragm aids ventilation – Limb and body movements also assist
Reproductive System Females – Both ovaries functional – Ova fertilized in uterine tubes Adapted from Smith, H. M. Evolution of Chordate Structure. Holt, Rinehart, and Winston, 1960.
Males – Erectile copulatory organpenis – Os penis or baculum variable – Testes usually held in scrotum FIGURE 08: Ventral view of penises of New Guinean murid rodents Adapted from Lidicker, W. Z., Jr., J. Mammalogy 49 (1968): 609-643.
Brain Neopallium highly developed Corpus callosum present (Hedgehog) Adapted from Romer, A. S., and Parsons, S. T. The Vertebrate Body. Saunders, 1977; (dolphin) adapted by Norris, K. S. Whales, Dolphins and Porpoises. University of California Press, 1966. FIGURE 09: Left sides of the brains of a hedgehog and a dolphin
Sense Organs Olfaction – Olfactory receptors distributed across the mucosal surfaces of the mesethmoid and vomeronasal organ areas FIGURE 10: Cutaway view of the nasal chamber of an Aberts squirrel
Hearing – High acuity (especially in nocturnal mammals) – Sound use in: Communication Orientation to environment Locating food Avoiding enemies – Infrasound to ultrasound – External pinna leads to external auditory meatus – Middle ear has three ossicles encased in bony bulla
FIGURE 11: Lateral view of the right middle ear chamber (anterior is to the right) of Aberts squirrel, with the auditory bulla largely removed Middle ear
Other Senses Vision – Similar to other amniotes – Tapetum lucidum in nocturnal mammals – Retina with photoreceptors (rods and cones) Rods enable vision in low light (grayscale) Cones enable color vision in brighter light – Eyes reduced in some fossorial mammals Tactile – Vibrissae on muzzle (mystacial pad)
Digestive System Salivary glands Simple esophagus Stomach simple or complex – Multi-chambered in many herbivores – Site of microbial fermentation in ruminants Caecum – May be site of microbial digestion
Digestive System FIGURE 13: The four- chambered stomach of a ruminant artiodactyl Adapted from Storer, T. L., and Usinger, R. L. General Zoology. McGraw-Hill, 1965.
Muscular System Limb muscles highly adapted to style of locomotion Trunk muscles aid postural control and breathing Superficial muscles allow skin to move – Assist in suckling – Facial expression – Ear movements
The Skeleton Simplification of skeletal elements – Metabolic savings – Lighter skeleton for quicker movements Greater ossification – Well-braced muscle attachments Determinate growth – Epiphysis fuses to diaphysis
Adapted Stock, C. Rancho. La Brea: A record of Pleistocene life in California, Science Series, no. 13. Los Angeles County Museum, 1949.
The Skull Akinetic skulls Single craniomandibular jaw joint Braincase large Sagittal and lambdoidal crest may be present Zygomatic arch usually present Secondary palate present Turbinal bones within nasal cavities Foramina pass cranial nerves and vessels Three middle ear bones transmit sound to cochlea Tympanic bulla Hyoid bones support tongue
FIGURE 17A: Side view of the skull of the African wild dog (Lycaon pictus), showing the bones, foramina, and teeth
Dental formula Specifies number and position of teeth on one side Incisors 3/3, Canines 1/1, Premolars 4/4, Molars 2/3 OR (3/3, 1/1, 4/4, 2/3) × 2 = 42 Individual teeth designated with upper case letters for upper teeth and lower case letters for lower teeth P3 is upper premolar 3 M2 is lower molar 2
Mastication Complex chewing movements – Initially crush and puncture food – Later sliced by shearing surfaces of molars FIGURE 21: Molars of the Virginia opossum (B) Adapted from Crompton, A. W., and Hiiemae, K., Discovery 5 (1969): 23.
Carnivory vs. Herbivory Carnivores – Cheek teeth become blade-like (carnassials) – Adapted for slicing flesh – Jaw action is scissor-like Herbivores – Cheek teeth become quadrate with hypocone – Jaw action is horizontal and transverse
FIGURE 22: Comparisons of the occlusal surfaces of the right upper cheek teeth of a carnivore Adapted from Crompton, A. W., and Hiiemae, K., Discovery 5 (1969): 23.
Tooth structure Cementum binds tooth to jaw Inner dentine Outer enamel FIGURE 23: Generalized sections of mammalian teeth, showing the internal structure
Terminology Brachydontshort-crowned teeth Hypsodonthigh-crowned teeth Ever-growinggrow continuously Diastemaspace between incisors/canines and cheek teeth Bunodontrounded cusps on molars Lophodontcusps form ridges Selenodontcusps form crescents
Cusp terminology Modified from Romer, A.S. Vertebrate Paleontology. University of Chicago Press, 1966. FIGURE 25: Basic cusp pattern of mammalian molars
Axial Skeleton Rib cage and sternum Vertebral column – Greater head movement – Dorsoventral flexion of spine – Five vertebrae types Cervical, Thoracic, Lumbar, Sacral, Caudal FIGURE 26: Vertebrae of the gray fox
Limbs and Girdles Main form of propulsion in most mammals Fore and aft movement of limbs Pelvis – Ilium, ischium, and pubis Shoulder girdle – Scapula and clavicle (clavicle may be reduced or absent)
Manus and Pes Manus (hand or forefoot) Pes (foot or hindfoot) Five digits is ancestralhighly modified in several lingeages Pollex (thumb) and Hallux (big toe) have two phalangesremaining digits have three phalanges