Mammalian Evolution and Adaptations

Euarchontoglires

• Comprises primates, rodents, and lagomorphs.

• Also includes tree shrews and flying lemurs.

Primates

• Possess a generalist body plan adapted for arboreal life.

Primate Phylogeny

• Aye-aye are closely related to lemurs.

• Tarsiers are more closely related to Anthropoidea than to lemurs and lorises.

• Prosimians are polyphyletic, meaning they do not form a natural group with a single common ancestor.

Dermoptera and Scandentia

• Dermoptera (flying lemurs) are the sister group to primates.

• Scandentia (tree shrews) are the sister group to primates.

• Euarchontoglires are grouped with Rodentia and Lagamorpha.

Laurasiatheria

• Includes Chiroptera (bats), Artiodactyla, Cetacea, Perissodactyla, Caniformia, Feliformia, and Pangolins.

Laurasiatheria - Ungulates

• Includes Artiodactyla and Cetacea, now classified together as Cetartiodactyla.

• Artiodactyla includes pigs, deer, and camels.

• Characterized by a fused radius and ulna.

• Also includes Perissodactyla.

Ungulates: Hooved Mammals (Laurasiatheria)

• Traditionally, the superorder Ungulata included two orders:

  • Order Artiodactyla – even-toed (approximately 240 species) with foregut fermentation.

    • Examples: Suidae (pigs - 19 species), Tayassuidae (peccaries - 4 species), Hippopotamidae (hippos - 2 species), Camelidae (camels - 7 species), Cervidae (deer - 56 species), Tragulidae (chevrotains - 10 species), and Bovidae (cattle and antelope - 135 species).

    • Giraffidae - Giraffe - 8 species?

Ungulates: Hooved Mammals (Laurasiatheria)

• Traditionally, the superorder Ungulata included two orders:

  • Order Perissodactyla – odd-toed (approximately 17 species) with intestinal fermentation.

    • Examples: Tapiridae (tapirs - 5 species), Rhinocerotidae (rhinos - 5 species), and Equidae (horses - 7 species).

Ungulate Classification Changes

• Molecular evidence indicates that Cetaceans are nested within the Artiodactyls.

• New fossil evidence from ear and ankle structure of early whales supports this.

  • Examples: Maiacetus (49-40 million years ago) and Pakicetus (47 million years ago).

New Taxon: Cetartiodactyla

• Cetartiodactyla: combines Cetaceans and Artiodactyls.

• All are related to carnivores, chiroptera, and pangolins within the superorder Laurasiatheria.

Cetacea

• Mysticeti: baleen whales, 14 species

  • Examples: Blue whale (33 m & 190 t) and Pygmy right whale (6.5 m & 3.5 t).

  • Diet: Euphausiid

Cetaceans - Odonticeti

• Odonticeti: Toothed whales & dolphins, 70 species

  • Examples: sperm whale (15 m & 45 t) and porpoise (1.5 m & 50 kg).

Carnivora

• Includes fur seals, seals, and walruses.

• Studied using "Critter Cam" by Prof. Terrie Williams at the University of Santa Cruz.

Whale Feeding Behavior

• Acevedo-Gutierrez et al. (2002) studied blue and fin whales.

• Blue and fin whales "lunge-feed," engulfing water and euphausiids, which make up approximately 70% of their body mass.

• Vertical speeds increased during lunges.

• Fin whale mid-point speed: 5 ms^{-1} (max 10 ms^{-1}) for 33 seconds.

• Multiple lunges cause a disproportionate increase in the time at the surface.

Laurasiatheria - Chiroptera

• Chiroptera: Bats; not all bats echolocate.

• Over 1,000 species, representing 20% of mammals.

• Diet: 70% insects, the rest mostly fruit.

• Example: Yellow-winged bat (Lavia frons).

Chiroptera Adaptations

• Anoura fistulata has a glossal tube and an 8 cm tongue.

• Micro-chiroptera (lack underfur) have different brains from Mega-chiroptera (good eyesight).

• Odd connection between the retina & superior colliculus in Mega-chiroptera, possibly related to loss of echolocation.

• Molecular evidence supports monophyly.

• Various feeding habits: Leaf-nosed bat, Vampire bat, Bulldog bat, Pollinators (Baobab tree, Agave), Seed dispersers (50-90% of tropical trees & shrubs), Frog eating, Fish eating.

Bat Sensory Abilities

• The little brown bat (Myotis lucifugus) can detect wires down to around 0.3 mm diameter.

• With a call at 100 KHz, the wavelength (\lambda) = 3.43 mm, so the bat can detect down to approximately 1/10th \lambda.

• Represents 35%.

Carnivora Suborders

• Feloidea: cats, viverrids, mongoose, hyaenas (suborder Feliformia).

• Canoidea: dogs (Caniformia).

• Arctoidea: bears/giant panda, raccoons/coatis/red panda, badgers.

• Pinnipedia: seals, sealions, walruses; evolved from bear-like ancestors 25 million years ago.

Diving Behavior of Northern Elephant Seals

• Female Northern elephant seal (Mirounga angustirostris) diving record:

  • Initial mass: 291 kg.

  • Duration at sea: 81 days.

  • Number of dives: 5,657.

  • Longest dive: 44.4 min.

  • Deepest dive: 1,093 m.

  • Diel pattern: deepest dives occurring at midday.

Eulipotyphla

• Includes:

  • Hedgehogs (Erinaceidae).

  • Moles (Talpidae).

  • Shrews (Soricidae).

  • Solenodon (Solenodontidae) - venomous.

  • Moonrat (gymnure).

• Formerly classified as "Insectivora".

Mammalian Characteristics and Adaptations

• Focus on key characteristics and adaptations of mammals.

Endothermy

• Not a defining characteristic of mammals as it is also found in birds.

• Associated with other uniquely mammalian features:

  • Hair

  • Sweat glands

  • Four-chambered heart (higher metabolism)

  • Specialized teeth

Energetic Cost of Endothermy

• High energetic cost.

• Negatively correlated with body size (when considering mass-specific energy cost).

• Energy cost increases with body mass (when considering absolute energy cost).

• Energy cost does not increase proportionally to body size; when log body size increases by 1 unit, log energy cost increases by 0.75 (‘Kleiber’s Law’).

Variations in Endothermy

• Not all mammals are fully endothermic.

  • Examples: elephant shrew, naked mole rats.

• Comparison of body temperature (Tb) relative to ambient temperature (Ab) in different animals: lizard, echidna, platypus, cat, opossum.

• Classification: Ectotherm? Heterotherm

Heterotherms

• Example: Pygmy possum.

Facultative Torpor

• Big brown bat (16 g, Eptesicus fuscus) in the USA hibernates in winter in the North.

• Mexican free-tailed bat (10 g, Tadarida mexicana) in the southern USA migrates South in winter.

• Body temperature (Tb) can drop as low as 7 °C when torpid (up to 40-fold energy saving).

Lactation

• Nipples are not always present; Monotremes secrete milk onto flattened milk patches.

• Number of mammary glands does not always correspond to the number of nipples.

• Most mammals produce litters with fewer young than nipples (but not all, e.g., opossums!).

• Humans have 20-40 glands.

Evolution of Lactation

• Monophyletic origin suggested by detailed similarity of glands in different groups (Capuco & Akers 2009 J. Biol. 8, 37).

• Lactation allows:

  • Delay in development of teeth.

  • Less burdening of the mother.

  • Independence from environmental resource availability.

Antimicrobial Properties of Milk

• Milk has antimicrobial properties.

• Includes maternal immunoglobulins (IgG) (“Brambell” receptors FcRB).

Lactation

• Secreting milk – evolved from specialised sweat glands.

• Only 1 species has lactating males.

• Milk consists of different proportions of:

  • Water

  • Fats (0.2-50%) (rhino-whale/seal)

  • Proteins (3-12%)

  • Lactose (40% of calories)

  • Minerals (CaHPO4, Vit B6, B12, K)

• Hormonal control by prolactin & oxytocin (stimulated by suckling).

• Dayak fruit bat (Dyacopterus spadiceus).

Extreme Milk Compositions

• Humans: 4% fat, 7% sugar, 0.9% protein

• Cows: 3.5% fat, 5% sugar, 3.3% protein

• Hooded Seal: 60% fat – Intermittent feeding in a cold environment (Oftedal et al. 1993)

• Black Rhino: 0.2% fat – Slow reproductive cycle in warm climate (Skiviel et al. 2013)

• Tammar Wallaby: 14% sugar – Asynchronous content lactation (Nicholas et al. 1997)

• Eastern Cottontail Rabbit: 14% fat, 16% protein - Intermittent feeding

Mammalian Skin Structure

• Epidermis (melanin+)

• Dermis (Armadillo dermal plates)

• Hypodermis (fat layer)

• Hair follicle (epidermal)

• Hair papilla (dermal)

• Components:

  • Sweat gland [Modified as mammary gland]

  • Pore of sweat gland

  • Sebaceous gland

  • Basement membrane

  • Hair

Skin - Exocrine Glands

• Exocrine: Secreting through a duct

• Located in the Dermal and subcutaneous layers.

• Main types:

  • Mammary: milk producing.

  • Sebaceous: oil secreting sebum [lipids and waxes].

  • Wax producing: in ears.

  • Sweat: thermoregulation, excretion, communication.

Distribution of Sweat Glands

• Presence/distribution of different types of sweat glands varies in different groups.

  • Sweat glands: paws in cats, snout in platypus.

  • Scent glands: temporal region in elephants, anal region in rodents and cats.

Evolution of Hair

• Meng & Wyss. 1997. Nature 385:712-714

  • Probably arose as sensory structures first.

  • Arose as insulation in primitive mammalian endotherms.

• Must have arisen in the therapsid lineage at some point.

  • Early therapsids lack scales, but no evidence of hair.

  • Hair had arisen by 210 mya*. Multituberculates Late Triassic.

Hair Characteristics

• Hair has “grain” (lost in moles and other burrowers).

• Some hairs are specialised:

  • Sensory function (Vibrissae): navigation (rats) and trail following (seals).

  • Defensive (Spines).

  • Seals – Dehnhardt et al. (1998) Nature 394

Akinetic Skull

• Refers to lack of movement between the upper jaw and braincase.

• Not a defining feature of mammals.

• Secondary palate formed from processes of the premaxillae, maxillae, and palatines.

• Probably a result of young mammals’ need to suckle.

• Allows precise, strong tooth occlusion [contact between teeth].