Carnivora

Carnivora (from the Latin caro, "flesh," and vorare, "to devour") is the order of placental mammals (Laurasiatheria, Mammalia) that comprises approximately 305 extant species across 16 families—including cats (Felidae), hyenas (Hyaenidae), mongooses (Herpestidae), civets (Viverridae), dogs and wolves (Canidae), bears (Ursidae), weasels, otters, and badgers (Mustelidae), raccoons (Procyonidae), skunks (Mephitidae), and all seals, sea lions, and walruses (Pinnipedia). Within the superorder Laurasiatheria, Carnivora is the living sister clade to Pholidota (pangolins), forming the clade Ferae (which diverged from ungulates and bats during the Late Cretaceous ~78–75 Ma).

Crucially, economists and ecologists distinguish between the taxonomic noun "carnivoran" (any member of the monophyletic order Carnivora, regardless of what it eats) and the ecological adjective "carnivore" (any meat-eating organism, including sharks, hawks, spiders, or marsupial thylacines). While felids, polar bears, and weasels are obligate hypercarnivores, the order Carnivora also includes versatile omnivores (raccoons, coyotes, brown bears), frugivores (binturong, kinkajou), termite-eating insectivores (aardwolf Proteles cristata, sloth bear), and 99%-bamboo-eating herbivores (the giant panda, Ailuropoda melanoleuca, and the unrelated red panda, Ailurus fulgens)!

1. The P⁴ / M₁ Carnassial Tooth Pair: The defining anatomical synapomorphy of crown-group Carnivora (and Paleocene–Eocene stem-carnivorans, the "miacids"†) is a specialized pair of opposing, blade-like cheek teeth called the carnassials—specifically, the fourth upper premolar (P⁴) and the first lower molar (M₁) (note that extinct Paleogene Hyaenodonta† and Oxyaenodonta†, formerly called "Creodonta," evolved convergent carnassial blades further back in the jaw at M¹/M₂ or M²/M₃!). Driven by a hypertrophied temporalis muscle inserting on a high mandibular coronoid process and stabilized by a cylindrical, transverse hinge-like mandibular condyle that prevents lateral jaw wobbling, the lingual blade of P⁴ slices past the buccal blade of M₁ like a pair of self-sharpening steel shears to slice tough tendons, hide, and muscle, or crack marrow bones (secondary modifications flatten the post-carnassial molars into crushing bunodont surfaces in omnivorous bears and raccoons, or reduce them into homodont peg teeth in fish-catching pinnipeds).
2. Fusion of the Scapholunar Wrist Bone and Reduction of the Clavicle: In the carnivoran wrist (carpus), the scaphoid, lunar, and centrale bones are fused into a single scapholunar bone providing a stable joint for grappling prey or high-speed cursorial pursuit, while the clavicle (collarbone) is vestigial or lost so the scapula can swing freely along the parasagittal plane to lengthen stride.
3. Auditory Bulla Construction (Feliformia vs. Caniformia): All crown carnivorans divide into two strictly monophyletic suborders that diverged in the Early Eocene (~50–45 Ma) and are diagnosed osteologically by the bony floor covering their middle-ear chamber (auditory bulla):

1. Why Cats, Sea Lions, and Hyenas Cannot Taste Sugar (Pseudogenization of Tas1r2): In mammals, sweet taste perception is mediated by a heterodimeric G-protein-coupled receptor expressed on tongue taste buds: T1R2 + T1R3 (encoded by the Tas1r2 and Tas1r3 genes), while umami (savory amino-acid/nucleotide) taste is mediated by T1R1 + T1R3. In 2005 and 2012, Xia Li, Peihua Jiang, Gary K. Beauchamp, and colleagues discovered that all felids (domestic cats, lions, tigers, cheetahs) carry a shared 247-base-pair microdeletion in exon 3 of Tas1r2 that introduces a premature stop codon, rendering them completely blind to sweet sugars! Extending their survey across Carnivora, Jiang et al. (2012) found that at least seven independent obligate meat- or fish-eating lineages—including Felidae, spotted hyenas, fossa, Asian small-clawed otters, fur seals, harbor seals, and Pacific walrus—independently suffered different frame-shifting mutations in Tas1r2 after switching to carbohydrate-free diets! Conversely, the bamboo-eating giant panda (Ailuropoda melanoleuca) kept its sweet receptor intact, but suffered an inactivating mutation in its umami meat-taste receptor gene (Tas1r1) ~4.2 million years ago (Li et al., 2010)!
2. Convergent Evolution of the "False Thumb" in the Giant Panda and Red Panda (DYNC2H1 and PCNT): As famously celebrated by Stephen Jay Gould in The Panda's Thumb (1980), because the five true digits of a bear or musteloid paw are aligned side-by-side for walking and cannot oppose one another to grip bamboo stalks, both the giant panda (family Ursidae) and the red panda (family Ailuridae, a distant musteloid relative separated by >43 million years of evolution!) independently evolved an opposable "sixth digit" ("false thumb") by enlarging a wrist bone—the radial sesamoid bone! In 2017, comparative whole-genome sequencing of both species by Yibo Hu, Fuwen Wei, and colleagues discovered that despite their 43-Myr separation, both the giant panda and red panda share identical convergent amino-acid substitutions under positive selection in two primary-cilia/osteoblast limb-patterning genes—DYNC2H1 (dynein cytoplasmic 2 heavy chain 1) and PCNT (pericentrin) (human mutations in which cause polydactyly and skeletal ciliopathies)—alongside convergent adaptive pseudogenization of the umami taste receptor!

Why does the giant panda still have the short, simple digestive tract and carnivore gut microbiome of a meat-eating bear?

Unlike true mammalian herbivores (ruminant artiodactyls or hindgut-fermenting horses)—which evolved multi-chambered stomachs or giant fermentation ceca lengthening their intestines to 10–25 times their body length—the giant panda (Ailuropoda melanoleuca) retains the short, straight, simple gastrointestinal tract of a carnivorous bear (only ~4–6 times body length, with no cecum and a rapid gut transit time of just 8 to 12 hours), lacks endogenous cellulase genes in its genome (Li et al., 2010), and hosts a gut microbiome dominated by carnivoran Clostridium and Escherichia/Shigella rather than specialized Ruminococcaceae fiber-degraders (Xue et al., 2015). How does a 110-kg panda survive while digesting less than ~17% of the dry matter of bamboo?

1. Selective Foraging for Protein-Rich Young Shoots and Leaves: As shown by Yonggang Nie, Fuwen Wei, and colleagues (2019) using nutritional geometry, pandas migrate seasonally up and down mountain elevations to eat young bamboo shoots and leaves whose digestible protein-to-carbohydrate energy ratio (>50% of calories from protein!) actually matches the macronutrient profile of a hypercarnivore diet!
2. Hypometabolism via Mutated DUOX2 Thyroid Hormone Synthesis: In 2015, Nie et al. measured free-living panda field metabolic rates using doubly labeled water and discovered that giant pandas expend only 38% of the daily energy expected for a placental mammal of their body mass (rivaling three-toed sloths!), driven by shrunk brain, liver, and kidney masses and a panda-specific missense mutation in DUOX2 (dual oxidase 2) that lowers circulating thyroid hormone (T₄/T₃) levels by ~50%!

How do sea otters (Enhydra lutris) stay warm in frigid North Pacific waters without insulating blubber?

Whereas whales, seals, and walruses insulate themselves in freezing oceans with a thick subcutaneous layer of adipose blubber, the sea otter (Enhydra lutris)—the smallest marine mammal and the heaviest member of Mustelidae—has virtually zero subcutaneous blubber! Instead, sea otters rely on two mechanisms:

1. The Densest Fur in the Animal Kingdom: Sea otter pelage packs 120,000 to 150,000 hairs per square centimeter (~1,000,000 hairs per square inch)—nearly ten times denser than a dog or river otter—whose interlocking barbed underhairs trap a dry insulating blanket of air next to the skin so liquid seawater never touches the epidermis (unless fouled by crude oil spills).
2. Skeletal Muscle Proton-Leak Non-Shivering Thermogenesis: As demonstrated by Traver Wright, Randall W. Davis, and colleagues (2021), sea otters maintain a resting metabolic rate ~3 times higher than terrestrial mammals of equal size by running high rates of uncoupled mitochondrial proton leak across the inner mitochondrial membrane of their skeletal muscles—burning up to 20–25% of their body weight in sea urchins, abalone, and crabs every single day simply to generate internal body heat!

1. Li, X., Li, W., Wang, H., Cao, J., Maehashi, K., Huang, L., ... & Brand, J. G. (2005). Pseudogenization of a sweet-receptor gene accounts for cats' indifference toward sugar. PLOS Genetics, 1(1), e3. doi:10.1371/journal.pgen.0010003
2. Rybczynski, N., Dawson, M. R., & Tedford, R. H. (2009). A semi-aquatic Arctic mammalian carnivore from the Miocene epoch and origin of Pinnipedia. Nature, 458(7241), 1021–1024. doi:10.1038/nature07985
3. Jiang, P., Josue, J., Li, X., Glaser, D., Li, W., Brand, J. G., ... & Beauchamp, G. K. (2012). Major taste loss in carnivorous mammals. Proceedings of the National Academy of Sciences, 109(13), 4956–4961. doi:10.1073/pnas.1118360109
4. Nie, Y., Speakman, J. R., Wu, Q., Zhang, C., Hu, Y., Xia, M., ... & Wei, F. (2015). Exceptionally low daily energy expenditure in the bamboo-eating giant panda. Science, 349(6244), 171–174. doi:10.1126/science.aab2413
5. Hu, Y., Wu, Q., Ma, S., Ma, T., Shan, L., Wang, X., ... & Wei, F. (2017). Comparative genomics reveals convergent evolution between the bamboo-eating giant and red pandas. Proceedings of the National Academy of Sciences, 114(5), 1081–1086. doi:10.1073/pnas.1613870114
6. Wright, T., Davis, R. W., Pearson, H. C., Murray, M., & Sheffield-Moore, M. (2021). Skeletal muscle thermogenesis enables aquatic life in the smallest marine mammal. Science, 373(6551), 223–225. doi:10.1126/science.abf4557