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Lecture 19

BIO153 Lecture 19.pdf

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Christoph Richter

2009 BIO153: Lecture 19 Birds and Mammals Birds: the last of the dinosaurs ▯ approximately 10,000 species alive today (largest group within the tetrapods) Characteristics of birds: Because birds are uniquely adapted for flight they have many synapomorphies, and much of their morphology and physiology can be interpreted in light of adaptations for flight: ▯ bipedal ▯ endothermic ▯ feathers (modified scales) ▯ have beaks & lack teeth (digestive modification: crop & gizzard) ▯ lay eggs ▯ social – have complex mating systems ▯ most fly – many physiological adaptations crop = storage organ for food gizzard = organ for grinding food (grinding comes from small stones swallowed by the bird) It’s not immediately obvious, but the crop and gizzard are flight modifications: absence of teeth allows the skull to be light; without teeth, something is needed to grind food; storage organ can be rapidly emptied if the bird needs to take flight, etc. Specific adaptations for flight: ▯ keeled sternum -- for attachment of massive flight muscels ▯ fused clavicle (furcula) - stores energy from the downward stroke of the wings; springs open and brings the wings upwards ▯ hollow bones ▯ many fused bones ▯ reduced # of bones ▯ feathers ▯ 4-chambered heart ▯ advanced respiratory system 1 Respiratory system of birds: ▯ the lung volume of a bird is ½ that of a similar sized mammal ▯ air sacs, air space in bones, large trachea ▯ total volume of the respiratory system 3X mammal ▯ no diaphragm: flow-through ventilation How it works: as the bird breathes in (Inspiration 1), only 25% of the inspired air passes over the lungs; the remaining 75% goes into the posterior air sac. As the bird breathes out (Expiration 1), the air sacs get smaller, and the oxygenated air from the posterior sac moves over the lung. The bird breathes in (Inspiration 2), which sends fresh air into the posterior sac, and fills the anterior sac with deoxygenated air. Expiration 2 pushes stale air from the anterior sac to the trachea and out; at the same time, oxygenated air from the posterior sac is moving over the lung. In effect, then, the bird’s lung always has a flow of fresh oxygenated air passing through the parabronchi. Bird migration: long-range movements (partly possible due to birds’ extreme respiratory efficiency!) Migration is usually a means of tracking changing resources: e.g. the Arctic tern spends the boreal (northern) summer in the Arctic and the austral (southern) summer in waters around the Antarctic. Longest continuous migration: bar-tailed godwit (10, 000 km over open water!) Bird senses: ▯ visual, auditory senses well developed ▯ predators have binocular vision ▯ raptors (hawks, eagles etc.) have ~8X the number of rods and cones in their retinas as a human ▯ most birds cannot move their eyes; move their heads instead (fixed eyes have sharper vision than mobile eyes, because they are not being constantly deformed by muscles pulling on them) 2 ▯ owls have asymmetrically placed ears to hear precisely Bird song ▯ syrinx instead of larynx ▯ split – can produce 2 sounds simultaneously ▯ territory defense, courtship, advertising genetic quality Reproduction: ▯ most lack an intromittent organ (there are notable exceptions!) ▯ females are the heterogametic sex (unlike mammals, where females have 2 X sex chromosomes, and males have XY, female birds have ZW and males are ZZ) ▯ have extended parental care Intelligence: ▯ highly intelligent: social learning, tool use, vocal mimicry…. Mammals: Mammals arose from therapsids (mammal-like reptiles) during the Permian (~270 mya). Mammals are the only extant (surviving) synapsid lineage. Mammal characteristics: ▯ 3 middle ear bones ▯ lower jaw is one bone: in mammals, the quadrate, articular and quadrate bones (which make up the joint between upper and lower jaw in non- mammalian amniotes) have become the 3 bones (ossicles) of the inner ear (the incus, malleus and stapes). In mammals, the lower jaw is a single bone called the dentary (because it bears teeth). ▯ complex teeth: with the exception of a few groups that have reverted to homodonty (e.g. toothed whales), mammals have heterodont teeth (an individual has differently shaped teeth for different roles – e.g. shearing, crushing, cutting, grinding etc.). Diphodonty is the phenomenon where an individual grows one set of teeth during the juvenile period, then loses these and grows another set. 3 ▯ fur: this is a synapomorphy – all mammals have fur/hair (even if it is reduced to a few vibrissae (whiskers), as in whales or naked mole rats), and only mammals have fur. ▯ mammary glands: these arose from sweat glands. Why are mammals so successful? Even though they are an old group (~270 mya), they really diversified after the extinction of the non-avian dinosaurs 65 mya. Although they are not as speciose as, for example, the birds, they are often the “keystone” species in many ecosystems (e.g. top-level predators, etc.) They have successfully invaded a wide variety of habitats all over the globe. Some reasons for their success: 1. Changes in feeding ecology: ▯ jaws: hinge forward; more complex movements (side to side chewing, etc.) ▯ heterodonts: complex types of teeth; able to be effective predators and break down large prey items (e.g. lions stripping a zebra carcass) 2. Endothermy: ▯ the ability to internally regulate their body temperature meant that they could be nocturnal (not dependent upon the warmth of the sun to be active) ▯ higher metabolic performance (see below) ▯ note: endothermy is not unique to mammals; birds are endotherms and fossil evidence suggests that many theropods were endothermic as well. 3. Locomotion: ▯ parasagittal gait: legs under body made for more effective locomotion (see notes on Carrier’s constraint) ▯ platypus (Monotremata) still has a sprawling gait Fig. 6.18 ▯ Endothermy: most biochemical thermoneutral zone processes work best and fastest in a narrow range of optimal temperatures (usually around 36-41° C). Maintaining a constant body temperature at or close 4 to this optimum has many advantages, but also comes with a high energetic cost. The figure at right shows the energetic costs (measured as oxygen consumption) experienced by a mouse over a range of temperatures. Except for a very small window of temperatures where thermoregulatory costs are minimal (defined as the thermoneutral zone), the mouse expends a lot of energy just regulating its body temperature. (Note that below its thermoneutral zone, a drop of a few degrees in ambient temperature can lead to a doubling of metabolic rate! There are all sorts of interesting behavioural and physiological adaptations to cut these costs, but we unfortunately don’t have time to discuss them here.) Costs and benefits of endothermy: benefits: ▯ high metabolic rate: a high metabolic rate permits greater “metabolic performance”: e.g., faster locomotion, faster reproduction (more offspring produced in a shorter period of time), larger brains (brains are very metabolically costly to maintain (draw a lot of energy), so you need a fast metabolism to run a f
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