is this a mammal? 2 - homeothermy.pdf · requirement for mammal than reptile of same body mass....
TRANSCRIPT
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Is this a mammal?
Is this an adaptation to the cold?
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High metabolic rate, endothermy, and temperature regulation are central, coordinating features of mammalian anatomy and physiology (see handout). But endothermic homeothermy, especially at high Tb is energetically costly. Remember: approx. 10X greater energy requirement for mammal than reptile of same body mass.
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Advantages of homeothermy: Esp. larger animal, less time to warm up Greater independence from short-term
fluctuations in temperature due to weather
Most important: allows coordination of temp-sensitive biochemical processes such as enzyme-catalyzed metabolic reactions. Homeostasis in general allows for more complex physiology because reactions can occur at predictable rates.
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Why high Tb? More rapid diffusion (facilitates biochemistry) More rapid responses of cells (e.g., neurons fire
faster, faster developmental rates) Muscle viscosity decreases at higher temp, more
rapid and forceful contractions of muscles For endotherms: conserves water! Easier to
balance heat loss against gain when environment is slightly cooler on average than Tb
Whether predator or prey, faster is better!
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When did endothermy appear? “Endothermy,” “homeothermy” and many of the traits associated with them don’t fossilize… But many skeletal features give us a strong signal
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The lineage leading to mammals was called synapsids, characterized by a single temporal cranial opening (red)
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Pelycosaurs Dominated the late Permian, gave rise to therapsids
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Therapsids
Still “mammal-like reptiles”
Rapid radiation in late Permian, around 270 MYA
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Cynodonts
Last remaining group of
therapsids, survived mass
extinction at the end of the
Permian. Persisted well
into Triassic and developed
many features associated
with mammals.
Only 1 lineage of
cynodonts survived
through the late Triassic,
and this group became
ancestors of mammals.
Late Permian to mid Triassic
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Pelycosaur
Cynodont (primitive)
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Non-cynodont therapsids
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More derived cynodont
Early mammal
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Axial bending in lizard does not promote air flow, but flexion-extension of galloping quadrupedal mammal is coordinated with gas exchange
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Cynodonts showed first development of secondary hard palate
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By cynodont stage, evidence for morphological changes that would have gone along with endothermy:
Limb posture more erect, longer legs Muscular diaphragm (inferred from reduced lumbar ribs) Secondary hard palate Lighter skeleton for more agile movement Teeth and jaw becoming more specialized for efficient
mastication (next slides) Late cynodonts, early mammals: development of
zygapophyses, double occipital condyles, better flexion and extension of vertebral column
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Crown complexity of teeth gradually increased from simple single-cusped teeth, to anterior-posterior elongation by addition of small cusp or ridge, to development of ridges that could become shearing blades. Teeth now could serve more than just piercing-grasping functions.
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Pattern of tooth replacement modified from reptile to mammal: Reptile: 1 3 5 7….. 2 4 6…. (alt. waves) Mammal: 1 2 3 4…. (molars usually not replaced at all) Allows development of occlusal battery that is continually coordinated, opposing crowns can meet in precision bite, possible to develop sets of crests and valleys that work together to slice, crush, or grind. Well-chewed food can be digested more quickly, release energy and nutrients faster.
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Gradual development of dentary-squamosal jaw articulation (see handout for details)
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1. Synapsid jaw muscles originate inside skull, attach to proximal region of jaw
2. Synapsid opening allowed temporal muscles to expand during contraction
3. Trend: gradual enlargement of opening, mov’t of temporal jaw muscles onto newly ossified braincase
4. Masseter muscles from zygomatic arch (remnant of old temporal shield) to dentary
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New set of jaw muscles unique to mammals: masseter muscles Increase force of bite Increase control of jaw movement Decreases stress on jaw joint Can focus force of bite through cheek teeth Lactation: feeding young milk allowed for delayed tooth eruption, growth of complex adult dentition (also diphyodonty) until jaw larger and stronger Hair: provides insulation critical for thermoregulation
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Surprisingly, no clear consensus on why or how endothermic homeothermy evolved in mammals Main hypotheses: Thermoregulation (miniaturization, nocturnality) Aerobic capacity (sustained, higher activity) Reproduction (faster development)
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Changing body size hypothesis (McNab 1978, Crompton 1978)
Inertial homeothermy
Nocturnality
Pelycosaur Therapsid
Cynodont Mammal
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Aerobic capacity hypothesis (Bennett and Ruben 1979) Came from line of predatory pelycosaurs Selection for high maximum metabolic rate (allow
sustained activity) linked to high resting metabolic rate (BMR)
Level of metabolic activity you can sustain without going anaerobic (running out of O2 in tissues) is about 10-15X BMR
Thus, increase BMR and increase range of activity levels you can maintain
Selection to increase effectiveness as predator (or escape as prey!) brings along BMR, raises Tb
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Parting thought: OK, so if its so great to be an endothermic homeotherm like a mammal, why did mammals spend the next 100+ million years as small, probably nocturnal mouse or shrew-like creatures?
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Study questions: 1. Relate selected early morphological developments in the
ancestors of mammals to the demands of endothermic homeothermy.
2. What are the advantages of homeothermy? What are the advantages of regulating Tb at a temperature just above average environmental temperature instead of saving energy by regulating at a lower Tb?
3. Two of the main hypotheses proposed to explain the evolution of endothermy emphasize thermoregulation (for homeostasis or nocturnality) and aerobic capacity. Discuss the progression of changes that took place during the transition from pelycosaurs to true mammals that may have promoted the evolution of endothermic homeothermy according to the body size hypothesis of McNab.