The Evolution of Hibernation in Mammals

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The Evolution of Hibernation in Mammals

• Keri Massie and Emily Williamson
• Biology 425, Spring 2009

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Introduction

• many theories about the evolution of hibernation
  (Kemp, 2005)

• None have extra-ordinary evidence supporting them
  (Kemp, 2005)

Were going to explore theories specifically
related to the evolution of hibernation in
mammals
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What is hibernation?!?!

• Reduced activity in order to battle extreme
  environmental conditions.

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Who hibernates

• Mammals
• Birds
• Insects
• Reptiles

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Causes in mammals

• Physiologically
• Decreases in body temperature, heart rate, and
  metabolism resulting in long-term dormancy
• Interrupted by brief awake periods and
  re-warming (Yuan et al., 2007).
• Genetically
• Different gene expression patterns during winter
• PRL-2 (Yuan et al., 2007)
• Genes for Pancreatic lipase
• Genes for PDK-4

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Genetically controlled hibernation (Geiser, 2002)

• PDK-4 helps control cardiac physiology allowing a
  prolonged dormant state
• Fig. 1 Expression of PDK-4 during the
  hibernation season.

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Theories of the evolution of hibernation
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Genetically controlled hibernation

• PRL-2 found in bats, highly conserved in many
  mammals
• Has different expression pattern during active
  times and hibernating times shown below

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The Evolution of Hibernation in Mammals the
Genetic Effect

• Mutation in hemoglobin causing higher affinity
  for oxygen lead to changes needed for increased
  O2 absorption and endothermy (Kemp, 2005)
• Genes that are highly conserved in species may be
  selected for
• PRL-2 (Yuan et al., 2007)
• Nonsynonymous and synonymous mutations both had
  less than 2 chance of becoming fixed over
  populations
• PRL-2 is thus passed down with small variability
  from generation to generation
• Physiological traits required for hibernation
  happen in invertebrates (Geiser, 1998)

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The Evolution of Hibernation in Mammals from
Endotherms (Kemp, 2005)

• Many believe hibernation has evolved from early
  endotherms
• Endothermy allows animals to survive varying
  temperatures and supports higher degree of
  organization (Kemp, 2005)
• have higher aerobic activity potential
• higher breathing capacity

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Ectotherm to Endotherm
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The Evolution of Hibernation in Mammals
Polyphyletically (Geiser, 1998)

• Some believe hibernation and torpor evolved
  separately in different mammals and in birds
• Geiser analyzed phylogenetic tree and compared
  torpor in bird and mammal species

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The Evolution of Hibernation in Mammals
Plesiomorphically

• Many believe hibernation has evolved from a
  common ancestor, and has only evolved once from
  daily torpor
• Evidence
• Small ancestral mammals are thought to be
  heterothermic and have a different pattern of
  torpor than in modern mammals with control of
  body temperature regulation. (Geiser, 2002)
• Hibernation was shown to be plesiomorphic in
  Eutheria, Marsupialia and Monotremata.
• Evolved from a common pattern of circadian
  heterothermy, ectothermic at first then
  facilitated by endothermic thermogenisis. (Grigg,
  2000)

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The Evolution of Hibernation in Mammals Size and
Diet Hypothesis (Geiser, 1998)

• Both mammals and birds that hibernate are small
  and rely on a varying diet
• Possibly all early animals that were small went
  through some form of hibernation or torpor.
• Most hibernated in order to conserve energy and
  food, necessary in order to survive extreme
  conditions in the winter.

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How does this effect the evolution of hibernation?

• Theory of evolution of hibernation still debated
  over
• Researchers believe it is not one evolutionary
  process acting on mammals, but has evolved in
  small steps over many generations(Kemp, 2005)

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Literature Cited

• Geiser, F. 1998. Evolution of daily torpor and
  hibernation in birds and mammals importance of
  body size. Clinical and Experimental
  Pharmacology and Physiology 25736-740.
•  
• Geiser, F., N. Goodship, C. Pavey. 2002. Was
  basking important in the evolution of mammalian
  endothermy. Physiological Zoology 89412-414.
•  
• Grigg, G., L. Beard. 2000. Hibernation by
  Echidnas in mild climates Hints about the
  evolution of endothermy. In Heldermeier, G.
  Klingenspor M (eds) Life in the Cold Eleventh
  International Hibernation Symposium. Springer,
  Berlin Heidelberg New York, pp 5-19.
•  
• Kemp, T.S. 2005. The origin of mammalian
  endothermy a paradigm for the evolution of
  complex biological structure. Zoological Journal
  of the Linnean Society 147473-488.
•  
• Yuan, L., J. Chen, B. Lin, J. Zhung, and S.
  Zhang. 2007. Differential expression and
  functional constraint of PRL-2 in hibernating
  bat. Comparative Biochemistry and Physiology,
  Part b 148375-381

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Photo sources

• National Museum of Natural History
• Microsoft Office 2007, clip art
• Google images
• http//www005.upp.so-net.ne.jp/JurassicGallery/Dim
  etrodon4.jpg
• http//staffwww.fullcoll.edu/tmorris/elements_of_e
  cology/images/chipmunk_hibernating.jpg