Keri Massie and Emily Williamson

Biology 425, Spring 2009

Introduction many theories about

the evolution of hibernation (Kemp, 2005)

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

We’re going to explore theories specifically related to the evolution of hibernation in mammals

What is hibernation?!?!

• Reduced activity in order to battle extreme environmental conditions.

Who hibernates

• Mammals• Birds• Insects• Reptiles

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

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.

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

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)

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

Ectotherm to Endotherm

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

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)

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.

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)

Literature CitedGeiser, F. 1998. Evolution of daily torpor and hibernation in birds and mammals: importance of body size.

Clinical and Experimental Pharmacology and Physiology 25:736-740. Geiser, F., N. Goodship, C. Pavey. 2002. Was basking important in the evolution of mammalian endothermy.

Physiological Zoology 89:412-414.  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 147:473-488. 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 148:375-381

Photo sources:• National Museum of Natural History• Microsoft Office 2007, clip art• Google images

• http://www005.upp.so-net.ne.jp/JurassicGallery/Dimetrodon4.jpg• http://staffwww.fullcoll.edu/tmorris/elements_of_ecology/images

/chipmunk_hibernating.jpg