how allometry and habitat affect primate hair reduction
DESCRIPTION
How Allometry and Habitat Affect Primate Hair Reduction. By: Rachel Cohen, Sarah Demeo, & Kaitlyn Robins. Questions on hair evolution. What genetic causes led to reduction in body hair? No experimental data Why does pigmented hair (terminal hair) only grow in certain locations? - PowerPoint PPT PresentationTRANSCRIPT
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How Allometry and Habitat Affect Primate Hair Reduction
By: Rachel Cohen, Sarah Demeo, & Kaitlyn Robins
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Questions on hair evolution
– What genetic causes led to reduction in body hair?
– No experimental data
– Why does pigmented hair (terminal hair) only grow in certain locations?
– not currently known
– What causes the change from un-pigmented hair to pigmented hair in primates?
– not currently known
• Does allometry have a role in hair reduction in primates?
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Allometry
• The study of changes in size and shape of organisms– Often presented by ratios
– Study the change in hair density in relation to the surface area/mass ratio
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Gary Schwartz
• Took data from AH Schultz– Research was based on surveys of hair density in
23 primate species
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Hypothesis:
• Negative correlation between hair density and total body surface area, as an adaptation in anthropoid primates
– Anthropoids – higher primates that are composed of Old World monkeys, Great Apes, and New World monkeys
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Method
• Sampled 1 cm2 area of scalp, back, and chest hair of non-human primates– All specimens analyzed in study lived in forest
habitats
– Calculated body surface area• bSA = mass^2/3
• Hairs/cm²/ bSA = relative hair density– bSA is (body surface area)
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Allen’s Law
– Changes in the surface to volume ratio with changes in body size
– The ratio of surface area to volume decreases as an object or body becomes larger
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Fourier’s Law
• Heat production is proportional to body mass and heat dissipation is proportional to exposed body surface area
• A larger body will have a higher ratio of heat production to heat dissipation than a smaller body
• Insulating ability of mammalian coats depends primarily on hair density
• Leaves characteristics open to natural selection
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Results (graph)
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Results
• Correlations were all significant.
• What does this mean?– As body surface area (bsa) increases, hair
density decreases• found to be true for both New and Old World
Monkeys
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Conclusions
• Allometric growth:
– Thermal constraints on primates follow principle
– Better indicator of hair density was based on surface area/body mass ratio
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Based on the conclusion
• Hair had to be lost before habitat change
• Primates living in the forest would have selection for hair loss as size increased in correlation to Allen’s law (SA/mass)
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Conclusion Cont.
• Schwartz, concluded, based on human ancestors, humans would have had lost their coats prior to entering a savanna habitat
• Exocrine sweat glands and skin pigmentation evolved as result of fixation of hair loss
• Lost coat would have been a disadvantage in savanna habitat do to high levels of radiant heat– Can be seen in Amaral study
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Amaral’s Hypothesis:
• Addressing the question of loss of body hair in primates by comparing hair covered and “naked” primates
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Table 1
Table 1
Skin condition Environmental temperature (°C)
Peak thermal load (Wh)
Naked 30 54.6
Fully furred 30 19.9
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Table 2
Table 2
Skin condition Environmental temperature (°C)
Peak thermal load (Wh)
Naked 35 330
Fully Haired 35 118
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Advantages on naked skin on savanna
• No real advantage regarding tolerance to peak thermal loads
• High sweating capacity of humans essentially compensates the higher thermal loads absorbed by naked skin
• Wind favor heat dissipation by naked skin only if the air temperature is lower than core body temperature
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Coping with heat stress in the Savanna
• Keep a hair covering
• Variable conductance
• Increase sweating capacity
• Not to have a naked skin
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Table 3Table 3
Skin condition Environmental temperature (°C)
Water consumption (kg/12h)
Naked 30 .62
Fully haired 100% 30 .94
Fully haired 80% 30 1.19
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Table 4
Table 4
Skin condition Environmental temperature (°C)
Water consumption (kg/12h)
Naked 35 1.34
Fully haired 100% 35 1.26
Fully haired 80% 35 1.59
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conclusion
• Naked skin found in humans would not have been selected for in Savanna conditions due to poor insulation against diurnal heat, and need for higher water consumption
• High hair density provides protection against diurnal heat
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Future Work
• Further research and experimentation into the causes of hair reduction in primates
• Better understanding into what caused humans to lose body hair
• Completion of non human primate genomes to compare and better support hypothesis with evidence
• Further comparison of allometry in primates that inhabit grassland/savanna
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References• Amaral, Lia Queiroz do. “Loss of body hair, bipedality and thermoregulation.
Comments on recent papers in the Journal of Human Evolution” Journal of Human Evolution
30:4, April 1996, 357-366 http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WJS-45MGY6D-20&_user=768496&_coverDate=04%2F30%2F1996&_rdoc=1&_fmt=high&_orig=s
earch&_origin=search&_sort=d&_docanchor=&view=c&_acct=C000042521&_versi
on=1&_urlVersion=0&_userid=768496&md5=c7cd94004e85d8dca497fc3e49532e2c&searchtype=a
• Schwartz, G. G. and Rosenblum, L. A. “Allometry of primate hair density and the evolution of human hairlessness”. American Journal of Physical Anthropology, 55: (1981) 9–12. http://onlinelibrary.wiley.com/doi/10.1002/ajpa.1330550103/abstract
• Ruff, C. B. “Morphological adaptation to climate in modern and fossil hominids” . American Journal of Physical Anthropology, 37: (1994),65–107. http://onlinelibrary.wiley.com/doi/10.1002/ajpa.1330370605/abstract