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?

– 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?

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

Gary Schwartz

• Took data from AH Schultz– Research was based on surveys of hair density in

23 primate species

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

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)

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

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

Results (graph)

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

Conclusions

• Allometric growth:

– Thermal constraints on primates follow principle

– Better indicator of hair density was based on surface area/body mass ratio

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)

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

Amaral’s Hypothesis:

• Addressing the question of loss of body hair in primates by comparing hair covered and “naked” primates

Table 1

Table 1  

Skin condition Environmental temperature (°C)

Peak thermal load (Wh)

Naked 30 54.6

Fully furred 30 19.9

Table 2

Table 2  

Skin condition Environmental temperature (°C)

Peak thermal load (Wh)

Naked 35 330

Fully Haired 35 118

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

Coping with heat stress in the Savanna

• Keep a hair covering

• Variable conductance

• Increase sweating capacity

• Not to have a naked skin

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

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

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

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

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

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• 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