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 cm 2 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 Skin conditionEnvironmental temperature (°C) Peak thermal load (Wh) Naked Fully furred3019.9

Table 2 Skin conditionEnvironmental temperature (°C) Peak thermal load (Wh) Naked35330 Fully Haired35118

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 3 Skin conditionEnvironmental temperature (°C) Water consumption (kg/12h) Naked30.62 Fully haired 100%30.94 Fully haired 80%301.19

Table 4 Skin conditionEnvironmental temperature (°C) Water consumption (kg/12h) Naked Fully haired 100% Fully haired 80%351.59

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 onrecent papers in the Journal of Human Evolution” Journal of Human Evolution 30:4, April 1996, MGY6D- 20&_user=768496&_coverDate=04%2F30%2F1996&_rdoc=1&_fmt=high &_orig=s earch&_origin=search&_sort=d&_docanchor=&view=c&_acct=C &_versi on=1&_urlVersion=0&_userid=768496&md5=c7cd94004e85d8dca497fc3 e49532e2c&searchtype=aJournal of Human Evolution 30:4 45MGY6D- Schwartz, G. G. and Rosenblum, L. A. “Allometry of primate hair density and the evolution ofhuman hairlessness”. American Journal of Physical Anthropology, 55: (1981) 9– Ruff, C. B. “Morphological adaptation to climate in modern and fossil hominids”. American Journal of Physical Anthropology, 37: (1994),65–107.