The study found that hindlimb length correlated inversely with both indicators of aggressiveness: Primate species with greater male-female differences in body weight and length of the canine teeth had shorter legs, and thus display more male-male combat.
There was no correlation between arm length and the indicators of aggression. Carrier says arms are used for fighting, but "for other things as well: climbing, handling food, grooming. Thus, arm length is not related to aggression in any simple way."
Verifying the Findings
Carrier conducted various statistical analyses to verify his findings. First, he corrected for each species' limb lengths relative to their body size. Primates with larger body sizes tend to have shorter legs, humans excepted. Without taking that into account, the correlation between body size and aggression indicators might be false.
Another analysis corrected for the fact different primate species are related. For example, if three closely related species all have short legs, it might be due to the relationship an ancestor with short legs and not aggression.
Even with the corrections, short legs still correlated significantly with the two indicators of aggressiveness.
The study also found that females in each primate species except humans have relatively longer legs than males. "If it is mainly the males that need to be adapted for fighting, then you'd expect them to have shorter legs for their body size," Carrier says.
He notes there are exceptions to that rule. Bonobos have shorter legs than chimps, yet they are less aggressive. Carrier says the correlation between short legs and aggression may be imperfect because legs are used for many other purposes than fighting.
Humans "are a special case" and are not less ag
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Contact: David Carrier
carrier@biology.utah.edu
801-585-7967
University of Utah
11-Mar-2007