Re: arm length

alex duncan (
17 Aug 1995 00:18:57 GMT

In article <40qdgo$> Dave Brown, writes:

AD>Not so. Homo sapiens has arms of the expected length for an ape our
AD>size. In fact, Homo, Australopithecus & Pan all have arms that are the
AD>same length relative to body mass.
DB>How are you figuring this? I've never heard anything like that before.

I erred a little bit. Allow me to clarify.

The subject of arm length can be examined from several perspectives. The
intermembral index is a common way of comparing relative lengths of the
fore and hindlimbs. Relative to the lengths of our hindlimbs, humans do
indeed have apparently short forelimbs. Our intermembral index is around
72, while the smallest ape intermembral index is around 100 (bonobos) and
approaches 150 in some gibbon species.

However, most biologists agree that it is important to look at the
variables of limb length relative to body mass. For example, we might
get the impression that a gibbon with an intermembral index of 140 has
quite short hindlimbs, and, relative to the length of its forelimbs, it
does. However, the IM index obscures the fact that both the fore and
hindlimbs of gibbons are elongate relative to the body mass. The
forelimbs are so much more elongate than the hindlimbs that the hindlimbs
APPEAR to be short by comparison.

Several people have looked at the issue of limb length vs. body mass in
humans and other hominoids. The following is from Aiello and Dean (1990)
"An Introduction to Human Evolutionary Anatomy" p. 250:

"The higher intermembral index in the large apes (and the low index in
modern humans) results primarily from the alternation [sic - I think they
meant "alteration"] in the relationship between lower limb length and
body weight rather than upper limb length and body weight. Not only
humans but also the large apes are consistent with the relationship
between body weight and upper limb length that is found in monkeys."

Bill Jungers and Tim White recently had a debate about this issue in
Nature (369:194). Jungers demonstrated that it is impossible to
distinguish between African hominoids (and A. afarensis) on the basis of
humerus length. (In fact, the humeri of common chimps are on average
shorter -- relative to body mass -- than those of modern humans, though
the difference doesn't appear to be statistically significant.) This was
a response to White et al's publication of the Maka A. afarensis sample
(Nature, 366:261-265), in which he claimed the short, robust humerus was
not consistent w/ arboreal adaptation.

In his response, White points out that Jungers failed to include orangs
and gibbons in his sample. It's not too clear why this should be
relevant. However, if we calculate average hominoid (sample including
gibbons and orangs) arm length relative to body mass, and compare humans
to the result, then we do indeed have relatively short arms, as the
inclusion of orangs and gibbons "biases" the results. So, I was
incorrect when I stated that humans have arms of the expected length for
a hominoid of our body mass. However, everything I've encountered
indicates that I was correct in noting that humans, chimps and A.
afarensis have arms of the same length relative to body mass.

Alex Duncan
Dept. of Anthropology
University of Texas at Austin
Austin, TX 78712-1086