Re: Dissecting the Aquatic Ape: Bipedalism

HARRY R. ERWIN (herwin@mason2.gmu.edu)
31 Jul 1996 12:57:25 GMT

Paul Crowley (Paul@crowleyp.demon.co.uk) wrote:
: In article <4th488$lc0@portal.gmu.edu>
: herwin@osf1.gmu.edu "HARRY R. ERWIN" writes:

: > : > The niche is large-bodied suspensory feeding in open-canopy forest.
: > : > [..] you have to climb down out of the tree to move to another tree.
: >
: > : Do you really believe this? Or are you just repeating a formula?
: >
: > Yes, I believe this. Published studies of Pan trog. and Pan paniscus show
: > that Pan trog. never bridges and Pan paniscus rarely bridges.

: I was querying: "The niche is . . . . ". It is not one currently
: occupied by any animal. It is an imaginary niche -- and therefore
: highly suspect.

But one apparently occupied in the past.

: > There have been semi-terrestrial hominoids for about 20 MYr, but our
: > ancestors came down to the ground sometime between 10 and 4 MYr ago.

: What real evidence is there for "our ancestors came down . 10 to 4 Mya" ?

Dryopithecus was still primarily arboreal at 10 MYr BP. We have bipeds at
4.2 MYr BP.

: > : > In relatively closed open-canopy forest, a bipedal stance doesn't
: > : > give you useful early warning of attack.
: >
: > : Again: Do you really believe this? Or are you just repeating a
: > : formula? Have the assumptions involved here ever been tested in
: > : the real world?
: >
: > This comes from study of human populations and the reason why they put a
: > lot of effort into cutting down underbrush in the areas they live
: > in--visibility. Also studies of infantry attack on tanks and the factors
: > that lead to success or failure. (I used to be an operations analyst
: > specializing in this area.)

: There is a vast difference between the reasons early hominids might
: have for walking tall while trying to look over the high grass to see
: lurking lions while also hoping the lions did not see them (I'm having
: some difficulty visualising this) -- and the reasons why modern Hss
: might cut down underbrush; let alone infantry tactics with tanks.

If you can see the threat at a distance, you won't be ambushed, whether or
not the ambushers see you. The large cats are ambush hunters and will lay
off alerted targets. Lee Talbot has a lovely film of this taken in the
Serengeti in the 1950s. There's a parallel discussion of this underway on
a HBES listserv that I participate in. I think I reflect the currently
evolving professional concensus on this. There are some papers on the
subject now in the process of publication that you should see shortly.

: > : Nearly all the lethal
: > : attacks on vulnerable chimps, baboons, or other primates appear to
: > : occur when a single victim is feeding or resting in a tree and the
: > : attacking group moves silently on the ground to cut off all avenues
: > : of escape.
: >
: > Not very applicable to leopard attacks, then.

: Leopards are mostly nocturnal. They're why chimps sleep in trees.

Point?

: > They're a good deal more comfortable in an area with little undergrowth
: > so they have good visibility.

: Now you're talking! Animals that do stand tall for extra visibility,
: such as meercats, do not live in forests; they inhabit open, fairly
: flat areas with good lines of visibility. Places where you can see
: significantly more with a bit of height. Even in such locations, I
: still can't believe that another foot or so would be enough to justify
: the adoption of a bipedal gait.

Visibility is complicated (8)). The critical parameter if you're looking
for something is the sweep rate, which is speed of advance times twice
your mean range of detection. You can see in a given direction to the
first obstacle that is approximately your height or greater, so range of
detection is a function of your height and the statistical distribution of
obstacle heights. If you're more concerned with ambush when stationary,
the critical parameter is the area you can observe, which is proportional
to the square of the mean range of detection. Assuming open terrain and a
Zipf's Law distribution of obstacle heights, you get sweep rate
proportional to height and ambush probability inversely proportional to
height squared. In closed terrain, where the obstacles are taller than
you, there is no dependency on height. In mixed terrain, you get a mixed
dependence.

--

Harry Erwin, Internet: herwin@gmu.edu, Web Page: http://osf1.gmu.edu/~herwin 

49 year old PhD student in computational neuroscience ("how bats do it" 8)

and lecturer for CS 211 (data structures and advanced C++)