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encephalization quotient (was Re: MORE ON SAUROPOD FEEDING HIGH & LOW)



Norm King <nking.ucs@smtp.usi.edu> asks:

> How do we equate brain size vs. body size here (is that called
> "encephalization"?)?

Here's the recipe:

Measure the weight of a lot of animals then remove their brains and
weigh them too.  Now take the logarithm of all of the numbers you've
just obtained.  Now try to fit a line (or lines) through the plot of
log(brain weight) vs. log(body weight).  The relationship that you've
just determined is:

brain weight = A*(body weight)^B

where B is the slope of your line and log(A) is the intercept.

If you were to perform the analysis only for birds and mammals, you'd
find particular values for A and B.  If you repeated the analysis only
for lizards, turtles and other poikilothermic animals, you'd find that
B is the same as it was for birds and mammals (about 2/3 I think -- I
double checked Christopher McGowan's discussion in _Dinosaurs,
Spitfires and Sea Dragons -- yes, it's 2/3), but A is about a factor
of 10 smaller (0.007 vs. 0.07).

Encephalization quotient is defined as the ratio between actual brain
weight and predicted brain weight where the prediction comes from the
above equation.  That is, to find an animal's EQ you take its body
weight, plug it into the above equation, then take its brain weight
and divide it by the number from the formula.  If the EQ is greater
than one then the animal had a relatively large brain.  If less than
one then it had a relatively small brain.

> This is the first place I've ever seen a discussion of selection
> pressure for _small_ brains--I mean, pressure to _decrease_ brain
> size.

At least some birds "shed" parts of their brains and re-grow them
every year.  The brain nuclei in question are related to the song
generating apparatus in male birds.  The argument put forth for this
seasonal loss of brain tissue is that flying is metabolically
expensive, and the animals are adapted to jettison as much excess
weight as possible.  It's a bit of a "just so" story, but it makes
sense.  If you're curious as to why birds have such large brains
(relative to poikilotherms), the "just so" story most people would
invoke is that flying requires a lot of sophisticated coordination of
musculature and visual and vestibular information processing.

Whether or not this says anything about sauropod brains I have no
idea, but since there is apparently selection pressure for the
ontogenetic loss of brain tissue in birds, it shouldn't seem like too
much of a stretch to find that there's been phylogenetic loss of brain
tissue in other animals.

-- 
Mickey Rowe     (rowe@lepomis.psych.upenn.edu)