[From Bruce Abbott (950807.1555 EST)]
Here are the body-weight and food intake data from Exp. 1 of Collier,
Hirsch, & Hamlin (1972) as measured from Figure 6:
Body-weight (grams) Food Intake (grams)
Ratio Rat 1 Rat 2 Rat 3 Rat 1 Rat 2 Rat 3
FF 334 360 302 22.9 24.9 21.9
1 378 404 334 20.0 23.2 17.7
5 423 423 330 22.0 20.9 22.9
10 405 432 347 19.9 20.9 16.7
20 416 442 356 20.9 20.9 15.8
40 421 455 366 18.8 20.9 17.6
80 425 452 377 19.8 21.0 17.7
160 419 427 377 22.9 17.9 22.1
320 430 401 382 17.7 12.9 15.9
640 430 --- 387 14.8 ---- 13.8
1280 416 --- 371 15.8 ---- 14.8
2560 397 --- 382 12.8 ---- 16.9
5120 402 --- 351 9.8 ---- 8.8
Certain of the ratio values were repeated "in a predetermined random order"
as a second phase of the experiment (the order was not specified). Each
ratio was assessed twice at 4 days per assessment. Below are the data for
the redeterminations (Figure 7). Bear in mind that the rats were a bit
older by then and tend to gain weight over time if allowed to. The random
ordering will tend to spread this gain evenly across the ratio values, which
was NOT the case in the first phase of the experiment.
Body-Weight (grams) Food Intake (grams)
Ratio Rat 1 Rat 2 Rat 3 Rat 1 Rat 2 Rat 3
5 441 454 396 16.8 20.9 17.9
10 430 447 382 20.9 23.0 22.6
20 440 450 392 20.4 21.1 19.7
40 435 452 391 21.1 19.8 20.4
80 442 460 396 15.9 19.9 17.4
160 435 440 394 19.0 20.0 17.5
320 432 432 392 17.9 23.0 23.7
640 438 437 386 13.9 11.7 12.4
1280 431 429 395 15.7 8.3 12.8
One thing that stands out about the intake data is the tendency for the
values to come out at xx.9. My guess is that the values were plotted to the
nearest gram or half-gram and that the deviations from these even values are
errors in positioning the point on the graph and in my measurements of those
positions.
Both sets of data indicate that these rats were able to keep their body
weights either increasing or holding constant up to a ratio of about 160 or
320; in the redeterminations, which tended to de-confound ratio requirement
and passage of time, body weight and food consumption seem to go up slightly
to FR-40 or FR-80 and then decline slightly with further increases. Recall
that increasing ratios occasioned a decreasing number of meals earned; the
rats compensated by increasing the amount consumed at each meal, until the
higher ratios were reached; after that I suspect they just couldn't hold
enough or process it efficiently when reduced to only one or two meals per
day. This contrasts with the findings in Experiment 2, in which there was
no opportunity to make up for number of meals by increasing the meal size
and food intake decreased essentially linearly with increased ratio requirement.
It would appear that rats are able to control body weight to some degree at
least if the decline in earned opportunities to eat can be compensated for
by an increased amount eaten per opportunity. Yet whether such compensation
is possible or not, increasing the response requirement ("cost" of a meal)
drives down the number of meals "purchased." This appears to suggest rather
strongly that rate of access to food is not a controlled variable in the
rat. Instead, error in the nutrient-control (body weight?) system seems to
establish something like the "value" or "attractiveness" of the food, which
is diminished by the effort and/or time required to obtain it. This in
effect establishes an error between having and not having the food which is
proportional to its "value," thus producing a given rate of output (lever
pressing) aimed toward obtaining the food. This system competes with other
control systems to the extent that their outputs require the same resources
(time, effort) which cannot be put into the service of the different systems
simultaneously. The time/effort spent in these competing activities aimed
toward different goals emerges dynamically; what we observe is that the
animal's actions appear to be allocated in the service of different goals
according to the relative "values" of the goals.
If there is another explanation for why the rats are able to control
body-weight by increasing the amount consumed/meal but not by increasing the
meal rate, I'd like to hear it. We will definitely have to take a look at
what happens when we disturb meal rate at a given ratio value in our
experiments.
Regards,
Bruce