[From Bruce Abbott (950706.1330 EST)]
In my last post (950805.1900 EST) I presented data from Collier, Hirsch, and
Hamlin (1972) on number of "meals" (accesses to food, Exp. 1) and number of
pellets (Exp. 2) as a function of ratio requirement. In this post I
describe data on weight and food consumption from Exp. 2. [Note: my
appologies to Ed Hirsch for misspelling his name last time.]
Two rats responded on the various ratio schedules; their pellet earnings
were presented in the last post. Two additional rats received ad libitum
food throughout; their food-consumption and body-weight data were presented
as an average across the two rats. The ad lib rats' average weight followed
an essentially linear function of time (one value given for each 10-day
block corresponding to the time each ratio rat was on a given ratio value,
beginning at about 390 g (while the experimental rats were on FR-1) and
ending at about 502 g (while the remaining experimental rat, Rat 1, was on
FR-240; Rat 2 was tested only up to FR-220). This represents a weight gain
of 112 grams over 140 days, or 0.8 g/day. The weights of the experimental
animals across ratio values must be evaluated against this steadily
increasing baseline. The weights of the experimental animals are presented
below:
Ratio Rat 1 Rat 2
1 387 383
5 395 400
10 409 414
20 416 430
40 427 436
60 425 433
80 435 431
100 431 419
120 435 413
140 441 408
160 430 404
180 425 395
200 425 365
220 422 350
240 389 ---
Food intake of the ad lib rats was essentially constant across the
experiment at about 23-24 g/day. Food intake of the two ratio animals
followed a linear decreasing trend, beginning at about 22 g/day on FR 1 and
falling to about 12 g/day at FR-240. You can compute the actual values from
the previously-posted pellet earnings by multiplying the number of pellets
times 0.045 g/pellet. The linear reduction in food input is equivalent to
about 42 mg lost earning per day per additional lever-press requirement, or
about one pellet (45 mg).
To summarize the weight data, the two experimental rats gained weight at
about the same rate as ad lib rats up to about FR-40; after that their
weights began to decline. By the end of the experiment Rat 1 was 2 g
heavier than at the start but 113 g lighter than the ad lib animals. Rat 2
was 33 g lighter than at the start by FR-220 and because of the weight loss
was not tested at FR-240; this is 152 g lighter than the ad lib rats at the
same point in time. Compared to ad lib rats, Rat 1 was at 78% ad lib weight
and Rat 2 was at 70% ad lib weight. At ratio values much above FR-40 these
rats were clearly failing to make up their losses via increased rates of
lever-pressing. Perhaps the problem is that the losses must be recouped by
expending the same currency: we're asking these rats to spend their way out
of debt.
Regards,
Bruce