Failure to demonstrate schedule-induced polydipsia in the degu (Octodon degus).

Abstract

When rats are food-deprived and placed on an intermittent focd reinforcement schedule, they develop a post-pellet pattern of drinking which is excessive in amount. This excessive drinking is called schedule-induced polydipsia ( 1 ) . One characteristic is the generalicy to a large number of different species (1, 2 ) . However, hamsters do not show schedule-induced polydipsia ( 4 ) . In the present study a South American rodent (found in Peru and Chile), the Degu (Octodon d e g w ) , was tested. Two male (208 and 298 gm) and two female (225 and 287 gm) Degu were fooddeprived and reduced to 80% body weight. Daily l-hr. test sessions were conducted in a srandard operant chamber with a food magazine and a water bottle (no lever was present). The operant chamber was housed in a sound-attenuated cubicle. Food pellets (45-mg Noyes Pellets) were delivered according to a fixed time 1-min. (FT l-min.) food reinforcement schedule, in which one pellet was delivered every minute independently of the animal's behavior. Test sessions were conducted for 14 days. All four animals were observed to eat the food pellets readily, with a fairly short latency after pellet delivery (never more than 10 sec.). However, none of the animals developed schedule-induced polydipsia. In fact, the animals never drank from the water bottle in the test chamber in any session. This contrasrs with the rypical patter0 for domesticated rats who usually make some contact with the water bottle during the very first session, and develop schedule-induced polydipsia within 5 ro 10 sessions ( 1 ) . The present data from the Degu and the data from hamsters ( 4 ) suggcsr that schedule-induced polydipsia may have some species limitations. Habitat variables must also be considered given that both the hamster and Degu appear to inhabit somewhat marginal, at least seasonally semi-arid biotopes ( 3 ) . Current explanations ( 1 ) must be revised to accounr for possible species limitations if future research supports these findings.