Young (7 days old), middle-aged (30 days old), and old (50 days old) Drosophila melanogaster males were used in an inhibitory conditioning of the proboscis-extension response. Two reinforcement schedules were applied, constant vs. partial. Partial reinforcement increased the number of trials needed to reach the learning criterion to the same extent in all age groups. Young flies needed fewer trials to reach the criterion than both middle-aged and old flies, which did not differ from each other. Extinction was delayed in the partial reinforcement groups by the same amount in all age groups. The slowest rate of extinction was observed in the old group and the fastest for the middle-aged flies. The results are discussed in connection with hypotheses that consider extinction an index either of behavioral rigidity or of strength of the initial learning.

Habituation of the proboscis extension response induced by sugar tarsal stimulation was individually studied in males of Drosophila melanogaster all along the adult life span (2–71 days of age at 25°C). A polynomial regression fits the best the results: the speed of acquisition of this kind of learning decreases between the ages of 3 and 35 days, a plateau being observed in flies older than 35 days. Similar results have been previously obtained using associative learning tasks of increasing complexity. These effects of age could be due to a memory impairment, but also to deficits in the process of central inhibition of irrelevant motor responses.

No abstract

Fourteen hundred and four mice from two inbred strains (Balb/c and C57BL/6), FI, F2, and backcrosses were subjected to four trials in a water-escape task and to a swimming test. Detailed analysis of mice behavior in these situations showed that the "Fi hybrid vigor" affected behavioral characters not directly related to physical vigor but of potential adaptive value. Their superiority was mainly due to more frequent adoption of an efficient behavioral tactic (direct or edge escape paths toward the exit) and more rapid progress with experience in this respect than other generations exhibited. These results clearly show that heterosis is not limited to physical vigor but may extend to behavioral and even psychological characters.Much work in behavior genetics shows and Jinks (1971): "Heterosis may be dethat genes influence behavior but not that fined as the amount by which the mean of an behavior, or any other phenotypic character, FI family (from the crossing of two "pure" is inherited as such. The genes inherited by lines) exceeds its better parent" (p. 119). an individual from its parents provide cer-Heterosis may be accompanied by reductain potentialities, whose expression de-tion in variability in the FI (Manosevitz, pends on interactions with the rest of the 1972; Rose & Parsons, 1970; Schlesinger & genome and with the environment in which Mordkoff, 1963;Smith, 1972) probably due development occurs. Sometimes a partic-to "developmental homeostasis" (Boesiger, ular type of heredity known as heterosis may 1974; Dobzhansky, 1950Dobzhansky, ,1954Lerner, 1954; appear. While several interpretations of Waddington, 1942), although this is not this exist (Lindzey, Loehlin, Manosevitz, & necessarily the rule (Winston, 1964; Rose & Thiessen, 1971), the most precise descriptive Parsons, 1970). definition, when measured from a mendelian This study, using a mendelian crossing crossing schedule, has been given by Mather schedule, is an attempt to make a detailed analysis of behavioral modifications implicit J. Medioni is a member of Equipe de Recherche No. in an y assumption of "hybrid vigor." The 700 Associee au Centre National de la Recherche main aim was to determine to what extent Scientifique: "La preparation a Faction dans le com-heterosis is not only the consequence of inportement animal." creased physical vigor. Indeed', any such J£SSSSJS^1K«£S?, assumption seems incompatible with the Universitede Tours. Pare de Grandmont, 37200 Tours, notion ot the adaptive value ot behavioral France.traits subject to heterosis as defined by