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Isolation-induced facilitation of male sexual behavior in mice De Catanzaro, Denys Anthony


The present study examined the effects of housing and social conditions on the sexual performance of male house mice (Mus musculus). Specifically, mice housed postpubertally in social isolation were compared to others housed in all-male groups. Following periods of differential housing, males were tested in the presence of ovariectomized females made receptive with exogenous, estrogen and progesterone. In five series of experiments, effects of isolation/grouping and related parameters were delineated, and physiological and social mechanisms underlying these effects probed. In the first series of experiments, the basic effects of isolation and grouping were examined. In Experiment 1A, animals housed 1, 3, or 12 per cage were given repeated weekly tests with females. Performance in isolates was consistently superior and reached an asymptote twice that of grouped animals, In Experiment 1B, reversal of housing conditions, reversed performance. Experiment 2 varied intervals, of isolation between subjects, finding facilitation at several intervals. Experiment 3 compared animals under different population densities. Density did not alter the effects of isolation and grouping. In all experiments, additional tests with target males indicated that aggressive and sexual performance were moderately correlated and responded similarly to parametric manipulations. In the second series, the strain and species generality of isolation/grouping differences in sexual activity was studied. Experiment 4 examined male Swiss-Webster, C57, and DBA mice housed individually or grouped for 2 weeks. Within each strain, social isolates showed more mounts, intromissions, and ejaculations and shorter latencies to first mount and intromission. Experiment 5 involved a similar comparison of isolated and grouped male rats, hamsters, and gerbils. Isolation produced no major effect in hamsters but reduced performance in rats and gerbils. Results suggest that facilitation of sexual activity by isolation is characteristic of the mouse species. Decrements accompanying postpubertal isolation in the rat resemble effects of prepubertal isolation in this species. These species differences may parallel differences in physiology and social behavior. In the third series, the minimum period of isolation required to produce isolation/grouping differences was established. Experiment 6 compared sexual performance of male mice isolated or grouped for periods of 1 day or 2 weeks. Isolation facilitated performance equally at both intervals; this differs from effects of isolation on other behavioral and physiological variables. Experiment 7 examined animals isolated for intervals ranging from 1 hour to 1 week. Isolates showed greater performance at all intervals exceeding 12 hours. Simple cleaning of grouped animal's cages increased their performance at 1- and 4-hour intervals. In Experiment 8, grouped males were observed continuously for 24 hours preceding testing. Intermale mounting was rare and neither it nor aggression correlated with subsequent sexual performance. In the fourth and fifth series, some possible physiological mediators of isolation/grouping effects were studied. In Experiment 9A, adrenalectomized and non-adrenalectomized mice were compared. Following adrenalectomy, grouped mice showed elevated sexual activity while isolates declined. In Experiment 9B, corticosterone treatment failed to reverse effects of adrenalectomy. In Experiment 10, ACTH treatment restores mating activity to preadrenalectomy levels in adrenalectomized isolates but had little effect on intact isolates. Results suggest an adrenal involvement in isolation effects but do not specify its nature. In Experiment 11, castrated males given replacement testosterone were compared to intact males. Isolation/grouping differences were present in intact but not testosterone treated mice, suggesting gonadal hormone involvement in the phenomenon. Social interactions among grouped male mice appear to suppress their subsequent sexual activity with females, Intermale aggression, particularly, may stress group members, producing physiological changes conducive to low sexual activity. Furthermore, the presentation of stimulus females may be relatively more novel for isolates and consequently produce higher levels of general arousal in these mice.

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