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R. D. LISK

Experimenters interested in determining the pattern of copulatory activity in the male rat invariably select a sample of animals found to be active copulators during a series of pre-experimental trials. Various investigators have observed that a certain percentage of seemingly healthy males refuse to copulate under standard testing conditions (Whalen, Beach & Kuehn, 1961). In my studies I define 'active copulators' as males which mate on a minimum of 4 out of 14 nights when in the constant presence of a stimulus female. In some samples, 40% fail to meet this criterion (Lisk, 1966). Grunt & Young (1952, 1953) found that changes in the level of androgens exceeding maintenance levels did not influence the sex drive in guinea-pigs. Larsson (1966) found this to be true for the rat also. Androgen therapy has
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N. C. Pratt and R. D. Lisk

Summary. Primiparous female hamsters were mated to proven breeders and stressed during early pregnancy. Females were housed singly throughout gestation except for Days 4, 5 and 6 when they were paired for 10-min intervals 3 times each day with another female matched for age, weight and day of pregnancy. Within each of the pairs, one female was consistently dominant to the other. Controls were exposed to a novel area instead of a conspecific. At parturition, all pups were counted, sexed and weighed. There were no significant differences between litter sizes or sex ratios (defined as % male) of control and dominant females. Litter sizes produced by control or dominant dams were significantly larger than those of subordinate dams, and litter sex ratios of dominants were significantly higher than those of subordinates. Subordinate dams produced fewer males than did dominant dams, but there was no difference in the number of females produced. Also, subordinate dams produced smaller pups than control dams. Examination of uterine implantation sites and fetal resorptions indicated that fetal loss occurred between Days 5 and 10 of pregnancy. These results suggest that subordinate dams produce smaller litters via selective resorption or spontaneous abortion of males in utero and that those males they do produce are smaller than those produced by dominant or control dams. We suggest that males are more susceptible in utero to effects of maternal stress in this species, and may require more maternal investment to survive to term.

Keywords: golden hamster; social stress; litter size; sex ratio; birth weight

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N. C. Pratt and R. D. Lisk

Summary. Primiparous females were mated to proven breeders and half received a subcutaneous implantation of a progesterone-filled silastic capsule on Day 3 which was removed on Day 14. The other half of the group received no treatment. Blood samples were taken from all individuals on Days 3, 7, 11 and 14. The females were paired for brief periods on Days 4, 5 and 6 with a conspecific matched for treatment group, age, weight and stage of pregnancy. Controls were exposed to a novel area instead of a conspecific. Within each of the pairs, one female was consistently dominant to the other. At parturition, all pups were counted, sexed and weighed. Among the untreated group, there were no significant differences between litter sizes or sex ratios (defined as percentage male) produced by control and dominant females. Subordinate females produced significantly smaller litters than control or dominant dams and significantly lower sex ratios than dominant dams. Subordinates produced fewer males than control or dominant dams, but there were no differences in the number of females produced. Among animals given progesterone implants, there were no significant reductions in litter size or sex ratio produced by subordinate dams compared with control or dominant dams. This study showed that female golden hamsters exposed to social subordination early in pregnancy produced smaller and more female-biased litters, and that these effects were accompanied by a significant reduction in circulating progesterone concentrations. When subordinate females were given supplementary doses of progesterone before exposure to stress, they did not show significant litter deficits. Therefore, it seems that progesterone plays an active role in mediating the effects of social stress on litter output in this species.

Keywords: golden hamster; social stress; litter size; sex ratio; progesterone

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K. E. Wynne-Edwards, U. W. Huck and R. D. Lisk

Summary. Females housed with their mates for 3 or 4 days before mating took place (i.e. early in the oestrous cycle at the time of introduction to the mate) were significantly more likely to litter than were females housed with their mates for only 1 or 2 days before mating. The duration of post-copulatory pair contact had a complex effect on pregnancy success. While only 41% of females littered when they had 24 h of post-copulatory pair contact, females exposed to either longer or shorter durations of post-copulatory pair contact littered at significantly higher rates. Exposure to a strange male 24–48 h after mating did not produce a strange-male induced pregnancy block. The critical parameter responsible for the decrease in the number of females littering was the absence of the mate, irrespective of the presence or absence of a strange male. If this pattern of pregnancy block is adaptive for females, it seems probable that females in the wild require substantial levels of paternal investment by their mates.

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U. W. Huck, R. D. Lisk and C. L. Guyton

Summary. Hamsters were mated repeatedly at 60, 180, 300 and 420 days of age or once during their lifetime at one of these ages. Copulatory stimulation was varied both for number of ejaculations (2 versus 14) and amount of vaginocervical stimulation (5 versus 50–60 intromissions). Two ejaculations provided sufficient spermatozoa to maximize litter size for all age and parity classes tested. Differences in fecundity depended on the amount of vaginocervical stimulation received. Higher levels of vaginocervical stimulation increased littering success at 300 and 420 days for nulliparous females and at 180, 300 and 420 days for multiparous females. Females which did not deliver litters did not show a cessation of oestrous cycles which characteristically follows the induction of a luteal phase. The decreased fecundity observed with increasing age or parity therefore resulted from a change in sensitivity to the stimulus conditions necessary to activate the neuroendocrine are for establishment of pregnancy.

Keywords: age; parity; pregnancy initiation; litter size; vaginocervical stimulation; golden hamster

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U. W. Huck, R. D. Lisk and C. Thierjung

Summary. When maintained under a 14L:10D photoperiod, the duration of behavioural receptivity in female golden hamsters was about 18–21 h depending on age and/or parity. The effectiveness of mating stimuli in initiating pregnancy was shown to be a function of when in the receptive period (early, middle, late) that mating occurred. During the 9-h period before ovulation, 5 ejaculatory series were sufficient to produce a nearly 100% pregnancy rate and maximum litter size. During the ovulation period, however, high pregnancy rates were achieved only when mating continued to satiety (12–15 ejaculatory series plus 10–24 long intromissions). Late in the receptive period even mating to satiety failed to result in a pregnancy. In general, pregnancy rates were significantly higher for young virgin than for older multiparous females when mating occurred during or after the ovulation period. The reduced fecundity of females mating during or after ovulation was due to insufficient vaginocervical stimulation to induce functional luteal activity and not to lack of spermatozoa. Females mating late in the receptive period did not show a cessation of oestrous cycles which characteristically follows the induction of a luteal phase. Greater amounts of vaginocervical stimulation during this time increased the number of females which delivered litters but had no significant effect on litter size. These results suggest that levels of male copulatory behaviour considered 'excessive' when mating occurs early in the receptive period are essential for pregnancy initiation when mating occurs later.

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U. W. Huck, N. C. Pratt, J. B. Labov and R. D. Lisk

Summary. Golden hamsters that were mated repeatedly from 55 days of age produced 6–12 litters. Litter size at birth rose between the 1st and 2nd litters, peaked on the 3rd, and declined steadily after the 5th litter. Offspring sex ratio (% male) at birth followed a similar pattern: increasing between the 1st and 2nd litters, remaining high through the 3rd, and becoming increasingly female-biased thereafter. Weaning success decreased sharply after the 6th litter and most dams failed to raise any young to weaning after the 9th litter. These sequential effects on litter size, offspring sex ratio and weaning success were also observed in females mated once at different ages, but they occurred considerably later in life, i.e. increasing parity hastened the effects of advanced age. These ageand parity-related changes in litter composition are consistent with the Trivers–Willard hypothesis that physiologically-stressed females would skew offspring sex ratios to favour daughters. However, since the observed changes in sex ratio were probably due to differential prenatal mortality, their adaptive significance is unclear.

Keywords: age; parity; sex ratio; weaning success; golden hamsters; litter size