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Darlene Frost and I. Zucker

Summary. Development of the reproductive apparatus was delayed in grasshopper mice maintained from birth in short photoperiods (10 h light/day). The inhibitory effects of short photoperiods on sexual maturation eventually waned and mice in 10L:14D became reproductively active. Adult mice transferred from long (14 h light/day) to short photoperiods underwent testicular regression after 10 weeks and complete gonadal redevelopment after 30 weeks. A similar phenomenon was observed in adult female mice; oestrous cycles ceased within 3 weeks and resumed after 13 weeks in the short photoperiod. The regressive effects of short photoperiods on the male reproductive system were mimicked by daily injections of melatonin administered to mice housed in 14L:10D. Responsiveness of the female reproductive system to melatonin was reduced among photorefractory as compared to photosensitive mice. We suggest that the initial rate of sexual maturation and the timing of seasonal breeding in adult mice are regulated by photoperiod; effects of short daylengths on the neuroendocrine—reproductive axis appear to be mediated by the pineal gland.

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E. L. Bittman and I. Zucker

Seasonal changes in daylength are involved in the phasing of the annual gonadal cycle of the golden hamster (Reiter, 1973). Ovarian cyclicity, as manifested in regular 4-day oestrous cycles, can be maintained indefinitely by exposing hamsters to daily light periods that exceed 12·5 h in length (long days). When the daily light period is reduced below 12 h (short days), or when hamsters are blinded, oestrous cycles cease after an interval of 3–8 weeks (Sorrentino & Reiter, 1970; Seegal & Goldman, 1975).

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M. H. Ferkin and I. Zucker

Summary. During the spring–summer breeding season, female meadow voles prefer odours of males over those of females, but in the autumn–winter season of reproductive quiescence this preference is reversed. Females housed in long (14 h light/day) and short (10 h light/day) photoperiods, respectively, had odour preferences comparable to those of spring and autumn voles, respectively. The preference of long-photoperiod voles for male over female odours was reversed by ovariectomy and restored by treatment with oestradiol. By contrast, neither ovariectomy nor oestradiol affected odour preferences of short-photoperiod voles. Long days appear to influence olfactory preferences by altering ovarian hormone secretion. The failure of oestradiol to affect odour preferences in short photoperiods suggests that the neural substrates mediating this behavioural response are refractory to oestrogens during the nonbreeding season.

Keywords: odour preferences; seasonality; photoperiod; oestradiol; vole

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Laura J. Beasley and I. Zucker

Summary. Adult pallids bats collected in April or May, were maintained in short or long photoperiods (10 or 14 h light/day) for 3–6 months. In August, the short-day bats had regressed testes, epididymal spermatozoa and fully developed accessory sex glands, corresponding to the autumnal reproductive condition of field animals; long-day bats had testes undergoing spermatogenesis, few epididymal spermatozoa and undeveloped accessory sex glands (summer reproductive condition). Bat's in each photoperiod manifested the expected autumnal reproductive pattern in October. We suggest that photoperiod influences the reproductive physiology of male pallid bats by affecting an endogenous circannual reproductive rhythm.

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Nancy G. Forger and I. Zucker

Summary. Male white-footed mice were maintained from birth in chambers equipped with latitudinal timers programmed to simulate the natural progression of daylengths at 42°N latitude. Mice were born into photoperiods starting on 1 April, 15 July, 15 September or 27 October. Testes and seminal vesicles of April- and July-born mice were mature at 60 days of age; puberty was delayed by several months in animals born into the September and October photoperiods but these mice became fecund at about 3·5 months of age, independent of current photoperiod. Therefore, increasing daylengths of late winter do not appear to synchronize the reproductive development of overwintering juveniles born at different times during the previous breeding season.

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Marie S. Carmichael and I. Zucker

Summary. Hamsters were entrained to a 23-h or a 24-h photoperiod, each with 14 h of light per cycle. Gestation length did not differ significantly for animals in the two photoperiods (378 and 383 h), and it is concluded that the duration of hamster pregnancy is not measured in circadian cycles.

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L. Smale, R. J. Nelson and I. Zucker

Summary. The influence of neonatal androgenization on behavioural receptivity was tested by treating female voles on the 3rd day of life with testosterone propionate or with the oil vehicle. After treatment in adulthood with urine or with oestradiol benzoate, androgenized voles were less likely than normal females to display behavioural oestrus and were more likely to engage in agonistic behaviour in tests with stud males. Uteri of androgenized and control females treated with oestradiol benzoate in adulthood manifested similar increases in weight; however, only normal females treated with male urine showed increased uterine weights. Males castrated in adulthood did not display lordosis after treatment with oestradiol benzoate. Sexual differentiation induced by neonatal testicular secretions appears to limit responsiveness of the adult neuroendocrine axis to chemosensory stimuli in male urine.

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Laura Smale, R. J. Nelson and I. Zucker

Summary. Short daylengths did not affect testes weight or spermatogenic index in male voles or uterine weight in female voles. Short daylengths did stimulate the growth of a winter pelage in both sexes; short-day voles had longer underhairs and guard hairs and a thicker, more dense pelage than did long-day voles. Plasma prolactin concentrations were five times higher in long-day than in short-day females and 25% higher in long-day males than in short-day males. The effect of short daylength on pelage was prevented by pinealectomy. We suggest that the growth of a winter coat is an obligate adaptation for winter survival, stimulated by exposure to short daylengths, but that changes in breeding activity are facultative and dependent to a greater extent on other cues for seasonal synchronization.

Keywords: vole; photoperiodism; prolactin; pelage; reproduction

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R. J. Nelson, J. Dark and I. Zucker

Summary. Male California voles were maintained in long (14L:10D) or short photoperiods (10L:14D) for 10 weeks and fed a standard diet of rabbit chow and water ad libitum. One additional group in each photoperiod received the standard diet plus supplements of spinach 3 times weekly. A fifth group was housed in 14L:10D and fed the standard diet, but for 10 weeks water availability was restricted to several hours each morning. Testes and seminal vesicles were heaviest in long-day voles fed spinach supplements and lightest in short-day voles fed only the standard diet; the latter animals manifested reduced testicular spermatogenesis. Testicular weights were also depressed in voles with restricted access to water. It is suggested that photoperiods that simulate those of winter induce regression of the reproductive organs of male California voles but the availability of green vegetation counteracts the inhibitory effects of short daylengths.

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T. M. Lee, L. Smale, I. Zucker and J. Dark

Summary. The influence of daylength on body mass and food intake of pregnant and lactating voles was tested by comparing animals housed in long versus short day-lengths. Pregnancy rates were approximately 50% in long-day females and in voles kept in short days beginning 2 weeks before mating, but were significantly lower in voles preadapted to short days for 8 weeks before mating. Body mass and food intake increased substantially during pregnancy and lactation and the magnitude of the increase was unaffected by daylength; by contrast, body weight was significantly reduced in non-impregnated voles kept in short as compared to long days. The suppressive effects of short photoperiods on body weight were completely counteracted during pregnancy and lactation. Voles that breed during the short days of winter face extreme energetic challenges but the advantages of early breeding appear to justify the costs.