In rodent species such as the Syrian hamster (Mesocricetus auratus) and the Siberian hamster (Phodopus sungorus), exposure to short daylengths, as would naturally occur during the autumn and winter months, induces a decreased function

auratus) and the Siberian hamster (Phodopus sungorus), exposure to short daylengths, as would naturally occur during the autumn and winter months, induces a decreased function at all levels of the reproductive hypothalamic–pituitary– gonadal axis (Hoffman and Reiter, 1965; Hoffmann, 1973; Arendt, 1995). In Siberian hamsters, it has been clearly demonstrated that this short-day inhibition of the reproductive system is correlated with a longer duration of nocturnal melatonin secretion (Carter and Goldman, 1983; Maywood et al., 1990; Bartness et al., 1993), but the precise mechanism by which the melatonin signal connotes daylength remains unknown (Gunduz and Stetson, 2001a; Gunduz and Stetson, 2001b). These and numerous other studies have established the importance of photoperiod and the secretion of melatonin from the pineal gland in regulating reproductive system activity in seasonally breeding species (Stetson and Watson-Whitmyre, 1984; Stetson and Watson-Whitmyre, 1986; Arendt, 1995). Typically, studies on rodent reproductive photoperiodism examine the influence of changing artificial daylengths using automatically timed light:dark cycles of room illumination provided by ordinary white fluorescent or incandescent lamps. Syrian hamsters exposed to natural, outdoor autumn and winter photoperiods exhibit similar reproductive responses (Reiter, 1973). In one study, groups of age-matched Syrian hamsters were exposed to short photoperiods in either the laboratory (artificial light) or an outdoor environment (natural light) for 11 weeks. The animals exposed to natural photoperiods exhibited reproductive inhibition significantly faster than the animals kept under artificial light (Brainard et al., 1984). Numerous environmental differences might have accounted for the different rates of reproductive change including light intensity, light spectrum, temperature, humidity, noise, odors and the like. Follow-up studies demonstrated that both light intensity and spectrum are important determinants of shortphotoperiod reproductive inhibition (Brainard et al., 1985; Brainard et al., 1986a). Surprisingly, those studies indicated that ‘non-visible’ near-ultraviolet wavelengths (UV-A) were as effective as visible wavelengths in modulating hamster reproductive status. Soon after, it was demonstrated that monochromatic UV-A light at 360 nm could rapidly suppress pineal melatonin production in Syrian hamsters, rats (Rattus norvegicus) and mice (Peromyscus leucopus) (Brainard et al., 1986b; Podolin et al., 1987; Benshoff et al., 1987; Brainard et al., 1994). Furthermore, it was shown that a daily pulse of monochromatic UV-A light at 340 nm delivered during the 2535 The Journal of Experimental Biology 204, 2535–2541 (2001) Printed in Great Britain © The Company of Biologists Limited 2001 JEB3320