Circannual Rhythms in the Ewe: Patterns of Ovarian Cycles and Prolactin Secretion under Two Different Constant Photoperiods1

Abstract

The purpose of this experiment was to determine whether circannual rhythms of reproductive activity and prolactin secretion are expressed differently in ewes housed under two different constant photoperiods and restricted temperatures. Eleven ovary-intact ewes housed previously under constant 12L:12D were used. One group (n = 6) was switched to 8L:16D while the other group (n = 5) remained on 12L:12D. Ovarian cycles and prolactin concentrations were monitored for more than 3 yr. The switch from 12L:12D to 8L:16D caused a change from cycles of varied length to regular 17-day cycles within 25 +/- 3 days in all 6 ewes. Subsequently, one complete circannual cycle (300.8 +/- 10.2 days) of breeding and nonbreeding activity was expressed in all ewes, but it was repeated in only 2 ewes. Ewes maintained on 12L:12D cycled at irregular intervals, and only 2 exhibited a distinct circannual rhythm of anestrus and breeding cycles. Episodic variations in ovarian cycle density were observed in most ewes in both groups. The period of this rhythm was similar between groups (290 +/- 19 and 278 +/- 16 days) and differed from 365 days (p < 0.05). Plasma prolactin exhibited a circannual rhythm under both 8L:16D and 12L:12D with a periodicity different from 365 days. We conclude that photoperiod length does not influence the expression of circannual reproductive and prolactin rhythms in the ewe. The absence of a circannual rhythm of breeding and nonbreeding activity, in the presence of circannual changes in ovarian cycle density, suggests that expression of an underlying endogenous oscillator controlling LH secretion was masked or damped.

Endogenous Circannual Cycles of Ovarian Activity and Changes in Prolactin and Melatonin Secretion in Wild and Domestic Female Sheep Maintained under a Long-Day Photoperiod1

“…Furthermore, when animals of both species were subjected to LP conditions, some animals with low prolactin levels throughout the study expressed alternating cycles of ovulatory activity, whereas others experienced no ovarian cycles despite having shown robust circannual changes in their prolactin concentrations. This uncoupling of reproductive activity from the prolactin rhythm agrees with the results of other authors [62], indicating that the prolactin concentration is unlikely to be related to reproductive activity; rather, they are probably independently regulated [9,42].…”

“…The present results show, for the first time, that both these species experience changes in ovulatory activity over the year when subjected to a constant photoperiod of long days. This suggests the existence of an endogenous rhythm of reproduction similar to that observed in other sheep breeds from higher latitudes when maintained under constant short (8L:16D) or 12L:12D photoperiods [9,[40][41][42]. Further, this rhythm was differently expressed in the two species, with the Manchega ewes showing much more sensitivity to the constant long-day photoperiod than the mouflon ewes.…”

Climate change and seasonal reproduction in mammals

“…Some mammals can detect and use annual variation in day length in the mid-tropics while others cannot. For example, a temperate breed of domestic sheep, the Suffolk, can detect and react reproductively to seasonal variation in photoperiod typical of 198N (Arroyo et al 2007) while, as expected, the same breed shows no response when exposed continuously to an equatorial 12 h of light per day ( Jansen & Jackson 1993). In contrast, one of the flying foxes (Pteropus rodricensis) breeds asynchronously when exposed in the laboratory to variation in photoperiod typical of their home at 238 latitude (West 1985).…”

Seasonality of reproduction and production in farm fishes, birds and mammals

“…with a free-running period close to 24 h) which generates endogenous circadian rhythms, a circannual 'rhythm', endogenous to animals, is likely to generate alternate periods of sexual activity with periods of sexual rest throughout the year. These alternate periods can be observed when animals are kept experimentally under constant photoperiodic regimens (constant short days, for example), over at least two consecutive years (mammals: Jansen and Jackson, 1993;Malpaux et al, 2001;Malpaux, 2006;birds: Gwinner, 2003;fish: Bromage et al, 2001). The physiological mechanisms underlying this circannual system remain largely unknown.…”