of Pelibuey ewes respond to the small variations in photoperiod that are present at 19 813
X
N, and that under natural conditions, photoperiod appears to be the main regulator of ovarian activity at
this latitude. However, other factors such as temperature or humidity may act as modulators, and their relative importance could increase at more equatorial latitudes. q 2000 Elsevier Science B.V.
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Keywords: Pelibuey sheep; Reproductive seasonality; Photoperiod; Subtropics
1. Introduction
Photoperiod is the main environmental factor that regulates seasonal breeding in Ž
sheep and goats from northern latitudes Yeates, 1949; Thwaites, 1965; Karsch, 1984; .
Chemineau et al., 1988 . It has been considered that the effects of photoperiod on Ž
. reproductive activity decrease in lower latitudes Hafez, 1952 , so that seasonal repro-
Ž duction is absent or greatly reduced in tropical or subtropical regions Carles and
. Kipngeno, 1986; Eloy et al., 1990 , or that if present, it is mainly due to seasonal
Ž .
variations in food availability Gonzalez et al., 1991; Chemineau et al., 1995
´
In the case of Pelibuey sheep, a breed of hair sheep of African origin, initial studies Ž
conducted in Mexico suggested that reproductive seasonality was absent Castillo et al., .
1972; Valencia et al., 1975 . Some degree of seasonality was later accepted, but it was Ž
. attributed to variations in food availability Gonzalez et al., 1991; Cruz et al., 1994 ,
´
Ž .
temperature or humidity Gonzalez et al., 1992 . However, evidences were soon found
´
that the reduced ovarian activity that occurs during spring is independent from nutri- Ž
. tional status Rodrıguez-Maltos et al., 1992; Velazquez et al., 1995; Martınez, 1998 .
´ ´
´
In the first direct studies of the effects of artificial photoperiod on Pelibuey sheep, we Ž
. found that it strongly influenced ovarian activity in intact ewes Porras et al., 1998 and
Ž .
LH profiles in ovariectomized ewes implanted with oestradiol Porras, 1999 . However, Ž
. the alternate artificial photoperiods used in those studies were classic long 16L:8D or
Ž .
short 8L:16D ones, which are extremes that are not naturally present at the latitudes where Pelibuey sheep are raised. Thus, the objective of this study was to evaluate if the
ovarian activity of Pelibuey ewes is affected by variations in photoperiod that mimic those found in tropical and subtropical regions of Mexico, where the difference between
the longest and the shortest day of the year is only about 2 h. In addition, 24-h melatonin profiles were characterized at different times of the year in order to evaluate the pineal
response to small changes in photoperiod, and the annual profiles of prolactin were characterized in order to evaluate the transduction of the photoperiodic signal to other
physiological end points.
2. Materials and methods
