Seasonal acclimation of prairie deer mice

Prairie deer mice responded to long nights by reducing their metabolic rates, core temperatures, thermal conductances and incremental metabolic responses to cold stimulus, while increasing their capacities for nonshivering thermogenesis. Some winter animals spontaneously entered daily torpor in the mornings and thereby further reduced their metabolic rates and core temperatures. Provision of exogenous melatonin (by subdermal implants) mimiced short photoperiod effects on metabolic rates and core temperatures of wild-caught, laboratory maintained animals. Provision of supplemental dietary tryptophan to laboratory animals conditioned to natural light cycles mimiced metabolic effects of long nights in summer animals, and further reduced metabolic rates of winter mice, but did not affect their core temperature levels. Newly caught, laboratory maintained deer mice responded to natural seasonal clues of shortphotoperiod and increased dietary tryptophan by reducing their resting energy requirements through both lower metabolic and lower core temperature levels. Short photoperiod and seasonal change also promoted gonadal involution, and resulted in more socially tolerant huddling by mice with reduced core temperature. Reduced 24-hour LH excretion rates were also observed in winter animals which were exposed to seasonal light cycles at warm (25°C) room temperatures. We propose that seasonal acclimatization involves pineal effects on sex hormone-influenced social behaviors and on resting metabolism. These effects serve to conserve resting energy expenditure and promote hypothermic insulation by wild prairie deer mice.     

Seasonal luteal cyclicity of pubertal and adult red deer (Cervus elaphus)

Reproductive failure of rising-two-year-old (R(2)) hinds and seasonal misalignment between calving and pastoral feed production are two factors limiting reproductive productivity of farmed red deer hinds in New Zealand. This study aimed to better understand processes around female puberty and breeding seasonality by describing the potential breeding season (i.e., oestrous cyclicity) of three red deer genotypes. A total of 27 hinds born in December 2005, representing Eastern European (Cervus elaphus hippelaphus), Western European (C.e. scoticus) and F1 crossbred (C.e. hippelaphus×scoticus) red deer, were blood sampled thrice-weekly for 7-8 months (February-September/October) across two years spanning the potential breeding seasons as R(2)'s in 2007 (i.e., puberty) and as adults in 2008. Plasma progesterone profiles were used to construct breeding cycle histories for each hind. Four R(2) hinds failed to initiate oestrous cycles (i.e., puberty failure). The remaining R(2) hinds, including all F1 hinds, exhibited between two and seven oestrous cycles. F1 hinds were significantly earlier to initiate, and later to terminate, cyclic activity, resulting in a longer mean pubertal breeding season (139 days) than for Eastern (86 days) and Western hinds (86 days). However, the data for R(2) hinds are confounded by live-weight, with the F1 hinds being significantly heavier than other genotypes. There were significant correlations between live-weight and seasonality parameters in 2007. All hinds were cyclic as adults in 2008, exhibiting between four and nine oestrous cycles, and a mean breeding season duration of between 132 (Western) and 137 (F1) days. For adult hinds there were no significant genotype differences in cyclic onset and cessation timing, and no observable relationships between live-weight and any reproductive parameter. However, the mean dates for the onset of the breeding season for all genotypes in 2008 were 2-3 weeks later than normally expected for adult hinds in New Zealand. The reasons for this are unclear but may relate to chronic stress of frequent animal handling. The study has demonstrated that puberty in red deer hinds is associated with a shorter potential breeding season than for adult hinds, and that perturbation of breeding activity appears to be quite common, leading to incidences of puberty failure and possibly other aberrant cyclic events. Live-weight×genotype interactions may influence puberty but do not appear to be strongly expressed in adults. However, the relatively late onset of oestrous cyclicity in the adult hinds may be an artefact of the study that has masked genetic influences on seasonal breeding patterns.     

Oestrous cycles and the breeding season of the Père David's deer hind (Elaphurus davidianus)

Reproductive cycles were studied in a group of tame Père David's deer hinds. The non-pregnant hind is seasonally polyoestrous and, in animals studied over 2 years, the breeding season began in early August (2 August +/- 3.3 days; s.e.m., N = 9) and ended in mid-December (18 December +/- 5.7 days; N = 8) and early January (6 January +/- 3.2 days; N = 11) in consecutive years. During the anoestrous period, plasma progesterone concentrations were low (0.2 +/- 0.01 ng/ml) or non-detectable. There was a small, transient increase in progesterone values before the onset of the first cycle of the breeding season. In daily samples taken during an oestrous cycle in which hinds were mated by a marked vasectomized stag, progesterone concentrations remained low (less than 0.5 ng/ml) for a period of about 6 days around the time of oestrus, showed a significant increase above oestrous levels by Day 4 (Day 0 = day of oestrus) and then continued to increase for 18 +/- 2.8 days to reach mean maximum luteal levels of 3.5 +/- 0.6 ng/ml. The plasma progesterone profiles from a number of animals indicated that marking of the hinds by the vasectomized stag did not occur at each ovulation during the breeding season and therefore an estimate of the cycle length could not be determined by this method. In the following year, detection of oestrus in 5 hinds was based on behavioural observations made in the absence of the stag. A total of 19 oestrous cycles with a mean length of 19.5 +/- 0.6 days was observed.(ABSTRACT TRUNCATED AT 250 WORDS)     

A study of oestrus and fecundity in a laboratory colony of Mouse opossums (Marmosa robinsoni)

Twenty wild caught Mouse opossums Marmosa robinsoni Bangs 1898 formed the basis of a laboratory colony which was maintained at the Brookfield Zoo during 1971–3. Approximately 250 young including 3rd-generation descendants were produced.
M. robinsoni is polyoestrous with a mean cycle length of 25.5±0.5 days. Vaginal oestrus, which normally lasts three days, is accompanied by a pronounced cycle of cornification and mucification. During this period large numbers of desquamated epithelial cells and mucus are found in the urine. Females are sexually receptive 24 hours before cornified cells first appear in the urine and receptivity lasts two to four days. Fertile copulation takes place throughout this period. Ovulation is spontaneous and occurs late in oestrus. The mean number of corpora lutea recorded was 19.8±0.9 ( n =23).
Anovular cycles, associated with atresia of mature Graafian follicles, were found in approximately 25% of the females. The sterile cycles, which had a mean length of 15.6±0.7 days, were not always accompanied by a behavioural oestrus. Observations on the behaviour of introduced pairs provided direct evidence of sexual incompatibility. Sexual incompatibility was found to have been responsible for at least 10% of the unsuccessful pairings recorded during the study.
There were no indications in the Brookfield colony of the decline in productivity among laboratory-bred animals reported in other Marmosa colonies. Litter-sizes and litter-rates in the F₁ and F₂ generations did not differ from those in the wild-caught generation. However, the fact that only one out of three pairings produced young suggests that productivity in the colony was lower than in the wild. The cause of this low productivity was not established.     

Rhythms of reproduction,metabolism and coat growth in deer: a model for all non-domesticated seasonal ungulates?

Seasonally-breeding deer living in cool temperate environments exhibit pronounced seasonal rhythms of voluntary food intake, growth rate and fattening and the growth and development of the coat. All of these rhythms are considered to be entrained by photoperiod and melatonin, although so far this has only been demonstrated to be the case in one species, the red deer. This paper reviewed current data on seasonal rhythms in several species of deer. A comparison of two species with different breeding seasons, the red deer and the Pere David's deer, indicated that seasonal reproduction, appetite and coat growth rhythms are linked and may be controlled by a single circannual rhythm generator. All of these seasonal rhythms should be considered as adaptive for species living in cool temperate environments with marked fluctuations in food supply and climatic conditions.     

Photoperiodic control of antler cycles in deer. VI. Circannual rhythms on altered day lengths

Groups of sika deer were exposed to light and dark periods of equal lengths but different from 12 hr. Light cycles were 4.94L/4.94D, 6L/6D, 8L/8D, and 21L/21D. In all experiments, deer underwent circannual cycles of antler replacement, testis size, molting, and coat color. The results indicate that the previously reported abolition of circannual cycles on 12L/12D was due to the 12-hr duration of the light or dark periods, not their equivalence. They also eliminate the possibility that the circannual cycle might be the sum of 365 circadian cycles. Circannual antler cycles appear to be expressed under artificial light cycles to which the deer cannot entrain.     

Genetic influences on reproduction of female red deer (Cervus elaphus) (1) seasonal luteal cyclicity

This study compared the onset and duration of the breeding season of female red deer (Cervus elaphus scoticus) and its hybrids with either wapiti (Cervus elaphus nelsoni) or Père David's (PD) deer (Elaphurus davidianus). In Trial 1 (1995), adult red deer (n=9), F1 hybrid wapiti x red deer (n=6) and maternal backcross hybrid PD deer x red deer (i.e., 14 PD; n=9) were maintained together in the presence of a vasectomised red deer stag for 12 months. They were blood-sampled daily or three times weekly so that concentration profiles of plasma progesterone could be used to identify the initiation, duration and cessation of luteal events. There was clear evidence of luteal cyclicity between April and September, with the transition into breeding associated with an apparent silent ovulation and short-lived corpus luteum (i.e., 6-12 days) in every hind. A significant genotype effect occurred in the mean time to first oestrus (P<0.05), with wapiti hybrids and 14 PD hybrids being 9 and 5 days earlier than red deer. Between six and nine oestrous cycles were exhibited by each hind, with no difference in mean cycle length (19.5-19.6 days) between genotypes (P0.10). The overall length of the breeding season was significantly longer for wapiti hybrids (143 days) than for either red deer (130 days) and 14 PD hybrids (132 days, P<0.05).In Trial 2 (1998), adult red deer (n=5), 14 PD hybrids (n=5) and F(1) PD x red deer hybrid (n=5) hinds were maintained together from mid-February (late anoestrus) to early May, in the presence of a fertile red deer stag from 1 April. Thrice-weekly blood sampling yielded plasma progesterone profiles indicative of the onset of the breeding season. Again, there was a significant genotype effect on the mean time to first oestrus (P<0. 05), with F(1) PD hybrids and 14 PD hybrids being 13 and 5 days earlier than red deer. However, conception dates were influenced by the timing of stag joining, and were not significantly different between genotypes. The results indicate genetic effects on reproductive seasonality. However, seasonality observed for PD x red deer hybrids more closely approximated that of red deer than PD deer.     

Effect of plane of nutrition on seasonality of reproduction in Spanish Payoya goats

The aim of this study was to determine if there is a seasonal pattern of sexual activity in female Payoya goats and if this seasonality could be modulated by nutrition. During the experimental period of 20 months, 43 non-pregnant adults goats were penned under natural photoperiod at latitude 37 degrees 15'N. At the onset of the experiment, the animals were allocated to three experimental groups differing in the level of nutrition and whether the animals were entire or ovariectomized does. The high nutrition group (H, n = 16 entire does) receiving 1.5 times maintenance requirements. The low nutrition group (L, n = 16 entire does) and an ovariectomized and oestradiol treated group (OVX, n = 11 ovariectomized does) received a diet supporting their maintenance requirements. The groups were balanced for live weight (LW) and body condition score (BCS) at the beginning of the study. In entire goats, oestrus was tested daily using aproned males, ovulation rate was assessed by laparoscopy 7 days after identification of oestrus and plasma samples were obtained twice per week for progesterone assay. OVX goats were isolated from the other groups and bucks, plasma samples were assayed twice per week for LH and there were four intensive sampling periods during the year to determine LH pulsatility. LW and BCS were recorded for all animals once a week. A clear circannual cycle in live weight change was observed in all experimental groups, being relatively stable or slightly decreasing in summer and autumn and increasing during winter and spring. The effect of exposure to high (H) rather than low (L) nutrition was to cause earlier onset of ovarian activity (5 versus 17 August; P < 0.05), and expression of oestrous (16 August versus 2 September; P < 0.01) and later cessation of reproductive activity (ovulation 11 February versus 17 January; P < 0.01). Consequently, seasonal anoestrus was 32 days shorter in does on the higher plane of nutrition (P < 0.01). The seasonality of reproductive activity was confirmed in the OVX does, with reduced LH concentrations during spring and summer, and increased LH concentrations in autumn and winter. There was no effect of nutrition on ovulation rate. These results demonstrate that the female Payoya goat exhibits marked reproductive seasonality which is modulated by nutrition but possibly not ovulation rate.     

Calving times of Red deer (Cervus elaphus) on Rhum

The distribution of calving times in a population of Red deer on the Isle of Rhum, Inner Hebrides, between 1971 and 1976 is described. Two-thirds of calves were born during a three-week period beginning in the last week of May, though calving extended from early May to late August. Except in 1976, when calving occurred late, the timing of births was similar in all years. Variation in calving time within years resulted from (i) individual differences in calving time, (ii) late conception in hinds which failed to conceive at first oestrus, and (iii) the effect of previous reproductive history: on average, hinds calved one week later after rearing a calf in the previous winter. Gestation lengths of wild hinds were longer than in a captive group. This may have been due to overestimation of a proportion of gestation lengths in wild hinds, but supplementary feeding could also have shortened gestation length in captive animals.     

Influence of plane of winter nutrition on plasma concentrations of prolactin and testosterone and their association with voluntary food intake in red deer stags (Cervus elaphus)

Blood samples were taken at weekly or 3-weekly intervals from 12 red deer stags, from 3 weeks of age until 3 years of age. Six of the stags were fed to appetite throughout the study and six were maintained on 70% as much during each winter and to appetite each summer. All plasma samples were analysed for testosterone and prolactin. Food intake was measured daily. Annual rhythms of testosterone, prolactin and food intake were evident. Peaks of prolactin, on an increasing photoperiod tended to occur slightly before peaks of food intake and troughs of food intake on a decreasing photoperiod occurred at or slightly after peaks of testosterone. The reduced plane of winter nutrition tended to delay rather than alter the amplitude of the annual cycles of testosterone, prolactin or food intake. Several of the unrestricted stags showed biannual rhythms of testosterone, prolactin and food intake accompanied by additional cycles of coat development and antler growth. These additional cycles tended to be 6 months out of rhythm with normal cycles. It is considered that this is due to the stags being stimulated by a particular day-length irrespective of whether it is decreasing or increasing and that plane of nutrition may influence the control of phase blockers.     

Age and seasonal coat changes in long haired and normal Fallow deer (Dama dama)

Coat changes from birth to the second winter coat in a long haired variety of Fallow deer are described and compared with normal deer. The long haired deer are characterised by a dense, curly crown and long tufts of hair growing from the ears. The long haired coat always contained a high proportion of growing hairs in contrast to the normal coat in which there were discrete generations of growing hairs. The difference between the long haired and normal deer was most marked in the first winter when the hairs of the long coat were longer and finer than normal and tended to be easily wetted in contrast to the normal coarse winter coat.     

Hormonal regulation of the annual pelage color cycle in the Djungarian hamster, Phodopus sungorus. I. Role of the gonads and pituitary

The Djungarian hamster exhibits an agouti pelage in the summer and a predominantly white pelage in the winter. This pelage color cycle is known to be regulated by the length of the daily photoperiod probably acting through the pineal gland, as is the seasonal cycle of reproductive function with which it is closely correlated ( Figala et al., '73; Hoffmann, ' 78b ). The possibility of a causal relationship between the decline in gonadal hormone secretion and the coat color change occurring in short photoperiod was examined. Gonadectomized and intact male and female hamsters were exposed to either long (16L:8D) or short ( 10L : 14D ) photoperiod for several months. Gonadectomy neither induced the change to the winter pelage color in long photoperiod-housed animals, nor prevented either the change to the winter pelage or the spontaneous return to summer pelage color in short photoperiod-housed animals. Chronic implants of testosterone in castrated males delayed and attenuated the short photoperiod-induced coat color change. Administration of ovine prolactin (100 micrograms/day) stimulated pigmentation in hamsters with the winter pelage, whereas administration of a alpha MSH (30 micrograms/day) was without effect. These results suggest that changes in pelage color may be regulated largely by changes in pituitary prolactin secretion and modified to some extent by changes in gonadal steroid hormone secretion.     

Effect of pinealectomy on seasonal changes in antler growth and concentrations of testosterone and prolactin in white-tailed deer

A 2-year study was conducted to determine under controlled conditions the role of the pineal gland in regulating the seasonal changes in antler growth and reproduction of male white-tailed deer. Blood samples were drawn from 6 pinealectomized (PX) and 18 control (C) deer at intervals of 2 weeks and analyzed for testosterone (T) and prolactin (Prl). Relative scrotal circumference and main beam antler length were recorded. Relative scrotal circumference was similar in PX and C groups, but the normal pattern was delayed 1 to 3 months in the PX deer relative to the C deer. The mean dates of beginning antler growth, velvet shedding, antler casting and pelage changes were significantly later in both years for PX deer than in C deer. Testosterone concentrations peaked 1 month later in the PX deer than in the C deer for both yearling and 2-year-old deer. Prl concentrations in C deer, but not in PX deer, were correlated highly with day length, and the PX deer were delayed relative to the C deer in showing the normal Prl pattern. Increasing levels of Prl in both groups coincided with beginning antler growth in both years. These results indicate that the pineal gland does not originate the seasonal cycles of male white-tailed deer but may synchronize cycles among individual deer, and regulate the circannual rhythm of Prl concentrations which may in turn influence other hormonal cycles.     

Effect of age and time of day on the timing of the surge in luteinizing hormone, behavioural oestrus and mating in red deer hinds (Cervus elaphus).

The oestrous cycles of fourteen red deer hinds (six yearling; eight more than 2 years old) were synchronized during the early breeding season by removal of a progesterone-containing intravaginal device and blood samples were taken at intervals of 3 h commencing 13 or 25 h later and continued for 54 h. The controlled internal drug release devices (CIDRs) were removed at 08:00 h (group 1; three yearlings and four adults) or 12 h later at 20:00 h (group 2; three yearlings and four adults). There was no significant effect of time of removal of CIDR on the interval to the onset of oestrus (group 1, 34.5 +/- 4.05 h; group 2, 42.14 +/- 7.8 h) on the time of peak concentration (group 1, 41.81 +/- 5.69 h; group 2, 41.71 +/- 7.81 h) or on duration of the luteinizing hormone (LH) surge (group 1, 15.00 +/- 0.95 h; group 2, 14.57 +/- 0.78 h). The six yearling animals exhibited oestrus and LH surge significantly later than the adults (55 +/- 4.2 versus 32 +/- 6.3 h for the LH surge for yearling and adult females, respectively). In a further experiment, 20 hinds were synchronized during the breeding season by removal of CIDR at two times of day 12 h apart and placed with a stag. Mating took place at a mean time of 42.1 +/- 2.4 h and 37.0 +/- 1.3 h later in the two groups. There was no significant effect of time of removal of CIDR upon time to onset of oestrus.(ABSTRACT TRUNCATED AT 250 WORDS)     

Seasonal differences between moose and roe deer in ability to digest browse

The digestive capacity of free-living moose Alces alces (L.) and roe deer Capreolus capreolus (L.) for four plant species was studied with in vitro technique. Moose were found to digest all species better than roe deer. Individual variations were also smaller for moose. In moose the level of digestibility was the same both summer and winter, while roe deer shot in winter digested winter samples of bilberry Vaccinium myrtillus L. and to some extent Scots pine Pinus silvestris L. better than did summer roe deer. This was taken as evidence for a seasonal adaptation in the roe deer rumen. Digestibility of the plant material studied showed a strong negative correlation to the fiber content. The results are discussed in relation to winter problems for roe deer and possible food competition between moose and roe deer.     

Effects of maintaining solstice light and temperature on reproductive activity, coat growth, plasma prolactin and melatonin in goats.

Interactive effects of light and temperature on aspects of seasonality were studied in female British Saanen dairy goats. Four groups of adult non-pregnant non-lactating goats (n = 5) were housed under the following conditions: controls (July-June): natural photoperiod and temperature; group 1 (July-December): long days (16 h light: 8 h dark) and natural temperature; group 2 (July-December): long days and average summer temperature (17.6 degrees C); group 3 (December-June): short days (8 h light: 16 h dark) and winter temperature (8.4 degrees C). Plasma prolactin and progesterone were measured once a week, circadian changes in prolactin and melatonin were determined in December and May, and coat development was assessed. Seasonal variation in prolactin was influenced by manipulation of both daylength and temperature. In group 1, prolactin concentrations decreased as the environmental temperature decreased, despite maintenance of long days. When light and temperature were maintained under summer (group 2) and winter (group 3) conditions, prolactin remained relatively constant, although at different high and low set points, respectively, but with indications of a seasonal rhythm. An asymptotic relationship between prolactin and temperature was maintained under all daylengths. The circadian pattern of melatonin was related to daylength and was not influenced significantly by temperature. Onset of oestrus was unaltered. In group 3 (maintained winter solstice light and temperature), anoestrus was delayed (P < 0.05) from a median control date of 17 March to a median date of 28 April. Winter coat development was delayed in group 1; group 2 showed premature moulting of the winter coat; and in group 3, development of the summer coat was delayed. The results imply that temperature modifies the influence of daylength on prolactin secretion and hair follicle growth by mechanisms that do not involve melatonin.     

The onset of seasonal quiescence in the female Bennett's wallaby (Macropus rufogriseus rufogriseus)

The breeding season of the non-lactating Bennett's wallaby terminates when animals enter the state of seasonal quiescence. To examine this transition, pouch young were removed from females at intervals which were 3, 4 or 8 weeks (6, 11 and 8 animals respectively) after the winter solstice. Within 48 days, 3, 1 and 1 females gave birth respectively, indicating that these animals were not in seasonal quiescence when pouch young were removed. Those animals which did not give birth were either in seasonal quiescence or had undergone a non-pregnant cycle. To differentiate between the 2 possibilities, techniques which would ensure the detection of pregnant and non-pregnant cycles were assessed in 8 females during the breeding season. As has been previously reported for the wallaby, changes in peripheral progesterone concentrations and the vaginal smear occurred during pregnant and non-pregnant reproductive cycles. In addition, mating was detected by marking the male with a mixture of coloured crayon and paraffin wax. It was concluded that reproductive cycles in female wallabies could be monitored by collecting blood samples 2 times each week for progesterone determination and daily examination of females for mating marks. These techniques were then used to study the onset of seasonal quiescence in 9 females. All animals continued to show reproductive cycles after the winter solstice and it was not until 10 weeks after the winter solstice that all animals were in seasonal quiescence. This represents an increase in the duration of the breeding season over that previously reported for this species.     

Pituitary response of prepuberal lambs to oestradiol-17 beta.

In two experiments 48 prepuberal Merino ewe lambs were injected with oestradiol-17 beta (E2) or saline to study the effect of E2 on their plasma LH levels and on oestrus and ovulation. In the three groups which received 30 (experiment I), 50 and 30 (experiment II) microgram E2 respectively, 27 out of 28 lambs showed an LH response, the corresponding mean LH peaks being 64.3 +/0 22.5, 153.6 +/-33.4 and 91.7 +/- 16.9 ng/ml at mean intervals of 11.1, 11.2 and 10.5 h, respectively, after injection. None of the 20 lambs in the control groups had an LH level higher than 18 ng/ml 12 h after injection. In the three E2 groups, 41.7, 62.5 and 37.5% of animals showed oestrus within 26 h of injection while in the control groups only one animal showed oestrus. Of 13 animals showing oestrus in the E2 groups, 11 failed to ovulate. The mean pre-injection plasma FSH level in experiment I was 102.7 ng/ml, and in four 5--7-month-old lambs over several weeks uas 155.3 ng/ml. Despite these high pre-injection levels of FSH, it appears that the follicles were unable to respond to the LH peak which followed the E2 injection.     

Plasma progesterone and LH concentrations in ewes after injection of an analogue of prostaglandin F-2alpha

Plasma progesterone and LH concentrations were monitored throughout a natural oestrous cycle in 12 Clun Forest ewes and compared to those following treatment with a single i.m. injection of 100 microng ICI 80,996, an analogoue of prostaglandin F-2alpha, given during the luteal phase of the cycle. After injection of the analogue there was a high degree of synchrony in the return of oestrus (440 +/- 1-9 h; mean +/- S.E.M.) and the timing of the LH peak (48-5 +/- 2-0 h) from injection. There were no significant differences in the plasma progesterone concentrations or in the height and duration of the preovulatory LH peak between control and treatment cycles. The technique offers the possibility of controlled ovulation in the ewe.     

Seasonal changes in the in-vivo activity of the luteinizing hormone-releasing hormone (LHRH) neural apparatus of male rabbits monitored with push-pull cannulae

Intact sexually mature New Zealand White male rabbits, raised under natural lighting and temperature conditions, were isolated and housed in air-conditioned quarters, in a 12-h light:12-h dark cycle. Push-pull cannulae were implanted towards the tuberal region of the hypothalamus, and animals were perfused with modified Krebs'-Ringer phosphate medium for an average period of 4 h. Most rabbits were repetitively perfused over an average period of 3.7 months. Perfusions were grouped into seasonal periods of about 40 or 80 days through the entire 1-year cycle: (A) 23 November to 31 December (winter solstice period: N = 6), (B) 1 January to 23 March (winter: N = 10), (C) 24 March to 13 June (spring: N = 9), (D) 14 June to 23 July (summer solstice period: N = 23), (E) 24 July to 13 October (late summer-early fall: N = 7) and (F) 14 October to 22 November (fall: N = 4). Maximal and minimal values of mean release, mean amplitude and mean frequency from every animal in each block were obtained. In the summer solstice group (D), maximal mean (+/- s.e.) LHRH release levels were significantly greater (14.42 +/- 6.62 pg/10 min) than for all groups (A, 0.76 +/- 0.27; B, 1.59 +/- 0.39; C, 1.34 +/- 0.22; E, 1.33 +/- 0.33; and F, 1.18 +/- 0.11) while during the winter solstice period (A), minimal mean LHRH release levels (0.48 +/- 0.04 pg/10 min) were significantly lower than in all other groups (B, 1.43 +/- 0.41; C, 0.96 +/- 0.11; D, 7.25 +/- 4.12; E, 1.18 +/- 0.37; and F, 1.18 +/- 0.11). Maximal values were highest during the summer solstice period while the minimal values were lowest during the winter solstice period. The amplitude and frequency of the LHRH pulses showed changes similar to those observed with the mean LHRH release. For 1 year, an estimated left testis weight was measured in 4-7 rabbits every 2-5 weeks. The percentage estimated testis weight peaked in early August and reached minimal levels during the winter months. These data demonstrate that the rabbit LHRH neural apparatus is very sensitive to seasonal influences although animals remained in a fixed photoperiod during the entire duration of the experiment.