Seasonal changes in plasma androgen levels and the gonads of the beach vole, Microtus breweri

The reproductive cycle of 108 mature male beach voles, Microtus breweri, endemic to Muskeget Island, Massachusetts, was investigated. Changes in body weight were seen through the year with animals heaviest (62.10 ± 1.38 g) in April and lightest (43.40 ± 2.84 g) in October. In April through June spermatogenic activity was maximum while testicular weight was greatest (1300 –1600 mg). In July spermatogenic activity remained maximum although testicular weight dropped (858.72 ± 152.00 mg). A decline in spermatogenic activity and testicular size followed during August through October. Testes were quiescent and weished approximately 300 mg from October through December. Plasma androgen levels were highest (>2 ng/ml) during spermatogenic activity in April through June and lowest (<1.0 ng/ml) in October when the testes contained only spermatogonia and Sertoli cells. The reinitiation of spermatogenesis in the male beach voles on Muskeget Island is synchronized with the spring increase in photoperiod and temperature, and testicular regression begins during the late summer.     

Prolactin and thyroid hormone interactions in salt and water balance in the newt Notophthalmus viridescens

Hypophysectomy lowered plasma sodium in summer newts to approximately 80% of control levels. Treatment of hypophysectomized animals with corticosterone, prolactin, or corticosterone + prolactin did not correct the pituitary deficiency.Thyroxine treatment (0.1 or 0.5 μg every other day for 2 weeks) of intact summer newts depressed plasma sodium to approximately 40% of control values. Thyroxine treatment of winter animals (0.1 μg every other day for 2 weeks) depressed plasma sodium to 90% of control values. In addition, thyroxine treatment of winter animals depressed urine sodium, more than doubled the rate of molting, and increased the osmotic permeability of the skin from 71 to 103 μl water/cm²/hr. Newts treated with both thyroxine (0.1 μg every other day) and prolactin (10 μg every other day) had levels of plasma sodium identical to controls in both summer and winter experiments. In the winter experiment, prolactin + thyroxine treatment lowered urine sodium, more than doubled molting rate, decreased osmotic permeability of the skin (from 70 to 48 μl water/cm²/hr) and decreased short-circuit current (from 30 to 16 μA/cm²) in isolated skin preparations. In addition, the skin of animals receiving both prolactin and thyroxine was more slippery than the skin of control animals or of animals treated with thyroxine alone, suggesting that they were secreting excess mucus.It is suggested that the effects of these hormones on the skin may at least partially account for their effects on plasma and urine sodium.     

N-acetyltransferase activity in the Harderian glands of the Syrian hamster, Mesocricetus auratus, is regulated by androgens and by hormones of the pituitary-thyroid axis

This study tested the hypothesis that activity of the enzyme N-acetyltransferase (NAT) in the Harderian gland of the Syrian hamster is regulated both by androgens and by hormones of the pituitary-thyroid axis. To test the effects of castration and hypothyroidism, intact or castrated male hamsters were given either tap water or methimazole in their drinking water for 3 weeks. Methimazole suppresses iodination of thyroglobulin, thereby decreasing circulating levels of thyroid hormones and increasing TSH levels. Hypothyroidism or castration caused elevated or depressed Harderian gland NAT activities respectively, compared with euthyroid controls. When castration and hypothyroidism were combined, the animals exhibited high NAT activity compared with castrated euthyroid males. To test the effects of castration and hyperthyroidism, male hamsters were given daily injections of thyroxine (T4) or diluent and were either castrated or left intact for 4 weeks. Intact animals given T4 had depressed Harderian NAT activity; serum thyroid hormone levels were elevated and TSH levels were depressed compared with those of intact controls. Castrated animals had depressed NAT activity below that of intact controls; serum thyroid hormone levels were normal but TSH levels were depressed. Castrated animals given T4 injections had NAT activity similar to that of euthyroid castrated hamsters; thyroid hormone levels were elevated but TSH levels were similar to those seen in euthyroid castrated hamsters. In another experiment, both T4 and tri-iodothyronine (T3) were equally effective in decreasing NAT activity in intact males. To determine the effects of the removal of pituitary influences, male hamsters were hypophysectomized. NAT activity in the Harderian glands of these animals was reduced compared with intact controls.(ABSTRACT TRUNCATED AT 250 WORDS)     

A year-long study of changes induced by thyroidectomy in the plasma lipid and lipoprotein spectrum in the European badger

Hypothyroidism is associated with hypercholesterolemia and increased risk for atherosclerotic disease. The European badger exhibits large seasonal changes in thyroid activity and the annual minimum of plasma thyroxine level in this species occurs at the same period of the year (i.e. late fall) as a pronounced hypercholesterolemia. We examined the plasma lipid and lipoprotein spectrum in a group of thyroidectomized male badgers every month for a year. Non-operated animals were used as controls. Our analyses included measurement of plasma lipid levels, density gradient ultracentrifugation of lipoproteins, electrophoresis of lipoproteins and apolipoproteins, and histological studies. Maximal differences between the two groups of animals were observed during spring, occurring concomitantly with the annual maximum of plasma thyroxine concentration in control badgers. Comparison with the latter animals revealed a permanent hypercholesterolemia and hyperphospholipidemia in thyroidectomized badgers, while their lipoprotein spectrum was characterized by the continual presence of elevated concentrations of cholesterol-rich lipoproteins of d congruent to 1.015 - 1.027 g/ml. The ratio of triglyceride/cholesteryl ester content in such lipoproteins remained constant throughout the year, resembling that noted in intact animals during late fall. Other features distinguishing the lipoprotein spectrum in thyroidectomized badgers were: (1) higher levels of lipoproteins with d 1.027 - 1.065 g/ml and d 1.065 - 1.100 g/ml, and (2) a cholesteryl ester enrichment of both these lipoprotein subclasses. The two groups of animals shared a heterogeneity of low density lipoprotein subfractions isolated on density gradients, together with the presence of apolipoproteins with molecular weights respectively typical of human apolipoproteins A-I and B throughout the low density range. Arterial walls and heart tissues from intact and thyroidectomized animals were free of atherosclerotic lesions at the end of the experimental period.     

Variations in thyroid activity and respiratory rate during a 24-hr period and role of testosterone and thyroxine on the oxidative metabolism of the water snake, Natrix piscator.

In intact adult male Natrix piscator testosterone and l-thyroxine stimulate oxygen uptake of skeletal muscle and liver homogenates to the same extent. The effects of the hormones are additive on skeletal muscle but not on liver. Testosterone and l-thyroxine are also able to restore the decline in respiratory rate of liver and skeletal muscle caused by surgical thyroidectomy. Variations in thyroid activity and oxidative metabolism during a 24-hr period were also studied. Compared with animals examined during the day, thyroidal uptake of ¹³¹I and intrathyroidal thyroxine were significantly lower and protein-bound iodine was higher than in snakes examined at night. Further, while no significant change was observed in the respiratory rates of liver homogenates, muscle samples of day animals (30°) had significantly lower respiratory rates when measured at the night temperature of 15°.     

Effects of glucose, insulin and adrenalin on glycerol metabolism in adipose tissue from hypothyroid rats

The in vitro metabolism of glycerol by epididymal fat pads from thyroidectomized rats daily injected with either 0, 0.1 or 1.8 μg of L-thyroxine/100 g body wt. was compared with that from intact controls. The basal as well as the adrenalin- or glucose-enhanced release of glycerol to the medium were similar in the tissues from all the groups. The effect of insulin in decreasing the lipolytic action of adrenalin was greater in the thyroidectomized animals treated with either 0 or 0.1 μg of thyroxine than in the other two groups. The utilization of (1-¹⁴C]glycerol for the formation of CO₂ and glyceride glycerol was increased in the thyroidectomized rats; this effect was smaller when the animals were treated with 0.1 μg of thyroxine and disappeared when they were treated with 1.8 μg. In the presence of glucose the difference in utilization of glycerol between the groups disappeared. The formation of fatty acids from glycerol was greater in the presence than in the absence of glucose and was similar in the hypothyroid animals and the controls. The effect of adrenalin in decreasing the utilization of (1-¹⁴C]glycerol was less in the tissues from hypothyroid rats than in the controls. The decrease of the action of adrenalin by insulin in the tissues from thyroidectomized rats treated with 0 or 0.1 μg of thyroxine was greater than in the controls. The increased capacity to form glyceride glycerol from glycerol in tissues from hypothyroid animals contributes to the high re-esterificatfon of fatty acids described for these animals. The effect of glucose is explained in terms of its competition with glycerol for the synthesis of -glycerophosphate.     

Temperature effects on thyroid function in the lizard Sceloporus cyanogenys

The effects of chronic and acute cold temperature exposure on thyroid function were investigated in groups of adult lizards, Sceloporus cyanogenys. Some animals were placed in constant temperature chambers at 16°C for 2 wk prior to testing, while those exposed to acute cold were placed in a 4°C environment for 0.5 hr preceded by a 14-day acclimation period at 32°C. Various parameters of thyroid function were measured, including total thyroid ¹³¹I uptake, thyroxine synthesis, and protein-bound iodine. Pituitary-thyroid relationships in both thermal situations were indirectly determined by comparing hypophysectomized and/or TSH injected animals to controls. Chronic cold reduced thyroid activity while acute cold stress apparently stimulated pituitary TSH secretion. Though TSH could not increase oxygen consumption in tissue slices from cold-treated animals, injection of this hormone did cause thyroid hormone uptake by peripheral neurons under similar conditions. Furthermore, serum proteins seem to act as peripheral storage depots for thyroxine during periods of cold-induced inactivity. TSH can apparently free the protein-bound hormone under appropriate conditions. A discussion of the possible role of thyroxine in reptilian thermoregulation is included.     

Hepatic mediation of hormonal and nutritional factors influencing the in vitro sulfur uptake by rainbow trout bone

Previous studies have shown that the mammalian liver mediates somatotropin (STH) actions via the release of peptides termed somatomedins. Our study indicated that the in vitro³⁵S uptake by gill arch bone of hypophysectomized (hypexed) rainbow trout, Salmo gairdneri, was significantly (P < 0.05) stimulated by liver slices from fed intact trout. Slices from trout starved for 2 or 5 weeks or from fish hypexed and starved for that period inhibited uptake. However, liver from feeding hypexed trout had no effect on uptake. A combination of thyroxine and STH stimulated uptake in the presence but not in the absence of liver tissue from long-term starved or hypexed animals. Neither hormone was active by itself. This study suggests a nutritional-hormonal control over skeletal sulfur metabolism and presents evidence for the existence of a “somatomedin-like” activity in livers of S. gairdneri.     

Thyroid and gonadal regulation of hair growth during the seasonal molt in the male European badger, Meles meles L

In the male badger we showed that hair growth and molt are related to plasma testosterone and thyroxine cycles. We established the action of testosterone by castration and subcutaneous testosterone implants, and the action of thyroxine by thyroidectomy and dietary supplementation with thyroxine. The following groups of animals were studied: controls, thyroidectomized, thyroidectomized and thyroxine-treated, castrated, castrated and thyroxine-treated, thyroidectomized and castrated and thyroxine-treated, castrated and testosterone-implanted, and intact testosterone-implanted. In control animals, molt and hair growth occurred during the summer, with a maximum growth in autumn. Molt ended at the beginning of winter when the plasma testosterone level had started to rise, and began again after this level had started to decline. Both the start of molt and the period of maximum hair growth coincided with high thyroxine levels of about 20 ng/ml. Castration advanced molt and hair follicle activity, whereas testosterone implants delayed both molt and hair growth. In thyroidectomized badgers, neither hair growth nor seasonal molt was observed. However, when thyroxine levels were restored to 20 ng/ml or more by dietary T4 supplementation, molting was resumed in animals that had undergone either thyroidectomy or thyroidectomy plus castration. In those that underwent castration only, the molt was advanced leading to early hair growth further stimulated by the suppression of testosterone. Testosterone had an inhibitory effect on the molt--since testosterone implants in intact control animals delayed it by 4 weeks--but did not inhibit it completely. On the other hand, a T4-enriched diet advanced the date of the molt. However, the molt could not be induced, nor could hair follicle growth be reactivated, at all times during the annual cycle. Thus, in castrated animals. T4 enrichment of the diet in early January, at the end of the molt, caused follicle reactivation only toward the end of May, despite the lack of testosterone. This 18-week latency period from January to May might therefore constitute a "refractory period" in this species. The above findings show that the regulation of the seasonal molt and hair growth in the European badger involves both the thyroid and genital axes. This regulation is discussed in terms of joint control by the hypothalamus and pituitary governed, in turn, by an external factor--the photoperiod--considered to be the main synchronizer.     

Hypothalamo-adenohypophyseal-thyroid interrelationships in the chick embryo. I TRH and TSH sensitivity.

The capabilities of the thyroid and adenohypophysis of the developing chick embryo to respond to exogenous thyrotropin (TSH) and thyrotropin-releasing hormone (TRH), respectively, were evaluated by means of radioimmunoassay measurements of plasma total thyroxine (T₄) levels. Chick embryos were treated with bovine TSH (2.5 mIU) on Days 6.5, 7.5, and 8.5 of incubation. The embryonic chick thyroid was shown to be sensitive to TSH as early as Day 6.5, as evidenced by an increase in plasma T₄ levels (P < 0.05). Additionally, the adenohypophyses of 6.5-day-old chick embryos were shown to respond to synthetic TRH (400 μg/100 g body weight) as indicated by a statistically significant rise in circulating thyroxine levels (P < 0.05). Possible underlying mechanisms to explain the increase in thyroidal activity that normally occurs during the Day 10.0–12.0 incubation interval in the chick embryo are discussed.     

Thyroid hormone regulation of the pulsatile discharges of luteinizing hormone in ovariectomized rats.

The pulsatile discharges of luteinizing hormone (LH) were characterized in ovariectomized rats in the presence or absence of thyroid hormone. LH secretion in ovariectomized rats with intact thyroid glands and thyroidectomized-ovariectomized rats receiving daily physiological doses of thyroxine (2 mug/100 g BW/day for 8 days) showed equivalent periodic discharges with frequencies between 15 and 45 min. Though the frequency of the plasma LH rhythm in untreated athyroid-ovariectomized rats was normal, the maximum and minimum concentrations were 2- to 3-fold higher than those of euthyroid-ovariectomized animals. On the other hand, treatment of athyroid-ovariectomized rats with a daily hyperthyroid dose of thyroxine (20 mug/100 g BW) for 8 days, attenuated pulsatile discharges of LH. The LH measured in the sera of these animals each gave dose-response curves by radioimmunoassay which were identical to the authentic rat LH reference preparation. Furthermore, neither the molecular profile nor the metabolic clearance rate of LH was affected by alterations in thyroid status. These results suggest that altered thyroid status does not influence the synthesis and metabolism of LH but does exert a profound effect on the secretion of this hormone by presumably acting directly on the hypothalamo-pituitary axis.     

Hormonal effects on mitotic rhythm in the epidermis of the red-spotted newt.

The mitotic indices of epidermal cells in the red-spotted newt were measured at 3- or 4-hr intervals throughout 24-hr periods under a variety of hormonal conditions. In intact control newts maintained on a photoperiod of 16 hr of light/8 hr of darkness, a peak in mitotic activity was observed during the dark hours and a second smaller (but not statistically significant) peak was seen during the light hours. Inversion of the light cycle induced a new and more rapid rhythm. The basic rhythm was accentuated by autografting the pituitary gland to an ectopic site and also by the administration of thyroxine to intact animals. Hypophysectomy and administration of the prolactin-inhibiting drug, ergocornine, to auto-grafted animals greatly reduced the rate of epidermal proliferation and apparently reduced the peaks until they were no longer detectable. Injection of ovine prolactin into hypophysectomized or intact animals elevated the overall mitotic index and obscured or altered the mitotic rhythm. Removal of the rhyroid gland or administration of thiourea also altered the rhythm, but treatment with ACTH produced no detected effect. Apparently the basic rhythm of epidermal mitosis can be overridden or altered either by hormonal factors that increase or by ones that decrease the overall rate of cellular proliferation.     

Effects of castration and thyroidectomy on the annual biological cycles of the edible dormouse Glis glis

The annual cycle of hibernation, body weight, and plasma testosterone and thyroxine levels were compared in 7 orchidectomized and 16 intact dormice under natural conditions, and in 21 thyroidectomized and 23 intact dormice exposed to a 12-hr light and 12-hr darkness photoperiod under natural temperatures. Hibernation was not affected by either orchidectomy or thyroidectomy, nor did thyroidectomy alter the annual body weight cycle, which, however, was obliterated by orchidectomy. As regards endocrine cycles, orchidectomy had no significant effects on the annual thyroxine cycle, whereas thyroidectomy obliterated the annual testosterone cycle, which was replaced by short-lived, infradian plasma thyroxine cycles. As the latter resembled the fluctuations previously observed in intact animals exposed to constant ambient temperatures, it is proposed that in edible dormice, the annual thyroid cycle, which is thermoperiodically entrained, in turn entrains the testosterone cycle.     

Effects of gonadectomy and replacement therapy on adrenal function in the domestic duck, Anas platyrhynchos

Adrenal function in ducks of both sexes was studied in intact and in castrated animals and in castrated animals receiving appropriate replacement therapy. No significant changes in adrenal weights were observed in the various groups of animals. In the males, in vitro production of corticosteroids from progesterone-4-¹⁴C by both the intact and castrated animals receiving testosterone phenylpropionate were significantly less than by the castrates, indicating an inhibitory effect of androgens on adrenal activity. No differences were observed in the three groups of female ducks.     

Plasma thyroid hormones in cyclostomes: Do they have a role in regulation of glycemic levels?

Plasma levels of thyroxine (T₄) and triiodothyronine (T₃) were measured by radioimmunoassay in intact Pacific lamprey (Entosphenus tridentatus) undergoing the period of natural fasting and in Pacific hagfish (Eptatretus stouti) maintained unfed in the laboratory. Plasma T₃ levels in both lamprey and hagfish were always severalfold lower than T₄ levels. The influence of thyroid hormones on glycemic level was studied following intraperitoneal injection of T₄ or T₃ (13–20 μg/100 g body wt), as well as after implantation of sealed Silastic capsules containing the goitrogen, 6-propylthiouracil (6-PTU), or after intraperitoneal injections of an antithyroglobulin serum (ATgS) exhibiting both anti-T₄ and anti-T₃ activities. Measured plasma T₄ and T₃ levels after hormonal injection were extremely high and could be considered pharmacological. The 6-PTU treatment decreased plasma levels of both T₄ and T₃ within several weeks. The glycemic levels in lampreys and hagfish after thyroid hormone treatment were lower than in control animals, whereas in animals treated with either 6-PTU or ATgS, hyperglycemic levels prevailed. It is concluded that thyroid hormones, possibly acting with other hormones, may participate in the maintenance of glycemic levels in cyclostomes and that their action is to reduce glycemic levels.     

Hormonal control of "delayed development" in Macrotus waterhousii. I. Changes in plasma thyroxine during pregnancy and lactation

Females in various stages of reproduction were collected from southern Arizona. Pregnant animals were placed in a controlled environment and given an ample diet of mealworms and crickets. This new environment had no effect on the gestation period, and pregnancy proceeded normally for 7 months. Plasma thyroxine concentration and histological sections of the thyroid were studied from animals in various stages of pregnancy and during lactation. Plasma thyroxine (T₄) was highest during the last trimester of pregnancy (5.4 μg/100 ml) and lactation (5.2 μg) and lowest during the first and second trimester (2.0 and 1.9 μg) of the 9-month gestation period. No correlation was found between thyroid follicle dimensions and the fluctuation in plasma thyroxine.     

Prolactin-induced modification of visual pigments in the eastern red-spotted newt, Notophthalmus viridescens

Ovine prolactin has been shown to initiate porphyropsin synthesis in “second metamorphosis” of the eastern red-spotted newt (Notophthalmus viridescens). Prolactin activity is demonstrable in both terrestrial juvenile efts and aquatic adults. In adults, thyroxine initiates molting, antagonizes prolactin-induced water-drive behavior, and yet does not appear to affect visual pigment synthesis. Adaptation of adults to aquatic and terrestrial conditions does not alter visual pigment composition; prolactin initiates porphyropsin synthesis in animals maintained in both experimental environments.     

On the interaction of thyroxine and of oxytocin with prolactin on the growth of the tail fin in the red-spotted newt

Changes in the height of the tail fin were followed over periods of 3 to 4 weeks in hypophysectomized or intact adult male, laboratory-conditioned red-spotted newts that were injected thrice weekly with doses of thyroxine (T₄), of ovine prolactin, of T₄ and prolactin in combination, or of oxytocin. In groups of intact animals, the highest dose of T₄ (10 μg) had a toxic effect when given alone. When given in combination with prolactin, it apparently antagonized the stimulatory action of prolactin in the growth of the tail fin. The animals of the groups receiving it, however, ate sparingly, if at all, suggesting that the effect of T₄ may have been one of general toxicity rather than one of direct inhibition. A very low dose of T₄ (0.001 μg) also appeared to be antagonistic when given to hypophysectomized newts in combination with prolactin (0.3 U), but was shown instead to be below the level necessary to maintain survival after removal of the pituitary gland. Doses of 1.0 and 0.01 μg T₄ with prolactin were adequate for survival and gave no evidence of antagonizing the stimulatory action of prolactin on growth of the tail fin. These findings suggest that among those urodeles that retain the tail fin as adults, the typical larval antagonism of T₄ (at physiological levels) to the somatotropic action of prolactin in the tail fin ceases once metamorphosis is complete. Oxytocin in doses of 100 μU did not alter the height of the tail fin, but antagonized the stimulatory action of 0.3 U of prolactin. It is suggested that the interaction is osmoregulatory in nature.     

Effects of oxytocin in larval Ambystoma tigrinum: Acceleration of induced metamorphosis and inhibition of the antimetamorphic action of prolactin

Larval tiger salamanders induced to metamorphose with thyroxine were injected with oxytocin (100 mU/day) in order to assess the potential effect of neurohypophyseal contamination on the antimetamorphic activity of mammalian prolactin preparations. Unexpectedly, oxytocin caused a marked acceleration of tail fin regression and body weight loss and a slight acceleration of gill resorption. An additional experiment revealed that the decreases in tail height and body weight observed in the initial experiment were induced by oxytocin in animals that were not treated with thyroxine. Prolactin was only partially effective in blocking these effects of oxytocin. In animals induced to metamorphose and treated with oxytocin, prolactin, or both hormones, tail fin regression and weight loss were accelerated by oxytocin and antagonized by prolactin. Prolactin partially reversed the effects of oxytocin on tail height and body weight. In this experiment, oxytocin failed to accelerate gill resorption while prolactin inhibited this process. Oxytocin was without effect on prolactin inhibition of gill resorption. Incubation of tail tissue with oxytocin in vitro induced water loss compared to paired control tissue pieces and the addition of prolactin to the incubation medium caused only a slight inhibition of the oxytocin-induced water loss. These findings are discussed in relation to the hypothesis that the antimetamorphic action of prolactin is osmoregulatory in nature.     

A year-long study of changes induced by castration in the plasma lipid and lipoprotein spectrum in the European badger

In man, an influence of male sex hormones on plasma lipid transport is well established; however, recent data on this subject in the literature are both relatively lacking and occasionally conflicting. The male European badger exhibits seasonal variations of large amplitude in its gonadic function. We have therefore attempted to establish the influence of male sex steroids on plasma lipids and lipoproteins in this species. For this purpose, we have examined the plasma lipid and lipoprotein spectrum in a group of castrated male badgers every month for a year, non-operated animals being used as controls. Our analyses included measurement of plasma lipid levels, density gradient ultracentrifugation of lipoproteins, electrophoresis of lipoproteins and apolipoproteins, and evaluation of plasma testosterone and thyroxine levels. The differences observed between the 2 groups of animals were maximal during the months when plasma testosterone was elevated in intact badgers (January to July). For this period, castration resulted in higher plasma concentrations of cholesterol, phospholipids and triglycerides, while the latter alone remained significantly more elevated in operated animals until the end of our experiments. With regard to lipoproteins, the main effect of castration consisted of a large augmentation in the concentration of lipoproteins with d approximately equal to 1.027-1.065 g/ml which were responsible for the transport of most of the increased amounts of triglycerides present in the plasma of castrated badgers. The proportion of apoprotein B in the protein moiety of these lipoprotein components was enhanced after castration. Other changes in the lipoprotein spectrum included (1) a moderate increase in the concentration of lipoproteins with d less than 1.015 g/ml and 1.019-1.027 g/ml, and (2) a modification of the respective proportions of high density lipoproteins with d 1.065-1.100 g/ml and d 1.100-1.162 g/ml. Finally, no considerable differences between the 2 groups of animals were noted in the respective percentages of the various chemical constituents in each lipoprotein subfraction assayed, except for those with d 1.023-1.027 g/ml, which, in castrated badgers, did not exhibit the enrichment in triglycerides usually noted during late winter and spring in intact animals.