Regulation of the breeding season in mammals.

Abstract

REGULATION OF THE BREEDING SEASON IN MAMMALS (Invited Paper) H. M. RADFORD* Summary The nature of seasonal breeding in both domestic and wild mammals has been known for centuries but only recently have we gained some insight into the mechanisms involved. Environment, encompassing climatic, nutritional, and biotic factors, plays a major role in synchronizing reproductive rhythms. Examples of the effect of environmental factors upon reproductive activity are discussed within the conceptual framework of central nervous control of pituitary secretion. I. INTRODUCTION Reproductive activity in the non-pregnant female mammal is manifest in periodic changes in the reproductive tract with associated times of sexual receptivity - the period of such changes being known as the oestrous cycle. Those species that experience only one cycle each year are monoestrous, those experiencing two or more each year are polyoestrous, and those in which the cycle(s) occupies a regular seasonal position are known as seasonal breeders. Seasonal breeding is apparent in many mammals, and seems to be an accompaniment of genetic selection whereby only those animals that produce young during a particular period are able to perpetuate themselves. Domestication, which involves artificial selection and provides comparatively uniform nutrition and housing, has removed the necessity for seasonal breeding and some domestic breeds have apparently freed themselves from the influence of seasonal changes upon their reproductive activity. None the less, seasonal breeding persists to a varying degree in some domesticated species and places limits upon the extent to which man can manipulate their breeding. Seasonal breeding in domestic and wild mammals has been recognized for centuries, but it is only during the past 30 years that man has gained some understanding of the underlying mechanisms. This has come in some measure from the results of numerous studies on the net effect of certain environmental variables upon reproductive function, but owes much to the concept of central nervous control of anterior pituitary function - a concept largely due to Marshall (1937) and developed by Harris (1955). While this concept was in part a result of studies of seasonal breeding in mammals, seasonal breeders have been little used in the numerous investigations undertaken over recent years into *Division of Animal Physiology, C.S.I.R.O., Ian Clunies Ross Animal Research Laboratory, Prospect, N.S.W. 19 the nervous control of reproductive phenomena (see reviews by Sawyer 1964; Everett 1964). In this review no attempt is made to consider more than a few examples of environmental control of reproductive performance. These are then discussed within the framework of central nervous regulation of pituitary secretion. II. ENVIRONMENT AND SEASONAL BREEDING Environment encompasses the full range of conditions to which an organism is exposed, and is a complex of clima,tic, nutritional and biotic factors. Within each of these classes there are variables that play a role in regulation of reproduction in some species. (a) Climatic factors (i) Light There is a vast literature, reviewed by Yeates ( 1954)) Hammond ( 1954), Amoroso and Marshall (1960), Clegg and Ganong (1959), and Ortavant , Mauleon and Thibault ( 1964)) on the effects of light on reproduction in both birds and mammals; there can be no doubt that light plays an important role in determining reproductive periodicity. In some mammals, light determines only macroperiodicity, i.e., the timing of the bree$ding season, while in others, events of the oestrous cycle are also regulated by light (Everett and Sawyer 1950). Many species commence breeding when days lengthen in the spring, and in these so called 'long-day' breeders, the experimental provision of increased lighting promotes the onset of reproductive activity. In others the onset of the breeding season is consequent upon decreasing daylength. Of the 'long-day' breeders, ferret and mink have been the most widely investigated, and the evidence is incontrovertible that the provision of increased lighting during winter months hastens the onset of oestrus (Bissonnette 1932; Hammond 195 1) . A similar situation exists also for the fieldmouse, Microtus agrestis, (Baker and Ranson 1932), raccoon (Bissonnette and Csech 1937)) domestic cat (Dawson 1941) and the horse (Burkhardt 1947). There is presumptive evidence from statistical studies in cattle (Mercier and Salisbury 1947) that increased fertility is associated with increasing daily photoperiod, but equally as much evidence to relate maximum fertility with decreasing daily photoperiod (Courot and Ortavant 1963). There is presumptive evidence from the time of occurrence of births that decreasing or low daily amounts of light stimulate the onset of the breeding season in the badger (Neale and Harrison 1958), the grey seal (Amoroso and Matthews 1952) and in a number of other species (reindeer, yak, warthog, bushbaby) kept in zoos (Zuckerman 1953; Brand 1963). However, the sheep and goat are the most widely known 'short-day' breeders and the sheep certainly the most widely studied. Yeates ( 1949), Hart ( 1950), and Hafez ( 1952) provided evidence that in many English breeds of sheep the breeding season starts after a change from increasing to decreasing hours of daylight, and Yeates ( 1956) demonstrated similar photosensitivity in Merino ewes. Subsequentl y Radford ( 196 1) showed that continuous light did inhibit but not completely suppress the development of normal sexual activity in Merino ewes and that 20 the response of mature Merino ewes to equatorial light was variable, some ewes exhibiting continuous sexual activity, others remaining seasonal. In Merino ewes subjected to equatorial light and reversed temperature seasons, WodzickaTomaszewska, Hutchinson and Bennett (personal communication) observed individual variation in occurrence of oestrus, but generally shorter and earlier breeding seasons; under the same conditions Southdown ewes exhibited seasons of oestrous activity shorter than those of controls. Thwaites (1965) has shown that Southdown ewes subjected to equatorial light exhibit some degree of seasonal oestrous activity for a year at least, although activity never exceeded 50% of control values and eventually became very low. The extent to which a photoperiodic mechanism operates to regulate seasonal breeding in sheep is obviously variable. In Australia there are many examples of the breeding season commencing while days are still increasing in length (Watson and Radford 1955). Results obtained from photoperiodic studies on sheep have been interpreted by most workers as consistent with the thesis that decreasing daily photoperiods stimulate and increasing daily photoperiods inhibit reproductive activity. An alternative, proposed by Ortavant, Mauleon an d Thibault (1964) is that there is an optimal photoperiod for reproduction, and values above or below this are inhibitory. A further possibility is that increasing light per day inhibits reproductive activity which, after a period of rest, recurs without further stimulus provided that the current light regime does not provide more than a maximum number of hours of light per day. This last explanation has the advantage that it can account adequately for the onset of the breeding season in Merino ewes before the longest day. Despite considerable experimentation, no one hypothesis based solely on photoperiodism serves to explain all of the observed facts on seasonal breeding in sheep. In photoperiodic studies many different light treatments have been used, ranging from reversed seasonal lighting, exaggerated seasons, sudden increases or decreases in light per day, and pulse type light : dark ratios. It is not possible here to summarize the results of such experimentation except to say that contrast detecting mechanisms seem to operate in all species studied. The experimental finding that a regime of 2 hours light 10 hours dark (i.e., only 4 hours light in any 24 hours) will bring ferrets into oestrus almost as effectively as days of 14 hours light : 10 hours dark suggests that the photosensitive mechanism takes account of only one light : dark ratio per 24 hours. Hammond ( 1954) believes that the 2L : 1ciD : 2L : 1 OD regime is in essence a 14 hour day, the ferret taking no account of one 10 hour dark period. (ii) Temperature and rainfall The evidence implicating temperature as a factor regulating the onset of breeding seasons in mammals is sketchy. It appears that prolonged cool weather rather than light is the stimulus for gonadal development in the ground squirrel (Wells and Zalesky 1940) and low environmental temperature can modify reproductive behaviour in the rat (Lee 1926). Further, Dutt and Bush (1955) involved temperature as a modulator of reproductive function in sheep when they showed that a lowered environmental temperature (40-45�F) induced oestrus some 50 days earlier than in controls at a temperature of approximately 90�F. 21 Marshall (1937) believed that low temperature is a factor in inducing full autumn rut in red deer. There is no evidence that rainfall per se exerts reproductive effects in i\ mammals. (b) Nutritimd factors Only in recent years has nutrition definitely been shown to regulate breeding seasons in mammals, some of the evidence coming from studies on wild rabbits and sheep. Myers and Poole ( 1962) have observed a correlation between availability of feed and breeding activity in wild rabbits - activity commencing very shortly after rains had promoted growth of feed. Suijendorp (1959) has also noted an association between occurrence of oestrus and feed availability in Merino ewes in Western Australia. Hunter ( 1961) too has shown an effect of nutrition upon occurrence of oestrus in ewes, and Smith (1962, 1966) has demonstrated that both current and previous nutritional status have an influence upon occurrence of oestrus in Merino ewes. While there is no doubt that malnutrition can interfere with reproductive function (Leathem 1961)) the extent to which nutrition actively contributes to regulation of the breeding season has not been investigated in any animal other than the sheep. (c) Biotic fa!ctors The presence of the male can profoundly modify reproductive activity in females of some mammals. There is little point in detailing here the well known effect of introduction of rams in synchronization of ovulation and oestrus in Merino ewes in the spring (Schinckel 1954; Watson and Radford 1960), or the effect of discontinuous association of the sexes in extending the breeding season in Merino ewes (Riches and Watson 1954; Lamond, Wells and Miller 1963). It is perhaps less well known that in Angora goats also the male can synchronize ovulation shortly before the normal time of onset of the breeding season (Shelton 1960). Signoret, du Mesnil du Buisson and Busnel ( 1960) have shown that recorded boar sounds influence the intensity of oestrus in sows. Reproductive phenomena related to the presence of the male are not confined to domestic species. This is evident in the effects of male rodents in synchronizing oestrus and interrupting pregnancy (Whitten 1956; Bruce 1960; Lamond 1959; Hughes 1964). Other social factors also can operate to modulate reproductive function. Changes in population structure and density have been invoked as explanation for change in breeding activity in wild rabbits (Myers and Poole 1962)) and in mice (Whitten 1959). The existence of hierarchies in many species automatically introduces a social factor in breeding activity; this has been observed in sheep recently by Mattner, Braden and Turnbull ( 196' 6). It should be emphasized that as yet little is known of the complexities of interaction both 'within and between sexes in the regulation of reproductive performance in mammals. 22 III. MECHANISM OF ACTION OF ENVIRONMENTAL FACTORS (a) Relation between pit&n-y and hypothalamus Thirty years ago it was considered that control of mammalian gonadal function rested with the adenohypophysis, and that cyclic regulation of gonadal activity was a function of a closed loop involving interactions between gonad and adenohypophysis (Moore and Price 1932). The feedback of gonadal steroids is still an integral part of hypotheses on control of the oestrous cycle, but it is now believed that the site of feedback is within the hypothalamus (Szentogathai et al. 196(2). The hypothalamus has now essentially assumed the role originally accorded the adenohypophysis. Owing largely to detailed studies by Green ( 195 1) there is no doubt that there are but few, and then only vasomotor, nerve fibres frolm the hypothalamus to the adenohypophysis. Hypothalamic control of anterior lobe secretion is exerted by the secretion of neurohumoural mediators - the so-called releasing factors - from hypothalamic nerve endings. These nerve endings lie near the primary capillary plexus of a system of blood vessels (the hypophysial portal system) which provides the major if not the sole blood supply to the adenohypophysis. A representation of this system is shown in Figure 1. The precise site of the perikarya of the neurons that produce the releasing factors is not known. However, cells of the arcuate nucleus and other areas that contribute axons ending in apposition to the primary capillary plexus of the hypophysial portal 23 system are probably involved. The releasing factors are apparently concentrated within the median eminence of the hypothalamus from -which they may be extracted chemically (Nikitovitch-Winer 1962; Guillemin and Schally 1963). With this transfer of endocrine control from a non-nervous to a nervous structure it is now possible to visualize the means whereby environmental sensory input can modify reproductive function, and the next part of this discussion refers ver y briefly to the relationship between the hypothalamus and other parts of the central nervous system. (b) Nervous pathways to hypothalamus This relationship has recently been reviewed in detail by Nauta ( 1963) and Gloor ( 1956). The hypothalamus receives much of its sensory influx from the brain stem or midbrain reticular formation. This is a highly complex multisynaptic system intimately concerned with consciousness in its broadest sense. Some correlates between release of gonadotrophin and electrical activity within the reticular formation have been observed and studied by Sawyer and Kawakami (1959). Another system in close functional and anatomical relationship to the hypothalamus in the rhinencephalon, including the olfactory bulbs and tubercle, septum, hippocampus and amygdala. This system is certainly involved in reproductive control. Lesions in certain parts lead to hypersexualism (Critchlow 1961) and stimulation of other parts can induce release of gonadotrophin (Koikegami, Yamada and Usei 1954) . There is evidence, too, that fibre paths between this system and the reticular formation relay extensively within the lateral hypothalamus and preoptic region (Nauta 1963). There is little evidence for direct cortico-hypothalamic projections, but every reason to believe that much of the cortical outflow is at various points both proximal to and parallel with both the reticular formation and rhinencephalon. Cortical control of hypothalamic neurons therefore remains a possibility. A representation of hypothalamic afferents is shown in Figure 2. (c) Mediation of environmental stimuli (i) Light It has been well established that the integrity of the eye is not essential for mediation of light effects, the hypothalamus itself being light sensitive (Benoit 1964; Lisk and Kannwischer 1964). However, an intact retinal ganglion layer has been shown to be essential in the ferret for light to influence reproductive activity (Thomson 195 1) , and it seems likely that a similar situation would exist in other mammals. However, experiments on blinded mammals have been confined to ferrets, and, since Ganong et all. ( 1963) have shown that light can penetrate to the brain in the sheep, dog, rabbit, and rat (Table 1) , some direct effect of illumination upon central nervous activity remains a possibility. If the eye is the receptor, the pathway from eye to hypothalamus still remains a mystery. There have been claims for the existence of direct retino-hypothalamic tracts (Critchlow 1963) but such findings are doubted by Nauta ( 1963) and Hayhow, Webb and Jervie ( 1960). An alternative is that the light input is channelled by accessory optic tracts from where it could be processed by the midbrain reticular formation and/or the rhinencephalic system. Visual projections related to endocrine function are considered in some detail by Marg ( 1964), Feldman ( 1964) and Gergen and MacLean ( 1964). Another possibility is that the hypothalamus, being so close to the optic tract, is sensitive to overall field effects arising in the optic tract itself as a result of incident illumination. Certainly, in sheep, electrical activity recorded fro$m the hypothalamus close to the optic tract TABLE I Amount of light reaching photovoltuic cell in brain when head to daylight (from Ganong et al. 1963) of animal exposed 25 does vary with intensity of optic stimulation (Radford, unpublished data). The effects of illumination upon reproductive activity take place gradually, implying some storage of energy arising from stimulation. No attempt has been made to determine just how such storage occurs. There has been a recent upsurge of interest in the pineal body as a possible mediator of light induced reproductive changes in the rat (Cohen et al. 1964 ) but there is no unequivocal evidence for the involvement of the pineal in the regulation of seasonal breeding. (ii) Temperature It is perhaps surprising that temperature has not been implicated more in the control of reproductive function. Temperature sensing mechanisms exist in the hypothalamus (Barbour 1921; Andersson and Larsson 1961; Hardy 1961) which is supposed to function as a computer in thermoregulation. These mechanisms lie in anatomical proximity to the preoptic area which can play a role in the release of pituitary gonadotrophin in the rat (Everett 1961) . Environmental temperature also has a marked centrally mediated effect on thyrotrophic hormone release (Von Euler and Holmgren 1956) and a similar mechanism for gonadotrophins is equally possible. The fact that environmental temperature can alter both thyroid and adrenal activity presents another possibility, namely that of altered thyroid and/or adrenal activity indirectly providing a stimulus for change in gonadotrophin secretion (Young 1961). (iii) Nutrition The mechanisms whereby nutrition can modulate ovarian activity are perhaps as conjectural as those already considered for light and temperature. The facts that undernutrition results in changed pituitary gonadotrophin content (Casida 1963; Srebnik and Nelson 1963) and that exogenous gonadotrophin can restore gonadal function in undernourished animals (Meites 1953) suggests strongly that nutritionally induced variation in gonadal activity is not due to gonadal change per se. However, there is no evidence to indicate whether the effect is directly upon the adenohypophysis or the central nervous system. The delayed effect of earlier nutrition upon oestrous activity in ewes (Smith 1962, 1966) suggests a storage mechanism, perhaps implicating the central nervous system, a system well adapted for storage, and certainly the hypothalamus has a function in regulation and recognition of food intake. On the other hand the effects of current nutrition upon oestrous activity (Smith 1966) or ovulation rate (Allen and Lamming l%l) may imply a more direct metabolic effect within the pituitary. (iv) Biotic factors There is no' question that reproductive effects initiated by association with the opposite sex are mediated by the central nervous system, and that the specialized sense organs for sight, sound and smell are to some extent involved. There would seem little doubt that the olfactory input is processed by rhinencephalic networks before influencing the hypothalamus (Green 1956), and indeed, electrical activity in olfactory structures has been related to release of pituitary gonadotrophin in rabbits by Sawyer (1955) and Radford and Sawyer (1960). The manner ~II 26 c ` .I < .I , which auditory stimulation operates to modify reproductive activity is completely u&known. i rl `I .. i,, IV. CONCLUDING REMARKS s ' -, -Within the preceding discussion it has been assumed that rhythmicity in breeding is largely controlled by external factors. This is not to say that such .. rhythmicity in all species and breeds is necessarily dependent upon *environmental change; There is no a priori reason why rhythmic breeding should not be endog&us,+ becoming seasonal only upon being synchronized by external environ. tt mental rhythms. * I t * L) . .(I _- Of necessity the foregoing has been but a brief discussion of seasonal bre$lingl ' An attempt has been made to fit known facts relating environmentj and .1 seasonal breeding to an anatomical :and physiological framework, (Figure. \, 3 ).p and from this the following conclusion would seem justified. Within mammals there exist appropriate mechanisms for the integration of a host of environmental variables. Within any species the final timing of the breeding season is almost certainly not controlled by one environmental factor but by a combinati .on o f environmental stimuli. 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