Sociosexual stimuli and gonadotropin-releasing hormone/luteinizing hormone secretion in sheep and goats

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Sociosexual stimuli and gonadotropin-releasing hormone/luteinizing hormone secretion in sheep and goats

P.A.R. Hawkena,b,*, G.B. Martina,b

a School of Animal Biology, University of Western Australia, Crawley, West Australia, Australiab UWA Institute of Agriculture (Animal Production), University of Western Australia, Crawley, Western Australia, Australia

Received 26 August 2011; received in revised form 18 January 2012; accepted 13 March 2012

Abstract

Sociosexual stimuli have a profound effect on the physiology of all species. Sheep and goats provide an ideal model to studythe impact of sociosexual stimuli on the hypothalamic-pituitary-gonadal axis because we can use the robust changes in thepulsatile secretion of luteinizing hormone as a bioassay of gonadotropin-releasing hormone secretion. We can also correlate thesechanges with neural activity using the immediate early gene c-fos and in real time using changes in electrical activity in themediobasal hypothalamus of female goats. In this review, we will update our current understanding of the proven and potentialmechanisms and mode of action of the male effect in sheep and goats and then briefly compare our understanding of sociosexualstimuli in ungulate species with the “traditional” definition of a pheromone.© 2012 Published by Elsevier Inc.

Keywords: Pheromones; Olfactory; Reproduction; Kisspeptin; GnRH; LH

1. Introduction

All mammals, particularly those that live in largegroups, are immersed in a rich and complex social envi-ronment that is full of the sights, sounds, and smells oftheir neighbors, mates, and offspring [1]. These sensoryinputs, the sociosexual signals, can profoundly altermany physiological and behavioral processes, includ-ing reproduction [2,3]. For example, in both female andmale ungulates, exposure to a prospective mate inducesan almost immediate increase in gonadotropin-releas-ing hormone (GnRH)/luteinizing hormone (LH) secre-tion, phenomena termed the “male effect” and “femaleeffect,” respectively (Fig. 1). Olfactory signals, often

called “pheromones,” are among the most potent of thesociosexual stimuli and can stimulate GnRH/LH secre-tion and induce ovulation in females, even in the ab-sence of additional sociosexual stimuli [4–6]. Changesin GnRH secretion into the hypophyseal circulationhave not been measured directly in female sheep orgoats exposed to males. However, the well-establishedrelationship between GnRH and LH secretion [7]makes the measurement of LH secretion in the periph-eral circulation a reliable bioassay of GnRH secretion.Nonolfactory stimuli also help to achieve the optimumneuroendocrine and ovulatory response of females tomales [8–10], but recent work has shown that thesestimuli play a relatively minor role and are unable tosubstitute for the full complement of sociosexual stim-uli in sheep or goats [10–12]. The field of pheromoneresearch has evolved dramatically since the initial con-cept of pheromones was proposed by Karlson and

* Corresponding author. M085, School of Animal Biology, Uni-versity of Western Australia, 35 Stirling Highway, Crawley, WA6009, Australia. Tel: �61 (0)8 6488 3588; fax: �61 (0)8 6488 1029.

E-mail address: [email protected] (P.A.R. Hawken).

Available online at www.sciencedirect.com

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0739-7240/12/$ – see front matter © 2012 Published by Elsevier Inc.http://dx.doi.org/10.1016/j.domaniend.2012.03.005


Lüscher [13], with the emergence of pheromone feed-back loops between GnRH neurons and regions of thebrain responsible for olfactory processing [14], increas-ing evidence of convergence between the main andaccessory olfactory systems [15], and the proposeddistinction between pheromones and olfactory signa-tures [16]. Even in the early 1980s, there was sufficientevidence for Martin et al [2] to question whether themale pheromone in sheep and goats “fits” the tradi-tional definition of a pheromone. In this review, we willupdate our current understanding of the mechanism andmode of action of the male effect in sheep and goatsand then briefly expand on the discussion outlined byDelgadillo et al [17] on how the male pheromone inungulate species compares with the “traditional” defi-nition of a pheromone.

2. Mechanism

It is logistically difficult to study the mechanismthrough which sociosexual stimuli impact GnRH secre-tion because of the sheer complexity of the phenome-non. For example, the time course of the neuroendo-crine response of females to males stems from an initial“acute” response (0 to 6 h after initial exposure tomales; Fig. 1) to the changes in LH and estradiolsecretion that precede the preovulatory surge of LH andovulation [2]. The nature of sociosexual stimuli is wide

ranging and female responsiveness is markedly affectedby the method of exposure (ie artificial simulation withvisual images compared with exposure of anosmic fe-males to males) [9,11] and is not necessarily driven bythe odors that we, as humans, associate with males [18].It can also be difficult to differentiate between stimu-lation associated with social stimuli (ie identification offamiliar or novel conspecifics or offspring) [review; 19]and sexual stimuli (ie male pheromone). This complex-ity is perhaps the reason why few studies have at-tempted to conclusively identify the mechanism drivingthe impact of sociosexual stimuli on GnRH secretion.Histologic assessment of neural activation is a particu-larly useful technique to study the impact of sociosex-ual stimuli on GnRH secretion because the neural re-sponse to an internal or external stimulus can bequantified using the immediate early gene c-Fos. TheFos protein is only present in the nucleus of recentlyactivated cells for up to 2 h after the initial stimulation,so we can use immunohistochemistry or in situ hybrid-ization to map the time course of neural activationrelative to a given stimulus [20–22]. In this section, wewill summarize our current understanding of the provenand potential mechanisms involved in mediating theeffects of sociosexual stimuli from males on the repro-ductive axis of sexually experienced female sheep andgoats (Fig. 2).

Fig. 1. Schematic representation of the impact of sociosexual stimuli from a prospective mate on the reproductive physiology of female sheep (the“male effect”) and male sheep (the “female effect”).

86 P.A.R. Hawken and G.B. Martin / Domestic Animal Endocrinology 43 (2012) 85–94


2.1. Main vs accessory olfactory system

Olfactory signals from the male can influence thereproductive centers of the female brain via pathwaysbeginning in either (i) the olfactory mucosa and mainolfactory bulb or (ii) the vomeronasal organ and acces-sory olfactory bulb (for a review, see [15]). In contrastwith the functional elements in rodents, the main olfac-tory system is the dominant pathway for the transmis-sion of olfactory signals in female sheep [23]. Thispathway is far more complex than the accessory olfac-tory system, with the main olfactory bulb both receiv-ing and transmitting information to numerous telence-phalic, diencephalic, and metencephalic regions of thebrain and cortical structures that extend rostrally fromthe anterior olfactory nucleus and piriform cortex andcaudally from the amygdaloid nuclei to the lateral en-

torhinal cortex [24]. This complexity is evident in theextensive pattern of neural activation elicited across thebrains of female sheep exposed to males (Fig. 2) com-pared with females exposed to female odor or isolatedfrom sociosexual stimuli from either sex [25]. Exposureto an intact male increases the number of Fos-immu-noreactive (Fos-IR) cells in key structures of both theaccessory and the main olfactory systems, including thecortical (CoA) and medial nuclei of the amygdala,multiple cortical regions, ventromedial nucleus of thehypothalamus, and medial preoptic area of the hypo-thalamus and dentate gyrus of the hippocampus (Fig.2). Male odor increased Fos activation in the CoA,basal amygdala, medial preoptic area, and ventromedialnucleus of the hypothalamus and only elicited more Fosactivation than exposure to the female odor in the CoA,

Fig. 2. Schematic representation of the main and accessory olfactory systems relative to their activation in female sheep exposed to males. Thesolid black lines represent the established role of the main olfactory system in mediating olfactory stimuli from males in sheep and goats—destruction (indicated by an X) of the main olfactory bulb and epithelium [9] or inactivation of the cortical amygdala [26] results in abolition ofthe luteinizing hormone (LH) response of female sheep to male odor (indicated by an X), but not the response to males. The accessory olfactorysystem (indicated by dotted black lines) is unable to substitute for the main accessory olfactory system [23], but exposure of sexually experiencedfemale sheep to males is associated with increased Fos activation (indicated by the photograph of Fos-immuoreactive cells) in several structureswithin this pathway [25]. Fos activation in the vomeronasal organ or bed nucleus stria terminalis was not reported [25]. Male exposure wasassociated with Fos activation throughout the main olfactory system and hypothalamus, with double labeling of a proportion of gonadotropin-releasing hormone (GnRH) neurons for Fos protein restricted to the preoptic area (POA) and organum vasculosum of the lamina terminalis(OVLT) [25]. The involvement of kisspeptin, gonadotropin inhibitory hormone (GnIH), and other neuropeptides in this pathway and the possibilityof cross-communication between GnRH neurons and regions of the brain involved in olfactory processing (gray lines) similar to that reported inrodents [14] require further investigation. MeA, medial amygdala; CoA, cortical amygdala; VMH, ventromedial hypothalamus; ARC, arcuatenucleus.

87P.A.R. Hawken and G.B. Martin / Domestic Animal Endocrinology 43 (2012) 85–94


mitral layer of the main olfactory bulb, and dentategyrus [25].

The main olfactory system is widely accepted to bethe primary pathway for processing olfactory stimulifrom males in female sheep because destruction of thevomeronasal organ and vomeronasal nerve does notprevent the LH response of female sheep to male odor[9], but inactivation of the cortical nucleus of theamygdala does (Fig. 2) [26]. Despite the clear inabilityof the accessory olfactory system to substitute for themain olfactory system in the perception and processing ofolfactory stimuli [26], many structures within the acces-sory olfactory system, including the medial amygdala, areactivated in sexually experienced female sheep exposed tomales (Fig. 2) [25]. Fos activation was limited to theaccessory olfactory bulb and ventromedial hypothala-mus (VMH) of female sheep exposed to male odor inthe absence of other sociosexual stimuli [25], but noactivation in this pathway was observed in sexuallynaive female sheep exposed to male odor or males [27].The functional relevance of neural activation in theaccessory olfactory system is unclear [28]. However,the lack of differences in Fos activation between femalesheep exposed to male odor and female odor in severalstructures [25] may indicate that some neural activationwas associated with the recognition of social rather thansexual stimuli. This concept is supported by prelimi-nary observations from our laboratory where femalesheep exposed to novel castrated males had moreFos-IR cells in the VMH than female sheep isolatedfrom novel social stimuli (novel castrated male, 11.6 �0.7 Fos-IR cells/mm2; control, 7.1 � 0.4 Fos-IR cells/mm2; Jorre de St Jorre, Hawken, and Martin, unpub-lished data).

2.2. Sociosexual stimuli and GnRH neurons

The GnRH neurons of the ovine hypothalamus arelocated in the preoptic area, anterior hypothalamic area,and mediobasal hypothalamus [29]. However, despiterecent interest in this field [25,26,30], it remains un-clear exactly how sociosexual stimuli from males acti-vate the GnRH neurons of the preoptic area or themediobasal hypothalamus to increase LH secretion.Boukhliq et al [31] identified a population of GnRHneurons in the mediobasal hypothalamus that were ac-tivated, as indicated by Fos immunohistochemistry, inmale sheep exposed to estrous females. However,Gelez and Fabre-Nys [25] only reported Fos activationin a small proportion of GnRH cells in the medialpreoptic area of female sheep exposed to males. Impor-tantly, the proportion of GnRH neurons double labeled

for Fos was consistent between females exposed tomale odor or to intact males [25]. The divergencebetween these 2 studies indicates sexual dimorphism inthe processing of olfactory stimuli from a prospectivemate, similar to that reported in rodents [14]. Alterna-tively, the divergence could be caused by differences inthe time of tissue sampling relative to exposure to thestimulus animal (1.5 h [25], 2 h [31]). Little is knownabout brain activity in females past the initial few hoursof male exposure, but the maintenance of high LHpulse frequency combined with a reduction in pulseamplitude and the associated changes in estradiol se-cretion [2] are likely to be associated with dynamicchanges in neural activation. A recent study in ourlaboratory reported that the number of cells immuno-reactive for Fos increased in the VMH and preopticarea was greater in females exposed to males for 6 hcompared with those exposed to males for 2 h prior totissue sampling (VMH 2 h, 14.8 � 0.9, vs 6 h, 18.4 � 1.8Fos-IR cells/mm2; P � 0.05; preoptic area 2 h, 16.8 �1.5, vs 6 h, 21.9 � 1.3 Fos-IR cells/mm2) [32].

2.3. Potential involvement of novel neuropeptides

The continued ambiguity over the exact mechanismthrough which sociosexual stimuli impact upon GnRHsecretion is possibly the result of continued uncertaintyover the control of pulsatile GnRH secretion itself.Many neuropeptides in the hypothalamus have bothinhibitory and stimulatory effects on GnRH secretion,which are often dependent on the gonadal status of theanimal [33]. However, a recent review article high-lighted the emerging roles of 2 novel neuropeptides,kisspeptin and gonadotropin inhibitory hormone(GnIH), in the control of GnRH secretion [34] that mayprovide clues to the mechanism through which socio-sexual stimuli have such a profound effect onGnRH/LH secretion. The product of the KISS1 gene,kisspeptin, and the kisspeptin receptor (GPR54) play anintegral role in the control of GnRH secretion [35–39],yet little is known about the involvement of kisspeptinin mediating the LH response of sheep to sociosexualstimuli. Okamura et al [30] argue that the activation ofkisspeptin neurons is a key step in the pathway throughwhich olfactory stimuli stimulate GnRH/LH secretionin female goats. This proposition is based on changes inelectrical activity in the brains of female goats that aremeasured through implantation of electrodes targeted atthe population of kisspeptin neurons in the caudal as-pect of the arcuate nucleus [40]. They detected regularvolleys in multiunit activity (MUA) that were accom-panied by a pulse of LH and found that exogenous



administration of kisspeptin increased LH secretion butdid not affect the timing of the next MUA volley [40].The authors interpreted this observation as evidencethat the MUA volley reflected electrical activity fromneurons other than the GnRH neurons themselves [40].They argue that the population of kisspeptin neurons inthe arcuate nucleus that coexpress neurokinin B (NKB)and dynorphin (Dyn) mediates the effects of male odoron GnRH secretion and also acts as a “GnRH pulsegenerator” through the combined action of NKB andDyn on the onset and succession of pulses of kisspeptinand, in turn, GnRH [30,41]. The concept of a GnRHpulse generator per se is still somewhat controversial[34] and one issue with electrophysiology is the inabil-ity to conclusively identify the cells responsible for thechanges in MUA. For example, before the work ofOhkura et al [40], the electrodes were placed in themediobasal hypothalamus and arcuate nucleus and ex-posure to the male pheromone still elicited an almostinstantaneous MUA volley and associated pulse of LHsecretion [42,43]. It is possible that both sets of elec-trodes detected electrical activity from the populationof cells targeted in later studies [40,41], but overall it isdifficult to conclusively identify the form and functionof the cells responsible for the changes in electricalactivity detected through this technique. GnIH is un-likely to play a role in stimulating the neuroendocrineresponse of females to sociosexual stimuli because, asthe name suggests, it appears to inhibit rather thanstimulate GnRH secretion [34]. However, if either kiss-peptin or GnIH is found to play a definitive role inmediating the onset and succession of the breedingseason as discussed by Clarke [34], then sociosexualstimuli would have to circumvent this circuit to inducean instantaneous, premature resumption of GnRH ac-tivity.

2.4. GnRH feedback

The transmission of sociosexual stimuli through theneural pathways of the main and olfactory systems doesnot appear to be a 1-way street. In transgenic mice, atransneuronal tracer has been used to identify neuronsthat are pre- or postsynaptic to GnRH neurons in anattempt to unravel communication between these neu-rons and the rest of the brain [14]. There were severalimportant observations from the perspective of thisreview: (i) approximately 800 neurons showed signs ofcommunication (pre- or postsynaptic) with approxi-mately 50,000 neurons in 53 brain regions; (ii) cells inthe medial nucleus of the amygdala were both pre- andpostsynaptic to GnRH neurons; (iii) there are direct

synaptic contacts between GnRH neurons and neuronsin the olfactory epithelium and several key regions inthe main olfactory pathway, including the piriform cor-tex and anterior cortical nucleus of the amygdala; and(iv) there are significant differences in GnRH circuitrybetween males and females [14]. The existence of thistype of cross-communication between GnRH neuronsand brain regions involved in the processing of socio-sexual stimuli in sheep or goats has yet to be investi-gated. However, maintenance of the neuroendocrineresponse is dependent on a continued male presence[44] and multiple regions of the brain are activated infemales exposed to males or male odor [25], so it isconceivable that a similar method of communicationexists in ungulate species.

3. Mode of action

3.1. Olfactory stimuli

The capacity of female sheep and goats to respond toram’s wool or buck hair, respectively, with an increasein LH secretion indicates that the olfactory stimuli frommale ungulates is a primer pheromone, capable of in-ducing long-term physiological changes in the femaleof the species [4,45–47]. Many pheromones have beenidentified in mammalian species as varied as mice (egmethanethiol) and Asian elephants (eg 1,5-dimethyl-6,8-dioxabicyclo[3.2.1]octane; for a review, see [16]),but despite significant advances in the identification ofpheromones and their receptors in mice [48], the exactchemical composition of the male pheromone has re-mained elusive in both sheep [4] and goats [49]. Iden-tification of the male pheromone in sheep has not ad-vanced since the mid-1990s, but it is widely acceptedthat the pheromone is a complex mix of substanceswithin the neutral and acid fractions of the fleece ex-tract [4]. By contrast, recent work has focused on iden-tification of the male pheromone in goats using changesin MUA as a “real-time” bioassay of neural activity infemale goats [49]. This approach has yielded severalimportant perspectives on the nature of the male pher-omone in this species: (i) pheromone production istestosterone dependent and is typically localized to thehead, neck, and shoulders of the male [50]; (ii) thepheromone resides in the lipid fraction of fleece extract[49]; (iii) fleece from male sheep induces a MUA vol-ley and associated increase in LH secretion in femalegoats [51]; and (iv) the chemical responsible for thestrong odor of male goats, 4 ethyloctanoic acid, doesnot elicit pheromonal activity unless left at room tem-perature for several months, at which time mass spec-



trometry identified the formation of several new sub-stances [18]. A recent review proposed that pheromoneactivity in many species, including humans, could re-sult from bacterial fermentation of secreted precursors,a perspective that may be a key in the identification ofthe male pheromone in both sheep and goats [16].

A common perspective that has emerged from re-search into the nature of the male pheromone in ungu-lates is that it does not appear to be a simple, singlemolecule and that there may be sufficient complexity toencode individual identity [28]. The importance ofmale novelty to the neuroendocrine response of femalesheep to males has been a focus of our laboratory andour work has yielded several key observations: (i) ex-posure of female sheep maintained in continuous con-tact with males to novel males is associated with arobust increase in LH secretion [52,53]; (ii) exposure offemale sheep maintained in continuous contact withmales to those same familiar males after 15 min, 1 d, or17 d of isolation is not associated with increased LHsecretion [53]; (iii) familiar males regain some but notall of their stimulatory effect on LH secretion in fe-males after 1 mo of separation [53]; and (iv) the LHresponse of females to novel males is associated withmore Fos-IR cells in the ventromedial nucleus of thehypothalamus than that of females reexposed to thesame familiar males (novel male, 17.8 � 0.97 Fos-IRcells/mm2, vs familiar male, 6.7 � 1.1 Fos-IR cells/mm2; P � 0.05; Jorre de St Jorre, Hawken, and Martin,unpublished data). These observations, combined withother studies using ovulation [54,55] or behavioral es-trus [56] as an end point, clearly demonstrate thatfemale sheep and goats can distinguish between indi-vidual males and tailor their neuroendocrine responseaccordingly. Furthermore, the LH response of femalesto novel males is associated with a 100% increase in therate of cell proliferation in the dentate gyrus of thehippocampus [52], a key structure implicated in learn-ing and memory [57]. It is perhaps important to notethat this region is activated equally in sexually experi-enced female sheep exposed to either males or maleodor [25]. The physiological relevance of the rapidincrease in cell proliferation is unknown, but Hawkenet al [52] suggested that it may be associated with theformation of a memory of the male that facilitatesdifferentiation of novel and familiar males reported intheir study [52] and other studies [53]. Male identity isimportant in rodents and enables females to discrimi-nate among males according to familiarity or domi-nance and modify their physiology or behavior accord-ingly (for a review, see [58]).

A recent review article highlighted the difficulties inthe current terminology surrounding pheromones andproposed that a distinction between pheromones andsignature mixes might help to facilitate our comprehen-sion of these complex phenomena. Wyatt [16] proposedthat pheromones occur in the background of signaturemixes that facilitate identification of individuals, whereasthe pheromone evokes a more species-wide effect on theirphysiology (Table 1). Wyatt [16] also proposed that theseadditional molecules delay the release of the smaller pher-omone molecule, thus extending its activity. The existenceof an individual signature mix that is intertwined with amore generic primer pheromone may help to explain thedifficulties encountered in conclusively identifying themale pheromone in sheep and goats.

3.2. Nonolfactory stimuli

There is some synergy between olfactory and nonol-factory stimuli in mediating the neuroendocrine re-sponse of female sheep and goats to males. For exam-ple, the proportion of female sheep ovulating inresponse to male stimuli is optimized when females areexposed to intact males [8] and the number of regionsof the brain that are activated, as well as the amount ofactivation within those regions, is greater in femalesexposed to males than in females exposed to male odoralone [25]. The question of whether nonolfactory stim-uli can substitute for olfactory stimuli appears to bedependent on the method of presentation. For example,female sheep that were rendered anosmic responded toa male presence with an increase in LH secretion sim-ilar to that observed in intact females [9] Similarly,intracerebral treatment of female sheep with lignocaineto inactivate the cortical nucleus of the amygdala

Table 1Definition of pheromones and signatures mixes from Wyatt [16].

Definition

Pheromone Molecules that are evolved signals, in definedratios in the case of multiple componentpheromones, which are emitted by anindividual and received by a second individualof the same species, in which they cause aspecific reaction, for example, a stereotypedbehavior or a developmental process (modifiedafter Karlson and Lüscher [13]).

Signature mixture A variable chemical mixture (a subset of themolecules in an animal’s chemical profile)learned by other conspecifics and used torecognize an animal as an individual (eglobsters, mammals) or as a member of aparticular social group, such as a family, clan,or colony (eg ants, bees, mongoose).



blocked their LH response to male odor but not theirresponse to intact males [26]. Pearce and Oldham [8]found a progressive increase in the number of ewesovulating when contact with males intensified from nocontact to opaque fence-line contact, to transparentfence-line contact, to full contact with males. However,when audiovisual or visual stimuli are provided throughprojected images, visual stimuli only appear to have aminor effect on LH secretion in female sheep and noeffect in male sheep [11]. In female goats, exposure toauditory stimuli from males in real time induced behav-ioral estrus but was only associated with ovulation in 2 outof 6 females [12]. The behavioral component of the maleeffect is difficult to study because sexual behavior islinked to testosterone production, which in turn is linkedto pheromone production [50]. However, Vielma [10]showed that female goats responded to sedated males withan increase in LH secretion, but that maintenance of theneuroendocrine response was only observed in femalesexposed to awake males.

4. Is the olfactory stimulus a pheromone?

Olfactory stimuli from male sheep and goats clearlyhave a profound effect on GnRH/LH secretion, but theolfactory signal shows considerable divergence fromthe original definition of a pheromone outlined by Karl-son and Lüscher [13] for invertebrate animals. Thediscrepancies between the male pheromone in thesespecies and the pheromone definition adapted for mam-malian species was initially challenged by Martin et al[2] and more recently by Delgadillo et al [17]. Here webriefly compare each aspect of the traditional definitionof Karlson and Lüscher [13] for invertebrate animalswith our current understanding of the male olfactorystimulus that stimulates GnRH/LH secretion in femalesheep:

1. A single (or very few) identified and character-ized compounds.● Complex mixture of compounds [4,18,49].● Not yet fully characterized in either sheep or

goats.2. Evoke innate responses that are not learned.

● LH response to olfactory stimuli is dependenton sexual experience [6].

● Incomplete patterns of neural activation in thebrain of sexually naive females exposed tomales or male odor [27].

● Acute LH response is associated with a rapidincrease in cell proliferation in the dentategyrus of the hippocampus [52].

● Potential role of olfactory learning implicatedby the divergent responses of female sheep tonovel or familiar males [52,53].

3. Specific for 1 particular action or effect in recip-ient.● Variability in the LH response and proportion

of females ovulating both within and betweenbreeds [59].

4. Not able to be substituted by any other stimulus.● Olfactory signal can be fully substituted by

nonolfactory stimuli if the stimuli are derivedfrom an actual male [9]. Artificial simulation ofvisual or audio stimuli is less effective [11,12].

5. Species specific.● Fleece from male sheep can stimulate electri-

cal activity and LH secretion in female goats[51]; this contrasts with observations in a pre-vious study [60].

● Male goat hair stimulates LH secretion in fe-male sheep [60].

Recent research clearly supports the view of Martinet al [2] that the male pheromone responsible for in-ducing ovulation in anovulatory sheep and goats doesnot fit well with the traditional definition of a mamma-lian pheromone. Interestingly, there is increasing focuson this issue, even in rodent species, where the mainolfactory system and olfactory learning are emerging ashaving a more integral role in reproductive processesthan was originally thought (for reviews, see [15,16]).

5. Future perspectives

The effects of sociosexual stimuli on GnRH secre-tion remains an area of interest, in part because, evenafter over 60 yr of research, there is still much thatremains unclear about this biological phenomenon, de-spite its profound effects on reproductive activity andits major role as a reproductive strategy [61]. Ourunderstanding of the endogenous control of GnRH se-cretion is still evolving [33] with the emergence ofkisspeptin as a key player in facilitating feedback be-tween gonadal steroids and GnRH/LH secretion [7].The strict segregation between the main and accessoryolfactory systems, in terms of both communication andfunction, also appears questionable, especially with re-spect to the recognition of and discrimination betweenindividual males [15,16]. However, in the context ofthe impact of sociosexual stimuli and GnRH secretionin sheep and goats, we propose that there are severalkey questions that must be answered:



1. How do sociosexual stimuli combine to increaseGnRH/LH secretion in females under the influenceof negative feedback from estradiol (anestrus) orprogesterone (luteal phase)? Which neuropeptidesare involved?

2. Research in rodents shows considerable dimor-phism between the olfactory pathways and GnRHfeedback loops between males and females—howdoes the mechanism and mode of action comparebetween the male effect and female effect (Fig. 1)?

3. How does a sexually naive female become asexually experienced female? Is it male exposure,mating, pregnancy, or lactation, and how do theseprocesses impact the brain regions activated bythe male or his odor?

6. Conclusion

The impact of sociosexual stimuli on GnRH secre-tion in sheep and goats is far more than a simpleresponse to the scent or odor of a male. For example,only full contact with a male can elicit the full neu-roendocrine response of female sheep and goats tomales in terms of LH secretion and neural activation,particularly in sexually naive females. The emergenceof the dual concept of pheromones and signature mixes,a far more comfortable fit with our current understand-ing of olfactory stimuli in these species compared withthat proposed by Karlson and Lüscher [13], may help inthe identification of the male pheromone in ungulatespecies. This conceptual change may also lead to abetter understanding of this biological phenomenonthat has great potential as a management tool in theproduction of sheep and goats worldwide.

Acknowledgments

The research from the University of Western Australiacovered within this review was supported under the Aus-tralian Research Council’s Discovery funding scheme(Project No. DP0558952) and Meat & Livestock Australia(Project No. B.MGS.0027) and the School of AnimalBiology, University of Western Australia. We also thankTrina Jorre de St Jorre for her contribution of unpublisheddata and T Esmaili and M Blackberry for their assistancewith laboratory work and data collection.

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Sociosexual stimuli and gonadotropin-releasing hormone/luteinizing hormone secretion in sheep and goats