The role of the Flehmen response in the behavioral repertoire of the stallion

Review of the Foundational Knowledge Required for Assessing Horse Welfare

A detailed understanding of what is usual for a species under optimal conditions is critical for identifying and interpreting different features of body function that have known impacts on animal welfare and its assessment. When applying the Five Domains Model to assess animal welfare, the key starting point is therefore to acquire extensive species-specific knowledge relevant to each of the four physical/functional Domains of the Model. These Domains, 1 to 4, address areas where objective information is evaluated and collated. They are: (1) Nutrition; (2) Physical environment; (3) Health; and (4) Behavioural interactions. It is on the basis of this detailed knowledge that cautious inferences can then be made about welfare-relevant mental experiences animals may have, aligned with Domain 5, Mental State. However, this review is focused entirely on the first four Domains in order to provide a novel holistic framework to collate the multidisciplinary knowledge of horses required for undertaking comprehensive welfare assessments. Thus, inferring the potential mental experiences aligned with Domain 5, the final step in model-based welfare assessments, is not considered here. Finally, providing extensive information on free-roaming horses enables a better understanding of the impacts of human interventions on the welfare of horses in both free-roaming and domestic situations.

Enhanced Understanding of Horse-Human Interactions to Optimize Welfare

Horses (Equus caballus) have been domesticated for millennia and are regularly utilized for work, sport, and companionship. Enhanced understanding of human-horse interactions can create avenues to optimize their welfare. This review explores the current research surrounding many aspects of human-horse interactions by first highlighting the horse's sensory capabilities and how they pertain to human interactions. Evidence exists that suggests that horses can read humans in various ways through our body odours, posture, facial expressions, and attentiveness. The literature also suggests that horses are capable of remembering previous experiences when working with humans. The interrelatedness of equine cognition and affective states within the horse's umwelt is then explored. From there, equine personality and the current literature regarding emotional transfer between humans and horses is examined. Even though horses may be capable of recognizing emotional states in humans, there remains a gap in the literature of whether horses are capable of empathizing with human emotion. The objective of this literature review is to explore aspects of the relationship between humans and horses to better understand the horse's umwelt and thereby shed new light on potential positive approaches to enhance equine welfare with humans.

The Nasopalatine Ducts Are Required for Proper Pheromone Signaling in Mice

The vomeronasal organ (VNO) specializes in detection of chemosignals, mainly pheromones, which control social communication and reproduction in many mammals. These pheromones must solubilize with nasal fluids before entering the VNO, and it was suggested that they are delivered to and cleared from the VNO by active pumping. Yet, the details of this pheromone delivery process are unclear. In this study, we first constructed a high-resolution 3D morphological image of the whole adult mouse snout, by using ultra-high-resolution micro-CT. We identified a net of micro tunnels starting from the nostrils and extending around and through the VNO. These micro tunnels connect the nasal cavity with the VNO and the oral cavity via the nasopalatine ducts (NPD). Other micro tunnels connect the nasal cavity to the main olfactory epithelium. We next demonstrated that physical obstruction of the NPD severely impairs the clearance of dissolved compounds from the VNO lumen. Moreover, we found that mice with blocked NPD display alterations in chemosignaling-evoked neuronal activation in brain regions associated with the vomeronasal system. Finally, NPD-blocked male mice exhibit reduced preference for female chemosignals, and impaired social interaction behavior. Taken together, our findings indicate that the NPD in mice are connected to both the nasal and oral cavity, serving an essential role in regulating the flow of soluble chemosignals through the VNO, and are required for proper pheromone-mediated social communication.

Grimace Scores: Tools to Support the Identification of Pain in Mammals Used in Research

The 3Rs, Replacement, Reduction and Refinement, is a framework to ensure the ethical and justified use of animals in research. The implementation of refinements is required to alleviate and minimise the pain and suffering of animals in research. Public acceptability of animal use in research is contingent on satisfying ethical and legal obligations to provide pain relief along with humane endpoints. To fulfil this obligation, staff, researchers, veterinarians, and technicians must rapidly, accurately, efficiently and consistently identify, assess and act on signs of pain. This ability is paramount to uphold animal welfare, prevent undue suffering and mitigate possible negative impacts on research. Identification of pain may be based on indicators such as physiological, behavioural, or physical ones. Each has been used to develop different pain scoring systems with potential benefits and limitations in identifying and assessing pain. Grimace scores are a promising adjunctive behavioural technique in some mammalian species to identify and assess pain in research animals. The use of this method can be beneficial to animal welfare and research outcomes by identifying animals that may require alleviation of pain or humane intervention. This paper highlights the benefits, caveats, and potential applications of grimace scales.

Sensory Abilities of Horses and Their Importance for Equitation Science

Vision, hearing, olfaction, taste, and touch comprise the sensory modalities of most vertebrates. With these senses, the animal receives information about its environment. How this information is organized, interpreted, and experienced is known as perception. The study of the sensory abilities of animals and their implications for behavior is central not only to ethology but also to animal welfare. Sensory ability, perception, and behavior are closely linked. Horses and humans share the five most common sensory modalities, however, their ranges and capacities differ, so that horses are unlikely to perceive their surroundings in a similar manner to humans. Understanding equine perceptual abilities and their differences is important when horses and human interact, as these abilities are pivotal for the response of the horse to any changes in its surroundings. This review aims to provide an overview of the current knowledge on the sensory abilities of horses. The information is discussed within an evolutionary context and also includes a practical perspective, outlining potential ways to mitigate risks of injuries and enhance positive horse-human interactions. The equine sensory apparatus includes panoramic visual capacities with acuities similar to those of red-green color-blind humans as well as aural abilities that, in some respects exceed human hearing and a highly developed sense of smell, all of which influence how horses react in various situations. Equine sensitivity to touch has been studied surprisingly sparingly despite tactile stimulation being the major interface of horse training. We discuss the potential use of sensory enrichment/positive sensory stimulation to improve the welfare of horses in various situations e.g. using odors, touch or sound to enrich the environment or to appease horses. In addition, equine perception is affected by factors such as breed, individuality, age, and in some cases even color, emphasizing that different horses may need different types of management. Understanding the sensory abilities of horses is central to the emerging discipline of equitation science, which comprises the gamut of horse-human interactions. Therefore, sensory abilities continue to warrant scientific focus, with more research to enable us to understand different horses and their various needs.

Signal Detection and Coding in the Accessory Olfactory System

In many mammalian species, the accessory olfactory system plays a central role in guiding behavioral and physiological responses to social and reproductive interactions. Because of its relatively compact structure and its direct access to amygdalar and hypothalamic nuclei, the accessory olfactory pathway provides an ideal system to study sensory control of complex mammalian behavior. During the last several years, many studies employing molecular, behavioral, and physiological approaches have significantly expanded and enhanced our understanding of this system. The purpose of the current review is to integrate older and newer studies to present an updated and comprehensive picture of vomeronasal signaling and coding with an emphasis on early accessory olfactory system processing stages. These include vomeronasal sensory neurons in the vomeronasal organ, and the circuitry of the accessory olfactory bulb. Because the overwhelming majority of studies on accessory olfactory system function employ rodents, this review is largely focused on this phylogenetic order, and on mice in particular. Taken together, the emerging view from both older literature and more recent studies is that the molecular, cellular, and circuit properties of chemosensory signaling along the accessory olfactory pathway are in many ways unique. Yet, it has also become evident that, like the main olfactory system, the accessory olfactory system also has the capacity for adaptive learning, experience, and state-dependent plasticity. In addition to describing what is currently known about accessory olfactory system function and physiology, we highlight what we believe are important gaps in our knowledge, which thus define exciting directions for future investigation.

Olfactory stimulation or inhibition of sexual behavior of stallions in non-breeding season

Stallions show decreased sexual responses and activities during short days in winter. To evaluate the importance of sexual olfactory communication in horses, we tested whether sexual responses could be stimulated through various sexual olfactory stimulations in winter. To this end, we presented stallions with various olfactory stimulations (urine from mares at different stages of the reproductive cycle, urine from stallions or geldings, or chemically defined synthetic odorant) during the non-breeding season and measured their behavioral responses through (1) a test of olfactory investigation (olfactory investigation and flehmen behavior) and (2) a test of sexual activity in the context of semen collection for artificial insemination. It appears that the duration of olfactory investigation and flehmen behavior is longer after presentation of urine (stallion, gelding, anestrous, diestrous and estrous mare) than after presentation of water or synthetic odorant. By contrast, geldings showed reduced flehmen behavior that did not differ from that after water presentation. It is of interest that during the mounting test, mare estrous urine was associated with significantly reduced latency to ejaculation when spread in the nostril of the stallion, compared to anestrous mare urine or water. Anestrous mare urine seems to even inhibit stallion sexual motivation as measured through a longer latency to reach mounting and ejaculation. It appears therefore that during the season of sexual rest, stallion sexual motivation can be stimulated by mare estrous urine and inhibited by mare anestrous urine. These results also suggest that the physiological state of the mare influence the content of urinary chemosignals.

Do olfactory behaviour and marking responses of Konik polski stallions to faeces from conspecifics of either sex differ?

The aim of the study was to assess whether adult stallions differentiate their olfactory and marking behaviour towards the excreta of their potential male rivals and mares. Four Konik polski stallions were individually exposed simultaneously to their own and each others' faeces as well as faeces of mares in oestrus vs. dioestrus. Five series of 30min observation sessions were conducted in a round pen each on two consecutive days from March to July, totaling in 5h observation per stallion. Stallions sniffed and displayed flehmen reaction towards mares' faeces significantly longer (P<0.01) than towards stallions' faeces. No significant differences were found in marking by defecations upon stallions' vs. mares' faeces. The stallions urinated exclusively on mares' faeces. Sniffing, flehmen, defecations and urinations performed towards faeces of mares in oestrus vs. dioestrus did not differ significantly. The results showed that stallions differentiate their olfactory behaviour towards excreta of conspecifics of different sex but perhaps not towards faeces of females in different reproductive status. This for the first time establishes that stallions exhibit different marking behaviour upon stallions' vs. mares' faeces. It is hypothesized that marking of other stallions' faeces by defecation may advertise presence of the stallion, whereas marking of mares' faeces by urination may serve to mask the presence of a mare to prevent potential rivals from locating the mare in an area where mares' faeces are found.

Olfactory behavioural responses in captive male southern hairy-nosed wombats (Lasiorhinus latifrons) exposed to oestrous and non-oestrous urine samples

Whilst flehmen behaviour has been observed in some marsupials, including the southern hairy-nosed wombat (Lasiorhinus latifrons), it is unknown whether it, or other male-specific behaviours are exhibited differentially in response to the detection of oestrous females, which if proven, could assist in the detection of oestrus in breeding programs for captive southern hairy-nosed wombats. Six captive male southern hairy-nosed wombats were exposed to urine samples collected from a single female that was confirmed to be in oestrus and mid-pregnancy. Urine samples were presented to the males simultaneously and subsequent behaviours recorded for five minutes. There were no significant differences in the number of times flehmen behaviour was displayed in response to oestrous and non-oestrous urine samples (P = 0.577), the number of times the male sniffed each sample (P = 0.595), or the time spent investigating each urine sample (P = 0.414). Our results suggest that neither male flehmen behaviour nor any specific male-related precopulatory behaviour was associated with the olfaction of urine from a female in different stages of the oestrous cycle, and is thus unlikely to be a reliable management tool for the detection of oestrus.

Facial Expression in Nonhuman Animals

Many nonhuman animals produce facial expressions which sometimes bear clear resemblance to the facial expressions seen in humans. An understanding of this evolutionary continuity between species, and how this relates to social and ecological variables, can help elucidate the meaning, function, and evolution of facial expression. This aim, however, requires researchers to overcome the theoretical and methodological differences in how human and nonhuman facial expressions are approached. Here, we review the literature relating to nonhuman facial expressions and suggest future directions that could facilitate a better understanding of facial expression within an evolutionary context.

p- and m-Cresols emitted from estrous urine are reliable volatile chemical markers of ovulation in mares

Urine samples from 14 mares, belonging to five breeds, were collected at estrus and diestrus to search for estrous specific volatile compounds which could be used to determine ovulation time. Around 150 volatiles were collected from urine head-space samples by solid phase micro extraction technique, and analyses were conducted by gas chromatography-mass spectrometry methods. Comparison of chromatographic profiles of volatile substances revealed that concentrations of m- and p-cresols were significantly greater during estrus while diethylphthalate was more abundant at diestrus. Monitoring of m- and p-cresols during the period of estrus and a few days before and after estrus revealed irregular changes in amounts of cresols until 3-4 days before ovulation when the concentration of the compounds began to increase with peaks 1 day before ovulation. On the day when ovulation occurred, amounts of the metabolites decreased sharply, almost to basal concentrations, and remained at these concentrations for 6 days - when sampling was finished. In four of the mares changes in the concentration of diethylphthalate were less pronounced and more temporally variable compared with those of cresols. Based on reproducible temporal changes in concentrations of m- and p-cresols, with respect to the time of ovulation, a noninvasive test to determine a precise insemination time could occur. This would save time, reduce costs and simplify the procedure.

A mixture of odorant molecules potentially indicating oestrus in mammals elicits penile erections in male rats

A common set of odorous molecules may indicate female receptiveness across species, as male rats display sexual arousal when exposed to the odour of oestrous faeces from rats, vixens and mares. More than 900 different compounds were identified by GC-MS analyses performed on faeces samples from di-oestrous and oestrous females and from males of the three species. Five carboxylic acids were found in lower concentrations in faeces from all oestrous females. We subjected 12 sexually trained male rats to a 30 min exposure to different dilutions of a mixture of these five molecules in the same proportions as found in female oestrous faeces. The behavioural responses of the rats were compared to those displayed when exposed to water (negative control) and faeces from oestrous female rats (positive control). Frequency of penile erections were found to be significantly dependent on mixture dilution, with two intermediate dilutions eliciting frequencies of penile erections that did not differ from those obtained during exposure to oestrous female rat faeces. Higher and lower dilutions did not elicit more penile erections than observed with water. These results support our hypothesis that a small set of odorous molecules may indicate sexual receptiveness in mammalian females.

Sexual behavior and ejaculate characteristics in Pêga donkeys (Equus asinus) mounting estrous horse mares (Equus caballus)

The objectives were to (i) characterize sexual behavior of donkey stallions (jacks; Equus asinus) during on-farm semen collection using estrous horse mares (mares; Equus caballus); (ii) compare behavior of young (less experienced) versus older (more experienced) jacks; (iii) determine whether semen suitable for artificial insemination (AI) could be collected using mares; and (iv) determine the suitability of using mares in field collection of semen from jacks. Six Pêga jacks (3.5 to 16 yr old), previously conditioned to breed mares, were used. Mount mares were confirmed in estrus by a teaser horse stallion (stallion) and a jack. Semen was collected with an artificial vagina, at intervals of 48 to 72h (180 collections). The mean+/-SD (young [3.5 yr] vs. old [14 to 16 yr]) were Flehmen response frequency, 7.4+/-5.8 (8.1+/-3.0 vs. 7.0+/-2.0); number of mounts without erection, 1.1+/-1.3 (2.1+/-1.4 vs. 1.2+/-0.4, P<0.05); latency from first exposure to mare to full erection on the ejaculatory mount, 18.3+/-17.7min (25.3+/-21.3 vs. 12.2+/-6.2, P<0.05); latency from erection to insertion, 5.1+/-3.5sec (5.3+/-3.8 vs. 4.8+/-3.2); and duration of copulation from insertion to dismount after ejaculation, 25.4+/-7.8sec (22.1+/-2.9 vs. 28.1+/-9.3). In all jacks, sexual behavior was generally normal, with the notable absence of open mouth behavior. Mare estrous behavior was markedly less intense than that in the presence of a stallion and usually absent. Semen characteristics were gel free volume, 47.3+/-28.7mL; gel volume, 71.8+/-54.8mL; total motility, 84.3+/-6.0%; progressive motility, 74.3+/-74.5%; sperm vigor, 3.9+/-0.5 (scale 1 to 5); sperm concentration, 253x10(6) cells/mL; and total number of sperm, 10.3x10(9) cells. Copulation duration was significantly correlated with gel free volume (r=0.9) and gel volume (r=0.7). We concluded that (i) the sexual behavior of jacks during semen collection using mares was similar to that reported for natural mating to jennies, (ii) precopulatory and copulatory behavior for the young (less experienced) jacks and older (more experienced) jacks were generally similar (except number of mounts without erection and latency to full erection); (iii) semen obtained using mares as stimulus and mount females was similar to that reported with estrous jennies; and (iv) semen collection from previously conditioned jacks, using estrous mares, was appropriate for field collection of semen.

Sexual behavior of mares

The mare is seasonally polyestrus, having an anovulatory period during the short light days of late fall and early winter, and beginning to ovulate as the days become longer during the winter. The complete estrus cycle is typically about 3 weeks, with 5 to 7 days of estrus and approximately 2 weeks of diestrus. When a mare lives within the natural social structure of the horse, i.e. a family band with several adult mares and one or more stallions, estrus is characterized by repeatedly approaching the stallion, frequent urination, deviating the tail away from the perineum, and standing still with the hind limbs spread apart. Diestrus is characterized by avoidance of an approaching stallion, and aggression toward the stallion, such as squealing, striking, and kicking, if he persists in attempting to court the diestrus mare. However, mares and stallions with long-term social relationships will often rest together, graze together and groom each other, all without sexual interactions. Hormonally, estrous behavior in the mare is initiated by estradiol that is secreted by the follicle, while estrous behavior is suppressed by progesterone, secreted by the corpus luteum. Mares are unusual among the ungulates in that they periodically exhibit estrous behavior during the anovulatory period. This is probably due to the release of estrogenic steroids secreted by the adrenal cortex. The display of sexual behavior by the mare throughout the year is thought to facilitate maintenance of the horse's social structure, in which the male remains with a group of females year round, in contrast with most ungulates in which the females and males only come together during the mating season.

Effect of breed and age on sexual behaviour of rams

The objective of this study was to highlight the problems that arise during the reproduction between thin-tailed rams and fat-tailed ewes. At the same time, particular emphasis laid on the influence of sheep breed, sheep age, time after ram introduction and day of the ewe estrus cycle on ram and ewe sexual behaviour. Rams were subjected to sexual performance tests by being individually exposed to 12 ewes for 3 h daily, 19 consecutive days. The 16 rams of the experiment were separated according to their age (9 and 21 months old) and breed (Chios and Karagouniki), and the 96 ewes of Chios fat-tailed breed, were divided by age (9 and 21 months old). The main characteristics of courtship behaviour, like sniffing, nudging, flehmen response and following were recorded and studied in detail. Mature Chios rams, which were the only one with previous experience of Chios ewes, exhibited higher rates of sexual interest per ewe than the other rams (P < 0.05). On the other hand, rams sniffed and nudged more young than mature ewes (P < 0.05), probably due to the fact that young ewes did not express intense symptoms of estrus. Young rams exhibited substandard sexual interest towards mature ewes, when they first came in contact with them (P < 0.05). In general, Karagouniki thin-tailed rams exhibited reduced rates of mating behaviour when they courted with Chios fat-tailed ewes in comparison with Chios rams (P < 0.05). Moreover, as the time after ram introduction passed, the frequency and duration of sexual behaviour components decreased (P < 0.001). Finally, the effect of the day of the experiment was only significant in the case of sniffing, which increased during the first 2 days and then declined and stabilized (P < 0.01). As it was demonstrated, ram age and ram breed played a fundamental role in the exhibition of sexual interest elements.

Morphological characteristics of the vomeronasal organ of the newborn Asian elephant (Elephas maximus)

The 6-week-old Asian elephant (Elephas maximus) has a well-documented precocious flehmen response to pheromones, suggesting that the pheromone-detecting vomeronasal organ (VNO) is functional very early in the life of this species. To further document this, the VNOs of two newborn elephants were examined in situ and analyzed by light microscopy (LM) to ascertain their structural maturity at birth. A tubular, cartilage-encased VNO was located along the anterior base of each side of the nasal septum. Its rostral end was connected to a duct to the roof of the mouth; the caudal end was attached to a well-defined vomeronasal nerve projecting toward the brain. LM revealed distinctive differences in the mucosae bordering the horseshoe-shaped lumen: a concave, sensory mucosa, and a convex, nonsensory mucosa. Small groups of receptor neurons were observed among ciliated columnar cells in the sensory epithelium. Numerous unmyelinated nerve bundles and blood vessels filled the underlying lamina propria (LP) and a small section of the vomeronasal nerve was conspicuous at one edge. The nonsensory mucosa manifested a thinner epithelium that principally consisted of ciliated columnar cells, some of which showed a granular cytoplasm, and a conspicuous row of basal cells. The LP was replete with acinar glands and ducts that opened into the lumen. This study shows that the VNO of the newborn elephant has reached an advanced stage of structural maturity, closely resembling that of the adult. Its composition supports the view that flehmen at 6 weeks delivers pheromones to a functional VNO.

Variations of equine urinary volatile compounds during the oestrous cycle

Equine urine was analysed by capillary gas chromatography. The volatile profiles from oestrous and dioestrous samples were compared to establish any qualitative or quantitative difference that may have potential value in olfactory communication. Forty-five different volatile compounds were detected. Of these, 17 major compounds were common to all chromatograms. The chemical profile of oestrous urine was distinguished by the presence of a unique peak that was not present in dioestrous samples. Numerous constituents exhibited endocrine dependence: while the concentrations of seix peaks increased at oestrus, the concentrations of another five peaks decreased at the same time. Since oestrous urine, but not dioestrous urine, has been shown to elicit sexual behaviour in the stallion, the unique peak, together with the peaks that were present in increased concentration at oestrus, may represent important chemical signals that stallions use to detect urinary 'oestrous odours'. Statistical analyses also indicated that the relative ratios (normalized peak areas) of many peaks changed significantly across the oestrous cycle: the rations increased in nine peaks, decreased in six peaks, and remained constant in two peaks at oestrus.