Reproductive behavior and libido of Nordestina breed stallions in an equatorial climateABSTRACT
The objective of this study was to evaluate the reproductive behavior of stallions of the Nordestina breed in an equatorial climate. Eight stallions aged 7 to 19 years, with reproductive experience in natural breeding, were used. Body weight (BW) was measured with the aid of a tape measure and the body condition score (BCS) was evaluated on a scale of 1 to 9. The live weight, body condition and libido of the animals did not show significant differences (P>0.05) between the dry and rainy seasons. The animals were under the same feeding system and presented libido above 4. The average reaction time of the Nordestino stallions was 25.43 seconds, and the older and younger stallions presented different reaction times of 16 and 30 seconds, respectively. The total mounting and ejaculation time was on average 62.05 seconds. Vocalization, Flehmen reflex and smelling are behavioral characteristics most used by stallions at the time of copulation. Nordestino stallions present sexual behavior similar to other equine breeds, however some pre-copulatory responses such as smelling and nibbling, kicking and vocalizing were more evident when compared to other breeds, which may indicate high libido and herd dominance behavior.
Keywords:
equine; flehmen reflex; native breed; vocalization
RESUMO
O objetivo deste estudo foi avaliar o comportamento reprodutivo de garanhões da raça Nordestina em um clima equatorial. Foram utilizados oito garanhões, com idades entre sete e 19 anos, com experiência reprodutiva em criação natural. O peso corporal (PC) foi medido com o auxílio de uma fita métrica, e o escore de condição corporal (ECC) foi avaliado em uma escala de 1 a 9. O peso vivo, a condição corporal e a libido dos animais não apresentaram diferenças significativas (P>0,05) entre as estações seca e chuvosa. Os animais estavam sob o mesmo sistema de alimentação e apresentaram libido acima de 4. O tempo médio de reação dos garanhões Nordestinos foi de 25,43 segundos; os garanhões mais velhos e os mais jovens apresentaram tempos de reação diferentes, de 16 e 30 segundos, respectivamente. O tempo total de montaria e ejaculação foi, em média, de 62,05 segundos. A vocalização, o reflexo de Flehmen e o cheiro são as características comportamentais mais usadas pelos garanhões no momento da cópula. Os garanhões Nordestinos apresentam comportamento sexual semelhante ao de outras raças equinas, porém algumas respostas pré-copulatórias, como cheirar e mordiscar, chutar e vocalizar, foram mais evidentes quando comparadas a outras raças, o que pode indicar alta libido e comportamento de dominância de rebanho.
Palavras-chave:
equino; reflexo de Flehmen; raça nativa; vocalização
INTRODUCTION
Horses were subject to the control of nature for around eight million years before they were domesticated (Waran, 2022). During this time, natural selection took place, in which some individuals showed better adaptation in maintaining life and reproduction than others, depending on their genes (Górecka-Bruzda et al., 2022). Natural and artificial selections have altered allele frequencies among animal populations, and, over the centuries, horse breeds have been selected mainly based on characteristics related to appearance and performance (Górecka-Bruzda et al., 2022), with populations shaped by natural selection, developing adaptive characteristics for specific ecological niches.
Ethology is important in natural selection, since the physical behavioral characteristics of a living being (phenotype) must be distinguished from the genetic makeup (genotype). However, the evolutionary process produces changes if the phenotype and genotype are involved in adaptations or differences in traits, which increases the adaptation of the carrier (Waran, 2022). Considering that sexual behavior is one of the stages of equine reproduction associated with social organization, courtship and copulation. The manifestation of these phases, as well as their intensity, can be affected by genetic, environmental and physiological factors and the type of management (Silva et al., 2015).
In addition, human interference in the breeding process can have negative implications for the well-being of stallions and mares, often related to the inability to allow animals to exhibit normal social and reproductive behavior (Gee, 2022). It is estimated that 25% of stallions go through some kind of experience that leads to problems in their sexual behavior, limiting their fertility (McDonnel, 1992). Stallions undergoing directed mounting have their libido and fertility diminished when they are mounted more frequently than once or twice a day. The resulting libido score can be used to predict the reproductive efficiency of stallions to select them in the breeding season (McKinnon et al., 2011). Other studies report that stallions freely covering mares every two hours during the day and night maintain their fertility (Bistrol, 1982; Henry et al., 1991; Steinjõrsson and Kristjansson, 1999; McDonnell, 2000).
The role of the stallion, his sexual behavior, as well as his interaction with the female and the environment in which they live are important points to be evaluated. However, there are few studies on the reproductive capacity of native breeds, such as sexual behavior (Souza and Costa, 2018; Lima et al., 2019; Rua et al., 2020). It is of great importance to study the reproductive behavior of horses from tropical regions to try to understand how they can adapt to potentially adverse environmental conditions, and it is also essential to develop studies so that the characteristics of native equine breeds are not lost over time. The Nordestina breed has been at risk of extinction for years, mainly due to frequent crossbreeding with other breeds (Ribeiro et al., 2024). Our hypothesis is that native animals have modified their reproductive characteristics due to their resistance, adaptability and management in semi-arid regions. The aim of this research was to evaluate the reproductive behavior of Nordestina breed stallions in the semi-arid region.
MATERIAL AND METHODS
The study was carried out at the Instituto Nacional do Semiárido - INSA, Campina Grande, Paraíba, Brazil, located at 7°16'S and 35°57'W. The region is characterized by a negative hydric balance which is resultant from annual average rainfall lower to 800 mm, insolation of 2800 h/year, year average temperature ranges from 23◦C to 287 ◦C, evaporation of 2000 mm/year and air relative humidity around 60% (Gois et al., 2017).
This study was conducted in accordance with the ethical principles of animal experimentation and under the approval of the Animal Ethics Committee of the Instituto Nacional do Semiárido (protocol number 0002/2022), Brazil.
Reproductive health and other assessments were carried out before the stallion was introduced to reproductive behavior and semen collection. The samples were collected in the dry season: in August - September 2023 (with 30 days difference between the two collections) and in the rainy season: May - June 2024 (with 30 days difference between the two collections). During the months of collection, the average temperature in summer was 28°C and in winter 26°C.
We used 8 stallions with reproductive experience in natural mating, aged between 7 and 19 years, with an average weight of 10.66±5.65. Body weight (BW) was measured using a tape measure and body condition score (BCS) was assessed on a scale of 1 to 9 to Henneke (1983), with stallions presenting scores ranging from 5 to 6. The body condition score was evaluated by a single evaluator.
Four observations were carried out on each stallion, totaling 32 observations, kept since birth in paddocks on the property, with ad libitum access to native pasture, concentrates in the trough and water, and remained in these conditions throughout the experimental period. All stallions evaluated received the same dietary management, thus not influencing the reproductive response
The stallions' sexual behavior and libido were measured by the same veterinarian. All stallions were observed and their behavior recorded for future libido analysis (Table 1). Reproductive behaviors were observed according to Rua et al. (2020) and Houssou et al. (2020) in the morning and recorded sequentially using a stopwatch.
The stallions were placed in front of a properly restrained female (to avoid accidents, bruises and kicks) once a day for 30 minutes, or until copulation with ejaculation occurred, in two seasons of behavioral observation in a 20 x 20 m corral. The females used during the behavior evaluation period were in heat and accepting the mount.
The other males remained outside the visual, olfactory or auditory field of the sexual courtship site, in an attempt to mitigate the effects of previous sexual stimulation. Two observers were used, always working in pairs and positioned outside the animals' field of action, to avoid any interference, where one made the notes on the ethogram and the other recorded the 30-minute period. The male was the focus throughout the observation period.
The experiment was conducted in a 2x2 factorial scheme with repeated measures, aiming to evaluate the reproductive behavior of stallions of the Nordestina breed. Two seasons of the year (dry and rainy) and two collection cycles (one and two) were considered. Repeated measures within the same subjects (n = 8, in both seasons and in both cycles) were included, allowing the comparison of the reproductive behavior of stallions in different seasons of the year in two collection cycles, totaling four measures per animal. This design allowed the analysis of the main effects of each factor, as well as the interactions between them, with each horse serving as its own control.
Statistical analysis was conducted using ANOVA for repeated measures, with the appropriate adjustments to compare variables within the subjects. The data were analyzed and showed no significant interaction between season and cycle, so they were analyzed considering only the mean for season. The behavior data were tabulated, and descriptive statistics were performed with the UNIVARIATE procedure and FREQ procedure in the SAS software (Statistical Analysis System, version 9.4). Spearman correlations were calculated by the CORR SPEARMAN procedure of SAS. Age, libido and weight data were analyzed using Student's test at a 5% probability level, considering the season (rainy and dry).
RESULTS
The reduced number of stallions is since we have a breed in danger of extinction where more than 50% of males are castrated (Ribeiro et al., 2024) and there are no studies on their reproductive behavior.
The live weight, body condition and libido of the animals showed no significant difference (P>0.05) between the dry and rainy periods (Table 2). The animals were under the same feeding system and had a libido above 4. The body score and libido of the animals’ change depending on the season, showing that the animals were under appropriate food management.
The average reaction time of the Nordestino stallions was 25.43 seconds, but the older and younger stallions had different reaction times of 16 and 30 seconds, respectively (Table 3). Total mounting and ejaculation time averaged 62.05 seconds. Other behavioral variables observed that showed high values were vocalization, Flehmen's reflex and smelling (Table 3).
Young animals spend more time sniffing the female's flank and vulva and using the flehmen reflex, while the older stallion spends more time vocalizing with the aim of stimulating the female. When approaching the female, the older stallion sniffs her female's vagina, vocalizes, externalizes the penis and mounts.
Older animals showed a higher frequency of vocalization, as well as a lower number of mounts without erection. The stallions showed low values for kicking, biting and licking. Flehmen's reflex showed a high positive correlation with sniffing, biting, RT and TME, i.e. the more he sniffed and vocalized, the faster the RT and TME (Table 4). Even though biting and kicking were present, they didn't correlate with any of the other behavioral variables, showing that they are calm animals. The higher the stallions' libido, the faster the RT and TME, and the less time the stallion spent courting the female with vocalizing, sniffing and Flehmen reflex behaviors.
Mean and standard deviation (SD) for the reproductive behavior events analyzed in Nordestina stallions
DISCUSSION
The stallions used in the research had a body score ranging from 5 to 6, which is ideal for the breeding period. Nutrition plays an important role in the reproductive performance of various species (Mantovani and Bauloni, 2011), and a minimum level of body condition is necessary to ensure adequate reproductive activity in stallions.
Rua et al. (2020) evaluated the libido of the Pantaneiro horse and observed that the animals had a libido of 4 to 5 and showed no difference between the seasons. Stallions' libido is constant throughout the year, with sexual desire peaking in the spring and gradually decreasing (but not disappearing completely) in the fall and winter (Sharp, 1999). During the breeding season, horses use various sensory cues to assess the reproductive state of their sexual partner, such as auditory, visual or olfactory cues (Stahlbaum and Houpt, 1989; Crowell-Davis, 2007).
Even though the Northeastern stallions used in the research were not subjected to managed reproduction, they had a low reaction time (25.43 seconds) when compared to the Pantaneiro. Rua et al. (2020) observed a reaction time of 78.1 seconds in Pantaneiro stallions. The Mangalarga Marchador's reaction time of 11.4 seconds is low, and this is justified by the fact that these animals are used for breeding all year round (Rua et al., 2015), with a high frequency of semen collection, making them more conditioned to reproduction (Sieme et al., 2004). The mounting and ejaculation time of the Nordestino stallion was considered low when compared to the Pantaneiro, which may have been because we have high temperatures and low relative humidity, which does not cause discomfort to the animals (Ribeiro et al., 2024). Compared to the Mangalarga Marchado (Rua et al., 2015), the value was similar, which is interesting, because Northeastern stallions don't have intensive management with regard to semen collection, but if necessary, they respond well to this type of management.
The native Brazilian breeds come from different ecosystems and generally show some different reproductive behaviors. For example, the Northeastern horse has a higher frequency of the flehman reflex and vocalization, which may be associated with the adaptation process of each breed. The stallion's pre-copulatory behavior includes smelling (Fig. 1), licking, nibbling or biting the head, scapular and axillary regions (Fig. 2), belly, flank and inguinal and perineal areas, in that order (McDonnell, 1992). Contact with urine, feces or vaginal fluids is usually followed by the flehmen reflex (Fig. 3).
The stallion usually “strokes” or nibbles the mare's flank and hocks, and she may turn her head back to allow muzzle-to-muzzle contact before mounting (McDonnell, 2016). If the mare urinates in the mounting area before copulation, this behavior increases the flehmen reflex response. When expressing the flehmen reflex response, the horse extends its neck, raises its nose, opens its mouth slightly and raises its upper lip (Nielsen et al., 2015). During the expression of this behavior, the air outlet is reduced, which results in an increase in air pressure within the nasal cavity, allowing the horse to analyze low-volatile compounds more accurately (Nielsen et al., 2015; Lima et al., 2023).
Some particularities were observed in the group of older stallions of the Nordestina breed: one of the 19-year-old stallions showed a pre-copulatory period with a lot of vocalizations, but this stallion did not show a flehmen reflex in any of the observations.
Another 16-year-old stallion showed pre-copulatory behavior of biting the mare, a behavior only seen in this animal, and he also showed the other behaviors such as vocalizing, smelling and kicking. During restraint, the stallion may have felt intimidated by the halter and artificial vagina, as well as by the approach of the veterinarian, which altered his reproductive behavior. The stallion would climb onto the mare and when the veterinarian approached, he would descend to the point of relaxing his penis and showing no interest; in the second observation, he showed faster responses to sexual behavior.
On the other hand, three young stallions (brothers) who were raised together with their sister did not show any reproductive behavior when we used their sister in heat as a mannequin for semen collection (Figure 4). Considering the father of the stallions and the mare, the semen was collected on the same day, and he showed all the pre-copulatory and copulatory sexual behavior, and we were able to collect semen from him using his daughter as a dummy, in which case the mare was raised separately from the father (Figure 5). We then changed the female, used a mare from outside the herd who was in estrus and the three stallion brothers showed all the pre-copulatory and copulatory behaviors, so we were able to collect semen from them (Fig. 6).
In the third behavioral analysis we used the mother of the three sibling stallions, even though they had been separated from her at weaning, one of the three stallions did not show reproductive behavior, and we were unable to collect semen from him using the mother as a mannequin.
Body odors in vertebrates are composed of a variety of substances, including fatty acids, steroids, nitrogen compounds and pheromones. Pheromones are chemical agents released into the environment, influencing the behavior of other members of the same species (Setchell et al., 2011). Odors emitted by vertebrates often contain information about the major histocompatibility complex (MHC), a set of genes responsible for encoding proteins involved in both the immune system and social and reproductive signaling in horses (Viluma et al., 2017; Tang et al., ., 2023; Vasoya et al., 2023).
Burger et al. (2015) demonstrated the ability of stallions to discern between the odors of females with different MHC types, adjusting their reproductive strategies. In addition, mares also demonstrated the ability to distinguish MHC type. Such findings indicate that both males and females tend to form social bonds with individuals that share similar MHC, while showing a preference for mating with those that have MHC different from their own (Burger et al., 2015).
A 16-year-old stallion mounted the mare from the side (Fig. 7), which we relate to his fear of the handling he was being given. When he got on the female, the equine arranged himself in the correct position, but when the veterinarian approached, he got off the female. The stallion is able to mount when he prances, but this can be accompanied by a side mount with a subsequent adjustment to the pranced position. The latter procedure is more common in young or inexperienced stallions (Souza et al., 2009).
Stallions of the Nordestina breed, brothers of the mannequin mare, showed no sexual behavior.
After mounting, the male embraces the iliac tuberosities with his forelegs and with his head contains the mare by holding or biting her mane (Fig. 8). It is an expected reproductive behavior for both experienced and inexperienced stallions to mount first without an erection. When the stallion is sure that the mare will stand for breeding, he mounts with an erection by placing his forelegs in front of the mare's hips. In erection riding, the penis is exposed and gradually becomes rigid. This type of riding usually leads to insertion and ejaculation. Insertion occurs after one or more seeking acts and ejaculation is achieved after several deep intravaginal thrusts.
The eight Nordestino stallions in the second and third collection cycles achieved ejaculation in a single mount, but in the first cycle only 2 experienced stallions achieved ejaculation in one mount and the rest did not. For copulation behavior, according to McDonnell (1992), most stallions achieve erection within one to two minutes and mount the female within one minute of erection. More than 70% reach ejaculation in a single mount, and more than 90% in one or two mounts. Approximately two thirds of stallions ejaculate after five to eight deep intravaginal thrusts. Most stallions are remarkably consistent over time in their pattern of pre-copulatory and copulatory behavior.
Knowledge of the sexual behavior of stallions is fundamental so that correct management measures can be adopted to help achieve good reproductive efficiency (Zúccari et al., 2016), and so that we can understand and manage them to facilitate semen collection and the organization of a germplasm bank.
CONCLUSIONS
Nordestino stallions show similar sexual behavior to other equine breeds, but some pre-copulatory responses such as smelling and nibbling, kicking and vocalizing were more evident when compared to other breeds, which may indicate high libido and herd dominance behavior, since most of the stallions studied are bred in an extensive system with the presence of other uncastrated males in the herd. Further research is needed into the effect, recognition and role of the MHC, which is important in establishing kinship and selecting sexual partners.
ACKNOWLEDGEMENTS
The authors wish to thank the Brazilian National Council for Scientific and Technological Development (CNPq)
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ZÚCCARI, C.E.S.N.; JULIANO, R.S.; SERENO, J.R.B. Aspectos reprodutivos de garanhões e de éguas. 2016. cap.14, p.348-371. Disponível em: https://www.researchgate.net/ publication/303804104 Acessado em: 16 jan. 2024.
» https://www.researchgate.net/ publication/303804104
Publication Dates
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Publication in this collection
14 July 2025 -
Date of issue
Jul-Aug 2025
History
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Received
13 Nov 2024 -
Accepted
07 Jan 2025
Source: personal archive (2023).
Source: personal archive (2023).
Source: personal archive (2023).
Source: personal archive (2023).
Source: Personal archive (2023).
Source: personal archive (2023).
Source: personal archive (2023).
Source: personal archive (2023).