Influence of prostaglandin F2 alpha on sperm production and seminal characteristics of the stallion

Six mature stallions were used to test the effect of prostaglandin F2 alpha (PGF2 alpha ) on sperm production and seminal characteristics. Semen was collected from each stallion twice weekly 1 hr following a 10 mg intramuscular injection of PGF2 alpha or a sham injection. A switchback design was used so that three stallions received PGF2 alpha and three served as controls during the first 9 weeks (period 1). Treatment regimens were reversed during the second 9 weeks (period 2). Treatment of stallions with PGF2 alpha resulted in an increase (P less than .05) in gel free seminal volume and a decrease in sperm cell concentration. Total spermatozoa, sperm cell motility, and percentage of primary and secondary sperm abnormalities of ejaculates were not significantly affected by treatment of stallions with PGF2 alpha before semen collection. All treated stallions exhibited a pronounced sweating response to the drug. During the experiment, two of the six stallions masturbated within 20 to 30 minutes after PGF2 alpha treatment without achieving an erection.     

Influence of prostaglandin F2α on sperm production and seminal characteriticss of the stallion

Six mature stallions were used to test the effect of prostaglandin F_2α (PGF_2α) on sperm production and seminal characteristics. Semen was collected from each stallion twice weekly 1 hr following a 10 mg intramuscular injection of PGF_2α or a sham injection. A switchback design was used so that three stallions received PGF₂ and three served as controls during the first 9 weeks (period 1). Threatment regimens were reversed during the second 9 weeks (period 2). Treatment of stallions with PGF_2α resulted in an increase (/prmP<.05) in gel free seminla voume and a decrease in sperm cell concentration. Total spermatozoa, sperm cell motility, and percentage of primary and secondary sperm abnormalities or ejaculates were not significantly affected by treatment of stallions with PGF_2α before semen collection. All treated stallions exhibited a pronounced sweating response to the drug. During the experiment, two of the six stallions masturbated within 20 to 30 minutes after PGF_2α treatment without achieving an erection.     

Cholesterol-to-phospholipid ratio in whole sperm and seminal plasma from fertile stallions and stallions with unexplained subfertility

Semen samples were collected from six fertile stallions and seven stallions with unexplained infertility. Percentages of motile sperm (77.5 +/- 11.3 versus 67.5 +/- 12.2, P = 0.2), and progressively motile sperm (70.8 +/- 13.6 versus 60.7 +/- 14.0, P = 0.2) were similar between fertile and subfertile stallions, respectively. Morphologic characteristics in ejaculates of control and affected stallions (% normal: 60.2 +/- 18.2 versus 52.9 +/- 11.3, P = 0.4; % abnormal heads 7.3 +/- 4.8 versus 12.1 +/- 5.0, P = 0.11; and % abnormal acrosomes 1.6 +/- 2.1 versus 3.0 +/- 3.4, P = 0.4) did not differ. After incubation with the calcium ionophore A23187, acrosome reaction rate of sperm from fertile stallions was 96 +/- 2.8% whereas only 2.9 +/- 2.5% of sperm from stallions with unexplained subfertility had acrosome reacted (P < 0.001). Molar amounts of cholesterol and phospholipid in whole sperm and seminal plasma did not differ (P > 0.1) between fertile and subfertile stallions. However, the molar ratio of cholesterol-to-phospholipid was 2.5 times greater in the seminal plasma (P = 0.09) and 1.9 times greater (P = 0.009) in whole sperm of subfertile stallions compared to fertile stallions.     

Sexual behavior and serum concentrations of reproductive hormones in normal stallions

Recently, it has been reported that impotence in the stallion has a physiological basis that involves decreased serum concentrations of luteinizing hormone (LH) and estradiol-17beta, but not testosterone. We have found such a hormonal profile in two of nine stallions studied during an ongoing investigation of the endocrinology of the normal stallion. Nevertheless, both of these stallions possessed vigorous libido and normal seminal characteristics. We conclude that the hormonal profile of low LH, low estradiol and normal testosterone, although it may accompany impotence in the stallion, is not predictive of, or causally related to, abnormalities in sexual behavior.     

Reproductive parameters of miniature stallions

Breeding soundness evaluation (BSE) of stallions is a routine component of stud farm practice. Guidelines for assessing satisfactory breeding potential have been developed using data derived from stallions of full-size breeds. In view of the increasing popularity of miniature stallions, knowledge of normal semen parameters of these stallions is important. Therefore, testicular measurements and semen parameters from 216 sexually rested miniature stallions were obtained. Semen was collected twice, 1.5 to 3 h apart, using an artificial vagina. Values were averaged over the 2 collections because of the sexual inexperience of the stallions. The smaller stallions (Group A, 72 to 86 cm; Group B, 87 to 96 cm) had smaller testicles (P<0.05), and Group A stallions had the lowest ejaculate volume (P<0.05) compared with Group C (97 to 104 cm) stallions. Thus, although there was no difference in the concentration of spermatozoa per milliliter between groups of stallions, Group A stallions had fewer total spermatozoa in their ejaculate than Group C stallions (4.31+/-0.47x10(9) vs. 5.41+/-0.30x10(9), P<0.05). Moreover, miniature stallions had smaller testicles and fewer total spermatozoa in their ejaculate than is commonly accepted as normal in full-size stallions. Average total scrotal width of miniature stallions was found to be 7.13, 7.38 and 7.95 cm for Groups A, B and C, respectively. The average total number of spermatozoa in the ejaculates of miniature stallions in this study was 4.94+/-0.22x10(9) cells, with 1.75+/-0.09x10(9) total normal, motile spermatozoa. When only stallions <96.5 cm in height were considered (conforming to requirements of the American Miniature Horse Association Registry), the average total number of spermatozoa in the ejaculates was 4.59+/-0.30x10(9) cells, with 1.70+/-0.11x 10(9) total normal, motile spermatozoa. Based on these findings, different criteria should be used to evaluate the potential breeding soundness of miniature stallions than are commonly applied to full-size stallions.     

Sperm morphology in Estonian and Tori Breed Stallions

The standard procedure for assessing the breeding potential of a stallion includes the parameter total number of spermatozoa classified as morphologically normal. This study investigated sperm morphology of fresh semen in randomly chosen Estonian (E, n = 8) and Tori (T, n = 7) breed stallions with proven fertility. Two ejaculates were examined from each stallion. An aliquot from each ejaculate was fixed in 1 mL formol-saline immediately after collection and examined with phase-contrast microscope at a magnification 1000× for all types of morphological abnormalities. Furthermore smears were prepared and stained according to Williams (carbolfuchsin-eosin) for a more detailed examination of the sperm heads with light microscope at a magnification 1000×. Analysis of variance was applied to the data, and results are presented as LSmeans (± SE). One T stallion that had a disturbance in the spermatogenesis and one 22-year-old E stallion were not included in the analyses. The T stallions had on average 57.5 ± 4.1% and the E-stallions 74.4 ± 3.8% morphologically normal spermatozoa (p = 0.012). In 4 of 7 T stallions and 7 of 8 E stallions both ejaculates had >50% morphologically normal spermatozoa. There was a significant difference between breeds in mean percentage of proximal droplets (17.3 ± 2.7% and 2.9 ± 2.5% for T and E stallions, respectively; p = 0.003).     

The influence of age and breed on stallion semen

This study reports on the variation in semen quality and in spermatozoal and behavioral characteristics of 168 stallions representing 9 breeds and ranging in age from 2 to 26 yr. Semen samples were collected into an artificial vagina and the number of mounts and urethral pulsations per semen sample were recorded. Semen characteristics were examined for total volume, gel-free volume, gel volume, color score, mass activity, nonmotile spermatozoa, dead spermatozoa, semen density, spermatozoa concentration, total number of spermatozoa and semen pH. Morphological characteristics of the spermatozoa included abnormal heads, abnormal mid-pieces, abaxial mid-pieces, protoplasmic droplets and abnormal tails. Sources of variation were evaluated and the overall means calculated by least-squares analyses of variance for nonorthogonal data. The significance of breed effects and between stallion variability were estimated using mixed-model procedures. All semen characteristics with the exception of color and urethral pulsations had significant variation due to age. Semen quality (gel-free volume, sperm concentration, total sperm numbers and sperm abnormalities) was poorest in stallions under 3 yr of age and over 11 yr. Significant breed variation was apparent in most characteristics except for pH, semen color, abnormal midpieces and urethral pulsations. It is recommended that both the age and breed of stallion be taken into consideration when evaluating stallion semen.     

Effect of controlled exercise on semen characteristics in two-year-old stallions

A two-phase study was conducted to evaluate the effect of controlled daily exercise on semen characteristics of 2-yr-old stallions. Eight nonexercised, sexually inexperienced 2-year-old Morgan stallions were randomly assigned to one of two groups. In Phase 1, four stallions were randomly assigned to receive daily controlled exercise for 16 wk. In Phase 2, these stallions were confined to box stalls for 16 wk. The remaining four stallions were confined to box stalls for 16 wk during Phase 1 and received daily controlled exercise for 16 wk during Phase 2. Individual semen collections were obtained from all stallions every 14 days. Daily sperm output (DSO) was determined by daily semen collections for seven consecutive days on Wk 0, 8, 16, 24, and 32. Mean DSO (10(9)) in Phase 1 was 3.52+/-.62 and 2.79+/-.75 for Wk O and 3.12+/-.39 and 2.61+/-.68 for Wk 16 for exercised and nonexercised stallions, respectively. Mean DSO (10(9)) in Phase 2 was 2.61+/-.68 and 3.12+/-.39 for Wk 16 and 2.48+/-.69 and 3.95+/-.51 for Wk 32 for exercised and nonexercised stallions, respectively. No effect (P>.05) of treatment was observed in either phase when parameters of spermatozoa concentration, total spermatozoa per ejaculate, DSO, total volume, gel-free volume, gel volume, percentage motility, percentage live, pH, and percentage normal were measured and analyzed.     

Zona pellucida binding and zona-induced acrosome reactions in horse spermatozoa: comparisons between fertile and subfertile stallions

Diagnostic tests that probe sperm function are needed to determine the potential etiologies of subfertility and to explore treatments of subfertility in stallions. Using epifluorescence and phase contrast microscopy, a comparison was made between ejaculates from 3 fertile and 3 subfertile stallions in which sperm-zona pellucida binding and acrosomal status were measured. Motile spermatozoa were selected by Percoll gradient centrifugation and were capacitated in vitro using TEST:TALP capacitation medium at 39 degrees C under humidified air containing 5% CO2. Concentration of motile spermatozoa was held constant during co-incubation with oocytes for fertile and subfertile ejaculates. The total number of zona pellucida-bound spermatozoa was higher for fertile stallions than for subfertile stallions (P < 0.05). Similarly, the percentage of acrosome reactions in zona pellucida-bound spermatozoa was higher for the 3 fertile stallions than for the 3 subfertile stallions (P < 0.05). These results indicate that spermatozoa from fertile stallions may interact with female gametes differently from that of subfertile stallions and suggest that sperm functions are measurable and may vary with fertility.     

Seminal traits, suitability for semen preservation and fertility in the native Portuguese horse breeds Puro Sangue Lusitano and Sorraia: Implications for stallion classification and assisted reproduction

The Puro Sangue Lusitano (PSL) is the major national breed of horse in Portugal, but no studies exist on its seminal characteristics, or on the possibility of conserving semen for future use. The aim of this study was to evaluate semen parameters, fertility and the aptness to semen preservation in Lusitano Stallions. In order to compare characteristics defined by a single or by multiple semen collections per stallion 152 ejaculates obtained from 152 Lusitano stallions presented at an annual breeding soundness examination as well as data related to 371 ejaculates obtained from 9 PSL were analyzed. These latter samples were also evaluated in terms of their possible use in assisted reproduction and were compared with 113 ejaculates obtained from 4 Sorraia horses, a rare and endangered Portuguese breed. The percentage of motile spermatozoa (PMS) was assessed after collection (AC), after semen dilution (AD) and at 24h of cool-storage. Mean values obtained for sperm motility and morphology and semen pH observed after semen collection differ significantly (P<0.05) between single collection/multiple stallions and multiple collections/limited stallions, and no age related effects were detected. Overall, Lusitano semen quality was comparable to that of related breeds, while Sorraia stallions had very poor semen quality. The response to cool-storage of diluted semen samples differed among stallions and breeds, and the best results for progressive motile sperm cells at 24h were in a range of 35-53% for PSL stallions and were lower for Sorraia stallions. Fertility rates obtained with artificial insemination (AI) averaged at 85% for PSL. With the exception of PMS AC, sperm vitality and semen pH no other seminal trait seemed to influence fertility rates in the Lusitano breed.     

Cytogenetic analysis of meiotic cells obtained from stallion testes

A normal course of meiosis and the associated course of spermatogenesis in males are very significant from the viewpoint of animal breeding, in particular animal reproduction. This takes on special significance when studying late-maturing animals such as horses. The aim of the study was to analyse meiotic cells, with particular consideration of synaptonemal complexes obtained from the testes of young stallions and cryptorchids, based on observations of the X-Y bivalent. The analysis was performed in successive stages of meiotic division using the FISH technique. The greatest diversity and most advanced meiotic stages were observed in the normal testis of a unilateral cryptorchid. No abnormalities were observed that could have caused cryptorchidism in the analysed horses.     

Testicular measurements and reproductive characteristics in stallions.

Factors affecting testicular measurements in situ and the relationships among the measurements and various reproductive characteristics were studied using data from 48 stallions. Mean values during the breeding season are provided for scrotal width, widths and lengths of individual testes, combined weight of testicular parenchyma, daily sperm production and daily sperm output. Testicular measurements were highly repeatable from day to day and for repeated measurements on a given day; technician provided the largest source of variation in the measurements of a given stallion. Age significantly affected all testicular measurements; testicular size for 2- to 3-year-old stallions did not differ (P greater than 0.05) from that for 4- to 6-year-olds, but was smaller (P less than 0.05) than testicular size of stallions greater than or equal to 7 years old. Scrotal width was correlated (P less than 0.01) with daily sperm production (r = 0.75) and daily sperm output (r = 0.55) and was generally the most repeatable measurement.     

A polymorphism within the equine CRISP3 gene is associated with stallion fertility in Hanoverian warmblood horses

Fertility of stallions is of high economic importance, especially for large breeding organisations and studs. Breeding schemes with respect to fertility traits and selection of stallions at an early stage may be improved by including molecular genetic markers associated with traits. The genes coding for equine cysteine-rich secretory proteins (CRISPs) are promising candidate genes because previous studies have shown that CRISPs play a role in the fertilising ability of male animals. We have previously characterised the three equine CRISP genes and identified a non-synonymous polymorphism in the CRISP1 gene. In this study, we report one non-synonymous polymorphism in the CRISP2 gene and four non-synonymous polymorphisms in the CRISP3 gene. All six CRISP polymorphisms were genotyped in 107 Hanoverian breeding stallions. Insemination records of stallions were used to analyse the association between CRISP polymorphisms and fertility traits. Three statistical models were used to evaluate the influence of single mutations, genotypes and haplotypes of the polymorphisms. The CRISP3 AJ459965:c.+622G>A SNP leading to the amino acid substitution E208K was significantly associated with the fertility of stallions. Stallions heterozygous for the CRISP3 c.+622G>A SNP had lower fertility than homozygous stallions (P = 0.0234). The pregnancy rate per cycle in these stallions was estimated to be approximately 7% lower than in stallions homozygous at this position.     

Aerobic Bacterial Flora of Semen and Stallion Reproductive Tract and its Relation to Fertility Under Field Conditions

This study was initiated in order to investigate the bacterial flora of the stallion genital tract by taking consecutive samples from normal stallions in regular use. The objective was to determine whether any growth of potential pathogens, particularly P. aeruginosa and K. pneumoniae, in fresh semen and urethra was associated with the presence of inflammatory cells in the semen and whether bacterial growth had any effect on sperm morphology and pregnancy results. Sixteen stallions, only used for A.I., housed at 3 different commercial stud farms, were used. A wide variety of microorganisms was found in almost all samples from fresh semen (total 115 samples). P. aeruginosa was isolated from 46/115 (40%) of the samples and from 12 of the 16 stallions. K. pneumoniae was isolated from the semen of one stallion. Samples taken from the distal urethra after ejaculation contained fewer microorganisms than samples from fresh semen. No bacteria were found in 51% of the extended semen samples. Most of the stallions had an acceptable sperm morphology, and very few of the ejaculates contained inflammatory cells. Pregnancy results among the stallions varied, but were acceptable for most of them. There was no correlation between the frequency of samples testing positive for P. aeruginosa in raw semen and pregnancy results. kw|Keywords|k]bacterial growth k]urethra k]Pseudomonas aeruginosa k]Klebsiella pneumoniae     

Effect of feeding a DHA-enriched nutriceutical on the quality of fresh, cooled and frozen stallion semen

Eight stallions were used in 2 x 2 crossover study to determine if feeding a nutriceutical rich in docosahexaenoic acid (DHA) would improve semen quality. Stallions were randomly assigned to one of two treatment groups (n = 4 per group). Stallions were fed their normal diet (control) or their normal diet top-dressed with 250 g of a DHA-enriched nutriceutical. Feeding trials lasted for 14 week, after which a 14-week washout period was allowed and the treatment groups were reversed for another 14 week feeding trial. Feeding the nutriceutical resulted in a three-fold increase in semen DHA levels and 50% increase in the ratio of DHA to DPA in semen. Sperm motion characteristics in fresh semen were unaffected by treatment. After 24 h of cooled semen storage in an Equitainer, total and progressive motility did not differ between treatment groups, but sperm from stallions fed the nutriceutical exhibited higher velocity and straighter projectory (P < 0.05). After 48 h of cooled storage, increases in the percentages of sperm exhibiting total motility (P = 0.07), progressive motility (P = 0.06) and rapid motility (P = 0.04), were observed when stallions were being fed the nutriceutical. For a subset of four stallions, whose progressive sperm motility was <40% after 24 h of cooled storage when fed the control diet, feeding the nutriceutical resulted in improvements in mean progressive motility of sperm after 24 h (P = 0.10) and 48 h (P = 0.03) of storage. Feeding the nutriceutical resulted in similar improvements in motion characteristics being observed in frozen-thawed semen. While it appears that feeding the nutriceutical may improve the motion characteristics of cool-stored stallion semen, it may be most beneficial for stallions of marginal fertility whose sperm do not tolerate the rigors of cooling and storage. The nutriceutical also appeared to improve the freezability of semen. More dramatic improvements in semen quality may be observed if modifications in the main fat content of the diet are incorporated with the DHA supplement.     

Collection of stallion semen without a mount

Semen collection was attempted from 18 stallions with an artificial vagina (AV) and without use of a mare or dummy mount. After the stallions were sexually excited by the presence of another horse (mares, and in some instances, geldings) an AV was placed over the erect penis. Thirteen of the 18 stallions manifested thrusting and ejaculation in at least one attempt at semen collection. Semen was collected from 12 of the stallions on the first attempt with all subsequent attempts at semen collection from 11 of these 12 stallions being successful. The ejaculate from one of the stallions was judged incomplete based upon spermatozoal concentration compared with the concentration of a subsequent ejaculate obtained with a mount.     

Effects of GnRH immunization in sexually mature pony stallions

Immunization against gonadotrophin releasing hormone (GnRH) was studied as an alternative for the commonly used surgical castration in stallions. Two GnRH vaccines comprising non-mineral oil adjuvants were evaluated for their potential to induce high antibody titers directed against GnRH and subsequent effects on reproductive characteristics. Twelve sexually mature male hemicastrated Shetland ponies were assigned to three groups. Group 1 and 2 were injected with 1mg peptide equivalent of G6k-GnRH-tandem-dimer conjugated to ovalbumin (OVA) in CoVaccine HT adjuvant (GnRH/CoVaccine) and in Carbopol (GnRH/Carbopol), respectively, and group 3 was injected with CoVaccine HT adjuvant without antigen (controls). After immunization no adverse effects were observed with respect to the injections sites or general health. Two weeks after the second vaccination antibody titers against GnRH increased rapidly in all animals of the GnRH/CoVaccine group, at the same time reducing serum testosterone levels maximally for the further duration of the experiment. In the GnRH/Carbopol group antibody responses and effects on testosterone levels were intermediate in two stallions and not apparent in the remaining stallions of this group. Semen evaluation showed that from 2 weeks after the second immunization onwards, sperm motility was affected in all stallions treated with GnRH/CoVaccine and one stallion treated with GnRH/Carbopol. Seven weeks after the second immunization, no semen could be collected from two stallions, one of each group, due to suppressed libido. Histological examination of the testes, 15 weeks after the initial immunization, demonstrated reduction in seminiferous tubuli diameters in all stallions of the GnRH/CoVaccine group and one stallion of the GnRH/Carbopol group. Furthermore, spermatogenesis was extremely disorganized in these stallions, as indicated by absence of the lumen in the seminiferous tubules, the absence of spermatozoa and spermatids in the tubular cross-sections and the impossibility to determine the stage of the tubular cross-sections. Testis size was also substantially reduced in three out of four stallions treated with GnRH/CoVaccine. The results demonstrate that two immunizations with G6k-GnRH-tandem-dimer-OVA conjugate in a suitable adjuvant such as CoVaccine HT caused a rapid and complete reduction of serum testosterone levels in sexually mature stallions, subsequently leading to reduced sperm motility and affected testis function, while no adverse reactions were observed after immunizations.     

Retrospective and statistical analysis of breeding management on the Italian Heavy Draught Horse breed

This study investigated some aspects of breeding management in the Italian Heavy Draught Horse breed, aiming at improving its efficiency at stud farm level. A first aim was to evaluate the risk of unsuccessful reproduction in mares after an early (3 years) or normal (4 years) age at first foaling, in interaction with different stud rearing systems. A second objective was the examination of the mean time length in which young 2-year-old stallions maintain a genetic superiority on older proven stallions, identifying a ‘genetic lifespan’ in which young stallions can be safely used for reducing the cost of services. Reproductive performance at first and second foaling of 1513 mares were used. Mares had a normal first foal at 3 (n = 745) or 4 years of age (n = 768) in stud farms on the basis of stable (n = 488), feral (n = 345) or semi-feral (n = 680) rearing systems. Logistic regression analysis was performed by modeling the risk of unsuccessful reproduction in the subsequent season (i.e., results at second foaling), as affected by the interaction of age at first foaling × rearing system (six classes). Genetic lifespan of young stallions was estimated by regressing the least square means from a mixed model analysis for repeated measures of individual differences in ‘total merit’ estimated breeding values (EBVs) between young stallions (mean no. of 45/year) and the mean EBV of all proven stallions in a given year of genetic evaluation (mean no. of 483/year). Young stallions born between 1999 and 2005 were used, following each generation (i.e., birth year) from 2 to 7 subsequent yearly genetic evaluations. In comparison with the best reproductive success of second foaling at 4 years in stable systems, the greatest risk of unsuccessful reproduction was at 3 years in feral (+167%) and 3 years in semi-feral conditions (+91%). Young stallions showed a 0.50 s.d. greater EBV at the first evaluation than proven stallions, with a mean annual decrease in EBV of 0.07 s.d./year on proven stallions. Optimal breeding management could be obtained in stud farms by limiting foaling at 3 years, particularly in feral and semi-feral rearing systems, and using young stallions for 3 to 4 years to maintain a perceptible selection differential with older proven stallions and to reduce cost of services. Later, the selection differential with proven stallions become less consistent and genetic improvement could be slowed down.     

In vivo fertilizing ability of stallion spermatozoa processed by single layer centrifugation with Androcoll-E™

A colloid with a species specific silane-coated, silica-based formulation, optimized for stallion (Androcoll-E™), enables a better sub-population of spermatozoa to be selected from stallion ejaculates. However, such a practice has not been critically evaluated in stallions with fertility problems. In this study we evaluate whether single-layer centrifugation (SLC) through Androcoll-E™ could be used to enhance fertility rates in a subfertile stallion. Ejaculates were obtained from two different stallions, one Lusitano (fertile) and one Sorraia (subfertile), with distinct sperm characteristics and fertility. Motility, morphology, plasma membrane structural (eosin-nigrosin) and functional integrity (HOS test), mitochondrial functionality (Δψm; JC-1) and longevity (motility after 72 h cooling) after centrifugation in Androcoll-E™, as well as pregnancy rates obtained after artificial insemination (AI), with and without (control group) SLC-treated sperm were assessed. The effect of SLC on sperm characteristics, and fertility results were evaluated by ANOVA and Fisher procedures, respectively. Our results showed that SLC-selected sperm did not differ from the raw semen in terms of viability, morphology, response to hypo-osmotic conditions (HOS test) and mitochondrial membrane potential (↑ΔΨmit; JC-1). Sperm motility in cooled samples was not improved by SLC treatment. Our data show that SLC through Androcoll-E™ has no effect on pregnancy rates in the stallions used in this trial.     

Dominance relationships and patterns of aggression in a bachelor group of Sorraia horses (Equus caballus)

The influence of individual factors on dominance rank and the relationship between rank distance and patterns of aggression predicted by models of evolutionarily stable strategies (ESS) of animal conflict were investigated in a managed bachelor group of Sorraia horses, Equus caballus. The group was composed of four to six stallions 3- to 12-years-old during the study period. The dominance hierarchy was significantly linear and rank was not related to age, weight, height or aggressiveness. Frequency and intensity of agonistic interactions were low, but higher-ranking stallions did not receive lower aggressiveness than lower-ranking stallions. There was some evidence that dominance relationships were more contested among close-ranking stallions, as predicted. Agonistic-related interactions among close-ranking stallions served similar functions to those among distant-ranking stallions, but the latter interacted more frequently than expected for access to resting sites and/or resting partners. Therefore, we found some evidence that agonistic-related interactions among distant-ranking stallions play a larger role in providing access to valuable and defendable resources than those among close-ranking stallions. Nevertheless, the fact that space to escape from aggression was limited and breeding access was independent from dominance rank may have reduced the benefits relative to costs of aggression and therefore limited the occurrence of contests over dominance and resources.