Visual and morphometric description of feral horse hooves from Sable Island National Park Reserve (2024)

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Abstract

Résumé

Introduction

A unique population of feral horses inhabits Sable Island National Park Reserve (SINPR). The crescent-shaped island (Figure 1) is composed of vegetative cover over sand. Located 290 km southeast of Halifax, Nova Scotia, Canada (43° 56′N, 59° 55′W), the island is ~40 km long, with a maximum width of 1.3 km, and a surface area of ~30 km (1,2). The Sable Island horses are descendants of horses deliberately placed on the island during the mid-to-late 1700s, some or all of which may have been horses that were confiscated from the French during the Acadian Expulsion in 1755 (3). Although the horses resemble light drafts and other breeds common to eastern Canada, herdlevel genetic diversity is comparable to that of other unique horse breeds within Canada (4). The population size is small and there has been an extended interval without introduction of new breeding stock. Adaptation has helped the Sable Island horses physically modify to the environmental conditions on Sable Island (5). The current population of ~550 horses exists as multiple, smaller bands of up to 10 horses each, with welldefined home ranges.

Sable Island horses are not managed at an individual or population level, with human interference prohibited by federal law since 1961 (3,4). The prohibition of human interference prevents any hoof trimming from being done. Hoof growth is continuous throughout the life of all horses, and feral horses experience natural abrasion on the ground surface of their hooves from the terrain. The growth of the hooves of the Sable Island horses is limited by health, environmental, and nutritional factors in the mostly sandy and grassy environment. Hampson et al (6) reported a high rate of hoof abnormalities in Australian feral horses and proposed that a lack of abrasive substrates in the environment contributed to decreased wear on the hooves. Feral horse hooves were once considered by many to be the standard of a normal and healthy hoof (7). However, based on current research, the feral hoof model is not as generally applicable to domestic horses due to the vast difference in lifestyles, and the assumption that feral horses innately have healthy, ideal feet may be false (6).

The conformation of the hooves of the Sable Island horses is of interest given their protected status and the environmental conditions of the island. To the authors’ knowledge, there have been no previous descriptions of the external hoof capsule of the Sable Island horse. Although a 2021 study evaluated the radiographic prevalence of laminitis from a subsample of the hooves studied herein, there was no evaluation of hoof conformation or morphometry (8). Therefore, a description of the abnormal hoof conformation of the Sable Island horse and comparison of these hoof abnormalities to other feral populations will add to the current literature on feral horse hoof conformation. In addition, welfare implications of these findings will be evaluated.

Materials and methods

An opportunistic sample of hooves was obtained from 101 horses of all ages, each with at least 1 abnormal hoof, as determined by a naturalist, and found recently deceased. Hooves were disarticulated at the level of the fetlock. This was done by an onisland naturalist (coauthor Z.L.) on Sable Island during regular surveys between 2007 and 2013. No information on signalment or body condition score was available. The number of hooves collected from each horse was chosen for practical reasons, with all 4 hooves collected unless postmortem changes were likely to prevent accurate evaluation. Based on the above criteria, 1 to 4 hooves were collected from each horse. Hooves were stored in a freezer at −20°C from collection to evaluation. The lateral, medial, dorsal, palmar/plantar, and solar surfaces of each hoof were digitally photographed against a white background with a scale (Figures 2, ​,3,3, ​,44).

Results

A total of 356 hooves from 101 horses were evaluated visually and morphometrically for hoof abnormalities. The number of hooves collected per horse varied. Most horses (79 horses) had 4 hooves collected, 2 horses had 3 hooves collected, 14 horses had 2 hooves collected, and 6 horses had 1 hoof collected. The numbers of abnormal hooves identified by visual assessment and morphometry varied. All 101 horses were classified as having at least 1 abnormal hoof on visual and morphometric evaluation. Most horses (97%) with all 4 hooves available for evaluation were classified as having at least 1 hoof abnormality from visual evaluation. From morphometric classifications of “abnormal,” 86% of horses with 4 hooves that could have measurements done were classified as abnormal.

Based on visual evaluation, long-heel conformation was identified in 85% of horses (Figure 5). This was similar to the 87% of horses with contracted heels indicated by morphometry (Table 2). Mediolateral imbalance was present in 45% of horses on visual evaluation (Figure 5), and 61% of horses had a mediolateral imbalance by comparing medial and lateral heel lengths on morphometry (Table 2). Many (44%) of the Sable Island horses had a medial flare, 46% had dorsal flares or dished dorsal hoof walls, and 78% had a lateral flare as determined from visual evaluation (Figure 5). Growth rings converging at the toe was an abnormality present in 43% of horses in our study, and a majority (62%) of the hooves visually evaluated in this study had cracks present in the external hoof wall (Figure 5). The mean (SD) toe length in our study was 80 mm (20 mm), with a range of 50 to 190 mm (Table 1), as determined morphometrically.

Discussion

The objective of this study was to visually evaluate and morphometrically describe a sample of abnormal hooves from Sable Island horses that died between 2007 and 2013. As collection of hooves was opportunistic and nonrandom, conclusions about the rate of occurrence of abnormal hooves in the Sable Island population cannot be made. Rather, the conformation of hooves from this unique population can be described and evaluated. It is not uncommon for feral horse populations to have high prevalences of hoof abnormalities when compared to domestic horses. In an Australian study of 100 feral horses, only 3 (3%) of the horses evaluated did not have hoof abnormalities (6).

In domestic horses, contracted heels are a poor prognostic indicator for future soundness if lameness is present. Domestic horses with contracted heels that are lame are 4× less likely to have the lameness resolved (9). The high proportion of horses with contracted heels in our study (87.1%), as determined through morphometry, may be due to an absence or decreased loading of the sole in horses with a long toe, flared hoof conformation when ambulating on sandy substrate (11). A contracted heel conformation was present in at least 1 hoof of 75% of Kaimanawa (New Zealand) feral horses, which also live on soft, wet substrate (12).

The high rate of mediolateral imbalances detected both visually and through morphometry may also be caused by the soft footing on Sable Island. An example of a mediolateral imbalance, which is a significant difference in length of the medial and lateral heel, is shown in Figure 4. The results of 45 and 61% of horses with mediolateral imbalance (by visual evaluation and morphometry, respectively) were similar to the results of an evaluation of feral horses in the Australian outback. Traveling on soft footing for 5 to 10 km a day, those horses had a 70% occurrence of mediolateral imbalance (13).

An ideal conformation for many breeds and uses of domestic horses is a hoof that is the same dimension in width and length (14). The shape of the hoof had a length greater than width in 100% of horses in this study (mean lengths of front and hind hooves: 13.3 and 11.7 cm, respectively; mean widths of front and hind hooves: 10.7 and 10.0 cm, respectively). A hoof that is longer than it is wide can also be due to the unique conformation of the horse and not have any resulting hoof capsule distortions. Breed variations, as well as environment, use, and age can shape the hoof, with many resulting normal hoof capsule shapes (15). It is possible that the unique environment of the Sable Island horse has resulted in a hoof conformation that is longer than it is wide.

In support of balanced hoof conformation, it has traditionally been considered ideal for hoof capsule alignment when toe angle is the same as heel angle (7); i.e., when the toe and heel tubules are parallel (10,16). However, it was shown that commonly accepted indicators of hoof balance, including heel and toe alignment, were rarely present in hoof capsule measurements of domestic horses in a prospective cohort study and a cadaver study (15). In the present study, the heel and toe tubules were not in parallel alignment in 56% of horses when visually evaluated.

The hoof flares in the Sable Island horses were similar to findings in feral Kaimanawa (New Zealand) horses. A medial flare can be seen in Figure 3. The Kaimanawa horses ambulate on a soft, wet substrate. In that study, based on evaluation of all 4 hooves of 20 adult horses, 50% had medial flares, 90% had a dorsal flare, and 85% had lateral flares (12). In an Australian study comparing the hoof morphology of 5 populations of feral horses, one of the populations, in which horses travelled 5 to 10 km a day (a relatively short distance) on soft substrate, commonly had horses with hoof flares and cracks. In that study, among horses traveling on softer footing for a shorter distance, 70% had with medial or lateral hoof wall flares, 80% had frog or bar abnormalities, and 30% had dorsal wall flares (6). By comparison, a significantly lower rate of these abnormalities was present in horses that travelled > 10 km per day on hard substrates. The high proportion of hoof flares observed in Sable Island horses appeared similar to other feral horse populations with low-mileage living on a soft substrate.

Growth rings on the dorsal hoof wall, present in 43% of horses studied, can be an indicator of laminitis or can be due to uneven growth of the hoof capsule, as seen in a club foot. Morphologic indications of laminitis include a dished dorsal hoof wall, elevated heels, and bars that connect to the frog apex, as well as divergent growth rings and flat or convex soles (9,17). Radiographic evaluation of a subsample of the same collection of hooves evaluated in this study indicated a < 9% prevalence of laminitis when the coronary extensor distance, capsular rotation, and ratio of dorsal hoof wall thickness to length of the distal phalanx were evaluated (8). The high rate of growth rings that converge at the toe is one indicator of laminitis in the hooves studied here. However, radiographic evaluation is a more sensitive measure of laminitis, and the earlier study that evaluated hooves radiographically detected a low prevalence of laminitis in this population (8). Growth rings converging at the toe are also present in horses that have distal interphalangeal joint flexural deformities (clubfeet), with a high heel conformation and asymmetrical growth rings due to more growth of hoof capsule at the heel than at the toe (16). Low heels were only observed in 11% of horses from visual evaluation and 1% of horses based on morphometric calculations. This study did not evaluate the prevalence of high heels in the population. However, the previous, radiographic study reported a 25% prevalence of a palmar angle > 8 degrees, which indicates a high heel conformation in the Sable Island horses (8). The low radiographic presence of laminitis and high palmar angle suggesting a high heel conformation in 25% of horses may indicate that the growth rings present on the hooves could be due to the conformation of the hooves rather than laminitis.

The lack of abrasive or hard surfaces that promote self-trimming may be the reason for some of the similarities between Sable Island horses and other feral horses living on soft substrate. Konik horses are a population of feral horses inhabiting forested areas of Poland that live and travel on grassy or sandy ground. A spontaneous self-trimming cycle has been described with these horses. During this time, hooves are initially observed to have hoof wall cracks 1 to 3 cm in length. However, within 1 to 3 mo, overgrown sections break off and sections of hoof fall off due to forces on the hoof from locomotion (18). Since this is a process that occurs over several months when traveling on soft footing, it is possible that the cadaver hooves studied here that were cracked may have been in the process of self-trimming when death occurred.

The mean (SD) toe length of Australian feral horses travelling shorter distances on sand was 87.0 mm (6.7 mm), compared to 84.6 mm (3.9 mm) in the high-travel, firm-surface group. The mean (SD) toe length in our study was 80 mm (20 mm), with a range of 50 to 190 mm. We observed a much larger SD in our sample compared to the Australian study, suggesting a less consistent hoof morphology in the Sable Island horses. This may be due to the Sable Island horses traveling even shorter distances than the group of horses traveling 5 to 10 km/d on sand in the Hampson et al study (6).

The deceased horses sampled here were harvested during a period of population growth. Data from Richard et al (19) collected from 2009 to 2013 indicated the Sable Island horse population increased at a rate of 1.053 (2009 population: 458 horses, 2010 population: 507 horses, 2011 population: 461 horses, 2012 population: 538 horses, 2013 population: 559 horses). Fecundity rates were 0.616 ± 0.023 and 0.402 ± 0.054 for adult mares and 3-year-old mares, respectively, during this same time. No other large mammals live on Sable Island, which removes interspecies competition for food; furthermore, there are no predators or human interference. Population dynamics are heavily determined by weather effects on forage supply, with harsh winters decreasing survival and increasing population density, causing increased competition for feed (19).

Feral horse survival rates in North America range from 70 to 95%, depending on external factors influencing each population (20). The probability of survival for adult female Sable Island horses from 2009 to 2013 was 0.866 ± 0.170 (SEM). This seemed lower than for some populations of feral horses studied in North America, with feral horses in Nevada having a 0.95 survival rate. However, management of the population occurred in that herd, which may have strongly influenced the dynamics of each age group (19).

Both visual evaluation and morphometry of the hooves studied here indicated a high rate of hoof capsule abnormality, with abnormalities determined by comparison to normal parameters for the domestic horse hoof and feral horses described by Hampson et al (6,12). Domestic horses, even with extensive hoof capsule abnormalities, can remain sound and capable of performing for variable intervals. However, in many cases, hoof capsule abnormality that persists over time can eventually result in performance limitation or lameness (21). Compared to domestic horses that are often stalled, pastured, and worked on a variety of substrates, Sable Island horses spend their entire lives on the island’s uniformly sandy substrate. It is possible that continual exposure to the same substrate type has allowed their hooves to remodel in response to environmental conditions more so than those of domestic horses. Another possibility is the unique environment of SINPR, where ample, seasonally based access to forage and water may provide the necessary nutrition for horses with abnormal hoof conformation and resulting impaired ambulation. The most important function of the hoof is to preserve the attachment between the distal phalanx and the wall so that the horse can have pain-free mobility (22). Given the low prevalence of painful conditions such as laminitis, combined with the sand substrate that the horses live on, it is likely that abnormal hoof conformation does not have a significant negative impact on the health of the Sable Island horse population.

The health significance of the hoof abnormalities in Sable Island or other feral horse populations is difficult to determine. Hoof condition is a measurable indicator of the physical health of the horse, which is Domain Three of the Five Domains Model of animal welfare status (23). When evaluating welfare of working equids, hoof horn quality, length of toe (too long or too short), and condition of the sole of the foot are included with observation of gait abnormalities (24,25). It is well-known that acute or chronic pain due to hoof abnormalities can cause difficulty ambulating (26,27). To determine if the physical components of hoof health affect negative or positive affective states through decreasing ambulation from pain, evaluation of the live horses for an abnormal or compromised gait would be required. Physical welfare indicators such as body condition and reproductive health of the Sable Island horses are positive. The population is stable and, during the summer, the horses appear to be in moderate-to-excellent body condition (28).

The collection of Sable horse hooves did not include all 4 hooves of each horse, with 78% (79/101) of sampled horses having all 4 hooves available for inspection. As a result, a complete picture of the hoof morphology present in each Sable Island horse cannot be provided. The method of selecting the hooves also presents biases due to the nonrandom sampling methods used. Visual evaluation of the hooves by a podiatry expert post-collection did confirm a high rate of hoof abnormalities in hooves collected. All specimens were collected from naturally deceased Sable Island horses, as any human interference with the horses is prohibited. If abnormal hoof conformation does have a negative effect on the horses’ ability to forage, the selection of abnormal hooves studied here may represent horses having a higher morbidity rate due to their hoof shape. Another limitation of the current study is long-term storage of hooves in a freezer, which may affect their morphometry due to postmortem changes such as dehydration. Due to the hard nature of the hooves, the hoof shape appeared to be well-preserved despite freezing and thawing. This study does not report a prevalence of the abnormalities described, simply a description of abnormalities observed in the hooves collected.

This study only allowed only a partial evaluation of the hoof health status of the Sable Island horses. A limitation related to the study of cadaver samples was the lack of information on how hoof condition affected movement. Evaluation of movement, posture, and other indices of discomfort or pain would be necessary to comment on the welfare impact of hoof conformation in the Sable Island horse population (29). Identifying Sable Island horses with abnormal hoof conformation and gauging the welfare impact of this is an area of future study for the authors.

In conclusion, Sable Island horses had hoof abnormalities such as hoof capsule flaring; long, contracted heels; and mediolateral imbalance. These abnormalities have been observed in other feral horse populations living on a soft substrate. The impact of hoof abnormalities on the welfare status of the Sable Island horses is difficult to determine without data on the prevalence of abnormalities and direct observation of horse gait and behavior.

Acknowledgments

Thanks to Andrea Floyd, for visual inspection of hooves; Alexa Avery, for performing physical measurements of hooves; and Marina Zadworny, for data cataloguing. Thanks to the Meteorological Service of Canada for logistical support on Sable Island. CVJ

Funding Statement

This work was supported by the Sir James Dunn Animal Welfare Society at the Atlantic Veterinary College and by the Sable Island Institute.

Footnotes

References