Keller, A; Clauss, M; Muggli, E; Nuss, K (2009). Even-toed but uneven in length: the digits of artiodactyls. Zoology, 112(4):270-278. Postprint available at: http://www.zora.uzh.ch University of Zurich Posted at the Zurich Open Repository and Archive, University of Zurich. Zurich Open Repository and Archive http://www.zora.uzh.ch Originally published at: Zoology 2009, 112(4):270-278.
Winterthurerstr. 190 CH-8057 Zurich http://www.zora.uzh.ch
Year: 2009
Even-toed but uneven in length: the digits of artiodactyls
Keller, A; Clauss, M; Muggli, E; Nuss, K
Keller, A; Clauss, M; Muggli, E; Nuss, K (2009). Even-toed but uneven in length: the digits of artiodactyls. Zoology, 112(4):270-278. Postprint available at: http://www.zora.uzh.ch
Posted at the Zurich Open Repository and Archive, University of Zurich. http://www.zora.uzh.ch
Originally published at: Zoology 2009, 112(4):270-278. Even-toed but uneven in length: the digits of artiodactyls
Abstract
In captive ruminants housed in small enclosures, characteristic hypertrophy of the outer hooves of the hind limbs is often observed. We hypothesized that the underlying cause is overload attributable to an asymmetry of the digits, especially with respect to their length. To test this hypothesis, the bones of the digits of four species of artiodactyls, which included 11 wild chamois (Rupicapra rupicapra), 11 captive fallow deer (Dama dama), 11 captive bison (Bison bison) and 11 European moose (Alces alces; 9 wild, 2 captive), were radiographed post mortem, and measured using a computer program. In addition, the dimensions of the outer and inner hooves were measured directly with a calliper that had an accuracy of 0.1 mm. The mean lengths of the epiphysis of the fourth metacarpal/metatarsal bone and the first and second phalanges of the fourth digit were greater than that of the third digit, whereas the third phalanx of the third digit had a greater mean length. The fourth digit of the forelimbs was an average of 0.5 mm longer in the chamois, 0.9 mm in the fallow deer, 3.0 in the bison and 2.9 longer in the moose. In the hind limbs, the fourth digit was an average of 3.0 mm longer in the chamois, 1.4 mm in the fallow deer, 2.3 mm in the bison and 5.3 mm in the moose. The mean total length of the fourth digit of the fore limbs was greater than that of the third digit in 73 - 95 % of specimens, depending on species. In the hind limbs, the fourth digit was longer in 91 - 100 % of the specimens. The hooves of the fourth digit were significantly broader than the hooves of the third digit, whereas the inner hooves of the third digits had a greater toe length than those of the fourth digit. The paired digits of artiodactyls are uneven in length, which suggests a different function during stance and weight bearing. It is conceivable that this asymmetry is the result of selection processes that favoured locomotion on soft ground. 1Even-toed but uneven in length: the digits of artiodactyls
2
3Anna Keller1, Evelyne Muggli1, Marcus Clauss2, Karl Nuss1,3*
4
51Clinic for Ruminants, Vetsuisse Faculty, University of Zurich, Winterthurerstrasse
6260, CH-8057 Zürich, Switzerland
72Clinic for Zoo Animals, Exotic Pets and Wildlife, University of Zurich,
8Winterthurerstrasse 260, CH-8057 Zürich, Switzerland
93Current address: Clinic for Ruminants, Faculty of Veterinary Medicine, Ludwig-
10Maximilians-University of Munich, Sonnenstrasse 16, D-85764 Oberschleißheim,
11Germany
12
13*Corresponding author: Karl Nuss, Clinic for Ruminants, Faculty of Veterinary
14Medicine, Ludwig-Maximilians-University of Munich, Sonnenstrasse 16, D-85764
15Oberschleißheim, Germany; Tel. 0049-89-2180-78850, Fax 0049-89-2180-78851,
16email: [email protected]
17
1811 text pages including references, 3 Figures, 5 Tables
19
20Key words: artiodactyla, digits, difference in length, hoof size
21 22Summary
23In captive ruminants housed in small enclosures, characteristic hypertrophy of
24the outer hooves of the hind limbs is often observed. We hypothesized that the
25underlying cause is overload attributable to an asymmetry of the digits,
26especially with respect to their length. To test this hypothesis, the bones of the
27digits of four species of artiodactyls, which included 11 wild chamois
28(Rupicapra rupicapra), 11 captive fallow deer (Dama dama), 11 captive bison
29(Bison bison) and 11 European moose (Alces alces; 9 wild, 2 captive), were
30radiographed post mortem, and measured using a computer program. In
31addition, the dimensions of the outer and inner hooves were measured directly
32with a calliper that had an accuracy of 0.1 mm.
33The mean lengths of the epiphysis of the fourth metacarpal/metatarsal bone
34and the first and second phalanges of the fourth digit were greater than that of
35the third digit, whereas the third phalanx of the third digit had a greater mean
36length. The fourth digit of the forelimbs was an average of 0.5 mm longer in the
37chamois, 0.9 mm in the fallow deer, 3.0 in the bison and 2.9 longer in the
38moose. In the hind limbs, the fourth digit was an average of 3.0 mm longer in
39the chamois, 1.4 mm in the fallow deer, 2.3 mm in the bison and 5.3 mm in the
40moose. The mean total length of the fourth digit of the fore limbs was greater
41than that of the third digit in 73 – 95 % of specimens, depending on species. In
42the hind limbs, the fourth digit was longer in 91 – 100 % of the specimens. The
43hooves of the fourth digit were significantly broader than the hooves of the
44third digit, whereas the inner hooves of the third digits had a greater toe length
45than those of the fourth digit. The paired digits of artiodactyls are uneven in
46length, which suggests a different function during stance and weight bearing. 47It is conceivable that this asymmetry is the result of selection processes that
48favoured locomotion on soft ground to provide better traction.
49 50 Introduction
51 Even-toed ungulates (artiodactyls) have an even number of digits (two or four) on
52each foot. Ruminants, which have two main digits, are the most prominent
53representatives of the artiodactyls. They have achieved and maintained a worldwide
54distribution mainly because of their efficient digestive system (Gentry, 1978). Some
55ruminants, which were domesticated long ago and have been used intensively, are
56still able to survive in ever-decreasing natural habitats. Wild ruminants such as fallow
57deer (Dama dama) and bison (Bison bison) are also farmed, but less commonly than
59 Domestic cattle are often housed on hard surfaces, which leads to the
60development of characteristic hoof lesions (Sogstad et al., 2005; Somers et al.,
612005). In the vast majority of cases the outer hoof increases in size
62disproportionately with age (Nuss and Paulus, 2006) and is predisposed to sole
63ulcers or white line disease. Similarly, when wild ruminants are kept in captivity,
64enlargement and abnormalities of the outer hooves of the hind limbs occur (Fowler,
651980). Although a similar, albeit small, difference in the size of the paired hooves was
66observed in a wild moose (Kendelbacher, 1935), a difference in hoof size has not
67been recognized in wild ruminants. It is conceivable that digit characteristics of
68ruminants are the result of a natural selection process that favoured locomotion on
69soft ground. This assumption is supported by a preference of domestic cattle for soft
70flooring and by the observation that diseased hooves recover when cattle have
71access to pasture (Flower et al., 2007; Hernandez-Mendo et al., 2007).
72 All ruminants have a metapodium that is composed of the two fused third and
73fourth metacarpal/metatarsal bones. This composite bone, also called the cannon
74bone, provides two separate epiphyses for the third and fourth digits (the acropodia),
75which themselves are composed of the three phalanges. Until recently it was 76assumed that the two digits and hooves of artiodactyls are equal in size (Manning et
77al., 1990) and that the plane of symmetry of each foot passes between the third and
78fourth digit (Fox and Myers, 2001). However, there are reports of differences in the
79length and width of the cannon bone epiphyses and the digital bones in cattle
80(Nacambo et al., 2007; Schwarzmann et al., 2007). Compared with the partner
81condyle, the condyles of the fourth digit were longer and those of the third digit were
82wider. These differences were also observed in cattle bones that were several
83thousand years old (Paral et al., 2004). A recent report described similar differences
84in the cannon bone and digits of three dromedaries (Camelus dromedarius)
85(Farhadian and Soroori, 2006). Based on these studies, our hypothesis was that the
86two digits of ruminants, a suborder of the artiodactyla, are anatomically different, i.e.,
87that the fourth digit is longer and has a larger hoof.
88
89 Materials and Methods
90 To test the hypothesis, the feet of artiodactyls, which included 11 chamois
91(Rupicapra rupicapra), 11 fallow deer (Dama dama), 11 bison (Bison bison) and 11
92moose (Alces alces), were collected immediately post mortem and frozen until further
93analysis (Keller, 2007). Different wild ruminant species were investigated in order to
94include different weight classes and habitats (Tab. 1). The chamois were wild and
95originated from the Swiss Alps. Nine of the moose were wild and originated from their
96natural habitat in Finland, and two had been kept in small enclosures in a wildlife
97park in Switzerland. The fallow deer had been kept in a wildlife park with grassland
98and forest. The bison came from various environments; four originated from large
99pastures on a farm, six from a wildlife park and one from a zoo. The feet were
100collected and examined post mortem. Radiographs were taken with a mobile X-ray
101unit (Gierth HF 200) and type 3A IP digital cassettes (Fuji Systems). The feet were 102placed on their dorsal surface and the beam directed palmaro-/plantarodorsally and
103centred between the digits at the level of the second interphalangeal joint. The
104exposure settings were 70 kV, 25 mA and 0.2 s for the moose and bison feet, and 56
105kV, 30 mA and 0.2 s for the chamois and fallow deer feet. The focus-to-film distance
106was constant at 1.15 m in all specimens. A radiodense scale placed at the level of
107the bones with the help of small blocks allowed exact measurements (Fig. 2). The
108radiographs were transferred to a personal computer and the dimensions of the
109bones were measured with a software program (Metron PX™, Epona Tech, Creston,
110CA, USA). The lengths of the bones were determined by measuring the distance
111between two points that could be exactly defined on the radiographs (Fig. 1 –
112longitudinal lines): The longitudinal axis of the cannon bone was determined by first
113drawing three lines across the width of the diaphyis of the bone and then drawing a
114perpendicular line through the centres of these three lines. At the level of the growth
115plate, a line was drawn perpendicularly to the longitudinal axis to serve as a base line
116for measuring the length of the cannon bone epiphyses (Fig 1). The length of the
117cannon bone epiphyses of the third and fourth digits were measured perpendicularly
118to the base line to the condyloid crest. The length of the first phalanx was measured
119from the most distal point of its sagittal groove to the most proximal point of the distal
120articular surface. The second phalanx was measured from its extensor process to the
121most proximal point of the articular surface. The length of the third phalanx was
122measured from the extensor process to the tip of the bone. The width of the bones
123was determined by measuring a line that ran perpendicularly to the longitudinal line
124and through the point of the bone with the largest distance from this line (Fig. 1).
125 The total length of the third and fourth digit was calculated by adding the lengths
126of the single bones of the relevant digits. The lengths of the three phalanges bones
127as well as the total digit lengths were compared between the third and the fourth 128digit, for the foreleg and the hindleg, respectively, within each species. The width of
129each bone of the third digit was also compared to the width of the relavant bone of
130the fourth digit. In addition, the longitudinal axis of the cannon bone was
131superimposed on the radiographs and extended to the level of the tip of the digits
132and compared with the axes of the digits (Fig. 1). The dimension of the hoof capsules
133was determined by measuring three representative variables, i.e., toe length, bulb
134width and sole width, as described for cattle (Nuss and Paulus, 2006) using callipers
135with an accuracy of 0.1 mm.
136 Data are presented as means and standard deviations of measurements of the
137third and fourth digits of the right and the left limbs. Statistical computations were
138made using SPSS 11.5 (SPSS Inc. Chicago, IL, USA). Descriptive statistics were
139used to calculate means and standard deviations. Differences between bone lengths
140within digits were analysed using a paired t-test. P≤0.05 was considered significant.
141
142
143 Results
144 There were significant differences in the mean length, and less consistently in the
145width, of the bones of the third and fourth digits (Table 2 and Table 4). The mean of
146the length of the cannon bone epiphyses of the fourth digit and the first and second
147phalanges were all larger than those of the third digit in the front and hind limbs. The
148mean length of the third phalanx of the third digit was slightly larger than that of the
149fourth, with the exception of the moose, in which the third phalanx of the fourth digit
150was longer than that of the third. The mean total length of the fourth digit was greater
151than that of the third digit (Table 2). In many specimens, the difference in length was
152apparent without radiographic examination upon closer examination of the feet (Fig.
1532). 154 In the front limbs, the fourth digit was longer by a mean of 0.5 mm in the chamois,
1550.9 mm in the fallow deer, 3.0 mm in the bison and 2.9 mm in the moose. In the hind
156limbs, the mean difference was 3.0 mm in the chamois, 1.4 mm in the fallow deer,
1572.3 mm in the bison and 5.3 mm in the moose (Table 2). Except for the bison, the
158difference was larger in the hind limbs.
159 There were also specimens in which the two digits were the same length or the
160inner digit was longer (Table 2). Overall, the outer digit was longer in 73 –95 % of the
161front limbs of all wild ruminant species and in 91 –100 % of the hind limbs, depending
162on the species (Table 2). The total length of the digits was larger in the hind limbs
163than in the front limbs; this difference was largest in the elk (2 cm) and smallest in the
164bison (2 mm). Age had no effect on the difference in length of the digits of the front
165and hind limbs.
166 Comparison of the mean widths of the bones of the forelimbs showed that the
167epiphysis and the first phalanx of the third digit were wider than those of the fourth
168digits (Table 3). The width of the second phalanx varied among and within species,
169and no significant differences were found. However, the third phalanx of the fourth
170digit was on average wider than the third phalanx of the third digit in all species
171examined. The differences in bone width were less often significant than the
172difference in bone length (Table 4). The cannon bone axis, when elongated distally,
173came closer to the fourth digit in 80 % of the 176 front and hind feet. Of the 35 other
174feet that did not show this pattern, 28 were forelimbs and 7 were hind limbs.
175 The hoof capsule of the hooves of fourth digits of the hind limbs of wild ruminants
176was wider because of larger sole and bulb dimensions (Table 4) except in the
177chamois, where no difference was found. With few exception of the bison, the hoof of
178the third digit had a longer horn capsule at the toe (dorsal wall length) than its
179counterpart (Table 4). In the forelimbs the hooves were larger with respect to bulb 180width and sole width than in the hind limbs. In one moose kept in a wildlife park and
181one bison kept in a zoo, the increase in size of the outer hoof was prominent (Fig. 3)
182and reminiscent of hypertrophic changes commonly seen in domestic cattle.
183Radiographs of the bison’s feet showed signs of arthrosis and diffuse aseptic
184pododermatitis, which were more prominent in the hooves of the fourth digits (Fig. 4).
185
186 Discussion
187 The results of this study supported our hypothesis that the fourth digit is usually
188longer than the third in even-toed ungulates. This has previously been reported in
189cattle (Muggli, 2007). These findings prompt a careful reassessment of the function
190of each of the digits during standing and during locomotion.
191 The radiographic measurements were done in a limited number of even-toed
192ungulate species that differed in age and gender, and thus, the data do not represent
193reference values. All fallow deer were intact males one year of age and thus,
194measurements derived from that group are not representative of fallow deer in
195general. Similar limitations applied to the claw measurements. However, the primary
196goal of this study was to investigate possible differences between the fourth and third
197digits. Because the measurements were done under standardised conditions for both
198the inner and outer digits, comparisons within animals were justified.
199 Radiographic measurements usually do not allow exact measurement of bone
200length because of size changes caused by projection of the radiographic beam on
201the screen. Schwarzmann identified an error of 3 to 7% in measurement of bone
202length using radiographs {2005 #7729}. Although an error may have occurred in our
203study, it can be assumed to be much smaller because of the radiodense scale placed
204at the level of the bones. The scale would have undergone the same proportional
205change in size as the bones, therefore allowing an accurate measurement of length. 206Macerated bones were not used for measurements because the goal of the study
207was to investigate differences in the length of the outer and inner digits and not to
208determine absolute bone length. The reference points for the radiographic
209measurements were identical for the bones of the third and fourth digits and were
210chosen for their ease of identification rather than to allow measurement of the exact
211length of the digits. The standardised conditions, use of modern digital equipment
212and validation of the method (Kummer et al., 2004) ensured that the measurements
213were reliable and the results comparable.
214 Our results showed that the paired digits of even-toed ungulates differed in
215length, and that the outer digit was, in the majority of cases, longer than the inner.
216This difference was due to the longer metacarpal/metatarsal bone epiphyses and the
217longer first and second phalanges of the fourth digit (Table 1). The third digit had a
218slightly longer third phalanx but this did not compensate completely for the difference
219in total length. The longer third phalanx correlated well with the longer dorsal wall
220length (toe length) of the hooves of the third digit compared to the hooves of the
221fourth digits. However, with the exception of the chamois, the outer hooves had
222significantly wider bulbs and soles in the front limbs as well as the hind limbs. This
223finding correlated well with the longer outer digit and the wider distal phalanx.
224 On some of the radiographs, the distal interphalangeal joint of the fourth digit was
225distinctly angulated, whereas the distal interphalangeal joint of the third digit
226appeared straighter. This angulation may have caused slight rotation of the third
227phalanx of the fourth digit, making it appear wider on radiographs. It is not clear
228whether this angle is present in the live animal or only after death. With high speed
229cinematography, such a difference could not be seen in cattle walking on a treadmill
230{Meyer, 2007 #7395}{Schmid, 2008 #16845}. We were unable to determine any
231abnormal conformation that accounted for this digital asymmetry. 232 A difference in the total length of the digits was observed in more than 91% of
233hind limbs and in more than 73% of the forelimbs, similar to reports in domestic
234ruminants {Schwarzmann, 2005 #7729}(Muggli, 2007). Age had no apparent effect
235on the difference in length of the paired digits in this study and in studies of ruminants
236(Schwarzmann, 2005 #7729; Muggli, 2007). Thus, it can be assumed that the length
237difference is a characteristic of artiodactyls, and that it possibly originated from a
238common ancestor with the same trait. It is reasonable to assume that the difference
239in length of the paired digits is the result of an evolutionary selection process that
240was advantageous to the animal.
241 A small number of the animals in our study had inner digits that were equal to or
242longer than the outer ones. It can be speculated that in the evolutionary process, a
243small percentage of individuals deviate from the norm so that adaptation to sudden
244changes in the environment and thus, preservation of a species, may be better
245assured.
246 There are several reasons why a difference in digit length and width could be
247adaptive for ruminants. The centre of gravity, located in the middle of the trunk of
248quadrupeds, is stabilised by longer outer digits, particularly on soft ground. In wild
249ruminants, the outer hoof was wider than the inner in the front and hind limbs; this
250difference was less distinct in ruminants kept in smaller enclosures (Fig. 3) or in
251domestic cattle (Nuss and Paulus, 2006). We hypothesise that the fourth digit serves
252to stabilise the centre of gravity to a greater extent than to bear weight, whereas the
253opposite applies to the third digit. In one heifer it was shown that on soft ground, the
254hoof of the third digit of a front limb absorbed an impact force higher than the fourth
255digit (Seebacher et al., 1980). If this was true for a larger number of animals, it would
256explain why the mean widths of the third metapodial epiphysis and the first phalanx
257of the third digit are larger (Bartosiewicz et al., 1997). 258 Under natural conditions, asymmetric digits and larger outer hooves may provide
259an advantage in terms of stability, because the longer outer digit provides better
260contact with the ground during movement and better stability while standing. It might
261be speculated that, when pushing forward, for example during a fight with a rival, the
262hind limbs are positioned at an outward angle which necessitates longer outer digits
263to maintain ground contact. When climbing a slope or during a quick turn to escape a
264predator, longer outer digits can be advantageous for the same reason. Spreading of
265the digits per se increases contact and frictional area and prevents slipping (Manning
266et al., 1990). The advantages of asymmetric digits and hooves may have contributed
267to the wide distribution of artiodactyls in different habitats such as alpine (chamois),
268marshy (moose), forest (fallow deer) and prairie (bison) environments.
269 A prevailing deviation in the direction of the foot axis was seen in wild ruminants,
270in which the axis of the cannon bone ran closer to the outer digit (Fig. 1). The
271incongruity between the axes of the cannon bone and the digits could possibly be
272caused by a deviation of the cannon bone axis at the level of the epiphyseal groove.
273In domestic cattle, such a clear-cut deviation of the limb axis could not be seen
274(Muggli, 2007). Measurement of the cannon bone axis should therefore be repeated
275using specimens in which the entire length of the cannon bone is seen on
276radiographs.
277 The anatomical differences identified in our study may aid in the interpretation of
278degenerative bone lesions and the differentiation of bone fragments in biological and
279archaeological specimens. The metapodia of modern ruminants differ only in size
280and not in anatomical proportion compared with their anchestors (Paral et al., 2004).
281Despite the disadvantages incurred by domestic ruminants kept in small enclosures
282and on hard surfaces during their domestication, the structural principles of the bones
283of the digits have not changed (Muggli, 2007). However, in domestic cattle, the 284natural selection processes have been disturbed by breeding programs and
285management interventions, such as claw trimming, a main goal of which is to
286eliminate asymmetries.
287 The hoof capsules of the hooves of the fourth digits were wider as evidenced by
288larger sole and bulb dimensions (Table 4). The horn capsules of the hooves of the
289front limbs were larger than in the hind limbs, presumably because the centre of
290gravity is closer to the front limbs. Except for moose, the dorsal wall was longer in the
291forelimbs than in the hind limbs; such differences among species may be related to
292the species’ habitat and require further investigation. Of special interest is also the
293question of whether the asymmetry of paired claws is related to a difference in the
294loads the two digits are subjected to.
295 In summary, our results showed significant differences in the length of the paired
296digits and hooves of even-toed ungulates. Whether and to what degree these
297differences are accompanied by corresponding differences in soft tissue structures
298and function requires further study. On hard surfaces, these anatomical differences
299may lead to hypertrophy and deformation of the outer hoof, which is evident in
300domestic cattle as well as in wild artiodactyls kept in captivity.
301
302 Acknowledgements: The authors would like to thank Kaarlo Nygrén, Ilomantsi
303Game Research Station, Finland, for the collection of the moose feet. 304References
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