- Email: [email protected]
Performance constraints in strength events in dogs (Canis lupus familiaris)
Behavioural Processes 86 (2011) 149–151
Contents lists available at ScienceDirect
Behavioural Processes journal homepage: www.elsevier.com/locate/behavproc
Rapid report
Performance constraints in strength events in dogs (Canis lupus familiaris) William S. Helton ∗ Department of Psychology, University of Canterbury, Private Bag 4800, Christchurch 8020, New Zealand
a r t i c l e
i n f o
Article history: Received 24 June 2010 Received in revised form 22 July 2010 Accepted 29 July 2010 Keywords: Dog Performance Skull shape Strength Structural trade-offs
a b s t r a c t Ecological performance in animals is considered to be constrained by trade-offs between structural and physiological features. Some of the best anatomical and physiological data in support of this perspective are based on findings from canines, in which researchers have demonstrated skeletal shape trade-offs between running efficiency and strength. There has, however, actually been little examination of the link between these structural differences and relevant performance. In this paper I analyze the performance of six breeds of dogs in standardized weight pulling competitions. More brachycephalic shaped (broad headed) dogs are indeed significantly stronger than more dolichocephalic shaped (narrow headed) dogs even when controlling for body mass. Morphological trade-offs between power and running ability may be an important constraint on the evolution of canines and other terrestrial vertebrates. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Ecological performance in animals is thought to be constrained by trade-offs between antagonistic pairs of structural and physiological features (Futuyma and Moreno, 1988; Van Damme et al., 2002). Some of the best anatomical and physiological data in support of this perspective are based on findings from canines (Passi and Carrier, 2003; Kemp et al., 2005). Domestic dogs are the most phenotypically diverse species (Lark et al., 2006). The dog, therefore, provides an excellent opportunity to uncover the links between genes and phenotypes. Researchers have demonstrated with quantitative genetic analysis a skeletal shape trade-off between running capability and strength in both dogs and foxes (Carrier et al., 2005; Lark et al., 2006; Karlamova et al., 2007). This is seen in dog breeds with a continuum between more dolichocephalic (narrow headed) and more brachycephalic (broad headed) dogs. More dolichocephalic dogs, like grey-hounds and huskies, have longer, narrower skulls and relatively small skulls in relationship to their bodies. More brachycephalic dogs, like bull terriers and mastiffs, have broader skulls and relatively large skulls in relationship to their bodies. There has actually been little examination of the link between these structural differences and relevant performance, because performance differences between more brachycephalic and more dolichocephalic dogs are simply assumed. Dogs do compete in a large number of regulated sporting activities making them an excel-
∗ Corresponding author. Tel.: +64 0 3 364 2902; fax: +64 0 3 364 2181. E-mail addresses: [email protected], [email protected]. 0376-6357/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.beproc.2010.07.019
lent model to more directly test relationships between phenotypes and physical performance (Helton, 2009). One set of these regulated canine sports involves a team of dogs or a solitary dog pulling a sled or person on skis (skijoring). These sled sports date back to at least 1886 to the first Saint Paul Winter Carnival. Sled sports are typically associated with dolichocephalic shaped huskies. These northern breeds excel in both the long-distance endurance races such as the Iditarod, which covers ∼1868 km in 8–15 days, and shorter distance ∼20 km sprint races. Huskies appear morphologically suited for endurance running (Coppinger and Coppinger, 2001). Amongst sled-pulling sports there is, however, an exception to husky dominance: weight pulling is a short distance (4.9 m) power competition. Weight pulling may be familiar to many readers as the activity was a feature of Jack London’s book Call of the Wild. The goal in weight pulling is for a single dog to pull a maximally weighted wheeled cart or sled down a short track. Although the original version of the sport was conducted on snow, wheeled competitions are more common and for this analysis wheeled versions were, therefore, examined. The closest human analogue sports for weight pulling would be power-lifting or world-strongest man competitions. Unlike the other sled-pulling sports, in weight pulling running endurance is sacrificed for raw strength and power. In these competitions, while traditional huskies do compete there are just as many more brachycephalic dogs, such as pit-bull terriers and mastiffs. This presents the opportunity to compare these different canine morphological forms, dolichocephalic and brachycephalic, in a regulated performance event. Here I analyze the performance of six breeds of dogs, three more brachycephalic and three more dolichocephalic, in standardized weight pulling competitions.
150
W.S. Helton / Behavioural Processes 86 (2011) 149–151
change in R2 and this was not statistically significant, F1,213 = 2.17, p = .14, R2 = .01, overall model R2 = .27. 4. Discussion
Fig. 1. The relationship between dog weight (kg) and weight pulled (kg) for the brachycephalic () and dolichocephalic (䊉) breeds. Dog weight (mass) is positively correlated with amount pulled as expected (p < .001). Brachycephalic dogs are, however, significantly stronger than dolichocephalic dogs regardless of weight (p < .001).
2. Materials and methods A data set was constructed for 217 dogs using sources available from the International Weight Pulling Association (http://www.iwpa.net/, January 2008). The data set included 108 brachycephalic dogs consisting of 73 American Pit-Bull Terriers, 19 American Bull Dogs, and 16 Bernese Mountain Dogs, and 109 dolichocephalic dogs, consisting of 19 Samoyeds, 20 Siberian Huskies, and 70 Alaskan Malamutes. These six breeds were selected as individuals of each of the breeds were well represented in competitions and formed two distinct samples. The United Kennel Club breed standards for the American Pit-Bull Terrier, the American Bulldog and the Bernese Mountain Dog all indicate a massive and very broad head relative to body size with a relatively short and broad muzzle (e.g. more brachycephalic). The breed standards for the Alaskan Malamute, the Samoyed and the Siberian Husky all indicate a head in proportion to body size with a longer muzzle also in proportion to skull size (e.g. more dolichocephalic). While none of the breeds represent the extremes of brachycephalicy (such as in the Pug) or dolichocephalicy (such as in the Borzoi) seen in modern dog breeds, they do form two distinct groups: one more brachycephalic in skull shape and one more dolichocephalic in skull shape. From the online repository, the dog’s weights and maximal pulled weights were collected. These were analyzed using hierarchical linear regression models with SPSS ©v. 17 software. 3. Results The dogs’ weights and maximal pulled weights are presented in Fig. 1. The brachycephalic dogs were less heavy (M = 31.8 kg, SEM = 1.1) on average than the dolichocephalic dogs (M = 36.8 kg, SEM = 1.2). Nevertheless, the brachycephalic dogs pulled on average more weight (M = 775.8 kg, SEM = 44.3) than the dolichocephalic dogs (M = 549.9 kg, SEM = 31.7). In order to examine the relationship between dog weight (kg) and maximal pulled weight (kg) a two step regression analysis was conducted. In the first step of the analysis dog weight and shape (coded brachycephalic = 1; dolichocephalic = 0) were entered into the model, F2,214 = 38.0, p < .001, R2 = .26. Both dog weight (B = 14.97; t = 7.38) and dog shape (B = 300.79; t = 7.38) were statistically significant, p < .001. In the second step of the analysis the interaction between dog weight and dog shape was entered into the model to test for a significant
An examination of Fig. 1 indicates while there is some overlap, generally more brachycephalic dogs are substantially stronger than more dolichocephalic dogs, when the weights of the dogs are equivalent. These results match expectations based on recent physiological and anatomical research indicating a functional trade-off between power and endurance (Passi and Carrier, 2003; Wilson and James, 2004; Kemp et al., 2005). An alternative explanation could be that these two breed-types are differentially conditioned. There is, however, no reason to believe this to be the case. More likely these results reflect the breed differences in anatomy and physiology found by other researchers (Passi and Carrier, 2003; Kemp et al., 2005; Carrier et al., 2005; Lark et al., 2006; Karlamova et al., 2007). The comparison between these breed-types for running endurance has not, however, been made. This is because there are no brachycephalic dogs competing in endurance sled events such as the Iditarod. Considering the considerable sums made from sponsorship and advertising contracts for winners of these races this may indicate these dogs are unsuitable for such events. The findings presented here strengthen the claims of other researchers indicating a fundamental trade-off between strength and running ability in terrestrial vertebrates. Comparative palaeontology provides additional evidence of these functional trade-offs in skeletal shape. In the canine lineage, for example, Borophagus secundus had a brachycephalic skull shape and filled the ecological niche of bone harvesting (Wang and Tedford, 2008). A more brachycephalic skull shape increases biomechanical bite force (Ellis et al., 2009). Increased bite force is advantageous when crushing bones, during intra-specific contests and for some predatory tactics (Slater et al., 2009). Hyper predators, for example cats, are usually more brachycephalic in skull shape for their respective weights (Wroe et al., 2005; Van Valkenburgh, 2007). Indeed within the extant canine family, the recently discovered coyote-wolf hybrids of the American northeast are more brachycephalic in skull shape and are oriented to hunting much larger prey than their more dolichiocephalic coyote relatives (Kays et al., 2010). Energetically efficient long-distance runners, like canines in general, usually have more elongated skull shapes (Wroe et al., 2005). Speculatively, these results may also be important for understanding human evolution. Researchers have suggested that long-distance endurance running was an adaptation of the homo lineage (Carrier, 1984; Bramble and Lieberman, 2004). Before the advent of projectile weapons, like spear throwers and bows, running prey to their deaths may have been the preferred homo hunting tactic, similar to the tactics used by extant African hunting dogs. The other African apes retain morphologies suggestive of the need for raw strength, not endurance running (Carrier, 2007). Perhaps these differences indicate that intra-specific conflict was not nearly as important in human evolution as hunting based on endurance running. Alternatively, the human adaptation to use weapons in intra-specific conflicts may have enabled increased morphological specialization for running efficiency. References Bramble, D.M., Lieberman, D.E., 2004. Endurance running and the evolution of Homo. Nature 432, 345–352. Carrier, D.R., 1984. The energetic paradox of human running and hominid evolution. Curr. Anthropol. 25, 483–495. Carrier, D.R., 2007. The short legs of great apes: evidence for aggressive behavior in australopiths. Evolution 61, 596–605. Carrier, D.R., Chase, K., Lark, K.G., 2005. Genetics of canid skeletal variation: size and shape of the pelvis. Genome Res. 15, 1825–1830.
W.S. Helton / Behavioural Processes 86 (2011) 149–151 Coppinger, R., Coppinger, L., 2001. Dogs: A Startling New Understanding of Canine Origins, Behavior, and Evolution. Scribner, New York. Ellis, J.L., Thomason, J., Kebreab, E., Zubair, K., France, J., 2009. Cranial dimensions and forces of biting in the domestic dog. J. Anat. 214, 362–373. Futuyma, D.J., Moreno, G., 1988. The evolution of ecological specialization. Annu. Rev. Ecol. Syst. 19, 207–233. Helton, W.S., 2009. Canine Ergonomics: The Science of Working Dogs. Taylor & Francis, New York. Karlamova, A.V., Trut, L.N., Carrier, D.R., Chase, K., Lark, K.G., 2007. Genetic regulation of canine skeletal traits: trade-offs between hind limbs and forelimbs in the fox and dog. Integr. Comp. Biol. 47, 373–381. Kays, R., Curtis, A., Kirchman, J.J., 2010. Rapid adaptive evolution of northeastern coyotes via hybridization with wolves. Biol. Lett. 6, 89–93. Kemp, T.J., Bachus, K.N., Nairn, J.A., Carrier, D.R., 2005. Functional trade-offs in the limb bones of dogs selected for running versus fighting. J. Exp. Biol. 208, 3475–3482. Lark, K.G., Chase, K., Sutter, N.B., 2006. Genetic architecture of the dog: sexual size dimorphism and functional morphology. Trends Genet. 22, 537–544.
151
Passi, B.M., Carrier, D.R., 2003. Functional trade-offs in the limb muscles of dogs selected for running vs. fighting. J. Evol. Biol. 16, 324–332. Slater, G.J.m., Dumont, E.R., Van Valkenburgh, B., 2009. Implications of predatory specialization for cranial form and function in canids. J. Zool. Lond. 278, 181–188. Van Damme, R., Wilson, R.S., Vanhooydonck, B., Aerts, P., 2002. Performance constraints in decathletes. Nature 415, 755–756. Van Valkenburgh, B., 2007. Déjà vu: the evolution of feeding morphologies in the Carnivora. Integr. Comp. Biol. 47, 147–163. Wang, X., Tedford, R.H., 2008. Dogs: Their Fossil Relatives and Evolutionary History. Columbia University Press, New York. Wilson, R.S., James, R.S., 2004. Constraints on muscular performance: trade-offs between power output and fatigue resistance. Proc. R. Soc. B (Suppl.) 271, S222–S225. Wroe, S., McHenry, C., Thomason, J., 2005. Bite club: comparative bite force in big biting mammals and the prediction of predator behaviour in fossil taxa. Proc. R. Soc. B 272, 619–625.
Contents lists available at ScienceDirect
Behavioural Processes journal homepage: www.elsevier.com/locate/behavproc
Rapid report
Performance constraints in strength events in dogs (Canis lupus familiaris) William S. Helton ∗ Department of Psychology, University of Canterbury, Private Bag 4800, Christchurch 8020, New Zealand
a r t i c l e
i n f o
Article history: Received 24 June 2010 Received in revised form 22 July 2010 Accepted 29 July 2010 Keywords: Dog Performance Skull shape Strength Structural trade-offs
a b s t r a c t Ecological performance in animals is considered to be constrained by trade-offs between structural and physiological features. Some of the best anatomical and physiological data in support of this perspective are based on findings from canines, in which researchers have demonstrated skeletal shape trade-offs between running efficiency and strength. There has, however, actually been little examination of the link between these structural differences and relevant performance. In this paper I analyze the performance of six breeds of dogs in standardized weight pulling competitions. More brachycephalic shaped (broad headed) dogs are indeed significantly stronger than more dolichocephalic shaped (narrow headed) dogs even when controlling for body mass. Morphological trade-offs between power and running ability may be an important constraint on the evolution of canines and other terrestrial vertebrates. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Ecological performance in animals is thought to be constrained by trade-offs between antagonistic pairs of structural and physiological features (Futuyma and Moreno, 1988; Van Damme et al., 2002). Some of the best anatomical and physiological data in support of this perspective are based on findings from canines (Passi and Carrier, 2003; Kemp et al., 2005). Domestic dogs are the most phenotypically diverse species (Lark et al., 2006). The dog, therefore, provides an excellent opportunity to uncover the links between genes and phenotypes. Researchers have demonstrated with quantitative genetic analysis a skeletal shape trade-off between running capability and strength in both dogs and foxes (Carrier et al., 2005; Lark et al., 2006; Karlamova et al., 2007). This is seen in dog breeds with a continuum between more dolichocephalic (narrow headed) and more brachycephalic (broad headed) dogs. More dolichocephalic dogs, like grey-hounds and huskies, have longer, narrower skulls and relatively small skulls in relationship to their bodies. More brachycephalic dogs, like bull terriers and mastiffs, have broader skulls and relatively large skulls in relationship to their bodies. There has actually been little examination of the link between these structural differences and relevant performance, because performance differences between more brachycephalic and more dolichocephalic dogs are simply assumed. Dogs do compete in a large number of regulated sporting activities making them an excel-
∗ Corresponding author. Tel.: +64 0 3 364 2902; fax: +64 0 3 364 2181. E-mail addresses: [email protected], [email protected]. 0376-6357/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.beproc.2010.07.019
lent model to more directly test relationships between phenotypes and physical performance (Helton, 2009). One set of these regulated canine sports involves a team of dogs or a solitary dog pulling a sled or person on skis (skijoring). These sled sports date back to at least 1886 to the first Saint Paul Winter Carnival. Sled sports are typically associated with dolichocephalic shaped huskies. These northern breeds excel in both the long-distance endurance races such as the Iditarod, which covers ∼1868 km in 8–15 days, and shorter distance ∼20 km sprint races. Huskies appear morphologically suited for endurance running (Coppinger and Coppinger, 2001). Amongst sled-pulling sports there is, however, an exception to husky dominance: weight pulling is a short distance (4.9 m) power competition. Weight pulling may be familiar to many readers as the activity was a feature of Jack London’s book Call of the Wild. The goal in weight pulling is for a single dog to pull a maximally weighted wheeled cart or sled down a short track. Although the original version of the sport was conducted on snow, wheeled competitions are more common and for this analysis wheeled versions were, therefore, examined. The closest human analogue sports for weight pulling would be power-lifting or world-strongest man competitions. Unlike the other sled-pulling sports, in weight pulling running endurance is sacrificed for raw strength and power. In these competitions, while traditional huskies do compete there are just as many more brachycephalic dogs, such as pit-bull terriers and mastiffs. This presents the opportunity to compare these different canine morphological forms, dolichocephalic and brachycephalic, in a regulated performance event. Here I analyze the performance of six breeds of dogs, three more brachycephalic and three more dolichocephalic, in standardized weight pulling competitions.
150
W.S. Helton / Behavioural Processes 86 (2011) 149–151
change in R2 and this was not statistically significant, F1,213 = 2.17, p = .14, R2 = .01, overall model R2 = .27. 4. Discussion
Fig. 1. The relationship between dog weight (kg) and weight pulled (kg) for the brachycephalic () and dolichocephalic (䊉) breeds. Dog weight (mass) is positively correlated with amount pulled as expected (p < .001). Brachycephalic dogs are, however, significantly stronger than dolichocephalic dogs regardless of weight (p < .001).
2. Materials and methods A data set was constructed for 217 dogs using sources available from the International Weight Pulling Association (http://www.iwpa.net/, January 2008). The data set included 108 brachycephalic dogs consisting of 73 American Pit-Bull Terriers, 19 American Bull Dogs, and 16 Bernese Mountain Dogs, and 109 dolichocephalic dogs, consisting of 19 Samoyeds, 20 Siberian Huskies, and 70 Alaskan Malamutes. These six breeds were selected as individuals of each of the breeds were well represented in competitions and formed two distinct samples. The United Kennel Club breed standards for the American Pit-Bull Terrier, the American Bulldog and the Bernese Mountain Dog all indicate a massive and very broad head relative to body size with a relatively short and broad muzzle (e.g. more brachycephalic). The breed standards for the Alaskan Malamute, the Samoyed and the Siberian Husky all indicate a head in proportion to body size with a longer muzzle also in proportion to skull size (e.g. more dolichocephalic). While none of the breeds represent the extremes of brachycephalicy (such as in the Pug) or dolichocephalicy (such as in the Borzoi) seen in modern dog breeds, they do form two distinct groups: one more brachycephalic in skull shape and one more dolichocephalic in skull shape. From the online repository, the dog’s weights and maximal pulled weights were collected. These were analyzed using hierarchical linear regression models with SPSS ©v. 17 software. 3. Results The dogs’ weights and maximal pulled weights are presented in Fig. 1. The brachycephalic dogs were less heavy (M = 31.8 kg, SEM = 1.1) on average than the dolichocephalic dogs (M = 36.8 kg, SEM = 1.2). Nevertheless, the brachycephalic dogs pulled on average more weight (M = 775.8 kg, SEM = 44.3) than the dolichocephalic dogs (M = 549.9 kg, SEM = 31.7). In order to examine the relationship between dog weight (kg) and maximal pulled weight (kg) a two step regression analysis was conducted. In the first step of the analysis dog weight and shape (coded brachycephalic = 1; dolichocephalic = 0) were entered into the model, F2,214 = 38.0, p < .001, R2 = .26. Both dog weight (B = 14.97; t = 7.38) and dog shape (B = 300.79; t = 7.38) were statistically significant, p < .001. In the second step of the analysis the interaction between dog weight and dog shape was entered into the model to test for a significant
An examination of Fig. 1 indicates while there is some overlap, generally more brachycephalic dogs are substantially stronger than more dolichocephalic dogs, when the weights of the dogs are equivalent. These results match expectations based on recent physiological and anatomical research indicating a functional trade-off between power and endurance (Passi and Carrier, 2003; Wilson and James, 2004; Kemp et al., 2005). An alternative explanation could be that these two breed-types are differentially conditioned. There is, however, no reason to believe this to be the case. More likely these results reflect the breed differences in anatomy and physiology found by other researchers (Passi and Carrier, 2003; Kemp et al., 2005; Carrier et al., 2005; Lark et al., 2006; Karlamova et al., 2007). The comparison between these breed-types for running endurance has not, however, been made. This is because there are no brachycephalic dogs competing in endurance sled events such as the Iditarod. Considering the considerable sums made from sponsorship and advertising contracts for winners of these races this may indicate these dogs are unsuitable for such events. The findings presented here strengthen the claims of other researchers indicating a fundamental trade-off between strength and running ability in terrestrial vertebrates. Comparative palaeontology provides additional evidence of these functional trade-offs in skeletal shape. In the canine lineage, for example, Borophagus secundus had a brachycephalic skull shape and filled the ecological niche of bone harvesting (Wang and Tedford, 2008). A more brachycephalic skull shape increases biomechanical bite force (Ellis et al., 2009). Increased bite force is advantageous when crushing bones, during intra-specific contests and for some predatory tactics (Slater et al., 2009). Hyper predators, for example cats, are usually more brachycephalic in skull shape for their respective weights (Wroe et al., 2005; Van Valkenburgh, 2007). Indeed within the extant canine family, the recently discovered coyote-wolf hybrids of the American northeast are more brachycephalic in skull shape and are oriented to hunting much larger prey than their more dolichiocephalic coyote relatives (Kays et al., 2010). Energetically efficient long-distance runners, like canines in general, usually have more elongated skull shapes (Wroe et al., 2005). Speculatively, these results may also be important for understanding human evolution. Researchers have suggested that long-distance endurance running was an adaptation of the homo lineage (Carrier, 1984; Bramble and Lieberman, 2004). Before the advent of projectile weapons, like spear throwers and bows, running prey to their deaths may have been the preferred homo hunting tactic, similar to the tactics used by extant African hunting dogs. The other African apes retain morphologies suggestive of the need for raw strength, not endurance running (Carrier, 2007). Perhaps these differences indicate that intra-specific conflict was not nearly as important in human evolution as hunting based on endurance running. Alternatively, the human adaptation to use weapons in intra-specific conflicts may have enabled increased morphological specialization for running efficiency. References Bramble, D.M., Lieberman, D.E., 2004. Endurance running and the evolution of Homo. Nature 432, 345–352. Carrier, D.R., 1984. The energetic paradox of human running and hominid evolution. Curr. Anthropol. 25, 483–495. Carrier, D.R., 2007. The short legs of great apes: evidence for aggressive behavior in australopiths. Evolution 61, 596–605. Carrier, D.R., Chase, K., Lark, K.G., 2005. Genetics of canid skeletal variation: size and shape of the pelvis. Genome Res. 15, 1825–1830.
W.S. Helton / Behavioural Processes 86 (2011) 149–151 Coppinger, R., Coppinger, L., 2001. Dogs: A Startling New Understanding of Canine Origins, Behavior, and Evolution. Scribner, New York. Ellis, J.L., Thomason, J., Kebreab, E., Zubair, K., France, J., 2009. Cranial dimensions and forces of biting in the domestic dog. J. Anat. 214, 362–373. Futuyma, D.J., Moreno, G., 1988. The evolution of ecological specialization. Annu. Rev. Ecol. Syst. 19, 207–233. Helton, W.S., 2009. Canine Ergonomics: The Science of Working Dogs. Taylor & Francis, New York. Karlamova, A.V., Trut, L.N., Carrier, D.R., Chase, K., Lark, K.G., 2007. Genetic regulation of canine skeletal traits: trade-offs between hind limbs and forelimbs in the fox and dog. Integr. Comp. Biol. 47, 373–381. Kays, R., Curtis, A., Kirchman, J.J., 2010. Rapid adaptive evolution of northeastern coyotes via hybridization with wolves. Biol. Lett. 6, 89–93. Kemp, T.J., Bachus, K.N., Nairn, J.A., Carrier, D.R., 2005. Functional trade-offs in the limb bones of dogs selected for running versus fighting. J. Exp. Biol. 208, 3475–3482. Lark, K.G., Chase, K., Sutter, N.B., 2006. Genetic architecture of the dog: sexual size dimorphism and functional morphology. Trends Genet. 22, 537–544.
151
Passi, B.M., Carrier, D.R., 2003. Functional trade-offs in the limb muscles of dogs selected for running vs. fighting. J. Evol. Biol. 16, 324–332. Slater, G.J.m., Dumont, E.R., Van Valkenburgh, B., 2009. Implications of predatory specialization for cranial form and function in canids. J. Zool. Lond. 278, 181–188. Van Damme, R., Wilson, R.S., Vanhooydonck, B., Aerts, P., 2002. Performance constraints in decathletes. Nature 415, 755–756. Van Valkenburgh, B., 2007. Déjà vu: the evolution of feeding morphologies in the Carnivora. Integr. Comp. Biol. 47, 147–163. Wang, X., Tedford, R.H., 2008. Dogs: Their Fossil Relatives and Evolutionary History. Columbia University Press, New York. Wilson, R.S., James, R.S., 2004. Constraints on muscular performance: trade-offs between power output and fatigue resistance. Proc. R. Soc. B (Suppl.) 271, S222–S225. Wroe, S., McHenry, C., Thomason, J., 2005. Bite club: comparative bite force in big biting mammals and the prediction of predator behaviour in fossil taxa. Proc. R. Soc. B 272, 619–625.