The Sacro-Iliac Joint of the Felidae and Canidae and Their Large Ungulate Prey: An Example of Divergence and Convergence

Abstract

AbstractThe aim of this chapter is to discuss the evolution of the shape of the sacroiliac joint in two carnivoran lineages (Felidae and Canidae) and their large prey (Ungulata) in the context of divergent and convergent evolution. The significant difference in the angle between the iliac wings of the pelvic girdle in the transverse plane (the interiliac angle) between the Ungulata (>100°) and both carnivoran lineages (30–40°) suggests a divergence in form that relates to the evolution of their feeding behavior over at least 75 Myrs. In the Canidae, the interiliac angle of around 40° and the inner C-shape of the iliac auricular surface congruent with the sacral auricular surface are not influenced either by locomotor nor predatory behavior. Hunting on small or large prey has had no impact on the sacroiliac joint of canids, even though solitary hunting of small prey switches to pack hunting of big prey. A hunting strategy based upon the harassment of large prey individuals could explain why the locking properties of the sacroiliac joint, determined by the interiliac angle, and the inner shape of the articular surface have not been influenced by prey selection. These joint properties are similar to those of felids that select prey with body-mass lower than their own. We suggest that the similarities recorded in canids and these felids result from convergent evolution due to prey selection even though their hunting strategies are different. In contrast, the interiliac angle is significantly smaller, and the locking properties of the joint are increased through a strong congruency of the W-shaped inner surface and the outer ridge in solitary big cats that are able to exploit prey with body mass greater than their own, These traits, resulting in a stiff sacroiliac joint, especially during recoil, are probably explained by attributes of the feeding behavior that require a sustained bite during the killing of prey. In lions, the interiliac angle is similar to that of canids, suggesting a relaxation of functional constraints relating to feeding behavior in a species in which individuals organize into social groups for pack-hunting of large prey. This chapter considers the role of divergent and convergent functional evolution of feeding strategies on the morphological traits of the sacroiliac joint that permit us to discuss the “form-function” relationship of this key articulation of the pelvic girdle in the Carnivora.KeywordsAuricular surfaceBitingCongruencyHunting strategyInteriliac angleKillingPostcranial systemPrey/Predator mass ratioRetreat movementTopography