Brains Versus Brawn Amongst Wild Canids
Continuing the theme from my last post, I'm going to cover a new study involving some of the carnivores that I'm observing and studying out here in Kenya. Last time we talked about mongoose, and this time we'll move on to one of my favorite mammalian families: the Canidae.
Few things are more important for a carnivore's survival than having a lethal bite. The critical mechanics underlying bite force have significantly influenced carnivore evolution--they determine morphology, hunting behavior, and prey selection. As such, these adaptations carry both evolutionary and ecological significance.
Another, less tangible aspect of carnivore life is also tightly linked to hunting behavior: social structure. Individuals that hunt in cooperative groups, such as wolves (Canis lupus), can subdue larger prey than if they were hunting solo. For example, a pack of grey wolves can take down an elk (Cervus canadensis), which weighs up to 5 times as much as a lone wolf.
Just as hunting behavior influences sociality, sociality is thought to have influenced the physical evolution of carnivores in turn. The “social brain hypothesis” suggests that social species need larger brains than solitary species. Within this framework, social species need more cognitive capacity to process the communication and coordination required to both hunt cooperatively and manage social dynamics within the group (Dunbar 1998; Holekamp 2007).
So, we have two significant lifestyle issues influencing carnivore evolution: physical structures needed to provide the power to subdue prey, and social dynamics that also influence physical structures by requiring larger brain volumes. How do these two influences on carnivore cranial structure interact with one another?
Some research has suggested that there are physical trade-offs between brain and brawn, with the need for strong jaws putting a constraint on brain volume. For example, Wroe and Milne (2007) examined carnivorous marsupials and eutherians and found a trade-off between brain volume capacity and the strength of primary jaw adductors. This effect raises the question of whether bite force, by constraining brain volume, can also influence the evolution of social structure.
Recently, a group of researchers from Brazil and the U.K. undertook a study to investigate this issue for a specific group of carnivores, the family Canidae (Damasceno et al. 2013). Canids are especially interesting in this framework, because they display a wide range of morphological and ecological traits as well as a broad spectrum of sociality. The researchers explored the relationship between bite force and brain volume and compared these measurements across all of the world’s canid species.
The authors used two metrics for both bite force and brain volume. For brain volume, they first used raw volume as calculated from cranial measurements. Because brain volume is known to be influenced to some degree by body size, they also calculated a “brain volume quotient” (BVQ) by dividing brain volume by total skull length.
Rather than just using jaw adductor strength, bite forces were determined via the “beam theory,” which uses the cross-sectional area of cranial muscles that are critical to the biting/holding motions used to subdue prey, in addition to the distances between some of the muscles and critical joints, such as the temperomandibular joint in the jaw. Once again, bite force is known to be influenced by body size, so a correction factor was applied to produce a “bite force quotient” (BFQ).
The analyses involved two main questions:
1) Do species with the largest absolute brain size also have the largest absolute bite forces, and are BVQs and BFQs similarly correlated?
2) Do species with the biggest brain volumes have the weakest bites?
The results were intriguing and not entirely what one might expect. First, although absolute brain volume and absolute bite force were correlated across the board, BVQ and BFQ were not. Interestingly, the only species that did show strong correlations between BVQ and BFQ were “group hunting hypercarnivores,” species that hunt cooperatively and eat nearly entirely meat (in contrast to the high degree of omnivory found in some of the smaller, less social species). These species included the dhole (Cuon alpinus), bush dog (Speothos venaticus), and African wild dog (Lycaon pictus).
Despite the expectations of the “social brain hypothesis,” which predicts that high sociality leads to larger brains, the grey wolf (Canis lupus) ranked 10th out of the 32 species in terms of BVQ. In fact, four small, non-social foxes ranked higher than the wolf on this measure. The wolf is both hypercarnivorous and a group hunter, though, so what’s going on here?
One of the biggest structural contributors to bite force is tooth row reduction (Van Valkenburgh 2007), which is strikingly apparent in other carnivore groups, such as the Felidae and Mustelidae. Most canids have 2 upper and 3 lower molars, but bush dogs have reduced their molar count 1 upper and 2 lower, dholes have 2 upper and 2 lower, and African wild dogs have the standard 2 upper and 3 lower. The wolf, however, has the same number of molars as the African wild dog, showing no significant specialization.
So, the African wild dog actually doesn’t deviate from the standard canid molar count, despite having one of the top BFQs, and the wolf has the same molar count as the wild dog, but has a pretty unimpressive BFQ. Besides tooth row reduction, how else, might bite force increase brain volume? An example of another adaptation that increases both parameters is the widening of the occipital bones, which both increases intracranial space and strengthens the neck muscles, simultaneously providing more “brain space” and increasing the ability to subdue prey. The other hypercarnivores with the highest BFQs in this study also have relatively wide snouts, deeper jaws, larger anterior teeth, and elongated trigonid blades on the first molars. All of these features distinguished the high-BFQ/high-BVQ hypercarnivores from the wolf.
Thus, the authors suggest that the failure of the wolf's high sociality to push it up in the BVQ or BFQ rankings suggests that skull shape, not sociality, might be the biggest determinant of BVQ. The authors also note that although the three species with the highest BVQs also happened to have the highest BFQs, the two metrics were not correlated across the board, showing that they are independent variables amongst canid species.
Another interesting insight that emerged from this analysis is that all of the canids with the very lowest BFQs are desert species: side-striped jackals (Canis adustus), kit foxes (Vulpes macrotis), Ethiopian wolves (C. simensis), and bat-eared foxes (Otocyon megalotis). The authors suggest that arid, resource-poor environments force carnivores to be opportunistic feeders, negating the benefits of skull morphology optimized for hypercarnivory. Bat-eared foxes are invertebrate specialists, and unsurprisingly had skull morphologies least like those of the hypercarnivores, with 4-5 molars.
The results of this study are fascinating and should spur further research with other carnivore taxa. Even the canids with the most forceful bites are no competitors for the bite forces of some non-canid species within the Mustelidae, Hyaenidae, and Dasyuridae (Wroe et al. 2005). Thus there still might be a brain-brawn trade off between families. In addition, measuring intelligence in animals (and humans, for that matter) is difficult at best, and the jury is out on how brain volume actually relates to metrics we would refer to as "intelligence."
The primary take-home message, though, is that these data strongly challenge the idea of such a trade-off within the Canidae, showing that once bite force and brain volume are adjusted for body size, increasing bite strength actually may facilitate higher brain volumes, the largest of which appeared only in species with the complex social systems required for cooperative hunting. In other words, these hypercarnivores are probably not dumb jocks. As we saw with the gray wolf, being social may not be enough to boost brain size on its own; structural features related to bite force appear to be key as well. Perhaps the wolf’s bark is worse than its bite after all?
Damasceno, E., Hingst-Zaher, E., & Astúa, D. (2013). Bite force and encephalization in the Canidae (Mammalia: Carnivora) Journal of Zoology DOI: 10.1111/jzo.12030
Dunbar, R.I.M. 1998. The social brain hypothesis. Evol. Anthropol. 6, 178–190.
Van Valkenburgh, B. 2007. Déjà vu: the evolution of feeding morphologies in the Carnivora. Integr. Comp. Biol. 47:147–163.
Wroe, S., McHenry, C. & Thomason, J.J. 2005. Bite club: comparative bite force in big biting mammals and the prediction of predatory behaviour in fossil taxa. Proc. Biol. Sci. 272:619–625.
Wroe, S. & Milne, N. 2007. Convergence and remarkably consistent constraint in the evolution of the carnivore skull shape. Evolution 61:1251–1260.