Chewing the Fat Facts: Thoughts on Carnivory, pt. 1

The whole of natural history fascinates me, but everyone has their own favorite topics, and one of the most intriguing subject areas (to me, at least) is predator/prey interactions. Herbivores are interesting in their own right, surely, but for me it is the predators that are the most thought-provoking and impressive. Given this proclivity to ponder carnivory , I've chosen to write up my term paper for a seminar course I'm currently taking (Topics in African Prehistory) on the entrance of hominins into the "Carnivore Guild," especially in terms of hunting small prey in forests, scavenging in grasslands, and evolutionary changes associated with meat-eating by the time Homo erectus appeared on the scene 1.8 million years ago. Indeed, a full discussion of predation or carnivory among mammals alone would take up multiple volumes and so I must narrow my focus a bit, but given some of the recent discussions on this blog about the habits of mammalian predators and meat-eating I thought it appropriate to take you, the reader, along for the ride as I work on my academic project.

Before we can launch into the complicated and contentious subject of hominin diet, hunting, and scavenging, we need to get a handle on what carnivory entails and what sort of adaptations we can expect to see in living terrestrial carnivores in Africa (we'll bring crocodiles back into the equation later on, but for now I think it's best to leave them aside). First off, no matter how obvious the words may seem, major terms needs to be defined for use in this discussion (after Shipman and Walker, 1989).

• Carnivory; Incorporating vertebrate flesh into the diet (either by hunting or scavenging)

• Insectivory; A diet primarily composed of insects and other invertebrates

• Herbivory; A diet primarily composed of plant material

• Omnivory; A mixed diet of flesh, insects, and plants

Note that the definition for carnivory isn't as strict as the others, allowing for animals in other ecological "guilds" to eat flesh, thus plotting out carnivory on a continuum from opportunistic feeding under stress (i.e. Hippos) to hypercarnivory (i.e. sabercats). How, then, did the habit of carnivory arise? We could probably trace back predatory heterotrophic interactions back over much of life's history on earth, different radiations producing different forms of carnivores and herbivores depending on what starting point you picked and what line you were looking at (i.e. sauropod dinosaurs evolved from bipedal, carnivorous ancestors). In the case of large mammalian carnivores in Africa, though, we should probably go back at least to the Miacids, a civet-like group that gave rise to the living Carnivora and the extinct Creodonts that lived between ~33-65 mya*. If we were to push back even further, though, we would likely run into an older related lineage of insectivorous mammals, the ability to find and capture prey being long established in mammals. If this is so, then the behaviors and tactics of modern predators are variations on common themes that have long been present, but there is more than one way for a cat to catch its dinner.

*It used to be thought that the extinct Creodonts were also descended from Miacids, but it no longer appears that this is valid. At present, Miacids are grouped within the clade are a clade Carnivoramorpha, which contains the Carnivora and the Miacoidea (the Miacidae and Viverravidae). (Thanks for the correction, Chris!)

Among large African carnivores (Lions, Leopards, Cheetahs, African Wild Dogs, and Spotted Hyena being of primary importance to us for this discussion), there are a variety of behaviors and modes of catching prey, but we can generalize the strategies into two different modes of prey acquisition. Some predators, like Cheetahs and Leopards, are solitary hunters*, usually relying on stealth and speed to acquire chosen prey and easily driven off their own kills by other predators. Some of the most famous African meat-eaters, by contrast, form social groups that allow them to capture prey much larger than themselves and overcome being relatively slower, coordination making up for less speed and agility during attempts to capture prey. Scavenging is also made easier in a group, too, a large number of carnivores in a group easily running solitary predators off their kills (interspecies and intraspecies conflicts over carcasses being common, as well). Still, there is a trade-off here too; food must be shared amongst the group, and social carnivores have hierarchies that determine who can feed when so participating in a kill doesn't automatically mean receiving a fair share of the rewards (thus generating a need to hunt or scavenge more often).

*Cheetahs sometimes form coalitions of two or three members, usually an all-male group that works to hold down a territory together. Leopards may also hunt in male-female pairs during their mating season, but are normally solitary hunters.

Once prey (or at least meat) is obtained, carnivores must process their food for digestion, starting with the teeth. Carnivores are marked by having relatively small incisors, large canines (which have previously gone to extremes in various meat-eating mammalian lineages), and most importantly a "carnissal shear" made up primarily of premolars. These teeth are specially adapted to slicing through flesh like scissors, and while most cats have all but lost their molars (a sign of hypercarnivory and a specialized concentration on flesh-eating), Wild Dogs and Spotted Hyena have more robust carnissals and molars to assist with cracking bones, also allowing them to have a more generalized diet in times of stress. Still, the carnissal shear is a very important aspect of organisms adapted to predation and carnivory, some lineages (like the extinct Thylacoleo carnifex) convergently arriving at a similar condition to slice through flesh. These differences in teeth allow for differing levels of carcass utilization (Wild Dogs and Spotted Hyena being able to get more out of carcass than cats due to their osteophagy), but once the prey passed the teeth the digestive system must take up the rest of the processing. It is also in this respect that carnivores differ greatly from herbivores in having absent or reduced fore- or midgut adaptations normally used to assist in breaking down plant food, carnivores having a longer small intestine to absorb nutrients gained from the meat. In the end, what is left often comes out as somewhat spiral shaped and very pungent.

As for the ecological niche of large carnivores, these animals typically make up only a very small part of the ecological diversity of a given area, this fact directly relating to 1) the availability of food and the success rate involved in catching it, and 2) territoriality. If predators equaled or outnumbered their prey (which also have to contend with disease, injury, and other factors of mortality) the system would not be sustainable; carnivores would eat themselves out of house and home. Likewise, given the fact that carnivores do not migrate with herds and hold down territories, there is only so much room in a given area for carnivores of a particular species, the high density of felid and canid carnivores now and in the past driving the two groups in different directions as to exploit different niches. Given this observation, it is often strange when we come across massive assemblages of carnivores in the fossil record; if the group doesn't seem to be a family or social group, why were they together? Some of these finds are "Death Traps" in which many carnivores are trapped and die in a small area (often a crevice, pit, or other such trap) over the course of time, but there are other instances where the reason for such a large number of predators in one place is more enigmatic.

Of further interest to us here is the early development of carnivores, especially in contrast to herbivores. Nature documentaries love to chronical the first moments after birth for various ungulates, baby antelope and giraffe getting to their feet and walking soon after birth, but carnivore offspring are far more altricial. They are usually born small, blind, and helpless, relying on the care of their mother (or larger social group) if they are to survive. On top of merely growing larger, they also must learn to hunt and even coordinate with other members in a social group, no small task for any animal. Indeed, cats are especially well known for kicking offspring out as they get larger, subadult males often getting the boot even in social carnivore groups.

This post is far from a rigorous scientific analysis of predation, but is serves to set the scene for what is to follow involving hominins and their interactions with predators. More detail will be added on as we go, but if nothing else I hope this has proven to be a somewhat useful primer or generalizations involving extant mammalian predators and how they make a living in Africa.

References;

Shipman, P. and Walker, A. (1989) "The costs of becoming a predator." Journal of Human Evolution, 18:373-392