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

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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

I believe the idea of a direct carnivoran-creodont connection has pretty much fallen by the wayside. I forget the exact details, but IIRC the carnassial shear in creodonts is composed of different teeth from in carnivorans, indicating that it probably arose independently. Miacids would then be only ancestral to carnivorans, not creodonts.

I've been waiting for a few years now for someone to work out where in the mammalian family tree creodonts do fit in - they're one of a number of fossil mammalian groups that recent restructuring of the understood relationships between living taxa has rather cast adrift.

Cool post. The gym I go to gets satellite, and I usually watch National Geographic while on the treadmill. Usually I see shows about large predators at the time of day I go, and I never get sick of it.

It is unfair that the large carnivores get all the attention (hey, the herbivores do lead interesting lives as well!), but let's just face it: we are all extremely fascinated with predators. Perhaps it's an artifact of a time when we were a lot more vulnerable, and paying close attention to the predatory animals made the difference between life and death.

Besides, watching wildebeest chew grass gets somewhat boring after a while. =)

Thanks for the reminder, Chris; I had forgotten that the Carnivoramorpha was formed and excluded the Creodonts. It all just keeps getting bushier and bushier...

>> 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

> I believe the idea of a direct carnivoran-creodont
> connection has pretty much fallen by the wayside.
> I forget the exact details, but IIRC the carnassial
> shear in creodonts is composed of different teeth
> from in carnivorans, indicating that it probably arose
> independently. Miacids would then be only ancestral
> to carnivorans, not creodonts.

And this is just the beginning of our troubles...
"The Miacidae is not a monophyletic group: it is a paraphyletic array of stem taxa.(...) The Miacoidea are regarded as basal carnivoramorphs. Some species of the genus Miacis evolved into modern day carnivores of the Order Carnivora, but only the species Miacis cognitus is a true Carnivora." (citation from the wikipedia entry on miacids). To confuse matters even more, some palaeontologists consider the Viverravidae (one of the two families that traditionally form the Miacoidae) the ancestors of the cat-like carnivores and the Miacidae (the other family) the ancestors of the dog like ones. In this case, it would rather make more sense to include the viverravids in the aeluroids and the miacids in the arctoids or cynoids then to lump them together in one superfamiliy Miacoidae.

This said, the miacids are essentially Laurasian critters, and I have never heard that they made it to Africa. The hyaenodonts, however, were clearly living in Africa before a land-bridge between Africa and Laurasia existed. In fact, Palaeogene Africa was hyaenodont heaven. They were the only large terrestrial predators - or at least the only large mammalian predators, I don't know about ziphodont crocs - there until Africa's geographic isolation ended in the Miocene. If Africa was their stronghold, it might have been their cradle, too - wich would probably mean that they were not creodonts at all, but either afrotheres or glorified cimolestans (the latter would place them outside Placentalia - cimolestans were Eutherians, but not proper placentals). This would leave the oxyaenids as the last remaining creodonts. But, of course, we find hyaenodonts on both sides of the Thetys, so the question of their African or Laurasian origin remains open. Phew...

Quote from johannes

"In fact, Palaeogene Africa was hyaenodont heaven. They were the only large terrestrial predators - or at least the only large mammalian predators, I don't know about ziphodont crocs - there until Africa's geographic isolation ended in the Miocene."

That might not be completely true; the Fayum fauna also contains some material from the enigmatic avian Eremopezus that appears to have been a cassowary-sized and probably flightless carnivore. Unfortunately, Eremopezus, which used to be classified as an aepyornithid (probably because of size and biogeographic reasons)is extremely mysterious and virtually nothing can be said of its lifestyle and even less of its appearance and phylogeny.

Thanks, Brian!

I have forgot about Eremopezus, although Darren Naish had mentioned it in his post about giant snakes (see here: http://scienceblogs.com/tetrapodzoology/2007/05/stupidly_large_snakes_t…, but be warned, it contains pictures from Anaconda, one of the worst movies of all time, and even mentions Jennifer Lopez). BTW, the madtsoiids mentioned in the same post might have been potential predators of the Fayum megafauna, but this depends on whether they were able to unhinge their jaws like modern snakes do when they swallow large prey (like J-Lo).

> J-Lo was able to unhinge here jaw for
> swallowing large prey...

Sounds possible, but would require some interisting modifications of the synapsid Bauplan (an extra set of pharyngual jaws to drag down prey?). :-)