Bears of the Pleistocene

[brotherbear]

www.prehistoric-wildlife.com/species/a/arctodus.html

Arctodus - predator or scavenger?
       With such an immense size and obviously powerful jaws it’s tempting to paint a picture of Arctodus as an apex predator that could kill anything it wanted. However, real science is not based upon assumptions that are established upon superficial glances but on in depth study of the available fossil material. With this in mind the analysis of Arctodus remains has led to a surprising but very plausible theory about its nature and behaviour.
       Many Pleistocene mammal fossils have been subjected to a process termed oxygen isotope analysis. This is based upon the principal that different environments have different isotope levels which get absorbed by the plants growing on them. As these plants are eaten the isotopes are absorbed and stored in the herbivores tissues as a marker that allows palaeontologists to establish which types of animal were active in which environment and what they were eating. In turn as these herbivores were killed and eaten by carnivores the isotopes get re-absorbed into the carnivores’ bodies which reveal roughly which animals were being eaten by which carnivores (for example the sabre toothed cat Smilodon seems to have had a preference for bison).
       The analysis for Arctodus shows that it was what is termed a hypercarnivore, an animal that has a diet where seventy to a hundred per cent of the eaten food is the tissue from other animals. However it also revealed that Arctodus ate all kinds of animals, and did not specialise in just one type of prey, something that is highly unusual for a predator, but quite normal for a scavenger.
       The skeleton also reveals hints to both the travelling and predatory ability of Arctodus, with special reference to the long limbs. These could be seen as giving Arctodus a significant reach advantage that allowed it to swipe at prey animals, but the problem here is that first Arctodus would have to get close enough to its prey to do this. In terms of speed the long legs with their broad strides are thought to have given Arctodus a top speed approaching fifty kilometres an hour, something that would have seen it able to comfortably match most of the available prey species.
       However these same legs are proportionally much thinner than they are in other running animals, and are considered too fragile to be able to support a heavy animal like Arctodus if it made a sharp turn when running at speed. This could mean an injury such as a break or dislocation that probably would have been serious enough to cause the death of the injured bear as it could no longer move about. But it is actually these long legs that further support the scavenger theory as since they are lightweight they would not require a great amount of effort to move. Additionally the long sweeping arc of the feet meant that Arctodus could comfortably cover more ground with each step, making locomotion such as walking or even running extremely energy efficient. This means that Arctodus could cover territories that spanned several hundred square kilometres on a reduced amount of food than would be required by a dedicated predator. This is a vital survival adaptation when you consider that a scavenger does not know when or where its next meal is coming from.
       Another clue comes from the immense size of the body. Arctodus simus was the largest carnivorous mammal currently known from Pleistocene North America, and possibly the largest carnivorous animal since dinosaurs like Tyrannosaurus disappeared at the end of the Cretaceous period. As such it’s extremely unlikely that the other Pleistocene predators would have put up much of a fight and risked injury or even death from a more powerful animal. This behaviour has been well documented in modern times where grizzly bears will walk in and steal kills away from packs of grey wolves that let the more powerful predator take what it wants rather than risking injury.

The final support for Arctodus being a scavenger comes from the skull. Arctodus is known as the short faced bear because its snout is proportionately shorter than that of other bear genera. These means that when food is placed in the mouth it is nearer the fulcrum (point of articulation) of the skull and mandible (lower jaw). This focuses a greater amount of pressure from the jaw muscles onto whatever is between the jaws, and seems to have been an adaptation that allowed Arctodus to crack open bones to get at the marrow within them. This is another key survival adaptation as Arctodus would inevitably come across carcasses that had already been picked clean, but the bones still in place because the predators that killed the animal were unable to crunch the bones open. Even more critical to survival is the fact that bone marrow can remain nutritious for months and even years after an animal dies, something that would help Arctodus to survive even when there were no fresh kills to steal. Also fossil evidence of large bison bones exist that look like they have been bitten open by an animal like Arctodus, a feat that would be beyond the scope of smaller predators like wolves.
       Arctodus also had a proportionately large nasal opening in the front of its skull which indicates that it was capable of sampling a larger volume of air for scents. This coupled with the bears larger size meant that it could sniff out and sample scents that were being carried higher up, possibly to the point of detecting a carcass from several kilometres away by smell alone. All of these factors combined point to the short faced bear Arctodus being a much specialised scavenging animal. Reply