In the last post, we considered the possibility that carnivores began as animals that ate invertebrates. An example of an insectivore that is closely related to carnivores is the sloth bear. The bears provide another example of a possible origin for carnivores. While sloth bears are insectivorous, other bears are principally herbivores. The classic example is the panda, but the South American spectacled bear, also called the Andean bear, is also principally an herbivore. While these two bears are herbivores, the polar bear is almost exclusively a carnivore. Could carnivores simply have begun as herbivores and then gradually shifted to being carnivores?
Before we answer that question, I do what to address a related issue. The panda is often cited as an example of an herbivore with carnivore teeth. The conclusion is often reached that teeth cannot reliably tell us what animals actually ate. This is not entirely true. Yes, pandas, along with the entire bear family, are put in the Carnivora, which is the order that contains all of the living mammalian carnivores. One of the defining characteristic of Carnivora are their carnassials. The carnassials are blade-like teeth. They are the last upper pre-molars and the first lower molars. These blade-like teeth slide past each other, which not only allows them to be used effectively for slicing flesh off their prey, but also keeps them constantly sharp.
First of all, note that it is pre-molars and molars that are used to identify Carnivora, not the canines. The canines are the long, sharp teeth seen when a dog bares its teeth or a cat hisses. While Carnivores use their canines to kill or subdue their prey, these canines are not the characteristic teeth of Carnivora and are not used for actually processing their food. Rather, that is the job of the blade-like carnassials. Second, while all members of Carnivora have carnassials, they do not all have the same style of carnassials. Compare a dog and a cat, for instance. Dogs have long mouths with several molars and pre-molars aside from the carnassials. In contrast, a cat has a shorter head and fewer teeth, so its large carnassials make up a much larger portion of the teeth. This is why cats are often described as hypercarnivores while dogs are simply called carnivores. With their various molars and pre-molars, a dog can process a wider range of foods than just meat. Indeed, one type of dog, the raccoon dog, is actually omnivorous. Cats, in contrast, have molars dominated by the carnassials and are pretty much restricted to eating meat.
When we consider pandas, they have carnassials, but their carnassials are small and they have other, large, broad crushing molars. While a panda can process meat (and they do occasionally eat meat), their broad, crushing molars are much more suited to crushing and chewing bamboo. The short end of the story is, while pandas do indeed have carnivore teeth, in the sense that they have carnassials, their teeth are overall better suited to grinding up bamboo, which is appropriate given their diets. We must consider all of the teeth in an animal’s mouth when considering its diet, and when we do that, we can actually make a pretty good guess about the normal foods it regularly eats.
Going back to the bears and the way they may have become carnivores, all living bears, including the panda, belong to the family Ursidae. This strongly indicates that they all belong to the same kind. Thus, if God first originally created a panda or a spectacled bear, then these herbivorous creatures would have fit in quite well with the original creation, which was composed exclusively of herbivores. However, these original bears would still have the genes necessary to make an omnivorous black bear or a carnivorous polar bear. After Adam’s sin, when death came into the world, is it possible that there were no changes made to bears, that God simply let the carnivorous genes begin to make an appearance, which gradually led to the herbivorous bears diversifying into omnivores, carnivores, and insectivores?
Such an explanation is possible. It works with bears. As mentioned before, there is an omnivorous dog, so it is possible to think of a completely herbivorous dog that gradually become carnivorous once its carnivorous genes began to be expressed. However, there are some carnivores that are simply difficult to envision as herbivorous. Take the cats, for instance. While cats will on rare occasions eat fruit, it is difficult to think of a completely herbivorous cat. That is partly because they are hypercarnivores: there are no known omnivorous or herbivorous cats. Could it be that there once was an herbivorous cat, but its genes are no longer expressed in any cat population? Maybe, but without any direct evidence of such an animal, its existence is simply speculative.
Besides, cats are not the most difficult animal to think of as herbivorous. That would be the pit vipers. The vipers are a family of snakes (Viperidae) that consists of venomous snakes with folding fangs. The pit vipers are a subgroup that not only have folding fangs, but also have heat-sensitive pits in their faces. Familiar pit vipers include the rattlesnakes, water moccasin, and copperhead. The pit vipers are exclusively carnivores, typically feeding on small, warm-bodied animals. Indeed, everything about the pit vipers seems to be designed to hunt and kill small mammals. The heat-sensitive pits allow the viper to detect the location of its prey. Even in the dark, small mammals give off a heat signature that can be detected by the snake. The fangs are typically folded up against the roof of the mouth, only folding out when the viper prepares to strike its prey. The fangs then act as hypodermic needles to inject a venom, which consists of a suite of enzymes that destroy blood and blood vessels, into its prey. Once the venom is injected, the viper typically lets its victim go, permitting it to die of the venom while running away. The snake then tracks down its fallen prey, using its tongue to pick up its sent. Once it has caught up with its now dead prey, it eats it.
How can this suite of traits, venom, hypodermic needles, and heat sensitive pits, be used to eat plants? Plants do not give off a heat signature, thus the pits would be useless. Plants have no blood, so the venom is useless (not to mention, plants don’t run away). The fangs are not used for holding or for swallowing the prey, so they are not useful for much other than to inject venom. How could any of these traits be used for eating plants? It may be possible to come up with creative ways these traits were used for eating prey, but with no known herbivorous or omnivorous pit vipers to show us how that might work, such an herbivorous viper is completely speculative. It is actually much easier to accept that the pits, fangs, and venom of pit vipers are specially designed for eating small animals, thus raising the question, how did something like a viper get to be so specialized for eating other animals? How could it have originally been an herbivore?
There are two more considerations when we think of the origin of carnivores, but we will leave that until the next post.
Thoughts from Steven