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Cold-blooded vertebrate carnivores


Spiders came to the rescue. They can be thought of as eight-legged, hairy lizards, for they are perfectly cold-blooded, operating at a very low metabolic level. Because spiders do not hunt over large territories—a few square yards are the entire dominion of a big wolf spider—spider predation is fairly easy to study in detail. Wolf spiders catch most of their prey on the move as they prowl their turf. (Robotic dinosaur)Hence the analogy between a wolf spider and a cold-blooded vertebrate predator such as a carnivorous lizard or a possibly cold-blooded Allosaurus is a good one.


If metabolism determined the abundance of predators, then spiders should produce huge populations compared to their prey. In Africa's game parks, the ratio of predator to prey among mammals is 1 percent or less—that is, there is roughly one lion or hyena for every one hundred large prey animals (zebra, wildebeest, warthog, bushbuck, etc.). (dinosaur factory)Spiders have such a low metabolism that they could, in theory, reach a ratio ten or twenty times higher. And so they do. Study after study showed spider populations achieving levels of 10, 15, and 20 percent of their prey populations, impossibly high by mammalian standards. There's no doubt, of course, that spider ecology is complicated, and that they suffer from the usual share of parasites, diseases, and disastrous dieoffs from bad weather. Nonetheless, their cold-blooded metabolism does, on average, show through this overlay of ecological noise. Predator-to-prey ratios work for spiders; they correctly indicate cold-bloodedness.


Would such ratios test equally well for big, cold-blooded vertebrate carnivores? In today's world there exists no predator-prey system in which both predator and prey are large, cold-blooded vertebrates. (walking dinosaur costume)Pythons (cold-blooded) feed on deer (warm-blooded), and Komodo dragon lizards (cold-blooded) kill pigs and tourists (warm-blooded), but nowhere does a giant lizard or snake feed on giant lizards or snakes as its principal prey. To test the predator-to-prey method of analysis for this case, I had to go into the fossil record, back to the earliest land vertebrates that evolved into the role of large top predator. A top predator by definition is a carnivore that eats the flesh of the largest available prey. It must therefore develop adaptations for dismembering the carcasses that are too large to swallow whole. The earliest vertebrates that evolved the requisite meat-slicing teeth were the finbacked reptiles, which first appeared very late in the Coal Age, about 300 million years ago.

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