Posts Tagged ‘Carnivore’

The Business of Eating

Sunday, November 14th, 2010


Though at first glance it may seem silly to use such an example to illustrate the relationship between poisonous snakes and other animals, it is not silly at all. Both the rattlesnake and a bison, say, are potentially dangerous to each other. Neither can get the slightest good out of contact with the other. On the other hand, both can profit immensely by staying cleanly out of one another’s way. What, then, is more logical than that the snake should evolve a warning device, and the potential enemy – the inadvertent trampler – the psychology to react to the warning? Even a carnivore that usually ate snakes – unless it was immune to snake venom – would logically be better off if it had a heritable ability to recognize, or to learn to recognize, harmful snakes. Then it could go about its business of eating harmless snakes without any trouble. The candy-stick coloration of coral snakes would surely entrench any such discriminatory capacity as might be found naturally in a coon or a hawk or any other snake-eating predator. So would the rattlesnake’s hair-raising song.

The idea of a poisonous animal evolving a warning device that will work only if a potential enemy also evolves the sense to react to the warning is hard for some people to accept. I do not know why this should be so. Besides logic, a great store of anecdotal evidence supports its reality. Nearly any mature Florida bird dog, for instance, reacts instantly to the sound of a rattlesnake. While it is hard to be sure what a dog has learned from previous experience or from other dogs, it can in most cases I know about be confidently said that the learning  process did not involve being bitten by a rattlesnake. A pointer I used to hunt with in central Florida, though it had never been bitten by a poisonous snake in its life, showed unmistakable evidence of associating the rattle with a particularly odious situation. In its quartering for quail, if you saw it suddenly jump into the air and you went to the spot to see what had scared it, one of two things was most often there – a coiled diamondback, or a bush of a certain species of Crotalaria, the dry pods of which rattle when disturbed, almost like a rattler’s alarm. Only one of the several species of Crotalaria sounds authentically like a snake, and only that species used to make my dog jump. But the effect of a collision with that was electric, and for all the years of its life the dog rose like a bird when it stirred the fearful noise from a diamondback or from the bush that I think sounded the same to the dog.

That is of course not a scientific observation. It involves a subjective judgment on my part, and the behavior of only one dog. And it in  any case leaves unanswered the question whether the reaction is innate in canines or is learned by associating the sound of the rattle with the bites or aggressive behavior of snakes in general – or is learned from other dogs. That dogs are or Old World, and rattlesnakes of American origin, makes it seem unlikely that the pointer was born genetically able to associate the sound with the snake. On the other hand, the buzzing of rattlesnakes is really just an elaboration of a tendency of many kinds of snakes to vibrate the tail when approached by a potential enemy. The vibration is often soundless, but in dry leaves it makes a little rattling or humming noise. Possibly dog ancestors evolved the capacity to associate such a sound with ill-tempered or dangerous snakes. But the important point is that the rattle of the rattlesnakes makes little sense unless it can be thought of as an agent of advantage to the bearer. And the advantage in not being stepped on by a bison or chopped up by the teeth of a wolf seems pretty clear. That the bison and wolf might go away poisoned and die would be little comfort to the snake. Its profit would come from preventing the encounter from happening.

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Advantages of an Erect Posture

Saturday, December 19th, 2009


The erect posture of dinosaurs is often said to be the key to their success. Why is this? An important reason is that an erect posture is mechanically more satisfactory than a sprawling one. The weight of the body is supported entirely from below. In a sprawler, the weight of the body is supported from the sides. While gravity effectively pulls straight down from the center of the body mass, in a sprawler this force has to be converted into a sideways component along the femur or humerus (upper arm bone), and then a vertical component down the tibia and fibula, and the radius and ulna (forearm bones), which causes great stresses are avoided if the gravitational force of the animal’s mass is transferred down through a straight, erect limb.

This mechanical advantage is important. Firstly, erect animals can run in a more sustained way: not necessarily faster, but with more stamina, because the effort of supporting the body weight is much less than in a sprawler. This would have been an immediate advantage to an archosaur chasing sprawling prey animals or escaping from a sprawling carnivore. Interestingly, the main plant-eating groups of the Middle Triassic, the pig-like rhynchosaurs and dicynodonts, were evolving semi-erect gaits at the same time. Indeed, the ancestors of the mammals, the cynodonts – which were moderate-sized carnivores at that stage – also showed similar advances.

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