Because of their metabolism. They need more oxygen.
And more oxygen means more radicals. More radicals means shorter life.

This is the general answer, there are always animals that are exceptions.

I was watching animal planet the other night (i know very credible but hey…thats not the point), and a zoologist was saying that kangaroo rats and african elephants have the same amount of heartbeats in their life its just that the kangaroo rats heart beats much faster…what do you guys think? I find it interesting.

Yup, I’ve also heard that thing and it’s said that a Kangooro rats can beat up their hearts more that 400 times per minute..:shock:

Small animals loose heat much faster than big ones. Maybe their hearts beat faster to supply more energy for heat production.

Just a guess though.

Weight specific metabolic rate increases as size decreases. This means that small animals have higher weight specific metabolic rates than larger animals and thus higher oxygen requirments. Note that the key here is "weight specific". A rhino obviously eats more food in a week than a mouse. However, if you pile up the weekly food intake of a mouse and a rhino side by side, you will note that the mouse food pile is many many times the size of the mouse. When comparing 1 gram of mouse tissue to 1 gram of rhino tissue, the mouse will have a higher oxygen requirment. The proportionality of this relationship comes into play because while smaller animals have much higher oxygen requirments per unit tissue, they do not have the larger hearts to match to scale. Therefore, the heart of smaller animals must increase cardiac output to meet oxygen and metabolic needs by increasing heart rate. (Also note…larger animals with much larger hearts are going to pump alot more blood during each contraction).

if we look at animals of increasing size, their volume and surface area increase. that "volume" of animal is of course made up of cells, organs, in short, living tissue. That tissue of course creates a lot of heat, and some of that heat must be dissipated. Exactly how much and how fast that heat is dissipated depends on the surface area of the animal.

So, for a LARGE ANIMAL: it has more volume (=produces more heat), less surface area (=less able to get rid of that heat). It therefore must have a slower metabolism (produce less heat in the first place) than a animal smaller than it.

Or, to look at that another way: imagine if we took a mouse and magically transformed it into the size of a cat, but kept its metabolic rates the same. The heat produced by the mouse would increase by about one hundred times (because of the increase in volume), but the surface area to get rid of that heat would increase by about 22 times. That would be one red-hot giant-sized mouse!

Is this true for all animals? If not, which animals is it not true for and what would cause that?

It is hard to say universally true. But generally, yes. Some animals simply have a slow metabolism because of their specific life stiles.

Is there any role of gene in the size of the animals? As i know the food intake depends on the activity of aminals, thats why the animals during hybernation utilize there stored energy.

Could you please rephrase your question, i don’t quite understand

I mean, size of a animal is also depend on its genetic makeup and matabolic rate is directly related to the amount of intake food and activities of an animals.
Thats why animals during hibernation or astivation have low metabolic rate.