What governs the size of animal bodies?

Question

Million of years ago there lived dinosaurs up to 36 m high. About 10,000 years ago lived Machairodontinae -compared to cats today they were huge. About 400,000 years ago lived Mammuthus trogontherii: it reached over 4 m at the shoulder. And these are just some examples.

Todays biggest living creatures look small compared to previous. So my question is, why are all creatures getting smaller? Why doesn't evolution generate any big creatures with size comparable to the dinosaurs?

The blue whale http://en.wikipedia.org/wiki/Blue_whale is the largest animal that has ever existed on Earth, and it is available to ride right now. — Alan Boyd, Sep 22 '14 at 12:26
For the record - both my comment and that from Mike Taylor make no sense now that the question has been edited. It originally mentioned wanting to ride a dinosaur. — Alan Boyd, Sep 22 '14 at 15:37
This question about dinosaur size might be helpful. African elephants would fit right in with many of the largest (non-sauropod) dinosaurs. — user137, Sep 22 '14 at 18:40
This post is so relevant to this question that it might almost be a duplicate. — Remi.b, Sep 22 '14 at 20:46
It's strange that nobody has mentioned human impact. Consider the following. — Resonating, Sep 23 '14 at 00:21

score 2 · Answer 1 · answered Sep 22 '14 at 14:44

I agree with other commenters that we are not necessarily living in a time of small body masses.
Here is a paper on a body mass distribution. It's quite mathematical, but the first author (Aaron Clauset) is good.
For individual body masses, you can look at Kleiber's law - this says that body mass and metabolism are related by a power law.

score 2 · Answer 2 · edited Dec 14 '14 at 13:34

There are lots of parameters that influence body size. Here is a non-exhaustive list:

visibility to predators
ability to hunt bigger preys or to manipulate smaller preys
running faster
be more impressive
fighting ability
energy consumption for maintaining a body
Quantity/quality of food to be found
homothermy (heat loss)
sustaining its own weight
Rooms for the organs
Attractiveness to the other sex
being more able to move freely (in viscous environment (water))
mechansisms for bringing nutrients and air to the tissues
density of the body (water)
excretion of wastes
moving in small habitats
Hiding in small areas
bearing big and complex structures
Parental care
flying better
artificial selection
etc..

Saying that today's species are smaller than previous species is maybe over generalize. For example, the largest species that is thought to have ever lived is currently extant, it is the blue whale (it can weight more than 200 tons). The tyrannosaurus is roughly the same weight that the african elephant (7 tons). However it is true that insects were typically bigger before than they are today. It is also true the the sauropoda are particularly impressive as some of them were 3 or 4 times taller than a giraffe and weight around 100 tons. [Bruhathkayosaurus](The http://en.wikipedia.org/wiki/Bruhathkayosaurus), the largest land animal that ever lived may have had a weight comparable to the blue whale!

A common argument to account for the big size of dinosaurs and especially of insects during the triassic is the oxygen concentration in the atmosphere (ref). It is particularly relevant for insects as they have no hemoglobin, meaning that the oxygen has to be dissolved into their body liquid (mostly water) to be brought to the cells. From Henry's law, the concentration of a gas such as oxygen in a liquid is a function of the concentration of this gas in the atmosphere. Therefore, the highest is the oxygen concentration, the easiest it is to bring oxygen to all the cells of a big body and therefore the lowest is the cost of having a big body.

What governs the size of animal bodies?

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