Why are animals today smaller

shadowcomplex

Then their prehistoric cousins? I mean imagine if the creatures that existed in the sea at that time existed today, deep sea fishing would be out and any boat smaller than a ship wouldnt be sea worthy

Capt'n Midnight

Cod used to be 5 foot long when Europeans first fished off Newfoundland
overfishing means they are a lot smaller now.

Megladon was one of the biggest carnivores ever and lived relatively recently.

Excluding the sauropods the largest mammals were as big as other dinos. Humans are probably the leading cause of the mega fauna extinctions, excluding Africa, of the last few tens of thousands of years. Roll the clock back a bit further and ice ages were the big killer.

Can't remember the names but IIRC about 5 million years ago there were members of the elephant family that were several times the weight of today's largest ones.


Blue whales are probably the largest animals that have ever existed.

Sperm whales are pretty tough buggers and have sunk whaling ships, but some how we managed in woooden boats for thousands of years

nozzferrahhtoo

shadowcomplex wrote: »

Then their prehistoric cousins? I mean imagine if the creatures that existed in the sea at that time existed today, deep sea fishing would be out and any boat smaller than a ship wouldnt be sea worthy

There is no easy answer to the reasons why animal sizes have changed over time. You will likely find very many.

There is, for example, no evolutionary reason for sizes to tend in one direction only. There may be advantages selected for that initially make being larger “better” but being larger also means needing more food. If after time the increase in food demands causes a food shortage… suddenly evolutionary advantage will switch poles and being smaller and faster will be “better” again.

Another difference is environmental factors such as, but not limited to, concentration of oxygen. Different levels of oxygen in the atmosphere lend different possibilities to animal sizes.

ShaShaBear

I would think (totally my own opinion) that today's creatures have evolved and adapted their size limits to allow for the fact that there is less space? Considering thousands of years ago, or even hundreds for that matter, animals in the wild would have had far more territory available to them. I know evolution can technically be considered an advantageous mutation in the genes, but if a giraffe can adapt the length of its neck to get more food, I'm sure other animals can reduce their size to make food last an entire herd of its kind

nozzferrahhtoo

ShaShaBear wrote: »

I would think (totally my own opinion) that today's creatures have evolved and adapted their size limits to allow for the fact that there is less space?

Common sense might tell you that sounds likely, but in fact in many cases the entire opposite is true. Look up and read about the Island effect, also known as Fosters Rule. When placed on islands, many species studied actually evolve into larger versions of themselves, rather than smaller or constant.

ShaShaBear wrote: »

if a giraffe can adapt the length of its neck to get more food

Actually it is often considered something of a myth that reaching food is the evolutionary reason for their elongated necks. More likely, some argue, their legs evolved in length as speed was an advantage. If this is so then the elongation of the neck is related to reaching water on the ground, not leaves up in the trees.

If your legs get long and your neck does not, you simply can not reach water any more, at least not without lying down and drinking sideways, which is certainly not advantageous to anyone.

Capt'n Midnight

ShaShaBear wrote: »

I would think (totally my own opinion) that today's creatures have evolved and adapted their size limits to allow for the fact that there is less space? Considering thousands of years ago, or even hundreds for that matter, animals in the wild would have had far more territory available to them. I know evolution can technically be considered an advantageous mutation in the genes, but if a giraffe can adapt the length of its neck to get more food, I'm sure other animals can reduce their size to make food last an entire herd of its kind

not sure if elephants have gotten smaller recently, but they are deffinetly loosing their tusks (iirc 30% are now tuskless) , as the benefits of having tusks are far outweighed by the risk of being killed for ivory

there was a time when all the continents were joined and there were big animals, but other times when the continets were joined the animals weren't as big


when two landmasses join or a barrier is removed you find that biodiversity goes down as some species will be out competed

on islands you may find animals evolving into niches where they have no competitiors and so be different sizes to those on the mainland




There are several rules of thumb for animal sizes / limb length
https://secure.wikimedia.org/wikipedia/en/wiki/Bergmann%27s_rule

Bergmann’s rule is an ecogeographic principle that states within a broadly distributed genus, species of larger size are found in colder environments, and species of smaller size are found in warmer regions.
...
The earliest explanation, given by Bergmann when originally formulating the rule, is that larger animals have a lower surface area to volume ratio than smaller animals, so they radiate less body heat per unit of mass, and stay warmer in cold climates. On the other hand, warmer climates impose the opposite problem: body heat generated by metabolism needs to be dissipated quickly rather than stored within. Thus, the higher surface area-to-volume ratio of smaller animals in hot and dry climates facilitates heat loss through the skin and helps cooling of the body.

RocketFalls

We eaten most of the big specimens, and the atmospheric oxygen levels are considerably lower than they were in the [distant] past.

NeuroCat

The topic of size in animals is actually very interesting. There is a programme I used to simulate evolution on a very simplistic level as part of my Evolution module. It showed on a very fundamental level that atmospheric oxygen, availability of food, temperature and other external factors all have a huge effect on the size of animals.

Island gigantism is very interesting aswell, shows the runaway processes which can occur when a population becomes isolated with little or no predation.

These points are quite fundamental and if you want a thourough read I would advise any 3rd level evolution, zoology or paleontology book.

Northumbria

• Increases in CO2 are linked to larger plants.
• Increases in oxygen are linked to larger animals.

They usually follow each other in a continuous rotation, the earth alternates between increases and decreases in temperature and CO2 and Oxygen levels.

As plants grow they take in CO2, the more CO2 the better plants grow - this slowly cools the earth.
As more plants grow they release more oxygen, the more oxygen the better animals grow - animals slowly destroy areas of vegetation, releasing CO2, warming the earth slightly.

Gradually larger animals develop and use up a lot of oxygen whilst the climate cools because the plants are taking in lots of the earth's atmospheric CO2.
In doing so, plants are using it up and so cannot sustain such levels of abundant growth leading to a fall in plant size. At the same time they are cooling the earth by taking in C02 making some areas around the poles less suitable for plants.

Then a gradual shift in the earth's axis together with a the decreased CO2 levels causes an Ice Age. Many of the plants die and much of the earth is unsuitable for them.
The earth still has abundant oxygen levels allowing for the large animals to thrive and continue to develop but at the same time there isn't as much being produced thanks to the fewer plants. And as the plants died and decayed they released CO2, the animals release CO2 and trapped CO2 from marshes and volcanoes goes into the atmosphere. This slowly warms the earth again.

Gradually the earth's tilt changes again and allows more sunlight where it is needed. Together with the increased CO2 this warms the earth, leading to the end of the Ice Age.

The pattern alternates continually as plants thrive and then decline and the pattern continually goes on.

This is a simplified version, CO2 isn't the only element involved and it is a lot more complex than written above.