First of all, I'm not sure that you want to use the word "reason" unless you want to invite the entry of conversation topics regarding who's reasoning you're talking about. "Reason" implies someone doing the reasoning, i.e., a "designer". I personally don't want to go there, so I'll mention a few things about the fossil record that you might find illuminating.
The oldest true land animals appear to date back to about 420 million years ago. By 300 million years ago, there were reptiles and the huge dinosaur die-off (referred to in the fossil record as the "K-T boundry") was 65 million years ago. So, for about 235 million years, reptiles ruled the land. In the subsequent 65 million years till the present, mammals have ruled the land. Throughout this history, there have been periods of advancing glacial ice about every 40,000 to 100,000 years and periods of essentially globalized ice as well.
The fossil record tends to show that the changes of living animal forms happen in steps of rapid change followed by stability. The selection factors which favor change can only be theorized about, at this point, but the observation of change can clearly be made.
In between times of rapid die off and rapid change, the periods of stability are often marked by steady increases in the size of the life forms. In many conditions, an individual being just a bit bigger than other members of the same species might be a large survival advantage, and over time this would select for continued growth within the population. Dinosaurs ruled the earth for 235 million years and had lots of time to slowly grow by this mechanism.
In the era of mammals, there have been periods of growth and die-off just like in the past before. During some periods, in some locations, the situation has been right for species to thrive for long periods and to select for individual growth. This has led to the existence of large animal species, some of which have subsequently gone extinct.
Factors that stimulate the genetics of growth include in-species competition for resources or mates, survivability in terms of predation and environmental exposure, and the ability to horde food resources.
Factors that tend to favor the die-off of large animals and make way for smaller ones include limited food resources, diseases, predation, and habitat change.
The bottom line is that species grow bigger when the environment is stable and the individuals compete amongst themselves for abundant resources. They then become vulnerable to environmental change. Large organisms tend to be supplanted by smaller ones when the conditions change and the selection pressures require them to compete for limited resources, and then organisms which need less do better.
2007-06-26 15:12:41
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answer #1
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answered by bellydoc 4
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Look at it differently...why have they gotten smaller?
Smaller animals require less food. When constrained, such as on small islands, animals such as the Kiwi bird and pygmy species of larger animals develop to avoid starvation. On the other hand, the blue whale is the biggest animal ever...its food supply has been no problem. The size of animals is constrained by its food supply. Humans are getting much larger because of our unlimited food supply. Dutch men average over 6'0" tall...200 years ago, they were 5'2".
Long ago, when animals first started conquering land, there was less competition for food...thus the animals were huge. Any time food supply issues occur, the larger animals are more vulnerable. For instance, if a meteor hit the earth and killed the dinosaurs, the smaller animals may have been able to survive off of the limited food supply. Cold-blooded animals also would survive since they can live long without food.
2007-06-26 15:07:37
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answer #2
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answered by Flyer 4
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The doc is close here. His information is basically correct. However I will make a couple of minor corrections.
Yes periods of long environmental stability lead to increased specialization. It allows animals to become very good at doing one certain specific thing (like feeding on a particular food or escaping a particular type of predator). This comes at the cost of being less able to live in other conditions.
However to get big you need a few other things as well. You need lots of food and wide open spaces. Imagine a large animal trying to run through a dense forest. They would get trapped by the trees and would have little to eat. Forests encourage animals to be aboreal. When they are not arboreal they tend to be small. Arboreal animals can eat fruit, insects, or leaves in the trees. Terrestrial animals must wait for food to drop, survive on what they can gleam from the undergrowth, or feed at the forest edges where leaves grow low enough to reach (Thats why whitetail deer prefer edge habitat).
It is the grassland where most large animals are found. In the grassland to survive you must be either small enough to hide underground or in the grass, fast enough to flee, or large enough that you do not have to worry about predators. Thus being large is a predator avoidance mechanism. However it requires long term climate stability and large exanses of open grassland. Most of the megafauna came from time periods when the earth had much larger areas of grassland than it does today.
2007-06-26 16:16:56
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answer #3
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answered by Jeff Sadler 7
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Paleontologists don't know for certain, but perhaps a large body size protected them from most predators, helped to regulate internal body temperature, or let them reach new sources of food (some probably browsed treetops, as giraffes do today). No modern animals except whales are even close in size to the largest dinosaurs; therefore, paleontologists think that the dinosaurs' world was much different from the world today and that climate and food supplies must have been favorable for reaching great size
2007-06-26 15:02:10
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answer #4
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answered by Chad S 2
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Very good question since humans were not
here during the dinosaur age.
And it can't be the evolution of living things
going from small to large.
I can only think of one answer: The atomic
structure of these large creatures met with less
creative resistance than other life forms that were to
come later, for whatever reason.
2007-06-26 14:41:41
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answer #5
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answered by kyle.keyes 6
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Humans have killed the big animals so smaller ones evolved. the smaller ones are hder to find, so more survived
2007-06-26 14:30:07
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answer #6
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answered by Alex 3
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evolution baby
2007-06-26 14:27:50
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answer #7
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answered by Anonymous
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good question lol
2007-06-26 14:32:12
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answer #8
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answered by person 1
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