Why Aren’t Animals bigger than they are?
As this is a biological question the answer is not as relatively straightforward and precise as an answer in physics or chemistry might be. Numerous factors affect the sizes of animals, and these factors are different depending on the exact physiology of the animal. For example, birds are the size that they are, as to be bigger would reduce the efficiency of flight, and compromise the overall ability of each species to survive.
There are several general points that apply to all animals. One of these is that of food supply. A larger animal requires more food than a smaller one. A habitat can only produce a certain amount of a certain type of food. Larger animals must be spread more thinly than smaller counterparts. They also have a larger minimum range to be able to survive, can only exist in large areas of their habitat, and cannot survive in isolated areas. When a species is spread too thinly its survival can be affected by deaths of only a few animals. With fewer animals there will be fewer potential mating partners, and so reproductive potential could be impaired. Being thinly distributed also leads to a lesser chance of finding a mate. Fewer animals also leads to a smaller gene pool, and a lower chance of survival when changes to the external conditions occur. Finally, larger animals are less able to survive periods of decreased food availability due to their high demands.
Some dinosaurs are known for their large size, and will make an excellent example. While our largest land animal today, the elephant, can weigh in at an impressive five tons, the largest dinosaurs were much larger than that. The largest complete skeleton of a Brachiosaurus (a herbivorous species) suggests a body weight of around 87 tons, but fragments have been found of an Argentinosaurus that could have weighed as much as 100 tons. Our modern elephants have very large external ears. These do not provide exceptional hearing, but have a large surface area. This is used for heat emission, to keep the elephant cool. The larger an animal gets if it is simply scaled up, the lower its surface area to volume ratio gets. This means that it will heat up easily, and not lose its heat. To keep a 50-ton dinosaur alive it would have been necessary to expend about half a ton of water every day just in keeping it cool. This would have meant very large dinosaurs needed constant access to large bodies of water in order to live, further restricting their range. Whether or not dinosaurs were endothermic is open to debate, but if they were not then temperature regulation would have been an even greater problem.
Another problem that giant sauropods such as Argentinosaurus would have had to deal with would be that of support. As length or height are increased linearly the volume and therefore mass of an organism increases cubically. This means that an animal twice as long in each direction as a cat would weigh about eight times as much as the cat. Therefore the force being placed on the leg bones of an animal goes up at an increasing rate with regards to its length. As energy must be expended in the production and maintenance of support for the body there must be a point at which it is inefficient to be any larger. In addition to this, when the forces involved are very large, bones can be broken. If there is an especially large animal attempting to be supported by a broken bone the forces involved will be too great to allow the bone to heal on its own. Therefore, if an animal is too large a fracture of a supporting limb is very much a lethal event. In the end, large sauropods died out probably due to their high degree of specialisation, and their inability to cope with change.
In more primitive animals, with less complex structures the situation can be somewhat simpler. If an animal does not have a developed system for distributing oxygen its size is limited by the maximum distance that oxygen can diffuse through its tissues. This limits the size of animals such as flatworms. For some smaller animals, support may again be a factor, but in a different way. One rather specialised type of animal is the pond skater, an insect which walks over water, supported by the surface tension. If it were to be much larger then the force exerted on the surface of the water would break the surface tension, and the animal would sink. Geckos are the largest animals that can walk up walls. They cannot be much large due to the fact that the adhesive surface area of the foot is proportional to the square of the gecko’s length, and its mass is proportional to the cube, so a gecko that is too large will simply be too heavy to support itself.
To conclude, for many reason animals cannot be larger than the sizes they currently are, given current circumstances, without causing problems in other areas, such as survivability, which is ultimately key to the evolutionary success of an animal species.
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