Re: I am teaching a third grade class and we have questions about adaptations.

Long or large ears that can be pointed up can receive sounds from far 
away. Animals that have fast predators, for example, rabbits hunted by 
foxes, benefit from having long ears that help them hear their predators 
before a predator comes within a striking distance. On the other hand, 
animals that spend most of their time underground, such as moles, don't 
have big ears, probably because they don't really have predators and, big 
ears would probably hinder movement underground. The long tails of some 
monkeys are used to hold onto tree branches, whereas the long tails of 
cats may be used for balance. Connected toes, like those of ducks, are 
good for swimming, whereas separate and long toes may be good for grabbing 
onto tree branches.

If one closely examines the life style and habitat of an animal one may 
figure out the adaptive value of the shape and size of a particular organ. 
However, one must keep in mind that opposing factors may have been 
involved during the evolution of an organ. For example, the long tail of 
an animal may help it hang down from trees, but at the same time, it may 
present its predator with something to grab onto. In cases like that 
natural selection may have resulted in a structure that balances the 
requirements of different factors.

Also keep in mind that every organ costs energy and material to build and 
maintain it. And, the energy and materials saved by not building an organ 
may be used to develop some other part of the body that is more essential 
for survival or to produce more offspring. Therefore, if an organ is not 
needed or used, it is likely to become smaller or totally disappear. Thus, 
sometimes the reason why an animal has short ears or a short tail may be 
not because short ears or tails are good for a particular activity, but 
because the animal doesn't need long ears or a long tail. For example, 
animals that live underground or in caves usually have poor eyesight or 
are totally blind. In their cases, being blind is not good for anything 
other than for saving resources that may instead be used to better develop 
some other part of the body or to produce more babies.