Animals identify each other in much the same way as humans do. They use sight, sound and smell. Most important is that they have become quite familiar with their species group as well as take note of the habits and quirks of their predators and prey.
Here's are some answers to finding identity in a seemingly identical world:
The best way to single out a particular animal from a look alike group is to hang with them. With careful observance, it doesn't take long to see that one is slightly bigger, one is always lagging behind, and the one seems to pick on the others.
There are countless mannerisms and body types that animals can use to identify each other. I once had two black female gerbils that only people with daily contact could tell apart. The differences between their weight, personalities, and energy made them seem like night and day, as long as you knew them.
Think all zebras look alike? Well, the second task of a newborn zebra after standing is memorizing the pattern of his mother's stripes. A zebra's stripes are like its fingerprint, no pattern is like another.
While shapes such as a blaze (stripe) on a horse's face, or patterns on an animals coat like a leopard's spots can be identity factors, so are color depth, texture, and sheen. Given enough time, even a human can see the hundreds variations in a herd of similar animals.
Vocal animals certainly have tone, and researchers are finding that some animals even have regional accents. Researchers of Pacific whales have found that they can track whales better at a distance by listening to the distinct songs and clicks made by individual pods (groups) of whales. Similar findings have been made in the study of wolves, dolphins, and birds.
Someday we just might be able to talk to the animals, but we probably don't want to know what they have to say about our treatment of the earth and its creatures.
At one time humans were able to use this sense quite well, but most of us have lost it. Not only can many animals identify one another through scent trails, but they can also distinguish whether the animal has the smell of distress or dominance. Dogs are currently being trained and tested to sniff out cancer and low blood-sugar levels in human diabetics, and the results are proving that they have high success rates.
Biology is all about chemicals. Sit next to someone who forgot their antiperspirant at a horror movie and you'll smell what I mean. However, identity is all about observation. Any animal could tell you that.
http://www.bio.davidson.edu/people/vecase/Behavior/Spring2003/statler/habitat.htm http://www.acousticecology.org/scienceresearch.html http://www.diabetes.ca/Section_About/type1.asp