Chanita -
Animals do vary.  Perhaps someone can correct me on this, but I believe that
some animals, like puppies, will nurse on whatever teat they can get to,
while other animals have a distinct nipple preference and always nurse on the
same teat each time.  I seem to remember kittens are like this, but I may be
mistaken.
Often an owner will supervise a litter of pups to be sure the smaller puppies
not only get to a teat, but get a "full" teat.  The dog owner also wants to
be sure that the bigger pups latch on to different teats so that each of the
teats gets maximum stimulation.
If somebody has more experience with animal litters than I, please jump in!
Cynthia D. Payne
LLL of Berkshire County Mass.