So far as i know there are three causes: 1) genetic:  some populations
are just small!! (as are some forms and varieties, perhaps because of a
genetic linkage - the same gene which causes the variety also hinders
growth or development.), 2)environmental factors, such as pollution
(Chlorine seems to do a good job!!), or colder or warmer water
temperatures than is optimal for a particular species, or naturally high
concentrations of some element or chemical (QUESTION: what about
salinity - can excess salinity cause dwarfism??), and 3) injury: when an
animal is injured when young, it often becomes stunted.
        uncertain about smoking - but it prabably stunts growth in preeteens at least!

Tenessee is cold already??,
-Ross.