Event.observe( window, "load", runHooks, false );

function runHooks(){
  showWeight();

  Event.observe( "applicant_sex_male", "click", showWeight );
  Event.observe( "applicant_sex_female", "click", showWeight );
}

function showWeight(){
  if ( $F( "applicant_sex_male" ) == "Male" ){
    $( 'weight_female' ).hide();
    $( 'weight_male' ).show();
  };
  if ( $F( "applicant_sex_female" ) == "Female" ){
    $( 'weight_female' ).show();
    $( 'weight_male' ).hide();
  }
  return false;
};