Different sidebar according to different categories through the single.php

I am trying to call a different sidebar, according to the category where the post is inserted through the single.php template.
In single.php template, I replaced the snippet

<?php include (TEMPLATEPATH. '/ sidebar_single.php');>

with this code:

<?php
if ( is_category() ) {
  $cat = get_query_var('cat');
  $args = array(
    'include' => $cat,
    'hide_empty' => 0
    );
  $categories = get_categories($args);
  if ( ($cat == 3) || ($categories[0]->category_parent == 3) ) {
    include(TEMPLATEPATH . '/includes/sidebar/sidebaranimals.php');
    } elseif ( ($cat == 4) || ($categories[0]->category_parent == 4) ) {
    include(TEMPLATEPATH . '/includes/sidebar/sidebarart.php');
    } else {
	include( TEMPLATEPATH . '/sidebar_single.php' );
  	}
}
?>

but when I open a post in a specific category their sidebar does not appear…
I know this code works in archive.php, but I need to open a different sidebar according to different categories through the single.php file.

My permanent link is set to: /%category%/%postname%/

What am I doing wrong?

Please help!