I should have said too that what makes raw polymer clay stick to itself (if there's enough contact) is first that the clay is a bit sticky so that alone generally holds it together till baking time. During baking though when the plasticizers in the clay harden (polymerize), the plasticizers in the two pieces will also join together across the two pieces as well as inside the pieces, creating a bond at the join.
You have two special concerns in your case though that will making bonding more difficult:
Sculpey, SuperSculpey, and Sculpey III are the weakest clays after baking in any thin or projecting
areas, so if a bond is stressed (say, pressure gets put on an ear) the projection is more likely to snap off at the join than if using a stronger clay like Premo for example, which takes stress better in those situations.
The other problem is that there's often little contact
between an ear and a head because the ear can be small, so the bond could end up weak for that reason too.
You'll probably want to do one or more of the things that makes a bond stronger unless you'll just be setting the finished item on a shelf and never handling it or making sure it will never drop, etc:
...use a stronger polymer clay
...make as much contact with the head as possible at the base of the ear
...don't make the ear too thin or too long
...use liquid clay or Diluent-Softener in the join, rubbed in
...let plain clays sit together overnight before baking, or even if using liquid clay or Diluent let the joined piece sit awhile so either of those can seep well into both sides
...use an armature between the two pieces... e.g, a short length of toothpick, wire, cardstock, etc., depending on the size of the ear --perhaps with white glue or liquid clay on each end
There are lessons on making simple sculpts on several pages of my site if you want to check them out for more tips too:http://www.glassattic.com/polymer/sculpture.htm
(...under the Websites
category, click on Whimsical
(... click on Sculpting
(...click on Simpler Heads
page and the Halloween/Spring-Easter/St.Patrick's Day/Thanksgiving/etc
page will also have lessons for simple sculpts like snowmen, Santas, angels, penguins, etc., for Christmas and winter, and bunnies, monsters, leprechauns, pilgrims, etc., for the other holidays:http://www.glassattic.com/polymer/Christmas.htmhttp://www.glassattic.com/polymer/Halloween_etc.htm