Why do dragons wear a medal?

All members of Class Hexapedal have a resistance to heavy-metal poisoning. The trait is quite noticeable even in the earliest members (the lead fish) of this group. Excretions of excess heavy-metals comes out through the scales which causes the colourful displays on many the members (metal-oxides). The displays have reached high complexities in the higher members of the Weir-drags, Sore-drags and the true Fire-dragons, particularly those that horde heavy-metals and can't help nibbling on them. The medals have developed along the lines of sexual dimorphic displays, chest emblems, flank emblems, etc. although there is still considerable variety in all individuals.