I had a full-size greyhound (desceased) and I would STRONGLY reccomend you getting a Martingale collar for Mario. You can find them at your pet store under the name Martingale collars or simply greyhound collars. If you want a leather one try going under a search engine like www.google.com or www.iwon.com and put in martingale collars and you can order leather ones. They are more expensive but they are very nice and should last forever.
If I ever get another greyhound, or get a doberman or any dog that their head is smaller than their neck I would definately get this type of collar.
They slip out of them so easily, these are made to stop that. I even used one of Shaianne's old ones on Keegan when she slipped out of her collar when she was smaller. I didn't want her to wear it as her regular collar b/c her head is bigger than her neck and its more difficult to get it off.
The one type of collar I would NEVER
get is the ones with the plastic clip. If you have a smaller dog they would be fine I suppose but Shaianne (my greyhound) broke one apart and was gone So fast I couldn't even say "WAIT". They are just not safe to me. I would get one that has a belt like buckle which is what Keegan has now.
I will admit when she was a smaller puppy she did have one that had a plastic clip but she was so small I couldn't find anything else.
Good luck with Mario he is a cutie!