I have two gliders I got from a petstore around Christmas time and there both not tame so I haven't gotten an opportunity to hold them until yesterday when I held him I could feel his rib cage, after I put him down I noticed his shoulder blades were quite defined as well. I have a female with a joey in pouch so I can't tell if she's skinny. I feed them two de thawed cubes of pp diet with vitamins and 2tbs of pp glider cereal. should I be concerned?

You need a digital scale that weighs in grams to get an accurate weight on them.

The pet store sold you a breeding pair????

Did they give an approximate age on them?

They said 5 months

a vet would be best place to go to get a answer on their overall health.

A $20 digital scale at store should measure in grams. I have an adult female at 80 grams very healthy. I have an adult male at 160 grams, a little porky.
but adult females go like 100 to 140 grams and adult males 120 to 160 grams usually.

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http://www.sugarglider.com/gliderpedia/index.asp?Weight