input: [It might be...] if your gerbil is getting older (2 years or so), and the change has been gradual, it might just be normal aging. However, a sudden change in behavior is suspect. Watch your gerbil closely for 24 hours and observe what you see. If you keep weight records, weigh your gerbil again. If something has definitely changed, behaviorally, in his/her appearance, or you observe a significant weight gain or loss, see the vet. [My Gerbil...] 
output: is lethargic, sleeps more, isnt running on the wheel