Is Exercise Enough?

I have one cat - a 16-year-old - who is at her proper weight; two cats - a
6-year-old and a 7-year-old - who are a bit underweight (healthy, just
petite); and one cat - a 2-year-old - who is about a pound or two
overweight. They all eat together and there is no way for me to feed them
separately*. They get about 1/4 cup (or less) of Science Diet Senior
Advanced Formula Dry - free fed (about 1/16th cup per day for each cat).
They also get 4 cans of Fancy Feast (4 different flavors placed in a buffet
of 4 different bowls) every 12 hours (two 3 oz. cans per day for each cat) -
but they don't eat it at the same time, they tend to browse on it for a
couple of hours, taking turns, each taking a few bites, going away, then
coming back to take a few more bites**.

* I'm afraid to pick up the food after a limited time (which would have to
be done if I fed them separately) - I wouldn't know if my two light-weights
would get enough food (especially the shy 7-year-old who waits for everybody
else to eat before she eats).

** They always have leftovers, leaving about 1/8th to 1/4th can in each
bowl.

My question is, since I can't reduce the food intake of my pudgy one, will
encouraging her to exercise more be enough to help her lose weight? She
loves to play with a feather on the end of a furry string on the end of a
pole toy - she'll jump up in the air after it time after time for 15 - 20
minutes at a time. When she is too tired she just quits, there is no
over-exertion, no panting. How many times a day should I "work her out"?

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Hugs,

CatNipped

See all my masters at: http://www.PossiblePlaces.com/CatNipped/