This article is from the WSSF 2009 AFRMA Rat & Mouse Tales news-magazine.

Editor, Karen Robbins
Q Re: Comment read regarding the amount of formula to give orphan rats when hand raising: “give 5% of body weight (in grams) as ml per feeding.” We had a question on what was the amount to feed orphan baby mice and there was no published info that Dr. Booth could find [see WSSF 2007 issue “Amount to Hand Feed Orphan Baby Mice”]. My question: “Is this the correct amount?” Any orphans I raised, I always fed them until their tummy looked full. Of course, finding a real rat to nurse and raise the babies is the best choice, not only to get the right amount and type of milk, but also for the psychological and emotional aspects.

Answer by Carmen Jane Booth, D.V.M., Ph.D.
A In the book, The Biology and Medicine of Rabbits and Rodents by Harkness and Wagner 1995, states the following for rats:
Food 12–18 grams/100 grams body weight/day
Water 15 ml / 100 grams body weight day
Milk composition 12.1 % fat, 9.0% protein, 3.2% lactose, the remainder is water.

According to this, if you assume 5 gram birth weight, 5g x 15 ml/100g = 3.75 ml, this would be 75% of the rat’s body weight per day if you go by just the water volume. I could not find any published paper for this in rats.

Answer by Margaret F. Smith, licensed wildlife rehabber
A The 5% of body weight (in grams) as ml amount is published in Shirley Casey’s magnificent rehabber handbook (which is only available to rehabbers), but also may be available on-line at their website (www.ewildagain.org). Because the 5% guideline is accepted by the International Wildlife Rehabilitation Council and the National Wildlife Rehabilitator’s Association, it also is published in their manuals (which must be ordered for a hefty price). Excerpts from my article, also published:

“Overfeeding can cause bloat, diarrhea, physical damage, and even death in an already stressed animal. Underfeeding can result in dehydration, poor bone development, and slow weight gain, among other difficulties.

“Feed in cc’s 5% of the animal’s body weight in grams. This amount is the stomach capacity for most small mammal species. You will need a digital gram scale to weigh the animal. To determine the amount to give per feeding each day, record in grams the initial intake weight, followed by an empty bladder weight every successive morning. If, for instance, the animal’s weight is 100 grams, you would give 5 cc’s per feeding, or if the animal weighs 340 grams, give 17 cc’s per feeding [if you are unsure of your math skills, try figuring 10% of the body weight first and divide this amount in half to derive 5% (e.g., 10% of 150 grams is 15 which, divided in half, is 7.5 cc)]. The stomachs of older juvenile and adult small mammals sometimes can accommodate a bit more than 5% of their body weight after hydration and stabilization (6–7% is a safe maximum).

“(Once hydrated) It then is time to start feeding dilute (half strength, or even less if the mammal is severely emaciated or sick) formula at 5% of their body weight per meal. Never prepare formula with isotonic (i.e., hydration) fluid; always use plain water. Gradually increase the strength of the formula, but maintain the feeding amount at 5%.”

It’s important to remember that 5% is a guideline, not a rule. Some animals require a little bit more (up to 6% . . . if the animal gets pudding poop, that indications the amount, if more than 5%, is too much) and some may need 15–30 minutes additional time between feedings if they’re unable to eat the entire 5% during one feeding (note that I never feed less than 5% of body weight per feeding).