Evaluating Milk Replacer for Wild Babies — Formula Guide — Out of the Woods Wildlife Rescue & Rehab

A rehabber reference

Sarah Howard, MS Animal Nutrition — Rehabilitator, Out of the Woods Wildlife Rescue & Rehab

Most formula advice in wildlife rehabilitation is handed down as a recipe. Someone gives you parts and ratios, you mix it, and it either works or it does not. This page is about the layer underneath that — what you are actually trying to match, and how to tell whether a recipe is any good before you feed it to something that weighs 40 grams.

The goal is mom, and mom is different for every species

Mammals have wildly varied diets, so it should not be surprising that their milk varies too. A cottontail's milk and a raccoon's milk are not solving the same problem. Food availability and the mother's own health shift it further.

We cannot make a perfect match. So the question becomes: which properties matter most, and which can we live with getting approximately right?

What to look at

Calories

One kcal is one Calorie is one thousand calories — the energy it takes to raise one kilogram of water by one degree Celsius. For growth to happen, calories consumed have to exceed what the animal burns simply staying alive. Underfeeding calories is the quiet failure mode: the baby is not obviously sick, it just does not gain.

Fat

Fat is energy storage, organ protection, and the transport system for fat-soluble vitamins. It provides 9 kcal per gram — more than twice what protein or carbohydrate give you, which is why fat is the lever that moves caloric density fastest.

What to consider: whether the source is plant or animal, and the fatty acid composition. These are not interchangeable.

Protein

Protein builds tissue structures, creates and maintains new cells, and transports nutrients. It can also be broken down for energy, at 4 kcal per gram — but an animal burning protein for fuel is an animal not using it to grow.

What to consider: plant or animal source, and the casein-to-whey ratio. Casein and whey behave differently in a neonate gut, and this is a real difference between products, not a marketing one.

Carbohydrates

Carbohydrates fuel muscle and the central nervous system, support immune response, and matter for brain development. Also 4 kcal per gram.

What to consider: source, and simple versus complex. Excess simple carbohydrate is a common cause of the GI upset that gets blamed on "a bad batch."

Consistency

Not a nutrient, but it decides whether any of the above reaches the animal. Formula that is too thick will not move through a Miracle Nipple; formula that is too thin drowns the calorie count.

You do not have to do the research yourself

The peer-reviewed literature on wild mammal milk composition is limited but growing, and combing it is not realistic in the middle of baby season. Fortunately other people have already done it.

Shirley and Allan Casey have been rehabilitating since 1986, founded WildAgain in 1992, started eWildAgain in 1994, and have published over 150 peer-reviewed articles and papers. Their wildlife formula calculator lets you enter a single formula or a blend and see how it compares against a target species' milk — percent of mom, macronutrient split, caloric density, and calcium-to-phosphorus ratio.

Use it before you adopt a recipe someone posted in a group. It takes two minutes and it will occasionally tell you something alarming.

What we run

Our own blends are modelled in that calculator and re-checked when manufacturers reformulate, which they do, quietly. A few of ours as of the last review:

Squirrel under 4 weeks — Esbilac with Fox Valley Ultraboost and water.
Squirrel over 4 weeks — Esbilac with Fox Valley 20/50 and water. We are moving off 20/50 as stock runs out in favour of an Esbilac, KMR and Ultraboost blend.
Opossum — Esbilac with KMR and water.
Cottontail — Goat's Milk Esbilac with Fox Valley Ultraboost and a little Lacto Bac 200.

We publish these as what we use, not as prescriptions. Your species mix, your local product availability and your animals' response should all move the numbers. Model your version rather than copying ours.

The things that are not in the recipe

A correct formula, handled badly, still kills animals. Worth as much attention as the macronutrients:

Vitamins and minerals, particularly the calcium-to-phosphorus ratio.
Mixing procedure. Stir, do not shake — bubbles become gas, and gas kills small animals.
Resting time. Let mixed formula sit in the fridge for a few hours so bubbles dissipate before first use.
Rancidity. Fat goes off. Powder that has been open a season is not the same product it was.
Storage. Discard mixed formula at about 24 hours.
Feeding regimen. Frequency, volume and position matter as much as the mix.
Availability. The best formula on paper is worthless if it is out of stock in April.

So which recipe is best?

That is the wrong question, and it is the reason this page exists. The best recipe is the one that comes closest to that species' milk, using products you can actually get, mixed and stored correctly, fed at the right frequency, and adjusted when the animal in front of you tells you to adjust it.

If you want to talk through a formula, email sarah.ootw@gmail.com or text 901-340-6534. We would rather answer a question than see another litter lost to a recipe that was never modelled.