You can find plenty lists of diets and recommended items. Leaving it at that would waste both of our time. Its more important I show you why I say what I do and that you understand how I got there.

This isn't the most spell/grammer checked section, may even be in different fonts. I will continuously be fixing it, but I wanted it up so that people could see that I do have (poorly worded) reasons behind what we do. 

The basis behind the food we recommend:

There is a completely reasonable misunderstanding that short-tailed opossums are omnivores. They can digest, more or less, a variety of food, but they are insectivores. Biologist look at the majority of a diet and if there are nutritional needs dependent on a certain diet. Otherwise almost every species would be classified as omnivores. Squirrels wont pass up a bone, wolves may eat some berries etc. This makes the word omnivore meaningless for us. No aspect of animal, individual or species, can be described in word. Descriptions are short hands for us.  

Animals have special adaptations for whatever diets they eat in the wild. Herbivores have griding teeth. Carnivores have shearing teeth. We are a fantastic example of omnivores, have "chomping teeth" with ridges and valleys to crush food and break it down to help digest it before it hits your stomach (chew your food). Note: canines aren't confined to  carnivore diets (google water deer) and ours are pathetic, back teeth are most reliable to tell diet. Insectivores have "chopping teeth", chopping insects into chunks. Insects very easy to digest and don't need ground up. This means insectivores don't have elaborate stomach to subsist eating too much harder to digest foods. Imagine a humming bird having to eat seeds. They would die before breaking down enough energy. Most insectivores are small, active, and need quick, easy to digest foods. They will get less nutrition out of most other foods that are harder to digest-- even meat.

Not all insectivores are the same. Getting a better idea of what each species eats is harder than you might think. Not a ton of funding for research to poke at poop or dissect tummies. I did manage to find some scientific papers. Currently, much of the diet portion of this guide focuses on one study (will expand). In that study[1], researchers collected wild individuals' poop to look at what the individuals ate by sex and between dry and wet seasons. The STOs ate almost exclusively arthropods (animals with exoskeletons). Every single STO ate arthropods, while only a few individuals ate vertebrates, seeds (probably started out as  fruit), and leaves.

I took the average nutrition values of as many related insects listed in the study as I could (while my own research laid deserted). Nutritional values of insects varies a bit from kibble which is based on carnivore diets. Even kibbles that are great for carnivores (like ferrets and cats) might fall a little short for insectivores.

(This is all one PhD student's desperate attempt to avoid their actual work, but I can argue that a misunderstanding of STO diets may behind at least some of the health issues we see in captivity. I use this information as a (rough drafted) blueprint for what I feed.)

Some random key points:
*The main goal of a set-up (other than safety) is to prevent stress
*Stress can related to illness, fear, and aggression
*Stress is reduced by letting them preform natural behaviors like nesting
*Not by having natural items
*STOs are terrestrial and found in dry forests
*Wheels are more important than you may realize

Why we recommend ~5-6 square feet or larger

Here's why:

In a small study using adult males (2) researchers recorded stress behaviors called *stereotypies. These stress behaviors are thought to be avoidable by allowing an animal to preform certain natural behaviors. The STOs were placed individually in the typical rat lab cage, then were given larger "enriched cages". In the lab cages each preformed one or more stress behaviors: tail-chasing, bar chewing, digging, head spin, and rolling*. They didn't show stress these after a week in the larger enriched cage. I suspect several other behaviors like being nervous or odd times of activity are also due to stress. 

The enriched cage was an impressive 13sqft, and the lab cage was 2.2sqft. There is a lot of caveats here! small sample size, only adult males, extra enrichment provided with hilariously taped up boxes (they just need to preform natural behaviors-- doesnt have to be done with "natural" things). What we can say is there is somewhere greater than 2.2sqft and at/before 13sqft on the cage size continuum that will help us reduce their stress.

*Stereotypies are repetitive behaviors like rocking, feather plucking, bar chewing, and many more, thought to be a response to stress. Maybe you’ve seen documentaries where captive wild animals pace in cages. Their foot prints fall in almost identical steps back and forth. Sometimes they sway their heads back as they turn. We have a rehomed ferret that came with pacing and the head sway. She’s better now, but it is forever burned in my memory (Lili from the “the freeloaders” page).

We cant base the needs of other species off what we would like. We are problem-solving, social apes. Each species has their own needs. Stereotypies are useful (but not always nor the only) indicators to see if we are meeting those needs.

We found that, with enrichment, we were able to eliminate stress behaviors by using our 200qt bin (~5sqft) and critter nation cages (6sqft). We've used the prevue 528 was (4sqft) and it was iffy for us... unsure. Might depend on the opossum. I'd just like to mention, for no particular reason what-so-ever, THE STRESS INDUCING CAGE WAS BIGGER THAN A 20 GALLON TANK. Just a random comparison, no idea where it came from... (read several other care sheets) 

Humidity and heat?!?

This topic only popped up recently and its getting weird. I think it comes from reptile people hybridizing with hedgehog people and, again, some misunderstandings of wild STOs and biology. I'm starting to gather that most people think STOs come from some tropical rainforest-like habitat. I don't blame them. They are from Brazil which is typically associated with rainforests, but the rainforests in Brazil are mostly in one large area in the northeast. STO's hangout southweast in drier forests with wet and dry seasons.

Reptiles almost always need certain heat and many a certain humidity. They are ectotherms*, they don't fully regulated their own temperatures within the body. Mammals are endotherms*, they are fantastic at this. Some mammals do have unique issues. We somehow broke African pygmy hedgehogs' ability to wake up from torpor (hibernation lite) if they get too cold.  Heat for us mammals tends to be more problematic: Chinchillas bake above 75F and ferrets get heat stroke very quickly in the upper 80's.

Similarly, we control our internal water better than reptiles and can rely more on drinking and digestion (not just moisture in food, digestion itself produces H2O). With water available, mammals compensate pretty well. I've fed ferrets both raw and kibble diets. On raw I have to remember to change the water. On kibble I have to keep up with filling it. This is part of what makes many mammals so good at invading new territories. Reptiles are limited to a range of similar humidity and temps. You'll never find wild iguanas in Canada (at least not for long). Brown rats are from Asia and now on every continent except Antarctica (for now...). Starlings were introduced to the USA from Europe (something to do with Shakespeare). In return we plagued them with squirrels.

*Biologists prefer the terms homeotherms for endotherms and poikilotherms for ectotherms so if you want to impress one you can use those terms (no, I don't know how to say poikilotherms, but most likely neither does the other biologist) 

"Natural"
I mentioned in enclosure sizes that the larger cage eliminated stress behaviors. Its important to mention that it was also "enriched" with items that let the opossums preform natural behaviors. That enrichment was hilariously taped up tubes and boxes. Animals in captivity need to preform natural behaviors, not have a natural items. It doesnt matter if its straw or fleece-- nesting is whats important; it doesnt matter if its wood or pvc pipe, climbing its what is important. A wheel will do so much more than branches. They don't give a darn if they are walking on dirt, just that they arnt getting sick from ammonia. Not going for natural, just safe, easy to clean things that let them preform natural behaviors.

There is a reason animals have different wild and captive lifespans. Those lifespans are based on animals that make it too adulthood. A female STO can breed 5x a year and have 60+ babies in her life. Out of those only TWO make it to breeding age. Many are dying slow deaths from starvation or infection, many others meet violent deaths. Not all predators make sure prey is dead before starting to eat.

Humans only romanticize nature because they don't have to live in it.  

Stereotypies
Has anyone given you grief for having exotics? kept from the freedom to be overbred then die horrible deaths as babies (should they be the lucky ~2 to survive to maturity)? As I mentioned in "natural", humans are way too gung ho about "natural". Hikes and camping are nice, but then you get to go back home. Personally I like my A/C, heath care, and lack of being eaten alive. I hope they do too.

BUT a miserable existance in captivity CAN be worse than being grison food (look them up, theyre cute—while youre at it, brush dogs too). So how do we know what they need and if its being met? I’ll teach you an imeaserably helpful term when keeping any animal: Stereotypy. Stereotypies are repetitive, unnautural behaviors with no apparent purpose. This includes things like rocking, feather plucking, and bar chewing. Humans do it too like pacing, rocking, and stimming. Maybe you’ve seen documentaries where captive wild animals pace in cages. Their foot prints fall in almost identical steps back and forth. Sometimes they sway their heads back and over at each turn. I have a rehomed ferret that came with pacing and the head sway. She’s better now, but it is forever burned in my memory. Its Lili from the “meet the freeloaders”.

             Its thought these behaviors are to keep a stressed animal within their psycological limits so how do we do it? We help them preform natural behaviors. It is extremely impotant not to do it from a human prospective. Other species have different needs. Our natural behaviors include most of us needing to socialize and problem solve. Hamsters need to tunnel (many of their stereoypies can be prevented with deeper bedding) and ball pythons like wait in holes for food or ‘adult fun time’ to come by.

There's nothing inherently special about "exotics" from "domestic" animals. All were wild once. The difference is that most domestic animals were bred to adapt to our needs and are easier to keep unstressed. With exotics we have to adapt the their needs. They can be kept just as unstressed, but its much harder to do so. 

Assessing kibbles
Kibble far more complicated than it seems at first sight. Companies pay a lot to trick people. People are busy, they have a lot of responsibilities and decisions to make in a day and evaluating kibbles takes time, a lot of research, and some recreational math.
Some tips:
- ignore absolutely everything on the bag except the nutrition label and name. Everything else is junk created by a PR team. Once you do it can be irritating how much nonsense you have to scroll past.
- vets don't often promote or recommend good foods. Vets do a lot of complicated work, but nutrition i'snt one many are well versed in. They are people just as much as you (it can be hard to remember that). Same goes for doctors, my mom was a nurse practitioner and I had to show her a label on peas before she believed me they had protein. Its not their field and that's okay. Unfortunately, some companies give a large portion of sales for vets to have them in their office.
- % protein is often misleading. For carnivores like ferrets, and most likely insectivores, plant proteins are a waste. A lot of brands increase protein by adding legumes like lentils and peas because they are cheaper. They might also cause kidney problems (research is being done for ferrets). These are a waste for species that don't digest plants well. To know the % protein you have to know how much comes from animals.
- all kibbles need some plant matter to bind them into the little balls, but the type is going to change how much it impacts blood sugar. If you know someone with diabetes, you know spikes in blood sugar are not good. It can actually lead to pancreatic cancer in ferrets. Starches will impact the animals blood sugar less than rice. Corn is basically pumping cupcakes in the kibble without the flavor. I'm not a huge fan of rice, but I wont touch kibble with corn (even if I wasn't very allergic to corn).

So we look for fewer "meat" ingredients, should just be a few first ingredients (so they arn't stretching to make it look better). Chicken meal is fine. Chicken by-product is, to many peoples surprise (was mine), also fine. I try to look for starch over grains or go with rice (STOS dont have the same problems as dogs with grain free) and avoid legumes (peas lentils etc).

I'm still learning how to describe our process so message me on FB if you want me to look at another kibble. May take some time to get back to you. Its a process and I will email the company if needed

[1] de Carvalho, R. F., Passos, D. C., & Lessa, L. G. (2019). Diet variations in short-tailed opossum Monodelphis domestica (Didelphimorphia, Didelphidae) due to seasonal and intersexual factors. Mastozoología neotropical, 26(2), 340-348.

 [2] Wilkinson, M., Stirton, C., & McConnachie, A. (2010). Behavioural observations of singly-housed grey short-tailed opossums (Monodelphis domestica) in standard and enriched environments. Laboratory animals, 44(4), 364-369.

Easiest way to find the papers is to copy and paste the titles into google search
If you find a paper you want, but its asking for money to read (researchers hate that too, we don't get any of that money), let me know. I'll do my very scientific voo-doo magic to try and make it appear. Also, scientific papers are written in code disguised to look like English. If parts aren't making sense let me know: I'll use the power invested in me by the great science overlords to turn jargony-wargondy into real people words.