What would cause creatures on a planet to favor large sizes over small sizes?

Question

On Earth, there are far more insects by biomass than any other creature type. Not only that, but they constitute a wider variety of species too. As a result, they also evolve interesting mechanisms unique from animals. That is, it is easier and more common for insects to be bioluminescent compared to large animals. If I wanted to create a planet that favored larger species and possibly non-insectoid species, how would I go about that? Would increased oxygen favor large animals? Or is it the type of biome that matters?

Criteria:

• Insects are NOT the largest by total biomass
• It is more efficient, possible, likely to be cat-sized than cockroach-sized
• Solution must be chemical, biological, or environmental in nature (no planetary changes outside of Earth's current systems)
• The planet must still be habitable for humans (non-advanced special equipment like fur coats is acceptable)

Bonus Points:

• The solution benefits non-insectoid species (fuzzy animals are more common)
• Does not involve "cuz evolution" (preferably a physical phenomena that causes the larger creatures, avoiding the claim that with evolution anything can happen)

Answer 1

If your sole purpose is to leverage the advantage towards larger creatures, then you have two pathways to do so:

1. Decrease the air oxygen content. Insects have very inefficient "lungs", that only really work by absorbing oxygen directly from the air. This severely limits the size that insects can attain. Animals with lungs have the ability to extract much more oxygen from the air, due to the dedicated, protected and complex internal structures of their lungs.
   So, lower oxygen in the air makes your insects smaller, and weaker. But would only make your lung-breathing complex animals less energetic.

2. Lower the temperature to just below freezing. Often. Insects are too small to survive a frost, other than by cowering in a communal nest or otherwise finding shelter from the cold. Larger animals have very little problem with the cold, as long as it does not get too severe or last excessively long.

The problem is that insects, and the myriads of smaller living thing people would just call "bugs", even down to the microscopic dust mites in their uncounted trillions in your home, serve a valuable ecological purpose. They are the scavengers, and the soil-turners, and the hair-devourers. Without them, we would eventually be up to our armpits in hair, and cellulose, and dead skin flakes.

If you remove or seriously reduce the numbers of such "bugs", you will need to invent something else that serves the same purpose. For all the myriad little tasks, in the little crevices and under the soil. Something small, that breeds quickly, that spreads everywhere. Something that is mobile, and burrowing, not just a sessile garbage eater, or you create fungus forests. Ideally something that can fly, to distribute itself to locations where it is needed. Something like... oh dear, you've re-invented Bugs again.

Answer 2

You need stability.

Evolution tends to favor larger size on average, all other thing being equal. Larger size leads to greater efficiency of resources and better robustness (sequestration) of resources even if the total amount of resources needed is greater. 100lbs of cow uses less energy than 100lbs of rat. This is why larger organisms have evolved over and over and over again.

However smaller animals will always drastically outnumber larger ones because there are more small (low energy acquisition) niches to fill, and there has to be more low energy niches due to how trophic level energy distribution work. Larger animals will never reach combined total mass greater than small ones. There just is no realistic way to make the energy distribution work.

Small size often also means faster breeding which leads to better aggregate survival in drastically changing conditions. This is why it is always small animals that survive mass extinctions. Dinosaurs and birds are a great example as long as conditions are stable dinosaurs kept getting bigger and bigger (we see the same thing with mammals) but it is always the smaller sized ones that survive mass extinction. Now don't get me wrong, more small animals die during mass extinctions than large ones becasue there are more of them, but becasue there are so many species and individuals of them and their resource needs are so low there are always some that survive.

You can the most large organism by having stable conditions, but they will always be the minority of the total mass. this is why life has evolved megafauna over and over again, as long as conditions are stable on the long terms large animals have an advantage because they can better survive short term fluctuation, but as soon as long terms conditions become unstable that advantage disappears. But even when conditions favor the large they still make up a minority of the biomass because even though they are more efficient they need more total resources and cannot change as quickly.

Heck single celled life out mass all multicellular life combined by several orders of magnitude for the same reason.

Answer 3

Megaladons were apex predators until climate changes affected the key aspects of their viability.

1. Food sources became more scarce; as tropical temperatures in waters became more scarce, the food sources either perished or adapted to colder climates.
2. Breeding grounds became less accessible; as ice caps moved in, shallow waters where a megaladon in heat could have its calf safely became more and more scarce.

If you look at the largest animals, they all come from singleton offspring situation. Large litters typically happen in "boom or bust" environments, wherein a large amount of offspring are yielded, but depending on how the environment is, very few will survive. The offspring in stable environments also tend to have longer "helpless" phase after birth (like a Megaladon for instance!).

Another interesting thought about this is that if you look at the general trend of humans, you see that they are getting (a) taller and (b) smarter. That isn't necessarily because of "evolution" either, probably more due to better nutrition and overall health. Stunted growth tends to affect populations that struggle with sustenance or some type of injury.

What I'm trying to get at is if your world is on an earth-like planet, the easiest way to affect the size of creatures is to create a world without any serious changes in the overall environment for some period of time (say a hundred thousand years). Life, uh, finds a way. Then, come up with a food of which there is an overabundance. The longer the climate remains static in such an environment, the more likely it is that the overall size of all animals becomes larger. The safer it is to care for your offspring, the easier it is to have a singleton approach, which also trends towards larger animals.

Hope this helps!

Answer 4

much more oxygen and energy, the oxygen, because the quantity of vegentation, or vegetation like creatures, and energy from a star powerfuller than ours, and, to survive predators, the animal becomes bigger and the predator becomes bigger to kill the prey, a lower gravity would be very usefull, as the lower the gravity, the less the weight, the less the energy needed to move it, something like this happened in our world, when there was a lot, and i mean a lot, more oxygen, bugs (yes, yes, you said no bugs) became the size of hummans and more, Meganeuropsis permiana, is one of them, they had to stop growing, or they couldn't support their exoskeleton, so here the lower gravity in your plnet comes in, they need less energy as the exoskeleton wheighs less, so they can keep growing

chemical, by using voids and/or hydrogen etc. the total surface area could be bigger than a whale, but still wheigh the same as one, also, continuing the hazards, etc. in this other answer, a hazard which makes animals develop so big would be the need to get away from land, but, you cannot develop wings, the answer searched by evolution would be flight, a bit like here Plausibility of Floating Whales thank you for your time, sorry for any misspelled words, spanish keyboard

Answer 5

Non-native life, ecological catastrophe and noxious chemicals:

Hazards: Perhaps your environment has harsh factors that damage life forms which are small. There could be waves of toxins that poison everything, but are absorbed through the surface. Small animals have a large surface area per unit volume. Perhaps survival is dependent on being able to survive acid exposure - the acid gets into all small spaces, and across the skin. Insect-like creatures would dissolve, but creatures with some kind of moisture-resistant coating are burned but live.

Mass extinctions: Even a single extremely large bottleneck could wipe out all existing tiny creatures, and it could take thousands or millions of years for tiny things to re-evolve from bigger ones. Let's say a virus that affected all insects killed all of them but had a reservoir in larger species. All of them would die, then all their eggs hatch and all of them die again. Mass extinctions happen. A major change to the environment could expose a weakness in the otherwise successful small organisms. For example, if the planet had never experienced cold, the small organisms might all freeze to death and have no mechanism to compensate, while larger organisms had more body mass and managed to not go extinct before the climate stabilized.

Migration: Or perhaps the life on the planet comes from somewhere else. The colonizing animals are either microscopic or large. No bugs made the journey. The body plans of many large animals might not be well adapted to becoming small. A rat might be able to eventually become a shrew, but a lion might find it a harder evolutionary process.

There are eventually going to be evolutionary advantages to small size, and mechanisms will compensate over time. But in the meanwhile, live large!