The Serina Series: Episode I: Cats

Question

"Serina" is a popular speculative evolution project in which, apart from a long list of fish, invertebrates and plants, the only terrestrial chordate to colonize this terraformed moon is the canary. The project's creator, "Sheatherius", then explored how this one species could branch out into multiple species occupying various niches.

There are fans of Serina, if not the very idea of it, but the one who worshiped it like a god is an Australian scientist who was vehemently fed up with the feral and invasive species plaguing his home soil. In the foreseeable future, transdimensional historians have found an alternate universe that has not one alternate Earth, but whole systems of Earthlike planets, all of which in the Archean stage of evolutionary development. One of these systems include nine habitable worlds, most of which are the size of Venus, orbiting a binary system of middle-aged red giants (each one having been red giants for 542 million years--time travel is a requirement for the Aussie's plans to succeed.) Unfortunately, the scientific community has banned all travel and study to the alternate universe--categorized as "AU #9"--because all of the alternate Earths have been terraformed illegally, thus ruining the scientific community's Prime Directive beyond hope of repair.

The ecoterrorist in question, being a fanatic of Serina, had decided that each of the terraformed alternate Earths should be colonized by plants, fungi, fish, invertebrates, and only one terrestrial vertebrate. This particular person had a real beef with the feral and invasive species of his native Australia, but the one serious beef he had were the house cats. There are 600 million of them worldwide, and they are responsible for the deaths of trillions of small animals every year. So in revenge, the ecoterrorist transported all of them--he never cared where he got each particular cat from--and dumped them into only one of the terraformed alternate Earths.

Like the canaries of the original Serina, the cats are the only terrestrial chordates in this terraformed world. So in the first of what I'd hoped would be a series akin to "Anatomically Correct", my first question would be this--Can the house cats, if they have ended up being the sole land vertebrates of this new world, occupy the niches of at least all of the other placental mammals?

Answer 1

Short answer: yep, it's likely

You see, something similar happened on our planet after each extinction event, with the old niches now open for taking, species suffered drastic mutations in order to fill them up, so we had cases of w single species differentiating to runners, climbers and many other in relatively short spans, with these periods of mutation overdrive basically dying once the new groups had occupied the previously vacant niches. In your scenario, it would not be hard for your cats to undergo drastic changes, with some species becoming more arboreal, some adapting to life underground and some growing to ocuppy large predator niches. The main issue here would be: what will they eat until then though?

You see, cats aren't exactly herbivores, neither are they known for having a frugivorous diet, so unless there was a second species that fed on plant matter occupying a herbivore niche or some other kind of food source they could hunt until they had time to fill other herbivorous niches, they'd likely either completely die out or some would be selected for being able to somehow adjust their diet to fruits/plant matter, with all other animals likely descending from those special cats.

Answer 2

Unlikely but not impossible

Cats are obligate carnivores. They cannot subsist on an entirely plant-based diet. From your description, the only sources of meat on your planet are fish or other cats. So by necessity, you're going to see a lot of cannibalism right at the start. That tends to not work out well for the long-term health of a species, so a giant chunk of your 600 million cats will probably die of either starvation or predation by other cats within a year or two at most.

The ones that have the best chance of surviving long-term are those that are able to find ponds or streams that they can reliably obtain fish from. I'm unsure whether cats would be able to subsist only on insects/invertebrates (The Lion King notwithstanding), but that might also be a source of protein.

So after a massive die-off, you might the survivors start evolving in two or possibly three main directions:

1. optimized to catch and eat fish
2. optimized to catch and eat insects and other invertebrates. (if possible)
3. optimized to eat other cats or cat-descendents.

From those, you might also see a fourth scavenger branch fairly early, optimized specifically for carrion-eating.

The fish-eating branch over a long enough period of time is likely to lead to cat-otters, cat-seals, and eventually cat-whales. Evolutionarily-speaking, the first steps of this is actually likely to happen very rapidly, since fish are going to be the main non-cannibalistic source of food on the planet. After a fairly small number of generations, you're going to see adaptations to improve their ability to swim and take prey underwater.

Some cats do enjoy eating fruits, grasses, or other vegetable matter, but their digestive systems really aren't able to process them well. I can picture some of the insectivore branch maybe supplementing their diet with some fruits or other plants, which over the very long-term might eventually lead to primarily herbivorous animals, but there would not be a direct route to get there from your standard domestic cat. Depending on which fish are present on the planet, you might actually see a terrestrial pure-herbivore come up out of the ocean before one could evolve from cats. On the other hand, the insectivore branch might eventually lead to aerial predators along the lines of bats.

The predator branch will lead towards more efficient predators. They're unlikely to be very large, since there would not be any large terrestrial herbivores to prey on. So, big enough to take insectivore-cats and pescavore-cats reliably, but not much bigger than that.

Of course, all of this assumes that enough cats survive the initial arrival to viably reproduce beyond a couple generations without hitting a genetic bottleneck that they can't recover from. Personally, I wouldn't put much money on that happening.