@Tobias:
Those big animals like whales and elephants had been body builders/ politicians etc who think very highly of themselves. Of course, I am not saying all bodybuilders and politicians will becomes whales and elephants. But one can such mentality common among some of them.
Those who are lazy and cultivate such thoughts (sankhara) are likely to be born snails, asuras, etc. These variations come from the mixing of various different gati.
Those very low lifeforms like worms (and other microscopic living beings) arise due to those who do very lowly things. Excess or deviant sexual tendencies may lead to births as worms, among other causes. Worms have both male and female reproductive organs. During sexual intercourse among earthworms, both sets of sex organs are used by both worms.
That is why there are innumerable variety of lifeforms. But we can guess some general trends. Many of Pali and Sinhala words for animals have these meanings embedded in the word itself (this is called “pada nirukti“).
For example, the Sinhala word for cow is “haraka”, which comes from “hara + ka”, where “hara” is “essence” or “goodness” and “ka” means destroy. They had intentionally and recklessly destroyed environment or common properties and now are paying back with their milk and meat.
By the way, bacteria is probably not sentient: “Beings that have no centralized nervous systems are not sentient. This includes bacteria, archaea, protists, fungi, plants and certain animals.”
That quote from: “What beings are not conscious“. I cannot say this article is 100% compatible with Buddha Dhamma. I just saw it. But the general trend seems to be correct.