Sexual dimorphism, that is the difference is size between the male and a female of the same specie, is common yet normally not extreme.

There are however species where the difference is extreme: I remember reading about a fish where the female is about 1 meter long, while the male is a couple centimeter long, essentially a testicle living inside the female body with the sole purpose of fecundating her eggs.

In my world I want for such extreme dimorphism to be the norm among multicellular organisms using sexual reproduction, at least in one biome.

Which biome would favor extreme (female size/weight: male size/weight >= 50:1) dimorphism to be the norm in the vast majority of the species occupying it?

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    $\begingroup$ You probably read about anglerfish. $\endgroup$ – ProjectApex Jun 4 at 17:16
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    $\begingroup$ It's not about sexual dismophism in the sense of genders but in the sense of reproductive roles. Human males are not just males, they are the protector caste, females are not just females, they are the whomb caste.... For other animals genders have different roles ....like for ants males are either sexual toys for the queen or prince drones, while females are either queens or soldiers/workers. extreme sexual dimorphism is not defined by habitat but by the level of monogamy.... The more monogamous an animal is, the lesser the dismorphism. And selective altruism must also be accounted for. $\endgroup$ – user75689 Jun 4 at 18:20
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    $\begingroup$ More altruistic animals tend to develop sexual dismorfirms but only in behaviour and not in physical shape. males being more suicidal and willingly to sacrifice themselves or females more willingly adopting children of other parents. It's strong dismorphism, but only on a mental level. $\endgroup$ – user75689 Jun 4 at 18:24

IMO, You're stuck with water.

50:1... That means (for example) a 2 meter human and her 4 cm (yeah... centimeter) husband. For us imperial measure users, that's 1.6 inches. Gravity is not your friend. Any condition where something can be stepped on or landed on (in the case of birds being fertilized by gnats) would cause serious problems that, frankly, no amount of intelligence could easily fix. It's morbid... but I can imagine the males of a land-based species being eaten by dogs and stepped on by female children at play.1

Obligatory Futurama video.

Off the top of my head, the only biome that permits freedom of movement without substantial consequences due to gravity are water-based. I think you're stuck with it. You'd still have a problem with the males being easy food for females of different species... but at least it could happen.

1We need to honest here... distraction would kill lots of males. Females walking down the street or simply bears wandering through the forest (whether your looking for an intelligent species or not). Just rolling over in your sleep would be disastrous. Gravity is not your friend.

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  • $\begingroup$ Another way to get the freedom of movement needed might be the air, providing the pressure is high enough and the gravity is low enough so that flight is trivial. $\endgroup$ – TheDyingOfLight Jun 4 at 17:55
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    $\begingroup$ A female can carry her husband in a pouch like a kangaroo :) $\endgroup$ – Alexander Jun 4 at 17:57
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    $\begingroup$ Ants survive fine enough being small... $\endgroup$ – user75689 Jun 4 at 18:39
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    $\begingroup$ The greatest disparity in sexual dimorphism on land I can think of (at least among non-arthropods) is the giant bandicoot (Peroryctes broadbendti) in which the male is three times larger than the female. That's nowhere near the maximum size disparity seen in aquatic environments. $\endgroup$ – user2352714 Jun 4 at 18:48
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    $\begingroup$ @JBH Agreeing with your answer that sexual dimorphism on land is not as extreme as you get in the water, and providing additional examples to back it up. $\endgroup$ – user2352714 Jun 4 at 19:59

You are probably thinking about anglerfish; they inhabit a biome which is very poor in nutrients, the abyssal zone of the oceans. I believe this adaptation minimizes the nutrition needs of the male, so that a family of bonded fish is more food-efficient than otherwise. By the way, those guys do bond - the male bites the female and fuses to her permanently, connecting her circulation to his.

Also in the abyssal zone there is the boneworm, ehich might be the animal you had in mind:

Between 50 and 100 microscopic dwarf males live inside the tube surrounding a single female and never develop past the larval stage.

These critters thrive by feasting on whale bones which are sparse and scarse. They need to be efficient. Again I think that males skip developing past a larval state in order to conserve energy.

Other biomes where food is scarce, such as deserts, don't have such extreme dimorphism. I believe it's because deserts change relatively quickly in geological time, lasting for millenia to dozens of millions of years. But the abyss has been the abyss before life even started, so it has had more time for fauna and flora (bacteria) to evolve extreme features.

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  • $\begingroup$ But continental drifts completely change the abyss by removing deep volcanoes, off which many organisms feed off...without them, the other animals have no prey to eat $\endgroup$ – user75689 Jun 4 at 18:33
  • $\begingroup$ I suppose...... $\endgroup$ – user75689 Jun 4 at 18:34
  • $\begingroup$ @Kyu not all the nutrients in the abyss come from the chemosynthesizing microbes. Far from thermal vents, most of it comes from above in the form of dead organisms. The boneworms I mention feed exclusively on whale carcasses. $\endgroup$ – Renan Jun 4 at 21:50

Apparently its not exactly about habitat. This kind of sexual dimorphism in which the female is several times larger is usually rare, being more observed in some spiders and in 5 out of the 11 anglerfish species we know.

In the latter, the males are essentially sexual parasites, using its eyes and/or nostrils to find the huge females. After that, they latch onto them, usually in the underbelly, and basically fuse with the female, connecting tissues and bloodvessels and becoming completely dependent on them to survive.

In spiders as this article speaks (unless I understood something wrong), the difference in size seems to be mostly due to opposing selecting pressures to each sex, but this can't exactly be considered the only reason, with other factors like sexual selection and gravity hypothesis (smaller males can climb better and reach the females more easily) also being applied. Other factors like why in some species there's virtually no difference and in others the males are clearly smaller aren't quite clear yet.

Overall, it'd seem like it's not a matter of biome, as these 2 cases clearly have little to no similarities other than the extreme size. This seems more related to behavioral patterns, sexual selection and, possibly, different lifestyles (see harpy eagles, although their size difference isn't nearly as pronounced, it exists, and allows for a difference in prey, with the smaller and faster male hunting smaller, yet more agile prey, while the heavier and stronger female hunts larger prey, with both facing very little competition from one another normally). But it'd seem like a development in which mature males act as sexual parasites towards the females, as we see with the angler fish, or in which both adapt for extreme differences in type food they eat should be a strong motivator (a comparative yet very unlikely herbivore example could be one in which smaller males exclusively eat grass while much larger females feed on the leaves of trees, although we'd need to watch for the necessary adaptations that'd allow for reproduction despite such a difference in size and weight).

The main issue here for your ecosystem to work is the 50:1 ratio. For this to allow you're essentially stuck in scenarios in which the male is much smaller than the female, and it's her which will limit the max size. It's unlikely we'll have a scenario in which the male is, for example, the size of a bull. And that is not mentioning how not even spiders usually have such drastic levels, so we'd essentially need an ecosystem in which basically all males attach to females after reaching sexual maturity and convert into a spermsack for sexual reproduction to work, or have other similar methods to transfer sperm.

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