What would cause this specific method of reproduction to become prevalent?

Question

I have a class of animals in my world that have a bit of an unusual method of reproduction. Members of this class produce pod-like eggs. These pods are buried in the ground to incubate until the first rainfall, after which they begin to develop their own root system or hijack the roots of neighboring plants. These pods continue to develop the animal inside until it reaches full size, at which point the animal will claw its way to the surface and emerge.

While I am less concerned with the realism of the method, I am wondering what circumstances would allow for this method to become as prevalent as mammalian reproduction is on Earth.

Answer 1

What you describe is a super common method of insect reproduction.

https://entomology.unl.edu/turfent/documnts/wgmanagement

Developing their own roots is not a doable deal because there is no energy produced - just nutrients taken up. "Hijacking" existing roots (by eating them or drinking the sap) is totally workable and it works for lots of insects.

Mammals and reptiles exist because of their special systems to deal with immature animals. Those systems would be disadvantageous for a circumstance where immatures lived underground and ate roots.

It seems less of a stretch for amphibians. What they do is already close.

Immature amphibians are vegetarian. They then metamorphose and emerge from the water. That is roughly what you are having your creatures do. Subterranean vegetarian immature tadpole equivalents will eat roots until maturity then leave the ground just as conventional amphibians leave the water.

Underground it is safe from predators. In circumstances of lots of plants and an amenable subterranean habitat I can imagine amphibians could do this. In fact it seems so workable I bet in the tropics some do exactly this. Let me dig around some...

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That didn't take long.

Biologists Have Discovered Underground, Burrowing Tadpoles

Answer 2

Some plants might find it useful and develop a form of symbiosis, so that both the animal and the plant gets benefit out if it.

Then, in a world with high infant mortality, this method would allow for large numbers of prole to be produced with same investment by the parents and ensure some of them reached reproductive age.

Answer 3

The tricky part for me is the roots. Placing eggs underground is perfectly normal, so I don't think it needs justification.

What are the roots for?

In plants, roots serve several roles: They are anchors that hold the plant to the ground. They gather water from the environment. They gather secondary nutrients (particularly nitrogen) from the environment. They have a few other roles, but these are the main ones. In particular, roots are not about "feeding", in the sense of gathering energy (and "plant food" isn't food for plants)

So what could be the reason for your pods to have roots? You might want to anchor the pod if it liable to be dislodged. But if the pod has no leaves and branches, this doesn't seem to be a likely concern.

You might want to collect water or nutrients if they are somehow scarce. But why is it not easier for the mother to simply put some of her water and a supply of nutrients in the pod? She'll be putting calorific nutrients in the pod to feed the embryo anyway.

Unless she isn't... If the reason to have roots is to grow plants, perhaps the animal isn't laying eggs, she is planting seeds...

Perhaps her strategy is to plant seeds that grow into plants, that grow roots, and branches and leaves to photosynthesise and grow as plants and which then produce in alternate generations animals.

This looks a little like the alternation of generations that is found in Ferns and cycads, but taken to an extreme:

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So the animals are gametophytes. They are formed entirely of gametes, cells with only one copy of the genetic material (unlike human cells that have two copies of each chromosome). They mate and the female produces "pods/seeds" that she plants, or scatters (she might produce only a few or many millions). From these seeds, a sporophyte grows. It is rooted to the ground, it looks like a plant and has no brain. The sporophyte, when it is mature, undergoes meiosis producing eggs (that may be held within its trunk, for example, or scattered) the eggs develop and from the eggs emerge the gametophyte animal that must seek a mate, and may be carnivorous or herbivorous. Perhaps eating the very same sporophyte plants of the alternate generation.

Why did this complex form of reproduction develop? It is a "best of both worlds" strategy. You are a plant for half the time and an animal for the other half. It is hardly less complex than other ways of reproducing - think "placental mammals" for a truly ridiculously complex reproductive cycle.

Some cycads already do this. They produce sperms cells that are tiny haploid animals that actively swim. They are only 0.5mm long, but you can take this model of reproduction to its artistic conclusion and get full size haploid animals.

Answer 4

Parallels with known animals

The basic description reads similar to how many insects place their eggs. Which is placement of eggs in/on food source. Which upon hatching they start consuming.

I would say among creatures that lay eggs, that have low parental care this is a very common thing.

Differences to known animals

The unusual thing for this question is upon hatching/activation the embryo acquires resources via vegetational absorption rather then eating and digesting.

Having two systems of nutrient acquisition is going to be costly.

Another way of describing it is these creatures go through a stage of being parasites. This would be the major rational for this method of living. Taking resources from another life would compensate for the costs.

Plausibility of prevalence

So essentially external implantation of embryos that then parasite resources until sufficient stage of growth for independent living.

This would become prevalent if there are sufficient hosts to parasitize, and the hosts do not have sufficient resources to keep them away.

I see this as competitively viable. I don't see it dominating as other systems such as simply lay eggs that hatch and eat are simpler thus have an edge in many cases.