Where are all the non-aquatic mouthbrooders?

Question

Some fish mouthbrood. Some amphibians mouthbrood. Crocodilians carry their babies in their mouths to water. Is there a reason why there are no instances (that I know of) of terrestrial animals that mouthbrood?

Why can't there be pelican babies in pelican beaks or chipmunk babies in chipmunk cheeks?

EDIT: I want to have plausible mouthbrooding terrestrial creatures, but worry there might be some glaring misadvantage that I am missing to having that trait in a terrestrial creature. The world of these creatures would be much like our own. The basic picture, for a big branch of terrestrial animals, would be a throat pouch where babies would grow until fully ambulatory. For the more primate-like animals, the baby would be held to rest against their chest, much like we front carry our babies with baby slings, leaving the parents' forelimbs free to swing from one treebranch to the next. Is that too ridiculous?

Answer 1

The best candidate would be a snake.

Mouthbreeders have big mouths, and generally catch prey whole. There are nothing that feeds by nibbling that holds young in the mouth. There are no insects or non vertebrates that I know of that have big mouths like this. I suspect evolution favored big, gulping mouths first and then selected for mouth breeding after.

The plausible terrestrial mouth brooder is snakes. Snakes catch and eat prey whole. I found a very old paper which really dug into depth about the possibility that snakes would take their young into their mouths to protect them.

Do Snakes Swallow their Young?

"Sirs, — A short time since I was in Condersport, Pa., in whortle- berry time, and a man who had been out berrying stated that he suddenly came across a Rattlesnake with her young, some twenty-six, * about her. She immediately opened her mouth, and instantly the whole family of little ones went down her throat. Do you believe it? Is that the nature of the Rattlesnake? — H. M. S."

Ultimately it would seem that snakes do not swallow their young. But it is not implausible. Snakes can swallow and then regurgitate live animals (as discussed in linked paper) so the young snakes could re-emerge. Rock pythons do provide some maternal care so that is not an impossibility for snakes. Probably these would be the viviparous snakes although not necessarily - crocodilians lay eggs and then provide maternal care for the hatchlings and so snakes could too. Also, young snakes have a good shape for this maneuver.

Answer 2

First of all it's a matter of size. Newborn fishes and crocodiles are small enough to fit in their parents' mouth.

Despite this, some terrestrial animals use their mouth to carry around their babies: never seen a cat or a lion moving around their cubs by gently biting behind their necks?

Still, those cubs are too big to fit in the mouth.

Answer 3

Caveat: I'm not an evolutionary biologist.

It probably has something to do with the fact that non-aquatic animals breathe through our faces, whereas most aquatic animals have gills. My (cursory) reading on mouthbrooders indicates that most either do not feed while brooding or feed very little. This indicates to me that the mouth is almost or completely blocked by the brood. Even if a non-aquatic animal could survive a brooding period without eating or drinking water, we can't live without air. Anything that could put pressure on or potentially block the throat is dangerous to us.

So, if you want non-aquatic mouthbrooders, you might have to give them gills. It would be difficult to justify this from an evolutionary standpoint, however, since we spend most of our lives not brooding. Any orifice in the body is an opportunity for infection, which is why evolution favors fewer rather than more; look at how many species have cloaca instead of separate orifices for urine, solid waste, and reproduction. Mammals are basically the exception here, all other vertebrates favor cloaca. So, to develop a separate orifice (or two) for breathing during the few months out of their whole lives when an animal might be brooding, rather than using the perfectly serviceable orifice* in our faces, would require some strange kind of evolutionary pressure that made either gills or mouthbrooding absolutely essential. Aquatic animals, of course, developed gills because they have to filter oxygen out of the water. Perhaps there is something in the atmosphere on your world that the non-aquatic animals need to filter. This might give justification to the development of gills and, in turn, the feasibility of mouthbrooding.

*I am here referring to the throat. Technically we breathe through three orifices; the mouth and two nostrils, but since they all connect to the throat, I am considering them as a unit.

Answer 4

There are 2 advantages that I see aquatic mouthbrooders having over terrestrial:

1) Aquatic mouthbrooders are buoyant. The added weight of the young in the mouth is not noticed by fish because the weight of young aren't any different than the water already in their mouths for "breathing" through their gills. A land animal would have to have strong neck/back muscles to continuously carry their young in their mouths. This limitation in land animals may also cause them to get caught off guard by predators if they could not turn their heads fast and readily respond to threats with their teeth.

2) Land animals have to drink water regularly. Aquatic animals consume a small amount of the water that they are "breathing" through their gills and process out the salts. Conversely, land animals, save for camels and some hibernating animals, have to drink water regularly as well as eat which would hinder raising young.

I think that the need to drink and fend off predators are two major reasons that mouth brooding in land animals isn't as plausible.

There are animals that do carry their young in a pouch, however, those are marsupials. Kangaroos have a pouch in their lower bodies for carrying young which frees up their heads for eating, drinking and scanning for predators. The pouch also puts the joey in a lower center of gravity where the strong leg muscles can easily carry the added weight.

Answer 5

As has been mentioned so far, the young of a mouthbrooder are born tiny and the mouth of the parent is large. So, if hippos had litters of tiny hippos or if they gave birth to tadpoles that needed time to grow larger before metamorphizing into baby hippos then mouthbrooding would be a good way to protect them. They could carry their brood in their mouths across the savannah during dry seasons.

I imagine momma hippos and daddy hippos humming tunes to their mouth-broods and gargling songs to teach the ways of the hippo.