# For what reasons would an animal species NOT cross a *horizontal* land bridge?

In my worldbuilding project, 2 continents located on roughly the same latitude are connected by a land bridge. Both sides of the bridge are surrounded by a temperate grassland, and the lack of seasons means the climate is steady year round, so that rules out winter as an option.

The sentient aliens in my world, which I've been calling Not Humans, (very original) evolved on Continent A, and eventually spread out to Continent B across the land bridge where they found a species that wasn't present on Continent A. (Comparable in function to a horse) I haven't decided what the terrain is like on the bridge, but I would like to keep both sides of the bridge as grasslands if possible.

Why would these animals exist on only one side of the land bridge rather than both?

• "Temperate grassland" and "lack of seasons" are mutually incompatible. ("Temperate" means the standard four seasons.) And there are many kinds of terrain which horses cannot easily traverse; for example deserts (horses need an lot of water), terrain infested with the tsetse fly, dense forests, steep mountains, etc. To give an example, horses never crossed from the Eurasian grasslands into the African savannah until they were brought there by humans. – AlexP Apr 10 at 12:52
• Wait...I thought a landbridge is just a bridge made of land, ie a small strip of land, isn't it? Reading some of the other answers I am now confused. They seem to assume a man-made bridge. – genesis Apr 10 at 13:17
• Quick question. How long has the land bridge been open for? – Ynneadwraith Apr 10 at 15:52
• I don't think food or predation are good answers because they just displace the question to a different level of the food chain, i.e. the question simply becomes why food or predators are unable to cross? – Chuck Ramirez Apr 10 at 19:04
• @AlexP First definition on Google: “relating to or denoting a region or climate characterized by mild temperatures” — Nothing about seasons. On our Earth temperate climate is correlated with the existence of (mild) seasonal variation but there’s no rule, even in a science-based Universe, why this should be a universal requirement. – Konrad Rudolph Apr 11 at 11:02

One obvious answer is that the land bridge itself lacks food.

Nothumans can cross the Bridge because they're smart and pack a lunch. Nothorses can't cross the Bridge because that region lacks fodder for the grass loving megafauna of Continent A to venture through.

Another possibility is utterly ungothroughsome territory. If the Bridge is very low lying & swampy, nothorses won't be able to get through because they'll become mired. Nothumans can pick a path where others fear to tread or can build boats or swampshoes to traverse the Bridge.

• She only want me for my swampshoes. I think I need to cut her loose. Yes I do. – Willk Apr 10 at 13:01
• The connection between Central America and South America (Darien Gap?) is a swampy wilderness that is extremely difficult to cross. There were many creatures that were exclusively found to the north or to the south. – Michael Richardson Apr 10 at 15:11
• @Willk Horses and swamps don’t mix, remember Neverendering Story? Poor Artax... – Liam Morris Apr 10 at 16:22
• @LiamMorris Too soon, man...too soon... – Quasi_Stomach Apr 11 at 9:42
• "ungothroughsome" +1 – BoomChuck Apr 12 at 12:17

Tides

Many land bridges are often covered by tides, making them only passable at certain times. As the tide comes in, the bridge is covered, it is uncovered again once the tide goes back out.

Perhaps your Nothorses are not smart enough to wait until the tides start to head out before quickly crossing. Your Nothumans on the other hand are smart enough to know when to wait and when to go. There is nothing physically stopping the nothorses from crossing (allowing them to be brought back to the Nothuamn’s continent), its just they are not smart enough to work out when they should cross. If timed wrong, the tides would come in and sweep either Nothorses or Nothumans out to sea.

Alternatively, you could have something like the Giant’s Causeway: https://www.ireland.com/en-gb/amazing-places/giants-causeway/ https://en.wikipedia.org/wiki/Giant%27s_Causeway

This was a land bridge that stretched from Ireland to Scotland, its really interesting to look at with all the natural hexagonal rock formations. Not only is it interesting to look at, you’d have a hard time trying to get Nothorses to walk over that. You could pull them across, of course, but i doubt they’d want to walk over it if they didn’t have to, given how uneven and unforgiving the terrain is, especially for a creature of their size.

• To add to your excellent answer, note that there is no reason for Nothorses to enter land lacking grass (food). Nothumans go out of curiosity and they know how to take food with them. – Ister Apr 10 at 13:53
• Note that the giant's causeway never actually formed a land bridge in the past 10 thousand years, even though both "ends" are part of the same lava flow which is what caused the legends. – Tim B Apr 10 at 14:42
• You had me at "natural hexagonal rock formations." Whoa. Those are cool. – Cyn Apr 10 at 20:17
• @TheDyingOfLight Firstly, thats a problem with the question as a whole, not just my answer. Second, a horse is not going to be able to swim to the other continent, it would drown before it did and be swept out to sea. Finally, why would a horse even bother to? Just to see if it can get to the other side? (which it might not even be able to see, you can only see about 3 miles in any direction at sea level, after that the curvature of the planet blocks your vision). – Liam Morris Apr 12 at 17:49
• You can lead a nothorse to water, but you can't make it cross at low tide! – Mason Wheeler Apr 12 at 17:53

Option 1: Difficulty traversing

As others have posited, perhaps the land bridge is particularly difficult for nothorses to traverse. Horses are pretty well adapted to open, rolling steppe. It wouldn't actually take that much of a change to make it unlikely for them to make the crossing.

Lack of water, swampy territory, heavy woodland, presence of poisonous plants that they're not adapted to (ragwort in real life is deadly to horses), lack of grazing, too much grazing (one of the issues horses have outside of the steppe is the high sugar content of lush European grass, which causes health issues).

Any of these, perhaps dialled up a bit, would do well to restrict their range.

Option 2: Predation

In addition to the other answers about physical land barriers or poisonous flora, predation could also work.

• Continent B has nothorses.
• Continent A has something that thinks nothorses are extremely tasty.

The small numbers of nothorses that make it across the bridge do not survive long enough to establish a stable population. It gets worse for nothorses when their nemesis makes the jump in the other direction, but at least they have a sizeable population already present so they can maintain their numbers more easily while they adapt (or don't, of course).

For a real-world example, you can look at the restrictions to the range of platypodes in Australia. Their range is curtailed to the West of the continent by the larger presence of crocodiles in the East.

Option 3: Time

There are two main ways land-bridges are commonly formed. The first is a collision of two landmasses due to continental drift. This tends to be relatively permanent on a biological timescale. The second is land beneath the waves that is exposed by falling sea levels, which tends to be more transitory.

Perhaps your land bridge is of the latter type, and has only recently become traversible (say, in the past couple of thousand years). It takes time for population pressures to develop and push an animal to expand their current range.

Nothumans, like their human counterparts, are curious. They seem to like exploring. It's likely that they will make the journey across to find new, untapped resources.

Nothorses, however, are not particularly adventurous. If they're anything like actual horses, 'not particularly adventurous' is a sizeable understatement.

There isn't actually anything at all preventing nothorses from moving across the gap. They just haven't yet.

• "It gets worse for nothorses when their nemesis makes the jump in the other direction" begs the question - why hasn't their nemesis already crossed the land bridge? Now we're back to square one. – CactusCake Apr 10 at 15:22
• @CactusCake No reason they couldn't have. It's just nothorses that we're restricting here, not nothorse-nemeses ;) – Ynneadwraith Apr 10 at 15:24
• But if nothorsenemeses had already crossed, wouldn't that result in one of the following two scenarios: 1. all the nothorses have been eaten (kinda ruins the story); 2. nothorses are capable of cohabiting with nothorsenemeses (so now we're back to the original question of why haven't they populated both continents). I like the idea in this answer, but I think it could use some fleshing out to detail why nothorsenemeses only constitute a threat to nothorses in continent A and not in continent B, otherwise it exhibits the same mystery (in reverse) that OP's question is trying to reconcile. – CactusCake Apr 10 at 15:40
• For another real-world example: blogs.discovermagazine.com/science-sushi/2018/08/27/… Around one island, lobsters are abundant, eating up all the mussels and whelks in the area. Around another, lobsters are completely absent: the abundant whelks overwhelm and eat up any lobsters that stray in. – Timbo Apr 10 at 21:07
• Maybe the nemesis was the nothumans? We know they didn't cross the bridge (for reasons unknown). We've hunted or out-competed lots of real species to extinction. – Robin Bennett Apr 12 at 9:03

The terrain of the land bridge is navigable by not-humans, but difficult or unpleasant to traverse by not-horses.

Refer to cattle grids, a man-made structure used to allow humans (and vehicles) to traverse a passageway, but not livestock.

Naturally-occurring, highly-uneven terrain between these two areas might sufficiently dissuade the not-horses from crossing, while still remaining traversable by the not-humans.

The animals are particularly susceptible to radiation poisoning (causing sterility at low levels). The background radiation on the bridge is unusually high due to an large scale natural nuclear reactor that has been pushed to the surface with recent geologic uplift.

The same explanation could apply to other naturally occurring toxins.

Toxins could also take the form of a terrible smell, or sound, etc. that affects the animals but not other species.

Something similar to this could serve as a reason

In short, during the Cold War, the fence/wall border between the East and West of Europe didn't just separate humans. It also kept the deer apart. Even though today there are no barriers to speak of between these countries and no deer alive today lived through the Cold war, the deer populations refuse to cross the (now imaginary) line.

Taking this and applying it to your case, you could easily have the land bridge once populated with a predatory semi-aquatic species, which wouldn't venture too far inland. This could lead the horse species to avoid the landbridge, even if this predatory species is long gone.

• The deer story doesn't have any information on what the deer were doing before the cold war's fences were built - maybe those deer just always stay in their own areas, fence or no fence. It could be argued that the current evidence supports this theory more than the idea that "future generations of deer won't cross an invisible old fence line." – Xen2050 Apr 11 at 9:01
• Or the predatory semi-aquatic species can still be there. It is no match to non-humans, just like there is no match to humans. – Jan Hudec Apr 11 at 12:06
1. Some plant which exists on the other side, not very noticeable to humans but toxic to the horses. Or turn it around something they need in their diet.
2. A reason which possibly doesn't exist anymore, like an extinct predator, but the horses learned to leave their home because of it, possibly using the magnetic field to judge where that is.
3. No reason to expand. If the population of the horses is controlled by something other than the supply of food and space they might not have had any reason to move.

...I'll try to think of more later.

Or whatever the term is under your latitudes (edit: cattle grids). In my native language, it designate a mountain bridge, with a fenced surface (so you can see through it). Cows are deathly afraid of the void and unless being forced, won't cross it. It has been designed to keep livestock in defined area, so it should fit the bill perfectly.

Edit: Ruadhan pointed in the comment that livestock does not cross a cattle grid not because of the void, but because of the shape of the bridge, that would lead their hoove to slip between the bars (and potentially causing injury). I've found conflicting sources online and can't really tell you the main reason. Shape of the bridge is treated in the second option I propose.

TLDR: you can see the void under the bridge and non sentient animals are too afraid to cross it

Note that it also could work with a rope bridge. Horses wouldn't be able to cross it while we have (almost) no problem using one.

Edit: Given the way you have worded the question, I assume the bridge must be as natural as possible. Perhaphs one of the two suggestions above is the result of a specie of vine/climbing plant that somehow thrive above seawater and thus is prolific on your coast, to the point two points bonded over the years. Maybe this seawater vine eat fish. Or need high-concentration of salt/iode/whatever. Who knows?

• For reference, the term 'land bridge' is usually used to denote an area of land that is normally beneath the surface of the sea, but as sea levels drop gets exposed allowing terrestrial animals to traverse it. Some examples during the previous ice age are the Bering land bridge between Eurasia and the Americas, and Doggerland which connected Britain to Europe. – Ynneadwraith Apr 10 at 13:31
• I was under the impression that a Cattle-grid works because the cow's feet tend to slip between the bars and they find it difficult to walk on, not particularly because they won't walk over a void. – Ruadhan Apr 10 at 14:22
• We have a fair few around my family's farm, cars can traverse a grid separated as far as several inches without issue. Cows definitely could step in the gaps. If it was just the void that was the methodology, the grid would probably be more of a mesh than a series of metal bars with wide gaps that risk injury if an animal actually stepped on them. – Ruadhan Apr 10 at 14:27
• @Ruadhan Thanks for pointing that out. If I remember, I'll look into it later and edit accordingly. – Nyakouai Apr 10 at 14:28
• @Ruadhan there are painted cattle grids. Cows are smart enough to know they'll fall into a real cattle grid, but not that they won't fall into a fake one. – Random832 Apr 10 at 16:41

Alergic Flora The animals could be allergic to a type of flora that thrives either along coastlines or along the specific geographical conditions of the land bridge (ex. high coastal winds, tides, salt, brine). This condition could surround both continents as well if the animals don't spend much time near the coast.

Technological Advantage If the humanoids have primitive sailing technology, then perhaps the land bridge is impassable for all species, and the humanoids crossed by sailing along the coastline.

Predators Perhaps there are particularly viscous predators that inhabit the land bridge or the waters nearby. The humanoids can use fire or other technology to fend off the predators, but the animals are largely helpless against them. Could be large birds like Rocs that nest in the cliffs or sea monsters.

• +1 for allergies. That was the immediate obvious choice IMO. I'm surprised it's not mentioned more. – kayleeFrye_onDeck Apr 12 at 1:37

HOW: Walace's Line.

Essentially, your continents may have been separated in the past, but due to lowering tides have become connected via this land bridge in more recent history.

Disclaimer: Someone more knowledgable may be able to expand on specific climate-related science to assist with Worldbuilding a cause for your Wallace Line. My answer addresses the effect of such a boundary.

The Merriam-Webster definition:

...hypothetical boundary that separates the highly distinctive faunas of the Asian and Australian biogeographic regions...

(Emphasis mine.)

A massive trench divided these regions, preventing any natural formation of land bridges for the duration of that era. In short, this led to differences in the land animals that populated these landmasses.

WHY:

Your landbridge may have formed well after the evolution of your not-horses, and so they have evolved in one location but were not found on the former continent, initially, for this reason.

• Your landbridge was crossed by not-humans when it became available to them

• Not-horses did not cross because they didn't immediately have any evolutionary pressures to

Fitting to their nature, they stayed in their region. Your nothumans, also fitting to their respective nature, are perhaps curious beyond any evolutionary pressures, and were accordingly quick to explore beyond them. (They can have had any number of reasons to explore, in fact)

If your not-horses are not unusally intelligent, as your not-humans would be, they are likely content with their familiar and robust territory.

Address the differences in the organisms you are comparing, as you have created them, and you have any number of causes for behaviour stemming from these traits and characteristics! Humans as we know them are curious, while horses may be safe, coy or timid.

I can think of two realistic ways.

1. the land bridge is very new, human will notice the change before anything else, it will take a great deal of time for animals to exploit it. Humans explore for exporations sake.

2. the bridge is not so much a bridge as a chain of islands, human on canoe can jump from island to island with ease but other animals will have a much harder time, and the larger the animals are the longer it will take.

The most believable way is to base it on real horses (our horses? actual horses?)

Why would these animals exist on only one side of the land bridge rather than both?

1. Because the land bridge contains a terrain similar to a staircase.
It is pretty easy to get a horse up a flight of stairs, but extremely difficult to coax them down a flight of steps.
Horses are strong swimmers, so you'll want to make the land bridge long and you'll want very rough water in the area.

2. You could also make it so there are bits of ocean to cross with a nasty current due to tidal forces - only between tides would it be relatively calm (but not actually calm).

                                         +--+               +---
|  |               |  |      +-+           Continant B
+--------------+    Continent A          |  |          +----+  |      | |           +-----------------+
+--------------+    +-----+  |          |       |      | |           |
Sea-level-high-tide-------+    |        +--+       |       |      | +---+       +-------High tide--
|    |           |       |       |      |     |       |
|    |           |       |       |      |     |       |
Sea level low tide--------------------------------------------------------+-+-----------Low tide---
|    |           |       |       +------+     | | |   |
+----+           |       |                    | | |   |
+-------+                    +-+ +---+

1. Maybe some marsh land on their side to keep the non-horses from even getting very close. Or even a slight slope that goes from high to low tide (in elevation) which means no vegetation (food) for a few hundred yards would keep them further from the land bridge.
• +1 Rivers (especially in the Amazon Basin) are known to create allopatric speciation. One species on one side of the river, a closely related one on the other, with no interbreeding because they don't cross the river. The intro of this paper gives lots of examples. So I wouldn't be surprised at all that the horses refuse to cross just because things are a bit scary. – B.Kenobi Apr 11 at 3:49

# Source-Sink Dynamics - the population near the bridge is not doing well

The area near the grasslands may be a "sink". There are horses there, but the birth-rate/survival rate is low, most of the population has come from somewhere else ("the source"), thus there is no pressure from a growing population to expand into new territories. The population would actually be extinct were it not for new individuals constantly moving in from more hospitable environments. Why is this area inhospitable? Perhaps the grass is very low in key nutrients that are necessary to carry a fetus to term or to nurse a young animal, so only a few animals make it to adulthood each year.

Related point 1: the species may not be doing that great anywhere in their range. Any species with a net-zero population growth wont be expanding anywhere.

Related point 2: perhaps the land bridge is towards the extreme edge of their territory. Many animals migrate back to certain places to bear their young/find a mate (due to warmer temperatures, fewer predators, more food, safety in numbers etc.) and maybe this is as far as they can reasonably get to before having to head back to give birth etc.

# Bioindicators and other Environmental Clues

Some examples below:

Bioindicator density: A complete lack of desirable bioindicators or a wealth of undesirable bioindicators will often cause fauna to either shun or congregate to certain areas previously unexplored. Some simple examples would be things like availability of food, certain densities of specific colors in flora that trigger unconscious fear responses, etc.

Biological irritants: allergic reactions to certain flora, species-specific pests or pests that can't affect the sentient species that crosses the bridge that animals shun, or even something as simple as a harsh or overwhelming smell

Regional predation: It could be as simple as there is a species on the bridge and possibly the sentient's continent that for whatever reason also doesn't congregate beyond their continent as well as the land bridge itself; sort of an inverse relationship if you will. That species then feasts on most if not all animals it comes across. If the sentient biological make-up has something inherently inimical to these predators then they will have adapted to either avoid them, or their failed attempts at preying on the sentients simply make them a non-issue only for the sentients. It could also be that these creatures are trivial for the sentients to handle due to their availability of tools in offensive or defensive scenarios.

Induced hydrophobia: the very sight of water could scare away some species in a specific region if dense with life that can infect other life and trigger hydrophobia. Just seeing the ocean could terrify all the animals from the other continent due to their infection or parasitism that triggers hydrophobia being ubiquitous for the region.

Intolerable atmospheric pressure: the animals might not be able to handle high altitudes very well whereas the sentients have little to no issue with it. If your land bridge was sufficiently elevated, it could be inimical to many forms of life if they're sufficiently susceptible.

Disruptive magnetoreception: there could be something very disruptive to the animals with the magnetic field around the land bridge that leads to that area being shunned or possibly even a death trap where creatures become disoriented when getting too close, causing fear/fleeing, or death by exposure/lack of necessary nutrition.

My personal favorite is the last due to imagined long-term consequences. Can you picture such a sprawling graveyard after a couple million years?

Maybe in some places it is a tunnel or a formation has formed over it that the nothorses can't fit through.

Or possibly the sand is incredibly sticky to the nothorses feet and it's simply the shoes that the nothumans wear that allows them to cross without getting stuck. Or the fact that nothumans have arms that they could use to grab onto trees to unstuck their feet as they walk. In this same vein, the ground could be a temperature that the nothorses can't handle on their notshoed nothooves.