What is Insular Dwarfism and How Does It Affect Animals?

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One of the biggest things that impact a species’ evolution is its environment, where animals live, eat, and mate. The ecosystem has everything to do withr a species’ diet, behavior, and physical characteristics!

Although most environments are interconnected, some ecosystems develop in isolation from the wider world, limiting their available resources, territory, and species. Being cut off from the rest of the world significantly impacts species’ evolution. This leads to a little something (pun intended) called insular dwarfism.  

When placed in isolated habitats, some normally large species shrink over time and become miniature versions of their mainland counterparts. This phenomenon has led to dwarf buffaloes, pygmy elephants, and even miniature humans!

Island consisting of steep, rocky cliffs towering over the water.

What Is Insular Dwarfism?

Insular dwarfism is a phenomenon in which a species living in an isolated environment will shrink over time. Keep in mind that this affects the entire population, not just a handful of individuals. This occurs after a species has spent a long time – multiple generations, in fact, in an environment isolated from the rest of the world and the rest of the given species. 

Some of the animals impacted by insular dwarfism have been found to be only a fraction of the size of their mainland counterparts. Mammals seem to be the most affected by island dwarfism, though it can also happen to other species. 

There are several reasons why this occurs, but the most common one is the limited available resources for a species. Unlike the mainland, there is only so much food on an island, territory to claim, and places to drink. To survive, a species must adjust its anatomy by becoming smaller.

Where Does Insular Dwarfism Occur?

Insular dwarfism occurs in isolated environments, primarily islands, which explains the name. Being isolated is essential for insular dwarfism to occur, because few outside forces can interfere in the environment, no new animals can arrive, and moving in and out of the domain is difficult, if not impossible. 

Beautiful illustration of islands with steep, snow-clad mountains with a sunset behind them.

Insular dwarfism happens so often in islands that some people have coined the term ‘rule of the island,’ which says that species living on an island can undergo extreme changes in size if left there for long periods. These changes result from the availability of resources and the other animals living there.

While insular dwarfism is most commonly observed in islands, it isn’t exclusive to them and has also been observed in desert oases, valleys, tundras, and caves.

Fact: One of the most famous examples of insular dwarfism outside islands is the Mbuti people, the famous pygmies whose average height is about four and a half feet. Although they make their home in mainland Congo, they live in dense jungles that essentially isolate them from the rest of humanity.

Why Do Animals on Islands End Up Smaller?

With resources and land limited on an island, this can become a problem for animals as no matter what they do; there won’t suddenly be more resources or more land. Unlike animals living on the mainland, insular animals can’t just pack up and leave, so they are stuck in that habitat. 

To survive, a species must adapt to these conditions by becoming smaller over time through insular dwarfism. There are two leading causes of island dwarfism occurring.

Vert small monkey on wooden floor-boards.

Species Become Smaller From Stunted Growth

The first cause of insular dwarfism is quite simple. If there isn’t enough food or if the food that is present isn’t high quality, animals won’t reach their peak sizes. Parents always tell their kids to eat plenty of food to grow big and strong – the same rule also applies to animals. 

At some point, the island’s population can become too big, or its resources become too strained to sustain the whole population. This leads to increased competition between species, and this is where insular dwarfism occurs. 

Without enough food, animals won’t be able to reach their maximum size as they become stunted and small. While they might find enough food to stay alive, they won’t thrive.

If the situation stays the same on the island, the same will be true for future generations as they will experience the same shortages, allowing the cycle to continue.

This is where genetics comes into play. Although with increased competition, it might seem like stronger animals are the ones who benefit, the truth is, however, that smaller animals have the advantage.

One of Charles Darwin’s most important principles of evolution is the idea of survival of the fittest. Whenever people hear this, many often associate it with being the biggest or the strongest, and while that is sometimes useful, it isn’t the whole story.

Animals who are best equipped to survive and those with the most genetically valuable traits are the ones who are most likely able to find mates, control resources, and thrive in an environment. In the case of isolated areas, when being able to survive with as few resources as possible becomes more desirable.

Smaller individuals in a species require less food and can maximize what resources there are. The small members of the species have the best chances of surviving. This also means they are more likely to breed and pass on their genes than the bigger, less successful members of the species.

The more this happens, the more likely the next generation will have these genes and pass them on, gradually making the entire species smaller.

Some Species Won’t Need to Grow as Big on An Island

Aside from lacking certain resources, island ecosystems can also lack certain species, affecting the growth of those present, particularly predators. Some animals only grow big because it allows them to protect themselves against large predators who might threaten them, but if there are no big predators to worry about, they don’t need to grow as big.

An elephant on the African savannah has to grow so big to defend itself against things like lions. But in the islands of Southeast Asia, where big predators are a rare sight, elephants don’t have to worry about that.

So instead of just being strong, the most desirable traits in a species shift to becoming more energy efficient. 

Think of the species like a car; if they’re big, like an SUV, no one will mess with them, but they also require a lot of fuel to keep going. Something smaller and more compact might be helpful in a place without many gas stations because it can last longer with less fuel. It is a tradeoff, giving size and protection for efficiency.

These animals become the most efficient individuals, so they’re more likely to pass on their genes. 

It is important to remember that these two causes are not mutually exclusive and are often tied to each other.

How Long Does It Take For Island Dwarfism to Occur?

Island dwarfism affects an entire species and it requires the passing of certain genes through successive generations, so it’s not something that happens overnight. Depending on the species’ lifespan and genetics, something like island dwarfism can take centuries or even longer.  

Island dwarfism only occurs when a species continues to pass down certain genes that encourage animals to become smaller. The big problem is if these traits remain useful for a long time. When we talk about genetics, the problem is that we can only discuss what is beneficial here and now. 

A century later, the habitats animals inhabit could be completely different, and what is important for survival can change with it.

Are All Animals Affected By Insular Dwarfism?

A beatiful tropical island with palm trees and a boat close by.

While an isolated habitat affects all the animals in it, it doesn’t affect them similarly. By nature, insular dwarfism only affects certain species, those too big to survive in the smaller ecosystem. Smaller animals who don’t need too many resources to thrive will most likely remain unaffected. 

Elephants will struggle to find enough food, but mice are already relatively small and can survive just fine as they are on an island. 

Mammals tend to be the group of animals most affected by insular dwarfism due to generally being larger, though other animal genomes, like reptiles, are also affected from time to time. 

Birds and sea life are also less affected because they can leave the island and travel elsewhere depending on the island’s conditions. Amphibians are generally small, too, which means they don’t use much territory and resources.

Some Animals Are Affected By Insular Gigantism

Speaking of small, the interesting thing about the island ecosystem is that becoming small isn’t the only possible change. Sometimes, living on an island can lead to the opposite effects and make smaller animals bigger. This phenomenon is known as insular gigantism and affects small species living in isolated environments. 

Small species can thrive and grow in isolated environments, especially if there aren’t any major predators. Small species don’t have to worry about competition as much since they are less likely to deal with resource shortages.

When they don’t have to worry about eating, animals like rodents, birds, and amphibians can reach greater sizes. Some species can grow several times the size of their mainland counterparts.

Fact: The most interesting case of gigantism is the komodo dragon because it is both a reptile and a predator, but these can grow to over 20 feet long. Scientists are still trying to explain this phenomenon with several theories. One is that being the apex predator, the komodo dragon fills a specific niche. The other is that komodo dragons are a pygmy species of a much larger monitor lizard.

Some Other Examples of Insular Gigantism

Just as there are plenty of animals that shrink down on an island, some species have the opportunity to grow larger too, and they don’t waste that opportunity either. Some animals have gotten truly massive when given a chance. 

Coconut Crabs Are the Largest Land-Dwelling Crabs.

Many people love crabs; they keep them as pets or as meals for dinner, but what if there was a crab large enough to make our pets a meal? 

Scattered across the islands in the Pacific and Indian Oceans is the world’s largest land-dwelling crab, the coconut crab. These crabs are relatives of the hermit crab, and like their smaller cousins, when they are born, they have no shells so they rely on whatever they can find for protection.

Within a year, though, many coconut crabs get so big that most shells do not fit them; instead, their exoskeleton hardens to protect them. On average, each of these crabs weighs 9 pounds and measures 3 feet long from end to end. That is the size of a small toddler!

But if the size alone is scary, then their claws are terrifying. A crab this size packs equally impressive pincers. Scientists have calculated that each pincer has a crushing force of 90 times the crab’s body weight which makes its pinch comparable to the bite of a lion.

With such powerful claws, coconut crabs have no problem breaking the shells of coconuts (hence the name.) Aside from coconuts, these crabs have been known to eat chickens, rats, and even house pets.

Fact: Some people speculate that coconut crabs ate the remains of legendary pilot Amelia Earhart. Three years after her disappearance, an investigation in her last known location discovered the remains of a human woman were found in an island full of coconut crabs, raising speculation on whether or not the crabs were responsible.

The Giant Tortoises are Found on Islands All Over the World.

A giant tortoise.

If we want to talk about gigantism, there is no better animal than the giant tortoise. These massive reptiles started all this talk of evolution when Charles Darwin studied them during his stay in the Galapagos.

Giant tortoises are found worldwide, from Africa and Asia to South America, and are said to be the remnants of an Ice Age species. Whatever they are, they share two characteristics. First, they are massive, with some reaching sizes of over 4 feet and weighing over 600 pounds. The second is that they are mostly found in islands like Seychelles, the Pacific Islands, and the Galapagos. 

Because of these two key traits, scientists attribute their large size to insular gigantism. Giant tortoises are also known to survive for long periods without food or water, making them less affected by the limited resources of the island habitats.

The Dodo Bird is an Extinct Giant Bird

Old illustration of a dodo.

If we’re talking about island giants, probably none are as well-known as the dodo bird. Native to the island of Mauritius, these birds were hunted to extinction by European explorers when navigating the world. But, though they have been gone for centuries, they continue to fascinate researchers due to their massive size.

Many researchers think that the dodos were a species or relative of the pigeon that had settled in Mauritius. The island lacked large predators and any major competition for the dodos when they arrived, meaning they had a food monopoly and few threats to worry about.

Without worrying about these issues, dodos grew larger over time, losing their ability to fly but reaching unprecedented sizes by any of their relatives. The average dodo was about 3 feet tall and weighed almost 50 pounds, compared to the average pigeon, which only weighs about 0.6 pounds.

But while they were big, this evolution made dodos completely unprepared for the Europeans who showed up on the island during the Age of Exploration. Being so large, they couldn’t fly away, and being unused to being hunted, the hunters quickly drove them to extinction. It just goes to show that sometimes, bigger doesn’t mean better.  

Do Animals Affected By Island Dwarfism Become A Separate Species?

Illustration of a cute baby pygmy hippo.

Given enough time, animals living on islands and isolated places can become separate species. This process is known as speciation, and in particular, island dwarfism is peripatric speciation when a small part of the species living in the periphery is cut off from the remainder due to a physical barrier (this case being an island.) 

The different environment means different unique traits of the smaller group get passed on than the rest of the species, making them more common and distinguished from the rest of the population.

When this happens, the two groups will develop differently according to their needs and environment, gradually separating genetically. As these two groups evolve differently, they will become entirely separate species. A group is considered a new species when it becomes so genetically distinct that it no longer considers members of its original species possible mates.

Fact: Speciation is one reason archipelagos such as the Philippines and Indonesia are hotspots for biodiversity. With countless islands, new species can emerge from each one, creating hundreds of unique animals. However, since many of these species are only endemic to certain animals, it also puts them at high risk of extinction. 

What Are Some Examples of Island Dwarfism?

Over the years, many species have been discovered with insular dwarfism. Some of them are so drastic that their size comparisons to their mainland cousins are mind-boggling.

Even Elephants Can Become Smaller

A small Borneo elephant among trees and bushes.

Also known as the Borneo elephants, these animals have become famous for being noticeably smaller than other Asian elephants. These elephants are about 8 or 9 feet tall on average, compared to mainland elephants which are over 10 feet tall.  

They are so small that their tails sometimes drag on the ground, while their ears are often overly large compared to the rest of their bodies, a remnant of their original size. Despite this smaller size, the Borean elephant remains the largest animal on the island.

These elephants arrived on the island more than 300,000 years ago, likely on the land bridges that connected these islands to the rest of Asia. When the land bridges sank, the elephants were cut off from the rest of mainland Asia.

Fact: Before research was done to determine their origins, a popular explanation was that these elephants were the descendants of a herd given to the Sultan of Sulu as a gift in the 17th century by the neighboring rulers of Java and the surrounding kingdoms.

The Dwarf Buffalo of the Philippines

A man riding a tamaraw, or dwarf water buffalo on a dirt road.

Still within Asia is the famous Tamaraw or dwarf water buffalo. Compared to the traditional water buffalo, the tamaraw is tiny, standing quite a lot shorter than humans, as the average height ranges from 3.5-4 feet at the shoulder. That is a third the size of a mainland water buffalo!

These animals are endemic to the Philippines and, at one point, could have been found in several islands, but are today only found in a few grassy plains in Mindoro island. 

The tamaraw might not even be the smallest buffalo on the island either, as scientists have discovered the remains of water buffalo that reached heights of only 2.5 feet! Even though these animals are now extinct, it shows the full extent of insular dwarfism that a species can reach!

The Honshu Wolf Was Once the World’s Smallest Wolf

At less than a foot tall, the Honshu wolf was the world’s smallest wolf and was even smaller than some domesticated dogs during its time. As the name suggests, the Honshu wolf was found on the island of Honshu in Japan, and it was one of only two species of wolves in the country, the other being the Hokkaido wolf. 

Being so small, the Honshu wolf mainly preyed on small mammals such as raccoons (which were an invasive species introduced by the Europeans) and hares, which made them well-regarded by farmers as they got rid of these pests. Unfortunately, the arrival of diseases like rabies and the rampant habitat destruction from industrialization brought them to extinction.

Fact: The Honshu wolves became an essential part of Japanese culture, most notably the inspiration for Okami, the wolf messenger of the gods and the protector of farmers.

The Dwarf Mammoths of California

An elephant's head in profile with atmospheric lighting on a black background.

Even the Ice Age had its cases of insular dwarfism 20,000 years ago. The ​​​​California Channel Islands were home to a species of pygmy mammoths. Back then, the sea levels were lower, making it feasible for mammoths to swim across the channel and head onto the islands.

When the sea levels rose, the mammoths on the channel islands were cut off from the rest of the population, leading to their shrinking.

At some point, the mammoths shrunk to about 6’5 feet at the shoulder. While that is big for us, that is only half the size of their cousins on the mainland. 

Some Humans Evolved from Insular Dwarfism

Even humans were not immune to the effects of island dwarfism. In Indonesia, the remains of one of our ancestors were discovered on of Indonesia’s many islands called Flores (the home of the Komodo dragon), but this ancestor was only three and a half feet tall.

This subgroup was called Homo Floresiensis for the island it was discovered on, but many have since called them ‘hobbit people’ in reference to the small people of J.R.R. Tolkien’s books.

One popular theory proposed by scientists is that Homo Floresiensis evolved due to island dwarfism to allow them to survive on the island with limited resources.

Author: Quade Ong

Hello there, my name is Quade. I have been a writer for three years but an animal lover for over two decades. I grew up in one of the most biodiverse areas in the world, which has given me the blessing of seeing all sorts of beautiful animals. Now I strive to learn not just about the animals I am from, but those all over the world!

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