Big or Small, God Loves Them All: Foster's Rule

When smaller island species that are related to larger mainland species exist, it is called “insular dwarfism.” Scientists think this may occur because living on islands limits the space in which animals can find food. So large animals shrink because they need less food to survive. While pygmy mammoths and dwarf elephants are sadly extinct today, we can still see examples of insular dwarfism in modern animals. For example, if you live in the Americas, you have probably seen white-tailed deer. Adult males stand an average of 4 ft. (1.2 m) tall and can weigh over 500 lb. (227 kgs). However, an endangered subspecies of the white-tailed deer, called the Key deer, lives in the Florida Keys. They only stand about 2.5 ft. (0.8 m) tall and weigh 45–75 lb. (20–34 kgs). Another example is the endangered Sumatran tiger, an island tiger subspecies much smaller than its mainland counterparts. While the Siberian tigers of northeastern Asia stand about 3.5 ft. (1 m) tall and can weigh over 600 lb. (272 kgs), Sumatrans only stand about 2 ft. (0.6 m) tall and often weigh under 300 lb. (136 kgs).

Key Deer. National Archives and Records Administration, Public domain, via Wikimedia Commons.

But animals don’t just get smaller on islands, some may get bigger too. In New Zealand, birds called moas once existed. Giant moas stood 12 ft. (3.7 m) tall and weighed up to 600 lb. (272 kgs)—that is 3 ft. (1 m) taller and 300 lb. (136 kgs) heavier than today’s largest bird, the common ostrich. Their closest relatives alive today are South American birds called tinamous. The largest species, the gray tinamou, weighs less than 5 lb. (2.3 kgs). Another famous extinct big bird is the dodo. Native to the island of Mauritius, dodos grew up to 2.5 ft. (0.8 m) tall and weighed up to 40 lb. (18.1 kgs). Their closest living relative is the Nicobar pigeon of Southeast Asia, which weighs less than 2 lb. (0.9 kg).

Dodo. Jebulon, CC0, via Wikimedia Commons.

When larger island species that are related to smaller mainland species exist, it is called “insular gigantism.” Scientists think this may occur because living on islands means there are fewer predators. So smaller animals can grow larger because there is less to fear. Even though giant moas and dodos are extinct now, we see many examples of insular gigantism today. Do you know anyone with a pet hermit crab? Those species can easily fit in the palm of your hand. However, they have a relative found on many Indian and Pacific Ocean islands called the coconut crab. They are the largest arthropods (a group that includes insects, arachnids, and crustaceans) alive today. They can grow over 3 ft. (1 m) wide and weigh up to 9 lb. (4.1 kgs). What about geckoes—have you ever seen these tiny lizards crawling up a wall? There are about 1,500 species today, and many of them are smaller than your hand. However, on the island of New Caledonia lives the New Caledonian giant gecko, the largest gecko in the world. They grow to 1.5 ft. (0.5 m) long and weigh over 1 lb. (0.5 kg).

New Caledonian Giant Gecko. Lennart Hudel, CC BY 4.0, via Wikimedia Commons

Insular dwarfism and insular gigantism fall under a theory called Foster’s Rule, also called “island rule” or “island effect.” The scientist who proposed it, J. Bristol Foster, believed that the unique conditions of island habitats changed the size of animals who lived there. Big animals got small to use less food, while small animals grew big because of the lack of predators. However, this theory is questioned because it does not consider other factors that affect the size of animals, like genetics, habitat features, or competition from other species. Also, some animal groups display both dwarfism and gigantism on islands. One example is the extinct prongdeer. They resembled modern deer, but instead of antlers, they had three horns arranged like those of Triceratops on their heads. Both dwarf and giant species of prongdeer lived on the same island off the coast of Italy at the same time.

While Foster’s Rule may not fully explain why some animals shrink or grow on islands, the many examples of insular dwarfism and insular gigantism in animals show off God’s design. When he first created the animal kinds 6,000 years ago (Genesis 1:20–25), he gave them lots and lots of genetic potential. He knew earth would be filled with many types of habitats, especially after the global flood. Because he cares about the animals (Psalm 104:24–28), he wants to make sure they can adapt and survive in challenging situations. We can see God’s glory and creativity reflected in the many unique designs of animals (Romans 1:20)—even the ones who are no longer with us today.