Why Do Galápagos Tortoises Have Saddlebacks? Shells Explained

What is a saddleback shell?

In a saddleback tortoise, the front of the upper shell, or carapace, curves upward in a high arch above the neck. Seen from the side, the shell looks like an old riding saddle. The shell is also often narrower and thinner than a domed shell, and the tortoise’s neck and front legs are noticeably longer.

A domed tortoise has a rounded shell that comes down low over the neck, like an upturned bowl. These tortoises tend to be larger and heavier, with shorter necks and legs.

Between the two extremes are intermediate forms. On some islands and volcanoes the shell is only slightly raised at the front. Shell shape is one of the main ways scientists and park rangers tell the island forms of the Galápagos giant tortoise apart.

Reaching food on dry islands

The leading explanation is about food. Saddlebacks come from small, low islands, generally under about 500 m (1,600 ft) high, such as Española and Pinzón. These islands are dry for much of the year and have little vegetation near the ground. Much of the available food grows higher up, especially the pads and fruit of prickly pear cactus (Opuntia), which can grow into tall, tree-like plants.

A raised front shell lets the tortoise lift its head and stretch its neck almost straight up without the shell getting in the way. Long front legs add extra height. The result is a much greater vertical reach, so the tortoise can browse cactus and shrubs that a domed tortoise could not reach.

On larger islands with humid highlands over about 800 m (2,600 ft), such as Santa Cruz, mist and rain keep grass, herbs and low plants growing for much of the year. Here there is no advantage in reaching up, and a heavy domed shell gives better protection and a body that copes with cooler, damper conditions.

Display and competition between males

A second idea is that the saddle shape also helps in contests between males. When two males meet, they rise up on their legs, stretch their necks as high as they can and open their mouths. Usually the tortoise whose head reaches lower backs away.

In saddleback populations these displays are especially common, and males are more aggressive and territorial. A tall reach would therefore help a male win mates as well as food. Many researchers think the saddleback shell is a compromise: a small body suited to scarce food, combined with the high reach needed for both feeding and display.

Why saddlebacks could evolve at all

A saddleback shell leaves a large, unprotected gap above the neck when the tortoise pulls in its head. On a continent with big predators, that weakness could be deadly. On the Galápagos, adult tortoises had no natural predators, so a less protective shell carried little cost.

Saddlebacks are also a good example of convergent evolution, where similar features evolve separately because of similar conditions. DNA studies show that saddleback shells appeared independently on several islands. Saddleback tortoises are therefore not one related group; each evolved its shape from domed ancestors in response to a dry environment.

All Galápagos tortoise hatchlings have domed shells. The saddle shape develops gradually as a tortoise grows, and it is usually most extreme in adult males.

Darwin and the tortoise shells

When Charles Darwin visited the Galápagos in 1835, the vice-governor told him that he could tell which island a tortoise came from by its shape. Darwin had read earlier reports by an American naval captain describing tortoises from some islands with shells turned up at the front like a saddle, and others that were rounder.

Darwin admitted later in his journal that he did not pay enough attention at the time and mixed up specimens from different islands. Still, the idea that each island had its own variety of tortoise, along with mockingbirds and other animals, became an important clue in his thinking about how species change over time.

Cactus and tortoises: a two-way story

Some biologists think the relationship may have worked in both directions. On islands with tortoises, several Galápagos prickly pears grow as tall plants with thick, woody trunks and pads held high off the ground, while on islands without tortoises they tend to be lower and more sprawling. One explanation is that cactus plants evolved height as a defense against browsing tortoises, while tortoises evolved reach to get at the cactus.

This idea is hard to test directly, and other factors such as competition between plants for light also affect cactus shape. Still, it shows how closely the shell of a saddleback is tied to the plants of its island.

Saddlebacks today

Several saddleback forms came close to extinction. Española was reduced to just 14 adults before a breeding program restored the population, and the saddleback Pinta tortoise disappeared with the death of Lonesome George in 2012. Saddlebacks are also smaller on average than domed tortoises; see How big do Galápagos tortoises get? for size comparisons.