Extra Practice Objective 18

Background:

Galapagos Tortoises

The Galapagos Islands (see image above) are home to giant tortoises that feed on green vegetation. Originally tortoises lived only on Isabela Island and, although there was some variation in shell shape, most of them had dome-shaped shells or carapaces (see figure 1 below). This is still true today. Isabela has a relatively wet climate and varied plant life including many low-lying grasses and plants.

Tortoises are buoyant and can float for long periods of time. They eventually populated other islands after being carried there by ocean currents. However, today typical tortoises on some of the islands look distinctly different from those on Isabela. Initially all tortoises on on small islands such as Pinta, Espanola and Hood all tortoises had dome-shaped shells (carapace) as they arrived from Isabela. Eventually a mutation arose giving some tortoises a 'saddleback' carapace (see figure 2). Today on islands such as Pinta, Expanola and Hood, 100% of tortoises have saddle-back shells.

The 'saddleback' carapace is elevated above the neck and flared above the hind feet. On these islands the climate is dry and there is almost no ground vegetation. Here the predominant plant species is the prickly pear cactus, a plant that is tree-like in form with a woody trunk that holds its fleshy green parts high off the ground.

Use this information to answer the questions to follow.

A key is posted In: Objective 18 Materials--> Case Study Keys