There is a very interesting relationship between tortoises and the various Opuntia species that inhabit the various islands. As young plants, the trunks are thickly covered with long, sharp spines which give way in older plants to a tough, tree-like bark. Thus, these Opuntias are inaccessable to the tortoise until the fruits, laden with seeds, fall to the ground, whereupon the tortoises eat them. The seeds pass unharmed through the tortoise's gut and are dropped as the tortoise wanders from place to place. There are several other interesting differences between the two morphotypes.
Domed tortoises tend to be larger than saddle-backed tortoises.
A second difference is that saddle-backed tortoises tend to be more aggressive and solitary while domed tortoises tend to be more gregarious. Aggressive displays are contests to see which of two combatants can raise their head the highest. On one visit to the Galapagos, I was sitting in the tortoise compound on a cool, cloudy morning. As they became aware of each other, they begain to raise their necks with gaping mouths, each trying to out-do the other. This aggressiveness might force the tortoises to become more solitary, thereby avoiding competition for food.