 In combination with proportionally longer necks and limbs , the unusual saddleback carapace structure is thought to be an adaptation to increase vertical reach , which enables the tortoise to browse tall vegetation such as the Opuntia ( prickly pear ) cactus that grows in arid environments . Saddlebacks are more territorial and smaller than domed varieties , possibly adaptations to limited resources . Alternatively , larger tortoises may be better @-@ suited to high elevations because they can resist the cooler temperatures that occur with cloud cover or fog . 
