Living large can become advantageous for an island-bound animal. Predators drop away as prey gain girth.
As we have already seen, island birds repeatedly supersized. Not being able to fly away became increasingly insignificant as predatory attack became more remote.
Already-large monitor lizards turned into Komodo dragons on Indonesian islands. But then, today’s Komodo dragons are a serious step down from earlier, extinct Australian relatives.
Conversely, shrinking to reduce nutrient requirements is an equally viable strategy. While rats on the island of Flores grew to a meter to establish dominance, elephants shrank to the size of large hogs on the 2 separate occasions they made their way to Flores. A hominid isolated on Flores became a pygmy, as did humans that arrived there within the past 50,000 years.
Flores the only example in the world where insular dwarfism has arisen twice in hominins. ~ American evolutionary biologist Joshua Akey
Settling in on Madagascar, cockroaches grew to 6 cm or more and started hissing to scare off potential troublemakers. Meanwhile, hippos on Madagascar became pygmies. This was not the only instance of hippos shrinking. Finding their way to Cyprus, hippos dwindled to the size of sea lions.
Timing matters. A rattlesnake that made its way to an island in Baja California grew into a giant. The rattlesnakes that followed got a case of adaptive intimidation: shrinking rather than face well-established stiff competition.
Islands often inspire animals to try something different. Small ones enlarge while big ones shrink. Whereas rodents tend to gigantism, carnivores, artiodactyls (even-toed ungulates), and rabbits deign to dwarf.
Food resources and the ability to self-limit population size determine the evolutionary vector on an island. In the interest of self-preservation, rodents can prudently control their population numbers in absence of predators; but hippos, deer, and other artiodactyls cannot. Early island generations may at first upsize from the new-found abundance, then be forced to shrink as food becomes scarcer.
Lizards typically dine on various animal fare. The Balkan green lizards that inhabit islands in the Greek archipelago found meat running scarce, so they are adapting to eating greens: their digestive tracts are lengthening, and they are developing pouches (cecal valves) to harbor gut microbes that feast on floral food. Such changes are common in herbivorous reptiles, as in the green iguana.