Island gigantism

Insular gigantism or island gigantism is the evolutionary response that occurs in animals that colonize remote islands, where the affected species gradually increase their size. It is the opposite process to insular dwarfism. Both being allopatric speciations. Since size is a highly variable characteristic, growth can occur rapidly, obtaining surprising results in less than a million years.

Causes

The causes of this process are fundamentally the following:

• Absence of predators: Many rodents and other animals are small and light because that allows them to quickly escape from their enemies and hide in the first crack to appear. If these predators are non-existent on the new island and the resources are abundant, that advantage disappears, and small animals are vulnerable to the larger animals of their own species (which would otherwise be eliminated by the natural selection exercised by predation). Over time, the primitive population evolves to growing species in size. As an example, giant rats can be cited (Papagomys armandvillei) of the island of Flores, in Indonesia, that reach 45 centimeters of body and up to almost 70 tail, and that served as food to the small native hominid Homo floriensisthe giant rats of the Canary Islands, among which stands out Canariomys bravoi, the giant rat of Tenerife; the erinaceido (Deinogalerix) and the lire (Stertomys) giants of the late Miocene of the paleo-isla of Gargano, Italy; liron (Hypnomys) and the musarana (Nesiotitesgiants of the Pleistocene of the Balearic Islands; the moa-nalos Hawái, the great swans of SicilyCygnus falconerior the famous giant turtles of the Galapagos Islands (Geochelone nigra).

• Absence of competitors: Lack of animals offering some Tope the size that a new species can get by occupying the corresponding ecological niche can lead to really strange adaptations. Thus, in New Zealand giant aptero grids such as those of the genus can be found Weta, of size and habits similar to those of rodents (originally absent from the islands), while in Madagascar (lack of higher primates) there was a lemur (Megaladapis) similar to the indri that came to have the size of an orangutan.

• Presence of large dams: Although it is very rare that a carnivore of a certain size can cross a wide sea arm to colonize an island (the marine crocodile would be one of the few exceptions), it is not so much that small predators get it on branches and trunks to the drift, especially those of insectivorous habits that can feed themselves from the insects of the wood while the journey lasts. Once on the colonized island you can find a whole range of new prey and a lack of predators or competitors that hunt them. Consequently, colonizing carnivores are diversified over time occupying all possible niches, and many of the species they generate grow to specialize in the capture of medium-large animals. This is the case, among others, of the dragon of Komodo (Varanus komodensis), the largest lizard that currently exists, which evolved from modest sized rods and specialised in the hunt of the dwarf elephant of Flores; the pit (Cryptoprocta ferox) of Madagascar, a viverrite with the size and habits of a feline; or the gigantic eagle of Haast (Harpagornis moorei) of New Zealand, which tripled the size of its Australian ancestors of gender Hieraaetus and specialised in hunting the huge moas.

Insular gigantism in bees

The species Megachile pluto, commonly called Wallace's bee, is the largest bee in the world, measuring 39 mm long. The female Megachile pluto has a wing width of 63 mm. Its massive head is 13mm from side to side and has enormous jaws. It was found in the jungles of Indonesia by the famous naturalist Alfred Russel Wallace. No further trace of it was found for the next 120 years, and it was considered extinct, although it was rediscovered to the astonishment of many after a long period.

The female Megachile pluto has its body covered with velvety black hair except for a white stripe on the front part of the abdomen. She has huge tweezers to collect resin.