Snoozing and cruising to new habitats – a new theory on how mammals colonised islands
When Madagascar became an island 88 million years ago it supported no species of mammal ancestral to the species found there today.The mystery of how the island came to be colonised by the mammals we see today has puzzled scientists for many years. Now, a new review presented in The Mammal Society’s scientific journal, Mammal Review, has shed light on how mammals may have reached and colonised Madagascar and many other islands.
Madagascar is the fourth largest island in the world and is separated from Africa by over 400km of ocean, a significant barrier to all but the flying mammals. Scientists from the University of Hamburg now believe that non-flying mammals arrived to the island on rafts of drifting vegetation and used heterothermy (the ability to reduce energy requirements by lowering body temperature and metabolic rate during torpor) to survive the journey.
Julia Nowack from the University of Hamburg says,
“This theory has been around for decades, but most scientists have doubted this idea, arguing that the ocean currents were facing in the wrong direction or that the ancestral species of today’s Malagasy mammals were too big to use heterothermy. However, today a lot of evidence actually indicates that this theory might be true after all.”
In their Mammal Review paper Julia and her colleague Kathrin Dausmann outline that all taxa of terrestrial, non-flying Malagasy mammals include representatives that are able to become heterothermic, either on Madagascar itself or on other continents. With journey time aboard a raft from Africa to Madagascar estimated at around four weeks, heterothermy would explain how small mammals with high energy and water demands, and low fat stores were able survive the journey over the ocean and colonise the island.
This theory is not only important in the context of the colonization of Madagascar, however. Just recently scientists have discovered primate fossils in Peru, indicating that African primates somehow crossed the ocean to reach South America too. Colonization processes of unoccupied islands and dispersal events over insurmountable barriers have occurred more than once.
For any mammal the difficult part is not only the journey to the new habitat, but also surviving and founding a new population after arrival there. Heterothermy might also have abolished the need for more than one individual of a species being needed for successful colonisation. Delayed parturition and prolonged sperm storage as a result of torpor would have allowed females to reach the new habitat with unborn offspring and establish a founder population. In a highly seasonal climate, such as is found in Madagascar, torpor might also have helped these founder populations survive.
Access to the abstract in Mammal Review can be found at the following link:
Photo credit to Kathrin Dausmann
The Mammal Society: www.mammal.org.uk
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