Posts Tagged ‘dwarf ground sloths’

Survival of the Fittest and Dwarfism–A Paradox

September 4, 2015

The concept of natural selection forms the basis for the Theory of Evolution.  Environmental forces select the fittest members of each population to pass on their genes.  Most people think survival of the fittest means selecting the biggest, fastest, and strongest; and that is often true.  However, insular evolution (the evolution of species on islands) shows that survival of the fittest can mean the opposite as well.  During the Pleistocene many species of megafauna became stranded on islands.  Islands are often devoid of large predators.  Megafauna evolved to a greater size in response to predation, so without the presence of predators, there was no longer selective pressure toward a larger size.  Smaller individuals were just as likely to survive.  Moreover, these smaller individuals had an advantage on islands where less food was available.  On continents megafauna could migrate to different regions when forage became scarce, but they didn’t have this luxury on islands.  Smaller individuals were more likely to survive during times of scarcity because they required less food.  This explains the tendency toward dwarfism among Pleistocene megafauna that lived on islands.

During the Pleistocene there were 8 species of dwarf ground sloths living on Cuba, 7 on the island of Hispaniola, and 1 on Puerto Rico.  Megaloncus rodens was 1 of the species that lived on Cuba, and it grew to 200 pounds.  By contrast the smallest species of continental ground sloth grew to 500 pounds.  Dwarf ground sloths survived in the Caribbean until as recently as 4700 BP, about 1200 years after humans colonized the region.  Humans must be responsible for their extinction because there was no major climatic change during this time period.

Skeletal mount of Caribbean dwarf ground sloth (Megalocnus rodens) next to a paleontologist in 1920.  All continental species of ground sloths were larger than men.

Full-sized Columbian mammoths (Mammuthus colombi) swam to the Channel Islands off the coast of California and eventually evolved into a dwarf species known as Mammuthus exilis.  Evidence suggests they arrived as early as 250,000 years BP and survived 2 complete glacial/interglacial cycles but became extinct when men found the island.  The late surviving population of mammoths living on Wrangel Island off the Alaskan coast were not a dwarf species, but those living on St. Paul’s Island off Alaska were.

Each island in the Mediterranean Sea had its own species of dwarf elephant or mammoth.  Dwarf elephants lived on Cyprus, Malta, Sicily, Sardinia, and Stylus (now underwater).  Dwarf mammoths lived on Crete.  Every time sea level fell in response to glacial expansion, new waves of elephants colonized these islands, and all evolved into dwarf species.  Distances were shorter between the mainland and the islands during glacial expansions, and it was easier for these large animals to swim there.  Some of these dwarf species were still large animals, weighing as much as 500 pounds, but others reached heights of just 3 feet tall.

Size comparison between human, full sized elephant, and 2 different species of dwarf elephants.

Some Mediterranean islands also hosted populations of dwarf hippos.  Sicily, Malta, Crete, and Cyprus were home to 4 different species of dwarf hippos respectively.  There were 3 different species of dwarf hippo living on Madagascar until 1000 years ago when humans wiped them out.

Hippo in Middle~Late Pleistocene Mammalian by sinammonite

Artist’s depiction of extinct Mediterranean dwarf hippos.  P. minor lived on Cyprus, H. melitensis lived on Malta, H. creutzburgi lived on Crete, and H. pentlandi lived on Sicily.

Stegodon floresiensis insularis was a dwarf species of elephant that lived on the Indonesian island of Flores.  It shared the island with a dwarf hominid, Homo floresiensis, known famously as the hobbit.  This hominid likely evolved from Homo erectus, but in isolation from the parent population became a dwarf species.  The hobbit grew to 3.5 feet tall and had the brain capacity of a chimpanzee.  Nevertheless, the area of their brain associated with language was well developed, and they may have been as intelligent as modern humans because their ratio of brain capacity to body size was similar.  They hunted dwarf stegodons.  Komodo dragons and even marabou storks were a threat, especially to juvenile hobbits.  Homo floresiensis lived as recently as 14,000 BP.

Artist’s depiction of real life hobbits (Homo floresiensis) battling marabou storks on the Indonesian island of Flores.

Ground sloths, elephants, and hippos were/are excellent long distance swimmers.  That explains how they colonized islands.  The ancestors of the hobbit either built rafts or accidentally rafted to the island on flood debris.  If they built rafts, they may have forgotten the technology after several generations.

There are over 200 medical causes of dwarfism, and some traits that cause it can be inherited.  A human dwarf is defined as an adult less than 4’10” in height.  Some kinds of dwarfism in children can be treated with growth hormones and drugs that stimulate appetite.  Parents fear their children will face societal prejudice, if they don’t reach a “normal” height.  Few realize the trait for dwarfism could potentially be favorable for natural selection, if environmental conditions on earth deteriorate.  The traits that favor future survival of the fittest on this island earth can’t be predicted.

What happened when James Lusted, who stands just 3ft 7in tall, took his fiancee for a romantic meal at a Harvester in Cardiff?

If the world suffered a severe shortage of food for millennia, human evolution would tend toward dwarfism.  Humans with smaller body size have an  advantage in environments with less food.

References:

Muhs, Daniel; et. al.

“Late Quaternary Sea-Level History and the Antiquity of Mammoths (M. exilis and M. colombi) on the Channel Islands National Park, California, USA”

Quaternary Science Review May 2015

Poulakatis, Nikos; et. al.

“Ancient DNA Forces Reconsideration of Evolutionary History of Mediterranean Pygmy Elephants”

Biology Letters 2006