‘Is Dwarfism exclusive to humans?’, my cousin recently posted on my Facebook wall. Evolution has changed species in various ways, for example the miniaturisation of dinosaur species into smaller birds. However in humans, dwarfism occurs within a species, miniaturisation tends to downsize and speciate animals proportionally over a very long time period. Do animals besides humans experience intraspecies dwarfism?
What Is Dwarfism?
Simply, Dwarfism is when an individual person or animal is short in stature resulting from a medical abnormality. The most common form of dwarfism in humans is achondroplasia, where limbs appear shorter than the trunk with a larger than average head, along with characteristic facial features.
Achondroplasia is an autosomal dominant disorder. Autosomal means it is not inherited on the X and Y (and Z) sex chromosomes; dominant means that the condition is cause by a single mutated copy of the fibroblast growth factor receptor-3 (FGFR3) allele on chromosome 4, where a glycine is substituted with an arginine. The FGFR3 mutation inhibits bone growth because it is constitutively active. If two copies are present in offspring, the condition is fatal.
The other main cause of dwarfism is growth hormone deficiency (GHD). The lack of growth hormone (aka somatotropin) causes stunted and even suspended growth and can delay puberty if it occurs in children. GHD can also be genetic or from damage to the pituitary gland.
Does Dwarfism Occur in Other Animals?
Yes! Last December, biologists in Sri Lanka documented evidence of dwarfism in a wild elephant. They discovered the male Asian elephant (Elephas maximus) measuring 1.5 m in height in conflict with a male of average size. He turned out to be a scrappy lil’ elephant as the dwarf seemed to be the aggressor! Similar to human dwarves, this elephant’s small stature is due to its shorter limbs.
“If you think about it, most animals, especially mammals, are either predators or prey. If you are either and are born with short limbs you would be at a very big disadvantage. A dwarf prey animal is very likely to be caught by a predator and similarly, a dwarf predator would find it very difficult to catch prey. So such individuals are very unlikely to survive in the wild. Elephants in Sri Lanka are unique (together with those in Borneo) in that they have no predators. So he was very lucky that he was born here!”
Prithiviraj Fernando of the Centre for Conservation and Research, and one of the authors of the paper
To see the elephant, follow this link: https://www.youtube.com/watch?v=KC-cE5XXwuE
Is Smaller Better?
It might be difficult to consider the selective advantage of smaller stature among one’s own species. Miniaturisation has benefits such as evading predators, thermoregulation and shortened developmental timeframes. But dwarfism is a little different in that these benefits don’t necessary apply in a human dwarf’s environment. Nevertheless, a good deal of geriatric research has suggested that people with hereditary dwarfism have protection from diabetes and cancer.
In a number of species, smaller individuals outlive longer ones, for example mice. Body size is largely determined by the action of GH; it’s absence in mice leads to significantly increased lifespans. A number of explanations have been provided for this observation including increased stress resistance, reduced insulin levels and reduced mTOR signalling.
As humans, we often associate taller stature with success, dominance, attractiveness and leadership; bigger is better. However, research is causing scientists to question this connection. Remember, survival of the fittest comprises two tenets: (1) surviving and (2) reproducing, propagating your unique genetic code. Taller stature definitely falls to the latter and possibly the former in a more violent time of war and conflict. Shorter stature on the other hand shows its evolutionary success through increased lifespan. In an environment where medicine and technology is more deprived then this may be less true, but in the developed world it is undoubtedly an advantage.