A recent study published in Nature shows how the African elephant started evolving morphologically nearly ten million years ago by switching from a leaf browsing (C3) to grass eating (C4) animals. C3 and C4 are cycles of carbon.
This transition was enabled despite the abrasive action of grit ingested with the grass on dental enamel as the elephant gradually adapted morphologically by emergence of longer teeth with thicker enamel capable of withstanding wear and tear.
The interplay
Dr. Adrian Lister, the author of the study investigates the interplay between habitat change, behaviour and adaptation in elephants in East Africa. Dr. Lister is from the Department of Earth Sciences, Natural History Museum, London.
The enamel increases in thickness with one layer of enamel overlain with succeeding layers and cemented together.
“The more [the] lamellae, the more enamel in the tooth, and more wear resistant [the tooth],” he notes in an email to this Correspondent. The lamellar advent allowed increased hipsodonty and lamellar multiplication, explaining the ultimate survival of the elephant species.
The very first Proboscideans (animals with a strong muscular trunk) appeared in Africa and had shorter molars (used for crushing and chewing the food).
As they evolved against a backdrop of aridification and replacement of C3 dominated woodland with C4 dominated grassland, lamellation and hypsodonty took effect and tooth size increased.
Elephants underwent a three-fold increase in teeth height as well as substantial multiplication of lamellae after about five million years ago.
Fossil data
Plant and early elephant fossil data show that the spread of C4 vegetation, indicative of grassland occurred gradually beginning about ten million years ago and accelerated four million years later.
There was a behavioural switch to C4 grazing at around eight million years ago. Significantly, most of the hypsodonty occurred after around five million years ago.
This indicates a lag of three million years after the adoption of a grass-dominated diet — a finding based on elephant tooth enamel studies. He explains the lag between behaviour and adaptation by stating that the behavioural accommodation placed the animal in a new niche where selection then led to adaptation. A second factor for this lag is that while the switch to grass-eating is indicated by a strong C4 signal within the enamel about eight million years ago, the quantity of soil-derived grit might have increased later.
No proxy data
“We do not yet have good proxy data for environmental grit, or its consumption by fossil mammals, although dental microwear is a potential tool,” he notes.
The behavioural switch to C4 grazing would have resulted in more immediate adaptive response if not for the fact that the environment included semi-wooded habitats with limited airborne dust.
Major adaptation
Early responses to grazing pressure are seen in minor dental advances five million years ago.
But the major adaptation, a marked increase in hypsodonty required a reorganisation of cranial anatomy to accommodate high crowned teeth, which the author notes, could account, in part for its delay in appearance.
Correction
>>A sentence in the first paragraph of “The dental factor in elephant evolution” (Science & Tech page, Sept. 5, 2013) read: C3 and C4 are isotopes of carbon. Actually, they are not isotopes but names of cycles which operate during the procedure of photosynthesis.
