Before my love of dinosaurs kicked in, I adored elephants. My four-year-old self spent hours on the couch watching elephant documentaries, pith helmet firmly affixed to my head and my faithful companion Koba at my side. (A black, plush elephant bigger than I was, Koba was filled with a cheap stuffing I was allergic to. I had chronic bronchitis for years before anyone figured out the connection and Koba abruptly disappeared from my bedroom.) And when I wasn’t watching them on the set, I imagined elephants roaming the sparsely-vegetated confines of my family’s suburban backyard. I was the shepherd of my fictitious elephantine charges, and, innocent to the fact that the elephants on television were running from the gunshots of hunters, I guided my imaginary herd from place to place by firing my more humane “elephant mover.”
I still have a fondness for elephants. The proboscideans are wonderfully strange outcomes adapted by millions of years of evolution – immense, walking fermentation vats with prehensile snouts, a great degree of intelligence, and intense emotional lives. If I tried to explain what an elephant is to someone who had never seen one or heard of one before, I couldn’t blame them if they thought that I had taken a few too many pulls from the bottle. Elephants are truly fantastic creatures, made all the more wonderful by the fact that there is still much about them we don’t know. How a tiny elephant fetus grows into the world’s largest land mammal is one patch of that still-mysterious territory.
Earlier this week, I wrote a short article for Nature News about a study which estimated the minimum time required for a mouse-sized mammal to evolve to elephant stature at about 24 million generations. There’s good reason for mammalian inflation to take so long. Living large requires a series of structural modifications, such as stout legs to support a heftier body, over the course of many, many generations. Yet, as magnificent as such evolutionary transformations can be, the early growth of modern elephants is just as spectacular. Every elephant that has ever lived started off as a single cell. Over 640 days or more, a developing African elephant fetus goes from a microscopic speck to a 200 pound newborn, and if that baby is fortunate it will grow to weigh over 3 tons and stand more than nine feet high at the shoulder as an adult.
But, until now, no one knew how the skeletons of those little elephants developed in the womb. Different aspects of elephant reproduction have been described before, but the series of anatomical changes elephant fetuses go through has not received much attention. In a Proceedings of the Royal Society B paper published this week, University of Cambridge zoologist Lionel Hautier and co-authors finally outline how the skeletons of fetal elephants grow.
The aim of the study was to discern the timing of ossification in baby African elephants – when the cartilage that makes up their developing skeletons is replaced by bone. Hautier and colleagues studied and took CT scans of 17 unborn African elephants which ranged in size from a little more than an inch to almost a foot long. The dramatic changes in the unborn elephants are easy to see. In a tiny, 99 day-old specimen, only parts of the jaws, upper arm, and upper legs are ossified, but by 118 days a greater part of the skull, many of the ribs, and additional parts of the limbs have transformed into bone. Two months later, around 176 days, the skeleton is well developed, and the major remaining areas of cartilage are found around the various limb joints.
In the details, though, African elephants develop in subtly different ways from many other placental mammals. Regarding the skull, African elephants are like other mammals in developing their jaws first, but differ in that particular skull elements – the periodic (an element surrounding the ear opening) and the basioccopital (an element which borders the hole at the base of the skull) – change to bone relatively late. And, in the rest of the skeleton, African elephants are distinct from other placentals in having parts of the hips, the upper arms, and the trunk vertebrae ossify early while the neck vertebrae and fingers ossify late.
By the time the fetal elephant is about a third of the way through the gestation period, ninety percent of the skeletal elements have at least started to change to bone. This early and rapid ossification contrasts starkly with the pattern of development seen in some other mammals. Hautier and co-authors point out that mice, hamsters, and other rodents only reach the same degree of ossification during the last portion of the gestation period, close to birth, although this may be a peculiarity of rodents. This uncertainty is due to the fact that only a few placental mammals have been studied well enough to know how their skeletons develop in the womb. In general, though, the new study appears to support the idea that placental mammals with long gestation periods have young whose skeletons begin to ossify relatively early.
The precocial nature of baby elephants might seem to be a good reason for such rapid development. A newborn elephant must quickly walk and keep up with the herd – they virtually hit the ground running. A well-developed, sturdy skeleton is a necessity. But Hautier and colleagues point out that the skeletons of our species begins to ossify at about the same time in gestation as fetal elephants despite the fact that we are born helpless and require a long span of time before we can even start moving ourselves around. Baby elephants certainly require a well-developed skeleton to keep up with their mothers, but the early onset of ossification may not be connected to their precocial lives. Now that we have a basic understanding of how little elephants begin to grow, perhaps we can start to address some of these remaining puzzles.
Hautier, L., Stansfield, F., Allen, W., & Asher, R. (2012). Skeletal development in the African elephant and ossification timing in placental mammals Proceedings of the Royal Society B: Biological Sciences DOI: 10.1098/rspb.2011.2481