Why are there no elephants in the mountains?
Well, mainly because it’s costly to climb when you’re an animal of that size. A previous study estimated that a 4 tonne elephant would have to eat for 30 minutes to compensate for a 100m climb. Ideas man Graeme Ruxton and his co-author David Wilkinson develop this further in their new paper. They ask whether avoidance of hilly areas is to be expected in general for animals of a large mass such as the sauropods. These are the long-necked dinosaurs that were the largest terrestrial animals that ever existed. Some of the upper mass estimates of, albeit poorly described, species are over 100 tonnes! Using simple scaling relationships relating to the energetics of movement, food intake etc. Ruxton and Wilkinson show that as a herbivore increases in size the fraction of time spent eating to balance the cost of climbing will increase. In the case of sauropods we can look to the fossil record for support and it does show the creatures preferred flat environments such as fluvial plains. Their footprints and nesting sites are often preserved in these areas. Of course, energetic concerns aren’t the only issue stopping these animals from populating the hills. The danger of falling would be much higher on a friable surface and the bigger you are…
Any thoughts of regaining your energy on the way down after a costly ascent can be dispensed with. An animal must expend energy to control the rate of descent especially to avoid falling. One benefit of being large is that you have energy reserves so it is possible to travel into the hills if absolutely necessary but these forays would be infrequent.
This result suggests steep areas should be depauperate with respect to larger herbivores. We could imagine highland islands of smaller herbivores alongside plants which are free from the pressures of huge plant-eaters. The conclusion of the paper asks us to explore extant ecosystems for such a pattern. This could be extended to Mesozoic ecosystems. Perhaps there would be an ontogenetic niche shift in the sauropods, moving from hilly areas to the flatlands as they developed.
Author: Adam Kane, kanead[at]tcd.ie, @P1ZPalu
Image Source: Wikicommons