Why are elephants so small?

I have long intended to write a paper entitled Why Elephants Are So Small, as a companion piece to Why Giraffes Have Short Necks (Taylor and Wedel 2013). I’ve often discussed this project with Matt, usually under the acronym WEASS, and its substance has come up in the previous post, and especially Mickey Mortimer’s comment:

I think it would be interesting to read a study on that — the order in which various factors restrict body size without transformative adaptations. Similarly, what the differences would be for an aquatic animal like a whale.

That is exactly what the WEASS project was supposed to consist of: a list of many candidate limitations on how big animals can get, some rough attempt to quantify their Big-O behaviour, some discussion of which factors seems to limit the sizes of modern terrestrial animals, and how dinosaurs (especially sauropods) worked around those limitations.

(Whales are different. I have in my mind a half-formed notion for a third paper, completing the trilogy, with a title along the lines of Why Whales Are Dirty Cheaters.)

What are those candidate limitations? Off the top of my head:

Biomechanical:

• Bone strength
• Cartilage strength
• Cartilage thickness
• Muscle strength
• Nerve length and conduction time

Metabolic:

• Blood pressure: column height and capillary length
• Lung capacity
• Tracheal dead space
• Digestive efficiency
• Metabolic overheating

Those are just some of the physical limits. There is anecdotal evidence that elephants are not very close to their mechanical limits in their usual behaviour: they could get bigger, and still work mechanically. (Follow the link at the start of this paragraph. You will thank me.)

There are plenty of other factors that potentially limit organism size, including:

Behavioural:

• Feeding rate
• Ability to navigate dense environments
• Predator avoidance with limited athleticism
• Difficulties in mating

Ecological:

• Territory requirement
• Time taken to reach reproductive maturity
• Reproductive rate
• Birth size
• Lack of selection pressure: when there are no predators bigger than a lion, why would elephants need to evolve larger size?

I’m sure I am missing loads. Help me out!

I am haunted by something Matt wrote a while back when we were discussing this — talking about how alien sauropods are, and how easily we slip into assuming mammal-like paradigms.

We are badly hampered by the fact that all of the 250kg+ land animals are mammals. We only get to see one way of being big, and it’s obviously not the best way of being big. Our perceptions of how hard it is to be big are shaped by the animals that are bad at it.

So having written this blog post, I am wondering whether it’s time to breathe life back into this project, started in 2009 and repeatedly abandoned.