Holy moley... is that supposed to be humer(o)us?

Feb 21 2011 Published by under Uncategorized

I apologise for that title pun I couldn't help myself. I also apologise for going into less detail than I'd hoped in this post - I found myself a bit strapped for time between responding to Intelligent Design types trolling here and at Zygoma, writing a talk for Skeptics In the Pub tomorrow evening and trying to sort out my laptop power lead after it broke (now fixed thanks to the helpful staff at Maplins who gave me the parts I needed to make repairs).

On Friday I set you a challenge in my post titled The rest of the iceberg - I wanted you to identify what this bone is and what it came from:

As it turns out it seems to have been a real challenge, but back at Zygoma my regulars had it identified within 11 minutes. Their comments provide to be a useful resource for those less familiar with bones.

The first to correctly identify both the type of bone and the species it came from was Cromercrox (of Nature fame), who gave a great rationale for his suggestion:

What a weird creature! I first thought it was some kind of vertebra until I looked more closely. The bottom end looks like a humerus, sort of, with that articular condyle. And look at those huge flaring crests. If so, it’s from a small but very robust animal perhaps adapted for digging – I’d guess a mole (Talpa europaea)

Moles have bizarre humeri (or upper arm bones; see below to get an idea of what cat and dog humeri are like) - they are very short, very wide and they have a huge area for muscle attachment [old but full description in pdf form].

Cat and dog humeri from St. Petersburg College, Florida

Cat and dog humeri from St. Petersburg College, Florida

Long limb bones are handy for increasing leg length, which in turn increases stride length, which makes an animal faster, since speed is determined by stride length and stride frequency. Funnily enough though, the fastest animals usually have quite short humeri (and femora), since the main muscles that work the leg for each stride attach to them and by keeping the bone (and therefore muscle) short the frequency of the muscle contractions (and therefore the stride) can be increased. The leg length instead comes from extension of the lower parts of the leg (even the ankles and toes).

It's probably needless to say that Moles don't have a short humerus for this reason. Instead, they burrow through soil with their forelimbs, which means that they need to generate a large amount of force (which takes a lot of muscle) and they need to keep their limb movements quite tight to their body, to avoid wasting force by moving more soil than necessary. The width of the humerus relates to this control as well - since it means that just a small amount of rotation can move the arm out from the body[pdf], making digging and underground locomotion more efficient.

Close-up of mole by Michael David Hill, 2005

Close-up of mole by Michael David Hill, 2005

While this digging may be efficient and biomechanically fascinating, it doesn't go down so well with gardeners and groundskeepers. Some have even been known to make a mountain out of a molehill (sorry!). Puns aside, one King of England died as a result of a Mole - William III (William of Orange) was thrown from his horse after it stumbled in a molehill and sustained an injury that led to his death in 1702. Apparently the horse was of Irish stock (all the best horses are of course) and Moles don't occur in Ireland, so the animal hadn't learned to avoid molehills. How's that for an apocryphal tale to finish on?

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