Friday mystery object #147 answer


On Friday I gave you this object to identify:

I chose this object because it gave me a chance to take a photo using my new phone camera and a hand lens (inspired by an article by Nigel Larkin in the latest NatSCA News). I thought it might be a bit of a challenge, but I was proven wrong once again, as Jake tentatively identified the part of the skeleton this bone is from straight away – it is indeed a baculum or os penis (that’s the Latin for penis bone).

The species was a bit more difficult, but Barbara Powell was quick to identify that it came from a Mustelid and then there was a bit of disagreement between Barbara, Ric Morris and Dawn about whether it was from a Stoat Mustela erminea Linnaeus, 1758 or a Least Weasel Mustela nivalis Linnaeus, 1758.

As it turns out, Dawn made a very important observation in relation to this question:

M. e. [Stoat] hasn’t got the funky hook. I’m in for M. n. [Weasel]

Stoat and Weasel bacula from 'The Carnivores' R. F. Ewer (1998)

The reason this observation was so important is that this bone comes from a complete specimen that had originally been identified as a juvenile Stoat, but Dawn’s observation made me reconsider the validity of that identification.

Normally I would have to wait until I could access the collections before reassessing an identification, but the Horniman has just made a big chunk of our British mammal collections available online, so I have been able to do a preliminary check on the identification of this specimen from the comfort of my own home.

What immediately struck me was that despite being identified as a juvenile Stoat, the specimen appears to have fully fused long bones and a well-developed sagittal crest, which I wouldn’t expect from a juvenile animal. From what I can tell from the smallish online photo, the skull of this specimen is 39mm long and according the the excellent Will’s Skull Page:

Any skull less than 42mm overall length is almost certainly a weasel.

To give you an idea of the degree of overlap that you see in the sizes of Stoats and Weasels here’s an example from study skins:

Stoats and Weasels

Two specimens on the left are Stoats, the two on the right are Weasels. Note the similarity in size between a small Stoat and big Weasel. Specimens from the National Museum of Natural History, Naturalis, Leiden, The Netherlands

These lines of evidence have given me sufficient reason to reassess the identification of this specimen, although I will need to see the specimen to be certain.

To my mind this is one of the real benefits about running this blog – the details that can get missed by one person may be noticed when viewed by many pairs of eyes. This is particularly useful when dealing with specimens that already have identifications, since there is often a willingness to trust the data you have available. So many thanks to all of the contributors on this blog and this week special thanks to Dawn!

2 thoughts on “Friday mystery object #147 answer

  1. Very interesting reasoning there, and I’ll certainly look out for ‘funky hooks’ in future. In my defence I’d say that I’ve never seen a baculum from either M nivalis or erminea, just from M putorius which definately does have a funky hook! So thanks to you guys I have learned something about a weasel’s penis I’d’ve been unlikely to have learned elsewhere!

    • Glad to hear that your knowledge of weasel genitalia has improved – mine certainly has as well. I have to admit that I learn an awful lot from the people commenting here and I really value it, so thanks for your input!

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