Of bears (old and new) and geometric morphometrics

Okay, first let’s explain what I’m going on about – geometric morphometrics (sometimes referred to as relative warp analysis), simply put, looks at the form – size and shape, angles, degree of slope or indentation of any morphological feature of strictly defined sites (or landmarks) on an organism.

For example, the distance measured between named points on given bones in a skull. The figure below (copied from an excellent text on the technique), gives an idea of how it works…

Source: Geometric Morphometrics for Biologists: A Primer. By Miriam Zelditch, Donald Swiderski, David H. Sheets and William Fink. Published by Academic Press, 2004.

 

Measurements are made of, and between, a variety of morphological features (landmarks) and graphically illustrated as a sort of  ”map”.

These analyses are pretty useful when looking at the fossil record. It’s possible, for example to compare the geometric morphometrics of points on the skull of, say the sabre-tooth tiger (genus: Smilodon) with it’s closest living (extant) relative, the African lion (Panthera leo).

The process could even be used – in the absence of genetic data – to attempt to identify the closest living relatives of some extinct species in the first place.

Now, the reason I’m waffling on, is because the current issue of the Journal of Zoology (Volume 277, Issue 1; January 2009), has an article on the morphological differences (differences in form) of my favourite family of mammals – the bears (Ursidae), based on their ecology and how that relates to the geometric morphometric analysis of their teeth and skulls (Figueirido et. al. 2009).

Sorry if I’m boring you, but anything about bears still peaks my curiosity. Even years after I stopped working with them.

Anyway, the full reference of the article is…

Ecomorphological correlates of craniodental variation in bears and paleobiological implications for extinct taxa: an approach based on geometric morphometrics. J. Zool. (Lond.) 277, 70–80. Authors: B. Figueirido, P. Palmqvist, J. A. Pérez-Claros, Published Online: Oct 7 2008 5:11AM.

The authors mapped more than 17 craniodental landmarks such as the zygomatic arch, occipital crest and points between carnassial and molar teeth, amongst others.

They compared these landmarks with the known ecology of extant bear species, finding specific features that correlated to three dietary categories (1) herbivores, feeding mainly on vegetation – the giant panda (Ailuropoda melanoleuca) and the spectacled bear (Tremarctos ornatus); (2) omnivores that eat a wide mixture of animal and vegetable material – the North American black bear (Ursus americanus), the brown bear (U. arctos), the Malayan Sun bear (Helarctos malayanus) and the Asiatic black , or moon, bear (referred to as U. thibetanus in the study, but I’ve always known them as a seperate genus – Selenarctos thibetanus); (3) faunivores (animal eaters) such as the polar bear (Thalarctos maritimus) and the sloth bear (Melursus ursinus).

Now, here’s where I have my only real problem with this study, we’ve got the polar bear, that has a diet almost entirely made up of seals, together with the sloth bear, that has a diet consisting of insects. The texture, consistency and the requirements of dentition for these food sources have some self-evident differences. Although, to be fair, the authors clearly point this out in their methodology, and are at pains to point out that they use a…

“methodology based on principal components analysis for exploring the craniodental morphospace of ursids and only use the ecological categories for interpreting the results.”

In other words, they only rely on the feeding ecology of the extant bears to put their morphological data into some sort of context.

They found that the craniodental morphology of the extant bears was far more closely related to their feeding behaviour than it was to their phylogeny – their relatedness according to classification. For example, they saw that the insectivorous sloth bear and the seal-eating polar bear shared similarities such as…

“A flattened and large cranium with frontalized orbits and downward directed zygomatic arches, a comparatively slender jaw with small moment arms for the temporalis and masseter muscles, and poorly developed upper and lower tooth rows.

However, there are also some differences between the polar bear and the sloth bear, because the former has smaller moment arms for the masticatory muscles, a deeper rostrum and longer diastemas between the canines and the cheek teeth in the upper and lower dentition. In contrast, the insectivorous sloth bear has a more developed symphyseal region, more outwardly directed canines and post-carnassial molars that are comparatively reduced.”

To cut a long story short, until now, anybody interested in bears (extant and extinct) had thought that the extinct species of the Short-faced bear (Arctodus simus) and the cave bear (Ursus spelaeus), were carnivorous and herbivorous respectively.

No longer. This research shows that the craniodental morphology of omnivorous bears is intermediate between those of faunivores and herbivores.

Meaning that animals that eat both animals and plants (omnivores), have a craniodental structure which is generalised and does not specialise in either meat or veg’.

This corresponds nicely with what they found in the fossil record of A. simus and U. spelaeus. In the case of U. spelaeus, this gives us a bear – about a third again as big as the largest brown bear alive today – which ate a lot more animal flesh than we had previously thought.

Skull of the giant cave bear (Ursus spelaeus) – take a long look at the size of the human hand here, and imagine the fur, flesh and muscle wrapped around that skull. This was a big bear. (Source: Paleo Direct, http://www.paleodirect.com/)

Whereas the short-face bear (that just happened to have some particularly long legs), would probably not have eaten quite as many of our ancestors as we first thought.

 

The short-faced bear (Arctodus simus), illustrating that he had anything but short legs (Source: http://www.animalpicturesarchive.com).

I liked this study, neat and pretty conclusive – but I suppose, only really of any interest to a bear-geek. At least I got the chance to prattle on about the world’s largest terrestrial carnivore. Er, omnivore. Sorry.

Actually, that doesn’t really cover the mostly flora and fauna eating species does it? And “order omnivora” just doesn’t sound right either. Not for the mighty Thalarctos maritimus certainly.

Classification! It’s a real Porcus omnivorous isn’t it? I feel a posting about Linnaean taxonomy coming on, better quit while I’m ahead…

 

References

B. Figueirido, P. Palmqvist, J. A. Pérez-Claros (2009). Ecomorphological correlates of craniodental variation in bears and paleobiological implications for extinct taxa: an approach based on geometric morphometrics Journal of Zoology, 277 (1), 70-80 DOI: 10.1111/j.1469-7998.2008.00511.x

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