Slijper's Goat

or

What a grotesque little beast

Sometimes during the 1920's, a darling little baby goat was born without forelimbs. It barely even had stubs. But we all know how inventive and irrepressible goats are, so it comes as no surprise that this particular goat taught itself how to walk on its hindlimbs alone. Actually, it taught itself how to hop around, something like a kangaroo does, only without a long, thick tail to use for balance. The animal lived for about year, and then it died in a freak accident.

I don't know how the goat got into the hands of Dr. E.J. Slijper, a Dutch veterinarian. It's not impossible that Dr. Slijper was so eager to dissect the goat that he bought it from the gentle farmer who had lovingly raised it, promised him that it would be treated kindly, and then gently shoved it down a flight of stairs. (Excuse me, it fell.) But I do know that upon dissecting the goat, Dr. Slijepr observed some fantastic differences between its anatomy and the anatomy of a normal goat. Slijper's goat exhibited:
  • Increased spinal curvature, resulting in an S-shaped spine; for a diagram, see <http://www.uhh.hawaii.edu/~ronald/393/Slijper-1.jpg>
  • Dorsoventral flattening and elongation of the ischium
  • Anterior extension of the gluteus muscle, reinforced by novel tendons
  • A broadened neck
  • Enlarged hindlimbs (obviously)
  • A cylindrical thoracic cavity, instead of one that is V-shaped in cross-section
All of these changes are associated with the evolution of bipedalism.

Slijper's goat raises interesting questions vis-a-vis the role of phenotypic plasticity in the genesis of morphological novelty. It exemplifies the fact that the gross morphology of an animal's bone structure and musculature is only indirectly predicated on that animal's genome. The proteins composing muscle and bones are genetically encoded, but they act in accordance with their physical properties, resulting in the following trends:
  • Muscles grow with use and atrophy with disuse
  • Muscle fibers, tendons and bones all grow along lines of tension
  • Bone mineral deposition rates increase with use
Note that Slijper's goat only seems to exemplify a case of the Lamarckian inheritance. The atypical pressures that this goat faced altered its development, but not its genome. However, phenomena such as these force us to revise our conception of selection. It's easy to think that bipedalism must have evolved because it was adaptive, and I'm not saying that it didn't. But it's not impossible that bipedalism evolved as a trait that hitchhiked on some actually adaptive trait, or even upon a maladaptive trait that became fixed within a population. Recall as well that bipedalism has evolved multiple times, among ornithiscians (the taxa of dinosaurs that includes birds), rodents, marsupials, hominids, and others. There is no reason to assume that in each case the relevant selection regime was not unique.


For further reference...

If you can read Dutch, more power to you. Here's the original paper:
  • Slijper, E.J. (1946). "Comparative biologic–anatomical investigations on the vertebral column and spinal musculature of mammals". Verh. Kon. Nederl. Akad. Wetensch. Amsterdam II 42, 1–128.
Otherwise, you'll just have to trust my sources:
  • McCune, A. "Development and Epigenetics". Lecture delivered at Cornell University, 15 April 2004
  • Myers, P.Z. "Two legged goats and developmental variation." Pharyngula, <http://pharyngula.org/comments/A53_0_1_0_C>, accessed 16 April 2004.

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