Neck and Neck

The tale of the porpoise and the giraffe

I know this is really obvious, but one of the (many) differences between a porpoise and a giraffe is that one has a long neck and the other doesn’t.  You can see the difference below just in case you need reminding.  Simply put, the giraffe has a very long neck and porpoises apparently have no neck at all (and humans have a medium neck).  So how can this be when all mammals are built from the same set of homologous parts, neck included?  By homologous, I mean that each of their parts is an evolutionary modification of the corresponding part of their shared ancestor.  For all mammals, this shared ancestor is a small insectivore-like creature that spent a lot of time not getting trampled by dinosaurs. During embryonic development, the parts of both the porpoise and the giraffe grow from the same set of embryonic “starting parts.”  The very early embryo of the giraffe looks very much like that of the porpoise.

Porpoise carving, Ziracote wood, Belize                                    Beaded giraffe, Namibia

            During embryonic development, these parts grow at different relative rates in the giraffe and the porpoise to produce the adult shape you see.  The parts that make up the neck of the giraffe grow much faster than the rest of the body, whereas in the porpoise they grow much slower .  My Found Object is the neck of a porpoise I found on the beach a long time ago, and for comparison, I have an image of the giraffe neck bones copied from the web.

Porpoise neck bones, left side view

Giraffe neck bones (http://vanat.cvm.umn.edu/museum/images/GiraffeCerVert1a.jpg)

Both the porpoise and the giraffe have seven neck vertebrae, as do all mammals (pretty sure of that).  So do we, and so did our cat.  The difference lies in the relative size of each vertebra (the absolute size also differs but is not relevant to shape, which is only about proportions).  During the growth of the giraffe embryo, the relative length of each vertebra grew much faster than its relative cross-section with the result that each vertebra is quite elongated.  In the porpoise embryo on the other hand, the relative length grew hardly at all relative to the cross-section.  So voila, the neck of the porpoise is like a stack of pancakes and that of the giraffe is like a set of connected sticks.  Therefore, the giraffe has a very long, mobile neck, good for neck-wrestling with other male giraffes, but the porpoise neck is completely stiff.

Associated with this difference in mobility, the vertebrae of the giraffe neck veritably bristle with extensions for muscle attachment and have disc-like surfaces that articulate with the adjacent vertebrae to allow the neck to flex.  Not only the overall shape, but each of these details is the result of differences in relative growth among the parts within each vertebra. 

Porpoise neck bones, viewed from anterior, showing the Atlas

The porpoise neck has none of this, and projections and joints are essentially absent because little musculature is involved.  Unlike the giraffe vertebrae, the different parts of each porpoise vertebra seem to have remained asleep during development and sketched out only the general cross-section but not the length. The first two vertebrae are fused, and the rest might as well be, for they are non-functional as a movable neck.  It looks like a cartoon version of a neck and seems to serve only as a spacer between the skull and thorax. 

Evolution does not readily lose parts completely, but it can turn them into rudimentary, non-functional parts, like our tails. Yet, the porpoise neck still articulates with the skull through its most anterior vertebra, the atlas (a reference to Greek mythology).  The joint between the skull and the atlas suggests that the porpoise head can be swiveled a bit in that joint, although no giraffe would be impressed with this mobility.

The first vertebra, the atlas, appears to make a rotatable joint with the base of the skull.

A basic metaphor in biology is that form follows function (or is it the other way around?).  The long neck of the giraffe certainly allows a whole range of giraffean functions.  In contrast, it seems likely that evolution has sacrificed the porpoise’s neck and its mobility to achieve a fusiform, streamlined body that can be driven through the water with greatly reduced turbulence and drag.  The outcome is that the porpoise is a very swift swimmer as many a mariner can attest.  The extinct ichthyosaurs underwent a similar transformation to a fusiform body, although not through the same skeletal modifications.  They must have been swift swimmers too.  On the other hand, one wonders about the long-necked plesiosaurs. Was that long neck an impediment to swimming fast?  How did they deal with it? It reminds us that evolution devises complex solutions to problems that are burdened with trade-offs.

Returning to my original theme of relative growth, the same principle of differences in relative growth that creates differences in shape apply across all living creatures, even to the details of homologous parts, as we saw.  Does the principal of relative growth apply even to us?  Of course, it does.  Human shape is greatly variable, and much of that variation was produced by differences in relative growth of the parts of our bodies, a little different in each person. Taking it to a much more subtle level, we can recognize each other as individuals because even our faces are the outcome of a magical, subtle and unique combination of relative growth rates.  It could not be otherwise.