What is it about?

The jaws of hard prey crushing stingrays have really different shapes. Despite this no shape is better than any other at crushing hard prey. This is unusual because we expect to see performance differences when there are large differences in shape. Consider for comparison the beaks of Galapagos finches - different shapes are better for dealing with different types of seeds. So if shape is not important why does it vary? Well, shape CAN be important, if it artificially inflates the size of the prey. Prey can act larger by having large spines or knobs or ridges. Predators must then be large enough to swallow the whole prey, including the spines. But this does not seem to be the case for these stingrays.

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Why is it important?

We used a suite of methods to isolate the effects of material, size, and structure to ask the question - how do different jaw shapes effect crushing different kinds of mollusks? We milled stingray jaws from aluminum using CNC automated milling technology (based on computed tomography scans of actual ray jaws), crushed both live shells and 3D printed replicas to tease apart the differing effects of how shell materials, structures, and shapes effect their ability to resist being consumed. Although shell materials varied considerably, with some requiring greater work (greater deformation of shells before cracking) versus other requiring greater outright force, stingray jaws were equally well-suited to crushing a variety of these mollusk shells.

Perspectives

There are many critters that crush hard prey, hyenas, for example. All are very specialized, with particular teeth, jaws, and big muscles. That sort of suggests a common evolutionary theme that molds these creatures along similar processes. Interestingly, there are several, unrelated hard-prey crushing stingrays, despite their having a cartilaginous skeleton. Most of these rays are tropical, with cownose and bullnose rays occurring as far North as Delaware Bay in the United States. In the Caribbean, Florida, and the Bahamas you can find spotted eagle rays (Aetobatus). But other types of eagle rays (Aetomylaeus) are found only in the Indo-Pacific, alongside the other three stingrays. How forceful or how 'hard' can some of these rays bite, you might ask? Very, very hard...a cownose ray (Rhinoptera) just 2 feet across can generate more than 110 pounds of force. A big eagle ray could generate over 1000 pounds. These are animals specialized for consuming stiff-shelled prey, with jaws made solely of cartilage.

Dr Matthew A Kolmann
University of Washington

Read the Original

This page is a summary of: Morphology does not predict performance: jaw curvature and prey crushing in durophagous stingrays, Journal of Experimental Biology, November 2015, The Company of Biologists,
DOI: 10.1242/jeb.127340.
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