Evolutionary Psychology

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Robert Kurzban

The Evolutionary Psychology Blog

By Robert Kurzban

Robert Kurzban is an Associate Professor at the University of Pennsylvania and author of Why Everyone (Else) Is A Hypocrite. Follow him on Twitter: @rkurzban

Why Do Seahorses Look Like Horses?

Published 27 January, 2011

Yesterday, Nature Communications published a paper titled “An adaptive explanation for the horse-like shape of seahorses” by Sam Van Wassenbergh, Gert Roos, and Lara Ferry. I thought it was pretty interesting, and I thought I’d summarize it here.

Note: This is not an actual seahorse.

The authors were interested in why seahorses have their particular shape. (See image.) First, a bit about their ecology. Seahorses are predators, eating small fish and shrimp, striking as their prey move by their hiding places. The authors describe their technique, called “pivot feeding,” as “a rapid upward rotation of the head is followed by suction to draw the prey into the seahorse’s snout.” For suction to get the prey item, the head has to be close to it; and, indeed, seahorses have biomechanical equipment that enables them to move their heads really fast.  (Check out the movie.)

The authors were interested in why seahorses look like horses; that is, why do their heads come out from their body like the top part of the letter S? In particular, they wanted to investigate the possibility that this shape contributes to pivot-feeding. Their hypothesis was that having the head come out from the body, instead of in line with it, would increase stability during the quick movement of the head. (Or, as they put it, “Our working hypothesis was that  a trunk oriented at a sharper angle with respect to the head has an increased inertial resistance to the reaction forces of the dorsal  rotation of the head, and would therefore improve head rotation  performance.”)

Anyway, what they did was to model different possible seahorse body configurations. By building mathematical models of different seahorse shapes, they were able to look at how fast they would be able to move their heads as a function of the different shapes. (Here’s another movie.) The model reveals that there’s a tradeoff. Having your head in line with your body (as pipefish do) increases mouth velocity, but decreases strike distance relative to having an angle between head and body. They argue that in seahorses, which typically wait for prey to pass by – as opposed to swimming after them –  the strike distance advantage offsets the speed advantage, which is why the sharp angles are observed among such species.

They conclude, then, that the function of the orientation of head to body is to increase their range for catching prey. An article in New Scientist quotes the lead researcher as saying, “This is the first step in establishing it’s an adaptation.”

At the risk of sounding repetitive, I know what you’re thinking… All they have done here is shown a close fit between physiology/behavior and the hypothesized function. This behavior doesn’t fossilize, and they didn’t do any genetics, let alone neuroscience… Not only that, but they had an N of… zero. They didn’t run a single subject; all they did was build a model. And don’t even get me started on the absence of data from multiple populations…

No, but seriously, pretty neat, I think.

image: From http://www.susanscott.net/OceanWatch2007/may18-07.html

Copyright 2011 Robert Kurzban, all rights reserved.

Opinions expressed in this blog do not reflect the opinions of the editorial staff of the journal.