Sunday, 29 November 2009

Wherefore the hammerhead?

Fantastic stuff, this. Well, for those of us who, when little and naturally curious about everything*, wondered why, exactly, it was that hammerheads had hammerheads. As it were.

Scalloped hammerhead shark, Sphyrna lewini, Hawaii, USA (Pacific ocean)
UW118-2, (c) Andrew Seale

There are many possible reasons for this. It could be that a (or the, depending on your preference) creator thought it looked kinda cool, and was going through a flat-headed creature phase. Or because it makes said sharks look, you know, really really menacing. Or, alternatively, really, really ridiculous.

And learned people, of course, being learned, have posited their own theory: that it gives hammerhead sharks great stereovision and depth perception. Or not. The thing, up until this paper released last week, no one had actually bothered to test this assumption.

It has been suggested that, in fact, since their eyes are on the sides of their heads (I'm talking about the sharks here), rather than facing forward, that they couldn't have binocular vision. However, other clever people and television shows have claimed eyesight benefits. Who is right?

It turns out, the guys who posited better vision. The paper's authors looked at a range of different shark species, bother hammerhead and pointy-nosed. Included were bonnetheads (narrow) and scalloped hammerheads (wide).

Bonnethead hammerhead (Sphyrna tiburo)

Scalloped hammerhead (Sphyrna lewini)
© Doug Perrine

So they got a whole lotta sharks and then tested their eyesight with a version of the whole 'moving the torch in front of your eyes' thing. They weren't testing for concussion, but were instead measuring the electrical activities of the sharks' eyes.

First, they looked at monocular vision, and found that hammerheads kick ass. As it were. The fields were wider than pointy-nosed blacknose and lemon sharks (for example), with the scalloped hammerhead having a 182 degree range, and the bonnethead a very respectable 176 deg.

They then took these measurements, and plotted them to see whether the monocular fields overlapped. Which they did. Apparently, the scalloped hammerhead had an overlap of some 32 degrees, and the bonnethead had 13 degrees. The most hammerheady of all, the winghead shark, had 48 degrees!

Then, they factored in some other stuff such as head and eye movements, and watched those overlaps grow.

In short, the television series (and clever people) were right. That weird head shape does indeed improve depth perception and binocular vision.

Mostly awesomely, it also gives some hammerheads a 360 degree rear view (or very close) - one of the few occasions where the television people have underrepresented the situation. Gosh.

* Note: I saw a paper recently (for the life of me can't remember where) that said that the whole 'why' phase small kids go through isn't actually an attempt to make mommy/daddy's brain drip out their nose, but is, instead, because kids actually do want the answer to their questions. Go figure.
(Postnote: Calvin's dad's answer to this situation is quite tempting, though. And here's some more inspiration.)


Lucia Malla said...

Please give credit to the first picture of this post to Hawaii-based photographer Andre Seale:

This picture was taken in Hawaii around HIMB, the institution also involved in the hammerhead study cited by your post.

aimee said...

Hi Lucia

Happy to! Unfortunately, there was no credit where I found the photo.

Lucia Malla said...

Do you remember where it was?

aimee said...

I'm afraid not. I'm perfectly happy to remove the photo, if you'd like.

Lucia Malla said...

No, don't worry. We usually don't bother with uses of our images at non-profit/non-commercial websites/blogs for free, as long as it is credited.
I'm actually glad I found your blog, because now I have one more interesting pitstop to put on my already packed feed reader.

(I'm also an ocean/travel blogger. I understand the winding road to write a post and search for the right image to illustrate it.)

aimee said...

Aah, no worries, then :)Welcome to the blog, in that case, And check out sciblogs as well (particularly Brendam Moyle's blog, called Chthonic Wildlife Ramblings, for some more lovely creature pics).