Tuesday, July 10, 2007

Skull density & diving / sonar

The bottlenose whale (Odontocetes, {Not tursiops}) has the densest bone (beak/rostrum) of any animal, it is a deep-diving squid-eater. Per a book on whales, from Crescent City Library.

From Marc V: AFAIK, dense skeletons are seen in slow (prob. fat) shallow divers in salt water, who dive(d) for sessile bottom foods: seacows, walrus, Odobenocetops, Kolponomos, some Thalassocnus spp & coastal Homo er. The bottlenose whale's dense rostrum does not clearly belong to this group.

Skull/skeleton densities have to do with buoyancy (cf.fat content: sea mammals have to be overall about as dense as the surrounding water, I'd think) & audition (bone conductance, incl.sonar). Mysticetes have rather low-density skeletons (I guess finwhales (fast) have even lighter bones).

--Marc

Cameron's link to a paper on it:

"I've read that the densest bone known to science belongs to Mesoplodon densirostris. This (free) paper believes the most likely function to be protection in intraspecific male combat":

https://secure.cisti-icist.nrc-cnrc.gc.ca/cisti/journals/rp/rppdf/z01-188.pdf

2 comments:

Cameron McCormick said...

I've read that the densest bone known to science belongs to Mesoplodon
densirostris. This (free) paper believes the most likely function to be protection in intraspecific male combat:

https://secure.cisti-icist.nrc-cnrc.gc.ca/cisti/journals/rp/rppdf/z01-188.pdf

"the Dude" said...

Thanks much Cameron, the paper is great, considering how little they had to work with.

I agree with their conclusions, preliminarily, with a caveat that perhaps during intra-specific combat, the males may be transmitting "roars" of some sort at each other (like the male walrus "bell sound"?), and then physically targeting the sound source above the jaw of their opponent, and that is why the skin tears are typically in that region. This might then incorporate some of the acoustic- bone density hypothesis, secondary to the skin tearing.

Regarding deep diving and density, I felt that the addition of dense bone in both adult male and female rostum might be a remnant of a non-chasing shallow diving ancestor's dense body bones, most of which are now filled with oils.

However that exclude the further ossification in males alone.

(Reminds me of the narwhal for some reason.)

DD

Btw, male dugongs have tusks which the females lack. I wonder if they tend to have battle scars as well (not including boat propeller scars), and whether their well known dense rib bones are matched with especially dense bones in the face.