Re: Great in the air, not so good underwater

[Michael Habib <mhabib5@jhmi.edu>]

> Size in terms of body mass, or size in terms of wing area?  My hunch 
> (without having access to more information) is that the authors were 
> referring to mass.  If so, that would imply that the Guillmots and 
> razorbills have larger wings than penguins of similar mass.  If so, it 
> is to be expected that when swimming, they would stroke at a lower 
> frequency than penguins, due to the additional loading on their 
> relatively larger wings.

I also suspect that is what they mean, though I'll have to go grab the 
actual paper to confirm.  One thing I do find odd in that summary is 
that the mass overlap between penguins and alcids is rather minimal; 
the guillemots in my dataset range from 906 grams to 1177 grams.  
They're among the largest alcids; razorbills are only slightly larger.  
The smallest penguins, on the other hand, fall at about 1.3 kg.

On Wednesday, December 6, 2006, at 09:26 PM, Dann Pigdon wrote:

> Marine animals may trade off their swimming efficiency against flying 
> ability, according to a novel study in which motion sensors were 
> attached to wild seabirds, whales and penguins to reveal how they move 
> underwater...

Using "marine animals" here is a bit odd in terms of wording; the only 
vertebrate amphibious flyers are birds.  Most of them are alcids, with 
a few shearwaters, dippers, etc. in there as well.  So really, the only 
living "tradeoff" examples are avian.

> [Katsufumi] Sato’s team [University of Tokyo] found that the 
> size-to-flap ratio does not apply to seabirds. Guillemots and 
> razorbills use their wings for both flying and swimming, and 
> underwater they stroke at a lower frequency than other seabirds of a 
> similar size, such as penguins, suggesting inefficiency for their 
> size...

Again, wording seems a bit odd.  I will have to get the paper tonight, 
but it seems like penguins are just about the only "other seabirds" 
that they could be using for the comparison, unless they got data from 
shearwaters as well (the only other avian aquaflyers in the same body 
size range as alcids).

> ... this study suggests that the different wing shapes and flight 
> styles that are optimal for either air or water are very different. 
> Increasingly efficient adaptations for an underwater flier tend to 
> move away from aerial flight.

That has been the traditional view on amphibious flyers (and the 
transition to full aquatic lifestyles from flying ancestors, as in 
penguins).  There is probably some distinct truth in it, but the 
situation is likely to be a bit more complicated than it seems.  Alcids 
have poor maneuverability, but they are very fast flyers.  They are 
also very good swimmers, really, as penguins set a high bar (and thus 
being less efficient than a penguin is not immediately "poor").

Penguins may be more efficient swimmers for a number of reasons.  One 
factor is absolute size: a number of parameters related to efficiency 
are easier to maximize at sizes above those reached by the largest 
alcids.  Generally speaking, aquaflying efficiency in birds will favor 
relatively large size, small wings with moderately high aspect ratios 
(see Lovvorn's work), and an active power upstroke (see Lovvorn, Sato 
et al.'s previous studies, and the classic Clark and Bemis paper).  The 
power upstroke is important not only for the obvious benefit of 
generating more thrust per wing oscillation cycle, but also because net 
drag decreases if the body speed stays high on the upstroke.  To quote 
Lovvorn (2001) in regards to murres: "When fuselage speed was 
relatively higher during upstrokes, lower net drag at the same mean 
speed increased the ability to glide between strokes, thereby 
decreasing the cost of swimming."

These factors tend to mean high wing loadings and large 
supracoracoideus mass.  Both of these are good up to a point for aerial 
flying *if* the animal is a rapid, lower maneuverability species (like 
alcids).  As such, I'm not sure if alcids are making as much of a 
tradeoff as it appears at first glance.  That said, there is still some 
tradeoff, because alcids cannot manage the most efficient body mass, 
wing loading, and relative supracora. mass relationships and stay 
volant (hence the loss of volancy in at least two alcid lineages, and 
the loss of volancy in penguins).

Interestingly, penguins rank as the most efficient fully homeothermic 
swimmers.  I cannot remember the proper reference for that bit of 
information right off the top of my head, but I have a copy of the 
paper stored away here and I'll send the citation along to this thread 
when I dig it out.  I believe efficiency was measured as mass-specific 
fuel consumption per unit distance.

Cheers,

--Mike