floating on water surface (was Wing Position) [ODO]

Dennis Paulson dpaulson@ups.edu
Tue, 18 Jan 2000 11:58:07 -0800

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Dave McShaffrey wrote:

>I agree with Nick's assessment that the idea of gauging stream velocity by
>floating is intriguing but problematic.  Let me toss out two other
>1.  The males are trying to demonstrate the relative density of fish in the
>stream.  Males that defend territories with a high density of fish may have
>all of their offspring eaten by those fish; thus that territory is not
>suitable for oviposition.  While getting eaten is probably not adaptive (for
>the male), if males are abundant then a few males getting eaten at
>territories full of fish probably won't hurt the population in the long run.
>Further, if the male is attacked immediately - but escapes - that may be a
>sign for him to move on to a site that would be more hospitable for his
>offspring (or his mate while she is ovipositing).

Dave, I would argue vigorously against this hypothesis, as it goes against
natural selection on individuals as the driving force behind evolution.  An
eaten male does not contribute his genotype to the next generation, thus
strongly selecting against males that perform such behavior.  It doesn't
matter whether the female benefits or not, there can't be selection for
such behavior (except in the context of the Handicap Principle).  Also,
from my observations (we used to feed the sunfish in our pond), I would say
that few insects that are struck by a fish at the surface escape, so it's a
very large risk.

Just to add some natural history to my argument, it may be very different
fish that attack floating insects and benthic insects.  Trout are competent
as surface feeders but probably aren't especially effective predators
against calopterygid larvae, which are very cryptic and nestled among
vegetation and rootlets.  Mark McPeek has stated that trout don't seem to
place the same predator pressure on damselfly larvae as the much more
effective centrarchids (bass and sunfish).

Your second hypothesis, about reflection fighting, is interesting.  I've
often wondered if odonates saw their reflections on the surface.  But most
of them are differently colored above and below, so a view of their
underside might not stimulate interaction.

What is of course fascinating is how very few times these behaviors have
been noted.  Are we not watching our favorite animals closely enough?  It
would be interesting to ask more Calopteryx researchers about their
observations.  Because nature photography is as difficult as it is, I would
guess that if they were able to film it, the Rueppell's must have seen the
floating display more than once.

Dennis Paulson, Director                           phone 253-756-3798
Slater Museum of Natural History                 fax 253-756-3352
University of Puget Sound                       e-mail dpaulson@ups.edu
Tacoma, WA 98416

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