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Re: Ceratonykus braincase described
Jura <email@example.com> wrote:
> Just a brief aside here: BMR says nothing about the aerobic capacity of an
> organism (e.g. Konarzewski et al 1997), despite the popularity of Bennett and
> Ruben's (1979) initial hypothesis.
> What seems to keep myrmecophages from being cursorial is the fact that an
> exclusive diet of ants and termites is exceedingly low in energy. One needs
> to eat a crap tonne of these guys to make it
> worthwhile and that requires a nice, capacious gut. It's that gut that seems
> to be in contention with a cursorial bauplan.
Thanks for the clarification regarding BMR. Nevertheless, because
myrmecophagous mammals have a low metabolic rate it allows them to
reduce their energy demands (per unit body weight) - which in turn
allows them to survive on low-energy foods.
However, I would perhaps quibble with the requirement of a capacious
gut. Because most myrmecophagous mammals do not chew their food very
much (or do not chew at all, in some cases) before its sent down to
the stomach, these animals tend to have large and very muscular
stomachs (especially the pyloric area) that helps grind down the hard,
chitinous food. (Echidnas, which have a spiky tongue, appear to be
the exception here - maybe something to do with evolving from aquatic,
platypus-like ancestors?) So although myrmecophagous mammals do
increase the size of the gut (especially the stomach), it's not to the
extent seen in folivores.
Aside from the low-energy yield of ants and termites, the other
problem in having a myrmecophagous diet is the amount of non-animal
material ingested along with the insects themselves. As the
myrmecophage hoovers up the ants or termites, a great deal of debri
(bark, dirt, stones, etc) is taken in too. In one case, the stomach
of an aardvark contained 47% (by weight) of inorganic material.
I agree that myrmecophagy is in at odds with a cursorial bauplan. For
example, the Miocene tubulidentate _Leptorycteropus_, which is
interpreted as an omnivore, had more cursorial proportions than the
modern aardvark (_Orycteropus afer_), which is larger and more
specialized for myrmecophagy.