Energetics - was Re: Spinosaurs ate pterosaurs

[Martin Baeker <martin.baeker@tu-bs.de>]

just de-cloaking to throw in some remarks on animal energetics:

1. Porpoising (jumping out of the water) is assumed to be energy-saving at
high speed because drag increases with speed and is especially severe near
the surface. There is an estimate that for a 50kg-dolphin the speed for
which Porpoising is more energy efficient than swimming is about 5m/s and
for a whale 11m/s (faster than it can swim). For fish, porpoising is not
so efficient, because there is no need for them to be close to the surface
(as dolphins have to be at least occasionally).

[Source: McNeill Alexander, Principles of animal Locomotion]

2. Considering parachuting to get at flying insects, I tried to do a very
rough back-of-the-envelope calculation:

The energy needed per attempt to catch an insect
is the potential energy needed to gain the
necessary height (mass * earth acceleration * height) divided by the
efficiency of the animal (which is probably less than 10%).

So a 100 gram animal needs 1J of energy to gain a height of 1m. Let's say
a typical starting height is 5m, thus it would need 5J divided by its
efficiency - something like 50J per attempt.

In addition, its metabolism requires a basic rate of about 0.5J/s (see the
book of Schmidt-Nielsen) if it is warm-blooded, so it needs about
40000J/day (that's 40kJ).
Compared to the 50J per attempt from above, if the animal makes 100
attempts per day, its energy need would increase only to 45000J/day.

100g of protein (or carbohydrates) contain about 400kcal or 1700kJ of
energy, so our critter needs about 3g of protein per day (51kJ). As
insects are not only protein, this may amount to about 10g of insects.

If every tenth attempt were successful, it would have to catch a 1g-insect
with each successful attempt. Question to those more knowledgeable than I:
how much does a fly or comparable insect weigh? I suspect it is rather
less that 1g (more in the range of 200 milligram), so our critter would
need to catch rather big insects or make a much larger number of attempts,
but it would also increase its energy needs.

To summarize, to me it seems rather doubtful that lying in wait for a
large enough insect flying by close enough to pounce on is a viable
strategy for a warm-blooded animal. A foraging strategy with the animal
then jumping onto the starting insect might be better, but I suspect that
insects can accelerate faster than a jumping animal.
In any case, the parachute can only be opened after the insect has been
caught, otherwise the air flow around the parachute would catch the insect
and carry it around the parachuter as well (that's why a fly flap has
holes, after all).

A cold-blooded one might be different,
as it would have to gain only about 4000J/day for its metabolism alone, so
it needs only about 0.3g of protein (plus the amount needed for all that
climbing, another 5000J for 100 attempts), totalling to 0.6g protein, or
about 2g of insects. However, I don't know whether it could sustain the
necessary activity.

Any comments?

Martin.


                   Dr. Martin Bäker
                   Institut für Werkstoffe
                   Langer Kamp 8
                   38106 Braunschweig
                   Germany
                   Tel.: 00-49-531-391-3073
                   Fax   00-49-531-391-3058
                   e-mail <martin.baeker@tu-bs.de>