Re: Rabbit ears
- From: "chris.linthompson@xxxxxxxxx" <chris.linthompson@xxxxxxxxx>
- Date: Tue, 07 Aug 2007 06:53:03 -0700
On Aug 7, 1:12 am, "rmj" <gle...@xxxxxxx> wrote:
Why do rabbits have big ears? I assumed that the reason was that big ears
provide greater hearing, but this would seem to be either incorrect or
insufficient as an explanation because the majority of animals do not have
large ears. A large ear certainly should be simple to attain by natural
selection since in every species of animals with ears, some individuals will
have smaller or larger ears than the norm for the species; thus if a larger
ear offers greater hearing acuity then it becomes difficult to imagine why
smaller ears would be selected.
Who says large pinnae (the external portion of an ear) don't
contribute to hearing? Many animals have no external ears, but the
animals with acute hearing generally have large pinnae. There are, as
has been pointed out, complicating factors, mainly having to do with
temperature regulation.
Lagomorphs almost all have large ears for their body size. Consider
the Black-tailed Jackrabbit (Lepus californicus). It's ears are about
twice the size of a Snowshoe Hare's (L. americanus). They make quite
efficient heat exchangers. But the Snowshoe Hare's ears are still
large compared to a mammal not known for it's hearing- say a Raccoon
(Procyon lotor). At the extreme end of ear size in the Lagomorpha you
can find the Pika (Ochotona princeps), which has "normal" sized ears
for a mammal, probably due to its alpine habitat.
But you need to find an exception to the rule. In general, the
smaller the mammal, the smaller you expect it's ears to be. Why?
Because of surface area-to-volume ratios. The smaller you are, the
more surface area you have relative to your volume, and if you're a
mammal, that means you radiate heat more rapidly, leading to a
decrease in body temperature (this is obviously the reason
Jackrabbits' heat exchangers- their ears- are long and skinny).
Can we find tiny mammals with relatively large ears? Do any of those
those mammals rely on hearing to any great extent? The answers are
yes, and yes. Look at bats. Every species of Microchiropteran- the
ones that echolocate the most, and the most efficiently- have outsized
ears. Here's a fine example:
http://zmmu.msu.ru/bats/rusbats/pictures/paus.jpg
Isn't he cute?
Note that even temperate bats- the ones most subject to stress induced
by low temperatures- have these enlarged ears (although in the coldest
months they probably migrate or hibernate).
The large pinna serves to focus sound into the ear canal (or, if you
prefer, the external auditory meatus). That large piece of projecting
cartilage, the tragus, serves the same purpose: it helps make sure
sound that reflects off the pinna isn't just redirected outside the
ear entirely. In fact, the shapes of bats' ears is probably optimized
for the frequency of ultrasound it uses to navigate and hunt.
At the other extreme of mammalian sizes and pinnae function, African
elephants (Loxodonta africana) have much larger pinnae than Indian
elephants (Elephas maximus). This is almost certainly due mainly to
thermoregulation rather than auditory requirements. While bats utilize
ultrasound, elephants apparently rely on extremely low frequencies- in
fact, seismic activity- that are collected not through their ears but
through their feet and trunk.
Chris
.
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- From: rmj
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