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By David F. Salisbury
April 20, 2001
Ken Catania studies the brains of some
of the strangest-looking mammals alive: the star-nosed mole and
the naked mole rat.
"I used to be a little defensive about
studying such weird-looking animals," the assistant professor of
biological sciences acknowledges. "But then I realized that what
makes these animals so strange is their extreme specialization and,
for that very reason, there is a great deal that we can learn from
studying them.
"This research strategy appears to
be paying off. Catania has just been awarded one of only 15 fellowships
given annually by the Searle Foundation, a highly competitive honor
that will provide him with $240,000 to use on his research for the
next three years. In addition, his latest paper appeared as the
cover article in this month's issue of the journal Nature Neuroscience.
It was Catania's interest in the sense
of touch that led him initially to the community of moles. In their
underground world there is little light, so vision is not very important.
Also, sounds are attenuated and hearing is not that valuable either.
That leaves the senses of touch and smell pre-eminent. His first
research subject was the star-nosed mole-an animal that looks very
much like an ordinary mole except that has a peculiar star of fleshy
appendages ringing its nose. More recently he has begun studying
the naked mole rat, an animal famous in animal behavior circles
because it lives in insect-like colonies organized around single
breeding females, or queens.
Catania got acquainted with the star-nosed
mole when he was an undergraduate at the University of Maryland.
He worked at the National Zoo, where his job was to collect small
mammals, including star-nosed moles. "I learned where to find star-nosed
moles and how to collect them, which is a skill not many people
have," he explains. Although they range from Canada, down through
the Eastern United States as far as Georgia, people rarely see these
unusual-looking creatures because they are the only mole that lives
in marshes and wetlands.
As its name implies, the star-nosed
mole has a very unusual snout, ringed with 11 pairs of pink appendages
that form a fleshy star. There have been a lot of different ideas
about the function of this distinctive feature. Some have proposed
that it is a super-sensitive olfactory organ that helps the nearly
blind moles to sniff there way around underground. Others have suggested
that serves as an extra "hand" for grasping prey or other objects.
Still others have argued that it serves as an antenna that detects
electric fields as the moles swim through muddy marsh water.
It was not until Catania studied this
structure in detail as part of his doctoral thesis at the University
of California, San Diego that the star's true function came to light.
Working with noted neuroscientist Glenn Northcutt, he showed that
these appendages serve as an extraordinary touch organ, covered
with more than 25,000 microscopic sensory receptors, called Eimer's
organs, that allow the hamster-sized mole to literally feel its
way around its subterranean environment.
"I just showed some pictures of the
nose around the lab and everybody got very excited," he recalls.
Catania was able to generate equally
high levels of interest in this unusual creature when he came to
Vanderbilt as a post-doctoral fellow in the lab of Centennial Professor
of Psychology Jon Kaas. He and Kaas determined that more than 100,000
nerve fibers run from the moles star nose to its brain, more than
six times the number that connect the human hand and brain.
"Given this structure, it is possible
that the mole can feel distinctive differences in the textures of
different materials at a microscopic level," Catania speculates.
He and Kaas also discovered that the
mole's fleshy nose rays develop in a way unlike any other animal
appendage. The basic strategy employed to make everything from human
fingers, insect legs, fish fins, and porcupine quills is to start
as an outgrowth of the body wall and grow directly outward. By contrast,
the rays develop first as fleshy ridges along the side of the nose.
Then they become little cylinders that separate from the body beginning
at the ends farthest from the nostrils. These ends move forward
to become the tips of the freed appendages.
Next, Catania took a closer look at
the how the nerve connections between the star-nosed mole's brain
and appendages develop. The region connected to each of the appendages
is clearly marked on the mole's cortex, which makes it very easy
to study, Catania says. The mole's star is made up of two different
kinds of appendages. The central, lowermost pair is much shorter
than the other ten and contains a much higher number of nerve fibers
per touch receptor than do the longer pairs. The area of the cortex
that is connected to this pair is also much bigger than that devoted
to the others, the neuroscientist has found.
"This organization has surprising parallels
to that of the human visual system," Catania observes.
Humans, like most animals that rely
primarily on sight, continually shift their eyes. When an interesting
or important image enters their peripheral vision, a person instinctually
shifts his or her eyes to move the image into the central part of
the retina, called the fovea. The fovea has a much higher concentration
of nerve fibers than the periphery of the retina, which allows it
to provide the brain with a more highly defined image.
Similarly, active star-nosed moles
continually wave their nose appendages around. When one detects
something of potential interest, such as an unfortunate earthworm,
then the mole moves its nose quickly to bring one of the central
rays into contact, giving it a superior tactile image of the object
so it can determine whether it is something good to eat. For small
prey the entire process from first touch to complete ingestion takes
about a fifth of a second.
In the April issue of Nature Neuroscience,
Catania set out to determine whether the central appendages gain
control of larger regions of the cortex because they have been genetically
programmed to do so or because they are used more intensively. By
looking at the development of the mole's nose and brain, he was
able to determine that the central appendages get a head start in
the development process. Largely because they get their major growth
earlier in development than do the peripheral appendages, the central
rays establish a larger number of nerve connections and interconnect
with a larger area of the cortex than its fellow appendages, he
reports.
In his application to the Searle Foundation,
Catania emphasized his desire to study the organization of the brains
of highly specialized animals like the star-nosed mole. "When you
stop and think about it, most scientific research is concentrated
on just a handful of species, so I got the idea of looking at other,
non-traditional species," he says.
In pursuit of this goal, he recently
arranged to obtain a colony of naked mole rats. The mole rats have
a much different tactile system than do the star-nosed moles. Instead
of putting the vast majority of their touch-sensors in one organ,
the mole rat has sensory hairs spread all over its body, including
its tail.
"The mole rats are known for their
ability to travel rapidly backward through tunnels. The sensory
hair on their tails probably allow them to do this," Catania says.
Mole rats also have an exceptional
set of front teeth. They have four incisors that are located entirely
outside of their mouth so they can close their mouth tightly when
they are using their teeth for digging. The moles also have the
ability to move each front tooth independently. For example, they
can spread them apart or move them together. An indication of just
how important these teeth are to the animal is the fact that fully
30 percent of its cortex is devoted to processing information from
the region where the teeth are located, Catania says.
"Are these just super-sensitive chop
sticks, or do they have other functions as well?" Catania asks.
That is just one of a number of questions that he hopes further
study of these unusual animals will help answer. - VU -
A
Star Is Born, American Museum of Natural History, June 2000
http://www.amnh.org/naturalhistory/features/0600_feature2.html
Snouts: A star
is born in a very odd way, ScienceNewsOnline, Oct. 23, 1999
http://www.sciencenews.org/sn_arc99/10_23_99/fob3.htm
Star of the
Swamp, National Wildlife Federation, Dec./Jan. 1997
http://www.nwf.org/natlwild/starnose.html
Journal of
Comparative Neurology paper, 1997 "Somatosensory fovea in the Star-nosed
mole"
http://www.psy.vanderbilt.edu/faculty/kaas/catania&kaas.pdf
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