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Few brain cells reproduce, 17-year cicadas escaped ice
Q:
How long does it take for the brain to produce new brain cells? (Jason,
Chicago, Illinois)
 A tangle of neurons that sends information from the brain
to muscles or glands. Most such brain cells don’t reproduce. [© 1995-2004 by
Michael W. Davidson and the Florida State University, used with permission]
A: My readers are knowledgeable. Yes, one part of the human
brain can grow new neurons. We discovered this capability in November
1998. Before then, neuroscientists believed that adult brains did not
make new neurons because the neuron was too specialized a cell to divide.
This seems to be the case for most of the brain. The neocortex
(planning, reasoning, and language), for example, just gets one set of neurons
to last a lifetime (2001 research). Apparently, something stops these neurons
from making new cells. We don’t know why or how, yet.
The hippocampus (learning and processing new memories),
however, makes new neurons at a steady, vigorous pace. Perhaps the hippocampus
(located roughly behind your ear) needs constant renewal to keep up with new
information. The hippocampus of rats and mice cranks out 1,000 to 3,000 new
neurons per day — a substantial fraction of each animal’s lifetime output.
Younger animals make more new neurons than older ones do.
To answer your question, "in an adult, it takes about 30 days
for a new-born cell to become a functioning neuron," says
Fred Gage, genetics
professor at the Salk Institute for Biological Studies. "What is remarkable,
though, is that it happens at all."
Also, the number of new neurons that are born and survive
depends on what we do. Exercise and a full, meaningful life increase cell count.
Stress and anxiety decease the numbers, says Gage.
Further Reading:
American Scientist: Depression and the birth and death of brain cells by Barry
Jacobs, Henriette van Praag, Fred Gage.
University of Washington: Neuroscience for kids – Neurons
17-year cicadas escaped ice
Q: Most cicadas live and die in a year. Why did 17-year
cicadas become so long-lived? (Shirley, Florida, New York)
A: Periodical cicadas (genus Magicicada) probably first
appeared almost two million years ago, when glaciers covered the land and the
North American climate was chaotic. They evolved a clever strategy to combat the
vagaries of weather — staying in the warm underground as long as possible.
 Adult cicadas mating [Leon Higley, University of
Nebraska-Lincoln]
"By living longer [as a nymph], the cicada ‘ducks’ more of the
bad years in its comparatively safe underground burrows," says
David Marshall, biologist at
University of Connecticut Department of Ecology and Evolutionary Biology. "The
cold is a threat to the adult phase only because cicadas can’t sing, fly, and
mate if it is too cold."
Although each cicada lives 17 years, they move through life as
a group, called a brood. All hatch, grow, emerge, become adults, mate, and die
together, synchronized with their brood mates. Almost no adult emerges in the
intervening 16 years.
To picture how they might have evolved to this life style,
imagine cicadas long, long ago when the climate bopped around chaotically and
suppose they did not live synchronized lives. Instead, broods stagger
their lives by a year — different broods emerging each year. So, 4-year cicadas
have 4 broods and 17-year cicadas have 17 broods. Which cicada species would
better survive— one with a 4-year life span or a 17-year span?
For a given time period, over four times as many 4-year broods
will emerge into the maelstrom as will 17-year broods. So, the odds are much
higher that a cold snap occurs when a 4-year brood emerges and, therefore,
clobbers that brood. But, maybe the next brood will get lucky and miss the bad
weather so the species will survive.
That depends, of course, on how long the bad weather lasts. If
the cold snap lasts for 4 years, the species is a goner. Each brood will emerge
into the cold and die. The climate, however, is less likely to stay bad for a
whole 17 years, which gives a 17-year cycle cicada a much better chance of
surviving. Natural selection would favor the longer-lived species. Also, nature
would select those cicadas that synchronized their lives and emerged only rarely
— at the end of their life span.
Randel T. Cox
of the University of Memphis and
C.E. Carlton of the Louisiana State Entomology Museum devised a mathematical
model of cicada survival probabilities based on a similar set of assumptions.
They postulated that, over 1500 years, the weather would get killingly cold 1
year, at random, out of 50.
The model shows that 4-year cicadas practically don’t survive
such climate — a survival rate of 0.4%. Whereas, 17-year cicadas have a 96%
chance of survival. Perhaps that is why 17-year cicadas evolved such a long
subterranean life.
Further Reading:
Cox, R.T. and Carlton, C.E., 1998, A commentary on prime
numbers and life cycles of periodical cicadas: American Naturalist, v. 152–164.
(Answered Aug. 13, 2004)
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