Some years ago, there was a TV
documentary that showed the aggression of mice living in a packed
condition under one controlled environment, and a more accomodating
and friendlier mice living with less than 10% of the packed lot in
the same space condition.
When even one of the packed lot was transferred to the loose lot,
the aggression remained.
Here is a reference piece that maybe of
interest:-
Population overload: mice advice - overcrowding in
laboratory mice
Population Overload: Mice Advice
In the story of the pied piper, a town is rescued from its
ballooning rodent population by a young musician who leads the
creatures away with his enchanting tunes. The burgeoning rodent
populations of psychologist John B. Calhoun at the National
Institute of Mental Health (NIMH), however, disappear on their own.
As crowding intensifies in their experimental colonies, the animals
reach a point of no return and self-destruct; behaviors necessary
for survival, such as mating and parental care of infants,
disappear.
What is more, Calhoun calculates that the human population is
entering a 200-year period of rapid growth that, based on his
animal model, may reach a similar point of no recovery in around 80
years. Since crowding effects strike at the most complex behaviors
of a species, he says, escalating human population density is
likely to progressively impair each person's ability to create and
use ideas necessary for adaptive changes, rather than reproductive
ability.
But in Calhoun's scenario, humanity is not expanding toward an
inevitable oblivion. He sees a way for people, in contrast to
experimental rodents, to reverse rapid population growth and bypass
the point of no return. The "pied piper' in this instance is an
increasingly sophisticated use of computer systems that will extend
the human capacity to solve problems and adapt to the
environment.
If such an "evolutionary' extension occurs, Calhoun predicts it
will lead to intentional efforts to put a brake on fertility and
set the stage for a rapid population drop beginning in the 21st
century. He contends that unless this conceptual adaptation occurs,
attempts to institute widespread contraceptive use will fail and
the population will continue to increase.
"I try not to be a "doomsdayer,'' says Calhoun, "but if there's
high probability of our entering a point of no return, then [my
animal model] is worth considering.'
Calhoun attracted considerable attention with his early crowding
and cooperation experiments on rats and even inspired an animated
movie about humanlike rats (SN: 8/7/82, p. 92). His willingness to
assign human significance to animal data has been scoffed at by
some scientists, but he remains undaunted. Calhoun described his
latest and most elaborate crowding study --this one with mice--at a
recent lecture in Washington, D.C., and used it to buttress his
human population predictions.
The project took place in "universe 133,' an 18-foot-wide,
four-level mouse house at NIMH in Bethesda, Md. The structure
contains eight cells of equal size, food and water dispensers, nest
boxes and platforms on which mice can climb. Each cell has 120
numbered locations to aid in recording behavior. The entire unit
was designed to house an optimum of up to 16 groups of about 12
adults, or around 192 mice.
Beginning with eight pairs of mice, the researchers allowed the
population of universe 133 to expand. As it progressed through
stages 1, 2, 4 and 8 times the optimum density, successive
generations of mice became less and less able to reproduce or
interact with others in normal ways. In late stages of crowding,
around 200 weeks after the study began, mating ceased altogether.
At that point, the population fell precipitously from what had been
a peak of nearly 1,600 mice.
Childhood and juvenile behavior, says Calhoun, extended
increasingly into adulthood (which begins at about 172 days of age
for mice) as new generations crowded into the living space. Adult
females often persisted in the juvenile behaviors of wandering from
cell to cell and following strange objects, such as the feet of an
investigator when he entered the structure. Males often continued
in an even earlier type of behavior, huddling together in small
groups on cell barriers. Some became extremely aggressive, clamping
onto other mice with their teeth and swinging them from platforms;
an attacked mouse rarely fled, and if it did, the attacker rarely
pursued.
In a previous experiment with rats, Calhoun had found that a group
trained in two cooperative behaviors--gaining access to water only
when two rats were present at a fountain and gaining access to food
only when two members of an experimentally designated "clan' were
present--was better able to adapt to increasing population density
than a control group. The training, he says, allowed the rats to
develop new social roles and maintain an optimum number of social
contacts for each individual, even as crowding became worse.
But left to their own devices, the mice in universe 133 reached a
point where all animals failed to develop relationships and
reproduce. Crowding was reduced, of course, but the colony rapidly
became extinct as older mice died. Calhoun estimates this point of
no return to have fallen somewhere between 2.5 and 4.0 generations
after the mice reached two times optimum density.
How does this apply to humans? The modern human population, which
started out about 43,000 years ago, has grown rapidly only in the
last two centuries, says Calhoun. According to a computer
simulation he and a colleague developed, a 200-year-long world
population transition period began around 1975, when density
reached approximately two times optimum. Assuming a 27-year
generational span for humans, the point of no return (2.5 to 4.0
generations after 1975) would fall between 2042 and 2083.
