The Calhoun rodent experiments: the real-life rats of NIMH

Calhoun’s prophet rodents and the creation of the “behavioural sink”

I first read Mrs. Frisby and the Rats of NIMH as a young’un. What I didn’t know at the time was that it was inspired by a series of experiments on population dynamics from the 1940s to the 1970s. The two studies, over a total of eight years, aimed to explore the effects of population density on behaviour.

John B Calhoun was a behavioural researcher and ethologist who spent the largest part of his career at the US National Institute of Mental Health (NIMH), in Maryland. His work spans six decades, from early experiments with rodents in the 1940s to his death in 1995. But he’s become recently spotlit for his work in 1958–62, starting with a quarter-acre pen of wild Norway rats, and a 1968–72 experiment with mice called Universe 25. Universe 25 was his research pinnacle – a “rodent utopia”.

Calhoun calculated that his pen of wild Norway rats (or brown rats, aka Parisian rats, Hanover rats, street rats, common rats, sewer rats, wharf rats… in essence, “rats” as you know them) could have a sustainable density of 5000, but the population strangely never exceeded 200. With extremely high rates of infant mortality, by the end of the 27th month of observation the population had stabilised at 150 adults.

Calhoun and his team were surprised by these unexpected results and so embarked on a more controlled experiment involving a population of domesticated Norway rats, conducted in a converted barn (a step up in luxury for the animals).

Calhoun’s new rat palace consisted of a 3m x 4.3m room, divided into four pens by an electrified fence. These pens contained everything a rat could possibly need: food, water, elevated burrows, winding staircases, nest boxes… a rodent wonderland. Ramps connected pen 1 to pen 2, 2 to 3, and 3 to 4, but not pen 1 to pen 4. This meant that rats tended to concentrate in pens 2 and 3, leading to the development of what Calhoun dubbed a “behavioural sink”: a sort of voluntary overcrowding that resulted from the original, involuntary overcrowding, and led to significant changes in the rats’ behaviour.

These changes weren’t at the good end of the scale.

Among the male rats, behavioural disturbances ranged from “sexual deviation”, cannibalism, and frenetic overactivity to pathological withdrawal.

As a result of the population congregation in the middle pens, the rats became unnaturally accustomed to the presence of others. Eating and undertaking other “biological activities” were transformed into social activities, which they would only do in the presence of other rats. Although fine for eating (who doesn’t love a dinner party?), this disrupted the sequence of other normal activity, such as courting, building nests, and nursing and caring for their young.

In the worst-affected rats in a series of these experiments, infant mortality was as high as 96%. Among the male rats, behavioural disturbances ranged from “sexual deviation”, cannibalism, and frenetic overactivity to pathological withdrawal.

Now decades into his rodent studies, Calhoun’s aim had morphed into extending lifespan by creating a “Mortality-inhibiting Environment for Mice”, also known as Universe 25. Universe 25, like Calhoun’s decked-out rat barn, was created to cater for the wee experimentees in every way. The 2.7m x 2.7m metal enclosure had 16 vertical mesh tunnels, each with four horizontal corridors opening off at the top, and each corridor leading to four nesting boxes.

With each of these 256 nesting boxes capable of housing up to 15 mice, Universe 25 was theoretically capable of accommodating up to 3840 mice. (I know what you’re thinking right now: “I bet it didn’t”. Me neither, dear reader.) And, of course, there was an abundance of fresh water, food, and nesting material.

This is where everything starts getting a little… Biblical.

John calhoun with rodent experiment
John Calhoun with one of his rodent experiments. Credit: Yoichi R Okamoto, republished by National Institutes of Health (NIH), via Wikimedia Commons.

Calhoun’s reduction of mouse mortality was based on identifying the rodent equivalent of the four mortality factors listed in Revelation – as in the Book of Revelation. Calhoun described the results of the Universe 25 experiment in a journal article titled “Death Squared: The Explosive Growth and Demise of a Mouse Population”. His text included a hefty number of biblical references for something published in the Proceedings of the Royal Society of Medicine.

His lead paragraph ends thus: “This takes us back to the white first horse of the Apocalypse which with its rider set out to conquer the forces that threaten the spirit with death. Further in Revelation (ii.7) we note: ‘To him who conquers I will grant to eat the tree of life, which is in the paradise of God’ and further on (Rev. xxii.2): ‘The leaves of the tree were for the healing of nations.’”

And so, along with its abundance of resources, Universe 25 was cleaned every four to eight weeks, contained no predators, had the temperature maintained at a delightful 20°C, and was colonised by the finest, disease-free mice selected from the National Institutes of Health’s breeding colony. The only limitation was space.

Unfortunately, what resulted was a turbo-charged rat barn all over again. The population of Universe 25 began to steadily grow from the four original pairs of mice, until it reached a population of 2200 by Day 560. At which stage the population began its decline, and by the end of the experiment (Day 1500), there were only 27 mice remaining. The last surviving birth occurred on Day 600, after which the incidence of pregnancies declined very rapidly with no young surviving. The last conception occurred around Day 920.

While dominant males initially thrived and fathered all the offspring they could muster, the less dominant were inactive and eventually became violent towards each other. Those who were attacked would later attack others. The female counterparts of the withdrawn males simply hid. Nursing females in the presence of territorial males began to exhibit aggression, attacking and wounding their own young and forcing them to leave the nest before weaning was complete.

The next generation of mice, having never experienced healthy mouse behaviour, had no concept of mating or parenting, or even territorialism.

Consequently, the next generation of mice, having never experienced healthy mouse behaviour, had no concept of mating or parenting, or even territorialism. Some – dubbed the “beautiful ones” – spent their time eating, drinking, and grooming themselves in seclusion. Elsewhere, mice formed gangs of cannibalistic, raping plunderers.

Gradually, with a cohort of mice who would no longer actively mate or engage constructively in society, the population died out.

Calhoun was concerned with not only the death of the body (the second death) but also the death of the spirit (the first death – see Revelation ii:11) – an unusual inclusion in an academic paper. His thoughts are summarised with a word equation, the penultimate entry in the article:

Mortality, bodily death = the second death
Drastic reduction of mortality
= death of the second death
= death squared
= (death)2
(Death)2 leads to dissolution of social organisation
= death of the establishment
Death of the establishment leads to spiritual death
= loss of capacity to engage in behaviours essential to species survival
= the first death
(Death)2 = the first death

I have considered this equation for a while, and I have a few problems from a first-year university algebra perspective.

Let’s say mortality (bodily death) is a function called M. Now, as stated before, according to Calhoun’s article, there are five mortality factors in the ecology of animals in nature: emigration, resource shortage, inclement weather, disease, and predation. So mortality would be a function of these factors: M = M(e, rs, wi, d, p). Now, the death of the second death would be the composition* of M with M, which we denote as M M. (*Function composition is when you apply one function to the result of another: for example, if f(x) = x + 1 and g(x) = x3, then (f g)(x) = f(g(x)) = f(x3) = x3 + 1.)

This is where we hit a snag, because if we assume that M is a function of real variables, what we’re looking at is the composition of a multivariable function with a multivariable function, which isn’t technically possible.

Calhoun thought that the death of the spirit, which here equates to social breakdown, is unequivocally linked to the literal end of life.

But I digress. I think the most we can conclude from this is that Calhoun thought that the death of the spirit, which here equates to social breakdown, is unequivocally linked to the literal end of life. That is: “Loss of these respective complex behaviours means death of the species.” Unfortunately, he also concludes that “for an animal so complex as man, there is no logical reason why a comparable sequence of events should not also lead to species extinction”.

Many scientists, both at the time and since, would disagree with the somewhat macabre conclusions of the experiment, arguing that Calhoun’s work was not necessarily about population density per se, but rather it about social interaction. And humans interact with each other differently to the way rodents do. As a result, “Death Squared” is predominantly cited in works about lab rodents and rodent populations, rather than in research projecting the collapse of humankind.

Maybe Calhoun himself tried to soften his conclusions. At the end of the several pages of Biblical gloom in “Death Squared” he again relied on the Bible for the article’s last passage:

“Happy is the man who finds wisdom,
And the man who gains understanding.
Wisdom is a tree of life to those who
lay hold of her.
All her paths lead to peace.
                (Proverbs iii. 13, 18 and 17, rearranged)”

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