Curt Richter's rat hope experiment: Why did the first nine rats survive for days?

Question

I can understand the part that the experimenter saved the rat just before it was about to die and then the rat lasted longer for the next drowning. But I do not understand what it means that before such intervention, when he first tried drowning, first 3 rats died in 2 minutes but the remaining 9 rats survived for days.Was it just a random chance that the first 3 rats happened to have little hope and the remaining 9 rats had naturally more hopes?

The first rat, Richter noted, swam around excitedly on the surface for a very short time, then dove to the bottom, where it began to swim around, nosing its way along the glass wall. It died two minutes later. Two more of the 12 domesticated rats died in much the same way. But, interestingly, the nine remaining rats did not succumb nearly so readily; they swam for days before they eventually gave up and died. ..... Richter then tweaked the experiment: He took other, similar rats and put them in the jar. Just before they were expected to die, however, he picked them up

https://www.psychologytoday.com/gb/blog/kidding-ourselves/201405/the-remarkable-power-hope

Answer 1

For more information on the experiment, there is Swamy (2020):

The conclusion drawn was that since the rats BELIEVED that they would eventually be rescued, they could push their bodies way past what they previously thought impossible.

and the source (Richter, 1957) can be downloaded in PDF

I had to re-read the Richter paper a couple of times to digest it, and from my understanding, the differences in swimming time was in the same (first) experiment. The tweak to the experiment was where, instead of using domesticated rats, they used hybrid rats ("crosses between domesticated and wild rats").

References

Richter, C. P. (1957). On the phenomenon of sudden death in animals and man. Psychosomatic Medicine, 19, 191–198. https://doi.org/10.1097/00006842-195705000-00004

Swamy, S. (2020). The Power of Hope: A Rat Experiment by Dr Curt Richter. LinkedIn https://www.linkedin.com/pulse/power-hope-rat-experiment-dr-curt-richter-santosh-swamy

Answer 2

It's not clear what caused this. This variation in swimming times is in fact what motivates Richter to continue tweaking his experiment:

The significance of this average curve was greatly reduced by the marked variations in individual swimming times. At all temperatures, a small number of rats died within 5 - 10 minutes after immersion, while in some instances others apparently no more healthy, swam as long as 81 hours. The elimination of these large variations presented a real problem, which for some time we could not solve. Then the solution came from an unexpected source - the finding of the phenomenon of sudden death, which constitutes the main topic of this communication.

The numbers you cite regarding the 3 versus 9 rats comes from his second run of the experiment where he tests whether trimming the whiskers in the rats would result in different times.

The first rat swam around excitedly on the surface for a very short time, then dove to the bottom, where it began to swim around nosing its way along the glass wall. Without coming to the surface a single time, it died 2 minutes after entering the tank. Two more of the twelve domesticated rats tested died in much the same way; however, the remaining 9 swam 40 to 60 hours.

It seems that it was just caused by random luck.

References

Richter, C. P. (1957). On the phenomenon of sudden death in animals and man. Psychosomatic Medicine, 19, 191–198. https://doi.org/10.1097/00006842-195705000-00004