Tuesday, 21 January 2014
The Coolidge Effect and the sex life of a rat
John Calvin Coolidge Jr. was the 30th United States president, holding office from 1923 to 1929. He came to prominence as the Governor of Massachusetts when he ended the 1919 Boston police strike by publicly supporting his Police Commissioner’s orders for three quarters of the police force to be sacked. Coolidge's presidency steered the US through the period of unprecedented economic growth that became known as the roaring twenties. A renowned leader whose reputation has remained strong to this day, one of Coolidge's more obscure legacies was the lending of his name to a psycho-biological phenomenon – thanks to a singularly trivial event.
The story goes that President Coolidge and his wife were being shown around a farm. For some reason they became separated, viewing different parts of the farm at different times. At the chicken yard, Mrs Coolidge observed a rooster mating very actively and asked how often this occurred. She was surprised to hear it was dozens of times a day and joked that the staff should tell the president when he came by. When the president's party later arrived, the farm staff duly recounted his wife's observations concerning the rooster. President Coolidge demonstrated a keen sharpness of mind when he asked the simple but revealing question of whether the cock was mating with the same hen every time. On hearing to the contrary the President suggested the staff might mention that to Mrs. Coolidge.
The Coolidge Effect, named after the 30th President of the United States, is concerned not with industrial relations, economics or outstanding leadership. Rather, it concerns an aspect of sexual behaviour. Specifically speaking, it denotes the observation, which holds for many species, that males will be more eager to mate with a new female, as opposed to one that is familiar. In technical terms, males have been found to display a shorter refractory period (that is, the time between one copulating session and the next) if a new partner becomes available. The research underlying the Coolidge Effect was written up by scientists from the University of California in a paper published in 1963.
The study falls fairly and squarely in the field of experimental psychology, such that several similar scenarios were set up and different behaviours were observed, counted and compared across these different conditions. The particular behaviour under observation was rats having sex. Male rats were required to pass a simple test in order to be selected for the study. They were placed with a female on heat for half an hour and those that copulated to ejaculation a minimum of two times were chosen. Interestingly, how the researchers were able to detect the occurrence of a rat ejaculation I can scarcely imagine, and sadly the paper doesn't explain. Maybe a reader of this blog could enlighten the rest of us!
For the main experiment male rats were paired up with females on heat and allowed to mate until they stopped for at least 30 minutes, at which point they were declared to be sexually exhausted. Then the female was removed. Next, some of the males were introduced to a new on-heat female while others were re-introduced to the same on-heat female that they had reached sexual exhaustion with. Sexual activity during the re-introduction phase was measured, in particular the number of times the female was mounted and the percentage of male rats achieving first and second ejaculations.
At first nothing in the rats’ behaviour was untoward. The number of mountings and first ejaculations was similar whether or not a new female was reintroduced. However, none of the rats re-introduced to the original partner ejaculated for a second time, whereas several of the rats with new partners did enjoy what might be termed a "second coming". In addition, three male rats that were not at all interested in a re-introduced familiar female changed their tune and were seen to copulate with a new partner introduced later in an additional phase of the experiment. Final evidence for the Coolidge effect was an observation made among rats that copulated in a further re-introduction phase. When the partner was new 86% of males achieved ejaculation, a much higher proportion than the 33% managing this when the same partner was reintroduced.
So there is the evidence for the Coolidge Effect. It was based on the finding that male rats become more interested in sex with a new partner rather than an existing mate. But what does it mean? Why might it exist? It is probably easiest to understand from the perspective of evolution, that is, by considering the benefits to the continuation of the species. The evolutionary advantage of the Coolidge Effect is that it encourages a wider circle of copulation partners, so increasing the chances of pregnancy and procreation. Think of it as Nature's way of guarding against putting all your eggs in the same basket, as the old saying goes.
This paper is a worthy addition to the Cool Psychology Blog for several reasons. For one thing, it’s cool for rat ejaculations to be used as a variable in a psychology experiment. It just is. Also this would have been a fascinating experiment to run on a day-to-day basis – for instance, I wonder if any of the rats gained particular reputations for their prowess (or lack of). It's fairly well known that psychologists are fascinated by many different aspects of behaviour but who knew this stretched to an interest in rats having sex? On the other hand, you could say that the research is somewhat sexist. In the narrative only the males seem to be active participants in the mounting, copulating and ejaculating. It takes two to tango so it is likely that the females were more active than portrayed.
This line of research is still ongoing, but has grown in sophistication. A Mexican team published a Coolidge Effect paper in 2012 that measured sperm count and erection occurrence as well as the number of mounts and ejaculations. While evidence for the Coolidge Effect was apparent, still this recent paper doesn’t reflect societal trends towards sexual equality – it is still all about the males, with the female rats considered as passive sexual partners. On the other hand, a paper from the mid-1980s did show evidence of a Coolidge Effect in female hamsters re-introduced to the same or a new male partner.
The reference to the original research described in this blog that lead to the coining of the term “The Coolidge Effect” is included below. Unfortunately the article is copyright protected and so only available at libraries that pay to subscribe to it – for example university libraries. This link takes you to the paper’s official web page where you can read a short summary.
Lester, G.L. & Gorzalka, B.B. (1988). Effect of novel and familiar mating partners on the duration of sexual receptivity in the female hamster. Behavioral & Neural Biology 49, 398-405.
Tlachi-Lópeza, J.L., Eguibarb, J.R., Fernández-Guastic, A. & Luciod, R.A. (2012). Copulation and ejaculation in male rats under sexual satiety and the Coolidge effect. Physiology & Behavior 106, 626–630.
Wilson, J.R., Kuehn, R.E. & Beach, F.A. (1963). Modification in the sexual behavior of male rats produced by changing the stimulus female. Journal of Comparative and Physiological Psychology 56, 636-644.