Coolidge effect

The Coolidge effect is a biological phenomenon seen in animals, whereby males exhibit renewed sexual interest whenever a new female is introduced to have sex with, even after cessation of sex with prior but still available sexual partners. To a lesser extent, the effect is also seen among females with regard to their mates.… an old joke about Calvin Coolidge when he was President … The President and Mrs. Coolidge were being shown around an experimental government farm. When came to the chicken yard she noticed that a rooster was mating very frequently. She asked the attendant how often that happened and was told, 'Dozens of times each day.' Mrs. Coolidge said, 'Tell that to the President when he comes by.' Upon being told, the President asked, 'Same hen every time?' The reply was, 'Oh, no, Mr. President, a different hen every time.' President: 'Tell that to Mrs. Coolidge.' The Coolidge effect is a biological phenomenon seen in animals, whereby males exhibit renewed sexual interest whenever a new female is introduced to have sex with, even after cessation of sex with prior but still available sexual partners. To a lesser extent, the effect is also seen among females with regard to their mates. The Coolidge effect can be attributed to an increase in sexual responsiveness, and a decrease in the refractory period. The evolutionary benefit to this phenomenon is that a male can fertilize multiple females. The male may be reinvigorated repeatedly for successful insemination of multiple females. This type of mating system can be referred to as polygyny, where one male has multiple female mates, but each female only mates with one or a few male mates. The term 'Coolidge effect' was first suggested by behavioral endocrinologist Frank A. Beach in 1958. He attributed the neologism to: The joke appears in a 1972 book (Aggression in Man and Animals, by Roger N. Johnson, p. 94). The original experiments with rats applied the following protocol: A male rat was placed into an enclosed large box with four or five female rats in heat. He immediately began to mate with all the female rats repeatedly until he eventually became exhausted. The females continued nudging and licking him, yet he did not respond. When a novel female was introduced into the box, he became alert and began to mate once again with the new female. This phenomenon is not limited to common rats. The Coolidge effect is attributed to an increase in dopamine levels and the subsequent effect upon an animal's limbic system. In a study conducted by Fiorino et al., male rats were used to study the role of the mesolimbic dopamine system on their sexual behaviour. In their experiment, microdialysis was used to monitor dopamine efflux from the nucleus accumbens during three stages of sexual behaviour, these included: copulation, sexual satiety, and the reinitiation of sexual behaviour. Behavioural testing for the Coolidge effect consisted of several phases including, copulation with a female, reintroduction to the same female, access to the same female, introduction to a novel female, and copulation with the novel female. During these phases, dopamine and its metabolites were monitored. Results from this study found that overall there was a significant increase in dopamine efflux in response to both the first female and the second female. During copulation with the first female, concentrations of dopamine in these male rats showed a significant increase, however, when the same female was presented again, a significant increase in dopamine was not observed. When a novel female was presented, initially, there was a small increase in the levels of dopamine, however, after continued copulation with the novel female, a significant increase in dopamine levels was observed. From these results, they concluded that an increase in mesolimbic dopamine efflux is associated with the appetitive and consummatory stages of sexual behaviour in male rats. Their data also suggest that stimuli associated with a novel female may increase dopamine transmission in a rat that is sexually satiated, and hence have a role in the reinitiation of sexual behaviour. Additional studies have also provided further evidence for the role of the nucleus accumbens on sexual behaviour in rats. In a study conducted by Wood et al., male rats were divided into three conditions, and were presented with a cotton ball laden in either saline (control group), estrous vaginal smear of a familiar female (experimental group), and estrous vaginal smear of a novel female (experimental group). In this experiment, the role of the nucleus accumbens was characterized through recording neuronal activity of single cells in this area of the brain. Results from this study showed a greater proportion of neuronal activation when initially presented with a novel estrus stimulus in comparison to familiar estrus stimulus. Subsequent presentations of the novel estrus stimulus did not show an increase neuronal activity. It has been observed that in certain species, males allocate sperm differently due to the Coolidge effect. The allocation is usually according to level of sperm competition, female novelty, and female reproductive quality. An experiment performed on an external fertilizing fish called Rhodeus amarus, also known as the European bitterling, was used to show that sperm can be allocated differently if a novel partner is around, but what also happens if there is male-male competition. It is important to know that the European bitterling mating system works by females depositing their eggs into the gill filaments of freshwater mussels by her long ovipositor and then males proceed by ejecting their sperm into the gills of the mussel hosting the eggs. This means fertilization and development of the offspring relies on the quality and survival of the mussel. When the Coolidge effect was applied to this system, the experiment showed that it is the mussels, or the site of fertilization, that the males prefer to be novel. However, the takeaway from the experiment performed was that in male-male competition of the Rhodeus amarus, the dominant male will allocate more sperm when a novel mussel is present, while the subordinate male conserved its sperm until a proper opportunity came where it had a better chance of fertilization. A similar result was found in fowls, Gallus gallus, where the male showed a sperm allocation due to the Coolidge effect. The experiment found that male fowls reduce sperm investment in particular females they've encountered already, but increase sperm investment instantaneously if they encountered a new female. Wedell et al. suggest a theory that when a male allocates sperm so that he can save sperm for novel partners, he limits himself and the mate by possibly investing too little sperm to their partners which in return can inseminate only a few eggs therefore making reproduction less successful. This could even possibly force females to seek more copulation to ensure successful reproduction. These types of evidence of sperm allocation would suggest that Coolidge effect will determine how much sperm is invested into females, and if possible, sperm will be allocated so that sperm can be evenly distributed for multiple mates. Overall, it is typically seen that allocation changes due to male-male competition and whether a novel partner is encountered or not.