In biology and psychology, the Coolidge effect is a phenomenon seen in animal species whereby males (and to a lesser extent females) exhibit renewed sexual interest if introduced to new receptive sexual partners, even after cessation of sex with prior but still available sexual partners. 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.
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Origin of the term
Behavioral endocrinologist Frank A. Beach first mentioned the term "Coolidge effect" in publication in 1955, crediting one of his students with suggesting the term at a psychology conference. He attributed the neologism to:
… an old joke about Calvin Coolidge when he was President … The President and Mrs. Coolidge were being shown [separately] around an experimental government farm. When [Mrs. Coolidge] 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 joke appears in a 1978 book (A New Look at Love, by Elaine Hatfield and G. William Walster, p. 75), citing an earlier source (footnote 19, Chapter 5).
Allocating sperm
It has been observed that in certain species of 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 is applied to this system, the experiment showed that it is the mussels, or the site of fertilization, that the males will 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.
Work done by Wedell et al. on various species such as the feral fowl, lemon tetra, and Drosophila suggests 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.
Absence of Coolidge effect
Coolidge effect is typically found in most animals, however, there are instances where Coolidge effect is absent. A study in decorated crickets, Gryllodes sigillatus, showed that even though females do display Coolidge effect, the males in this species have no preference for novel mates. Because the females in this species control copulation, to test the Coolidge effect in males, two dead females were used. One female was already previously mated and the other was a novel female. To measure Coolidge effect, the variables examined were the amount of courtship for the preferred mate and the size of the spermatophore transferred to the female. The size of the spermatophore was measured by weighing the mass after it was transferred to the female. The outcome of the experiment showed that there was no difference in the latency to re-mating of males confined with novel females and those paired with previous mates. There also was no difference in mass of the spermatophore. This experiment would suggest that the Coolidge effect is not applicable since the males of the Gryllodes sigillatus do not prefer novel females. Further research done on spiders also supports the possibility of absence of the Coolidge effect in certain species. A 2007 study focusing on the Coolidge effect in simultaneously hermaphroditic species confirmed the validity of the Coolidge effect in freshwater snail Lymnaea stagnalis. Biomphalaria glabrata, another simultaneous hermaphrodite freshwater snail, does not exhibit sex-specific effects of partner novelty, and thus there is either no Coolidge effect in the species or no difference between the degrees to which the effect is expressed in the respective sexes.
Means of sexual recognition for mates
Though there is no single reason for why males will choose a novel partner, there have been experiments that show that the major determining factor for detecting a novel partner is through olfactory preference. An experiment using a Long-Evans rat, showed that odor played a major role in distinguishing the difference between a novel partner and familiar partner. Another study also examined the olfactory preference, but what part of the brain targeted the olfactory preference. The experiment revealed that the perirhinal-etorhinal cortex region of the brain in golden hamsters is crucial for the recognition of familiar conspecifics and certain social behaviors. The conclusion from this experiment was also consistent in rats and monkeys, since damage to this region of the brain impaired standard recognition memory, which would suggest that the hippocampal region of the brain is not crucial in social behavior memory, but rather, the perirhinal-etorhinal cortex does.
Sexual satiety and the Coolidge effect
The Coolidge effect states that males typically will prefer novel mates every time the opportunity is present, however there is a physical limit to the sex drive. An experiment performed on rats showed that when left to reproduce to sexual satiety, the motor ejaculatory behavior, intromission, and dislodging seminal plugs were all possible after multiple mates, however little to no sperm would be produced during ejaculation. The experiment also concluded that males that reached satiety and non-satiety males both had the similar amounts of intromissions and time spent dislodging the seminal plug. Another study performed on rats showed the same results, but found data that concluded that reaching optimal chances of impregnating their mates happened after resting for 15 days. These experiments would suggest that the one of the major limitations on the Coolidge effect is the physical boundaries of gamete production.
Empirical evidence
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.
While the Coolidge effect is usually seen demonstrated by males—that is, males displaying renewed excitement with a novel female—Lester and Gorzalka developed a model to determine whether or not the Coolidge effect also occurs in females. Their experiment, which used hamsters instead of rats, found that it does occur to a lesser degree in females, where the evolutionary advantage of mating with multiple partners is less straightforward.