The Coolidge effect is a biological phenomenon seen in animals, whereby males exhibit renewed sexual interest whenever a new female of reproductive availability is introduced, even after 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 evolutionary benefit to this phenomenon is that a male can fertilize multiple females. This type of mating system can be referred to as polygyny, where one male has multiple female mates, but each female mates with only one or a few males.
Origin of the term
In a 1974 letter, behavioral endocrinologist Frank A. Beach claims to have introduced the term "Coolidge effect" in either 1958 or 1959. He attributed the neologism to an old joke about Calvin Coolidge when he was President of the United States.
The joke appears in a 1972 book (Aggression in Man and Animals, by Roger N. Johnson, p. 94).
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 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 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). 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 that it also happens if there is male-male competition.
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.
Absence
The Coolidge effect is typically found in most animals; however, there are instances where the Coolidge effect is absent. Because the females in this species control copulation, to test the Coolidge effect in males, two dead females were used. Another study that focused 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 not just a single reason why males will choose a novel partner, there have been experiments that show that in several species, the major determining factor for detecting a novel partner is olfactory preference. An experiment using Long-Evans rats, showed that odor played a major role in distinguishing the difference between a novel partner and familiar partner.
Another study also examined not only olfactory preference but also what part of the brain targeted the olfactory preference. 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-entorhinal cortex.
This effect of olfaction has also been observed in invertebrates such as flies. In a study conducted by Tan and colleagues, the role of olfaction in species recognition was examined using flies, Drosophila melanogaster, which processed a mutated Orco gene, which codes for a co-receptor that is important for olfaction. 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 one of the major limitations on the Coolidge effect is the physical boundaries of gamete production. Research examining female rats has lent its support, wherein female rats showed a higher incentive motivation for unknown males over ones they had just copulated with.