The Coolidge effect is a biological phenomenon seen in animals, whereby males exhibit renewed sexual interest whenever a new female is introduced, even after sex with prior but still available sexual partners. [1] [2] [3] [4] To a lesser extent, the effect is also seen among females with regard to their mates. [3]
The Coolidge effect can be attributed to an increase in sexual responsiveness, and a shortening of the sexual refractory period. [5] The evolutionary benefit to this phenomenon is that a male can fertilize multiple females. [6] The male may be reinvigorated repeatedly for successful insemination of multiple females. [7] 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. [5] The Coolidge effect has been demonstrated to occur in humans across cultures and in both sexes. [8]
Behavioral endocrinologist Frank A. Beach claims in a 1974 letter [9] to have introduced the term "Coolidge effect" in either 1958 or 1959. [10] He attributed the neologism to [11]
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 1972 book (Aggression in Man and Animals, by Roger N. Johnson, p. 94). [12]
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. [13] He immediately began to mate with all the female rats repeatedly until he eventually became exhausted. [13] The females continued nudging and licking him, yet he did not respond. [13] When a novel female was introduced into the box, he became alert and began to mate once again with the new female. [13] This phenomenon is not limited to common rats. [14]
The Coolidge effect is attributed to an increase in dopamine levels and the subsequent effect upon an animal's limbic system. [15] 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. [15] 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. [15] 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. [15] During these phases, dopamine and its metabolites were monitored. [15] 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. [15] 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. [15] 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. [15] 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. [15] 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. [15]
Additional studies have also provided further evidence for the role of the nucleus accumbens on sexual behaviour in rats. [16] 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). [16] In this experiment, the role of the nucleus accumbens was characterized through recording neuronal activity of single cells in this area of the brain. [16] 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. [16] Subsequent presentations of the novel estrus stimulus did not show an increase in neuronal activity. [16]
It has been observed that in certain species, males allocate sperm differently due to the Coolidge effect. [17] The allocation is usually according to level of sperm competition, female novelty, and female reproductive quality. [17] 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. [17] 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. [17] This means fertilization and development of the offspring relies on the quality and survival of the mussel. [17] 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. [17] 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. [17] A similar result was found in fowls, Gallus gallus , where the male showed a sperm allocation due to the Coolidge effect. [18] 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. [18]
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. [19] This could even possibly force females to seek more copulation to ensure successful reproduction. [20] 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. [19] [21] Overall, it is typically seen that allocation changes due to male-male competition and whether a novel partner is encountered or not. [22] [23] [24]
The Coolidge effect is typically found in most animals; however, there are instances where the Coolidge effect is absent. [25] [26] [27] [28] A study in decorated crickets, Gryllodes sigillatus , showed that even though females do display the Coolidge effect, the males in this species have no preference for novel mates. [25] Because the females in this species control copulation, to test the Coolidge effect in males, two dead females were used. [25] One female was already previously mated and the other was a novel female. [25] To measure the Coolidge effect, the variables examined were the amount of courtship for the preferred mate and the size of the spermatophore transferred to the female. [25] The size of the spermatophore was measured by weighing the mass after it was transferred to the female. [25] 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. [25] There also was no difference in mass of the spermatophore. [25] This experiment would suggest that the Coolidge effect is not applicable since the males of the Gryllodes sigillatus do not prefer novel females. [25] Further research done on spiders also supports the possibility of absence of the Coolidge effect in certain species. [26] Another study that focused on the Coolidge effect in simultaneously hermaphroditic species confirmed the validity of the Coolidge effect in freshwater snail Lymnaea stagnalis . [27] 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. [28]
Though there is not just a single reason why males will choose a novel partner, there have been experiments that show that the major determining factor for detecting a novel partner is olfactory preference. [29] An experiment using Long-Evans rats, showed that odor played a major role in distinguishing the difference between a novel partner and familiar partner. [29] In their experiment, Carr et al. paired each male rat with a female rat and allowed them to mate. Male rats were then tested for preference through the use of an apparatus that had two cylinders that were attached to their home cage and contained the familiar female and the novel female in each cylinder. [29] Caps at the end of these cylinders prevented access to the females but had a hole in them to allow their odors to pass through to the male's cage. [29] Before the testing phase, the females were removed, and then the caps were removed to allow the male to explore both cylinders. [29] From this experiment, they found that males preferred the scent of the novel female. [29] While these males did not have access to these females to demonstrate mating preferences, this odor preference is believed to reflect promiscuous behavior, and therefore be important to the male mating strategy. [29]
In an earlier experiment, also conducted by Carr et al., they found that, unlike male rats, female rats preferred the odor of a familiar partner rather than the odor of a novel partner. [30]
Another study also examined not only olfactory preference but also what part of the brain targeted the olfactory preference. [31] In this study, male hamsters were given lesions to either the hippocampus or the perirhinal-entorhinal cortex or received a sham treatment. Then the hamsters were allowed to mate with a female hamster until they became satiated. All subjects were then presented with two anesthetized females, one of whom was the female they had previously copulated with, while the other was a novel female. [31] Hamsters with sham and hippocampal lesions investigated the anogenital region of the novel females for a significantly longer period in comparison to the familiar females. [31] Males with lesions to the perirhinal-entorhinal cortex did not show a preference for either a familiar or novel female and spent a similar amount of time investigating the anogenital region of both females. [31] The results from this study revealed that the perirhinal-entorhinal cortex region of the brain in golden hamsters is crucial for the recognition of familiar conspecifics and certain social behaviors. [31] 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. [31] [32]
This effect of olfaction has also been observed in invertebrates such as flies. [33] 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. [33] The result of this type of mutation renders these flies unable to discriminate between phenotypically familiar (i.e., from the same family and/or environment) and phenotypically novel (i.e., unrelated and from a different environment) females. [33] When presented with a phenotypically familiar and phenotypically novel female, mutant flies did not show a preference towards either female, while wild-type flies (i.e., flies without the mutated Orco gene) did. [33] The results from this experiment suggest that the Orco gene is important in discriminating between mates, and suggests an important role of olfactory cues in phenotypic familiarity. [33]
Although males typically will prefer novel mates every time the opportunity is present, there is a physical limit to the sex drive. [6] 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, but little to no sperm would be produced during ejaculation. [6] 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. [6] 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. [34] These experiments would suggest that one of the major limitations on the Coolidge effect is the physical boundaries of gamete production. [6] [34]
While the Coolidge effect is usually demonstrated by males—that is, males displaying renewed excitement with a novel female—the body of research into the phenomenon continues to explore the potentiality of the effect in females, specifically rodents. [35] [36] [37] 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. [37]
In a study conducted in 2013, researchers conducted an experiment to test if the Coolidge effect could be observed in female rats. [36] In their experiment, they assigned female rats to one of two conditions: paced or non-paced. In the paced condition, females were able to control the timing of copulation, while in the non-paced condition males regulated the timing of mating. [36] Females in the paced condition were placed into an arena that was divided into two compartments which contained holes that only the female could pass through. [36] The female was placed on one side of the divided arena, and the male was placed in the opposite side; this allowed the female to enter and exit the male's side depending on the circumstances, therefore regulating the timing of copulation. [36] In the non-paced condition, both the female and male rats were placed in an undivided arena and were allowed to mate. [36] In both conditions, females were exposed to a constant, familiar male followed by a novel male. [36] Their results found that females showed a significant increase in proceptive behaviours when a novel mate was present, but only if the female could regulate mating. [36] Repeated mating with the same male also resulted in a decrease in proceptivity, which is thought to suggest sexual satiety. [36] These results suggest that the Coolidge effect may also be present in female rats. [36]
Lester and Gorzalka developed a model to determine whether or not the Coolidge effect also occurs in females. [3] Their experiment, which used hamsters instead of rats, concluded that it does occur to a lesser degree in females, where the evolutionary advantage of mating with multiple partners is less straightforward. [3] [4] It is possible that the presence of the Coolidge effect in females could serve to optimize the female's reproductive success, and prevent mating with a sterile male. Mating with multiple males may reduce the probability that the female's gametes will not be successfully fertilized. [3]
Sexual attraction is attraction on the basis of sexual desire or the quality of arousing such interest. Sexual attractiveness or sex appeal is an individual's ability to attract other people sexually, and is a factor in sexual selection or mate choice. The attraction can be to the physical or other qualities or traits of a person, or to such qualities in the context where they appear. The attraction may be to a person's aesthetics, movements, voice, or smell, among other things. The attraction may be enhanced by a person's adornments, clothing, perfume or hair style. It can be influenced by individual genetic, psychological, or cultural factors, or to other, more amorphous qualities. Sexual attraction is also a response to another person that depends on a combination of the person possessing the traits and on the criteria of the person who is attracted.
Sperm competition is the competitive process between spermatozoa of two or more different males to fertilize the same egg during sexual reproduction. Competition can occur when females have multiple potential mating partners. Greater choice and variety of mates increases a female's chance to produce more viable offspring. However, multiple mates for a female means each individual male has decreased chances of producing offspring. Sperm competition is an evolutionary pressure on males, and has led to the development of adaptations to increase male's chance of reproductive success. Sperm competition results in a sexual conflict between males and females. Males have evolved several defensive tactics including: mate-guarding, mating plugs, and releasing toxic seminal substances to reduce female re-mating tendencies to cope with sperm competition. Offensive tactics of sperm competition involve direct interference by one male on the reproductive success of another male, for instance by mate guarding or by physically removing another male's sperm prior to mating with a female. For an example, see Gryllus bimaculatus.
Animal sexual behaviour takes many different forms, including within the same species. Common mating or reproductively motivated systems include monogamy, polygyny, polyandry, polygamy and promiscuity. Other sexual behaviour may be reproductively motivated or non-reproductively motivated.
The ventromedial nucleus of the hypothalamus is a nucleus of the hypothalamus. In 2007, Kurrasch et al. found that the ventromedial hypothalamus is a distinct morphological nucleus involved in terminating hunger, fear, thermoregulation, and sexual activity. This nuclear region is involved in the recognition of the feeling of fullness.
The sexually dimorphic nucleus (SDN) is an ovoid, densely packed cluster of large cells located in the medial preoptic area (POA) of the hypothalamus which is believed to be related to sexual behavior in animals. Thus far, for all species of mammals investigated, the SDN has been repeatedly found to be considerably larger in males than in females. In humans, the volume of the SDN has been found to be 2.2 times as large in males as in females and to contain 2.1 times as many cells. The human SDN is elongated in females and more spherical in males. No sex differences have been observed in the human SDN in either cell density or mean diameter of the cell nuclei. The volume and cell number of the human SDN considerably decreases with age, although the decrease in cell number is both sex and age-specific. In males, a substantial decrease in the cell number of the human SDN was observed between the age of 50–60 years. Cell death was more common in females than males, especially among those older than 70 years of age. The SDN cell number in females can drop to 10-15% of that found in early childhood.
The preoptic area is a region of the hypothalamus. MeSH classifies it as part of the anterior hypothalamus. TA lists four nuclei in this region,.
Sexual conflict or sexual antagonism occurs when the two sexes have conflicting optimal fitness strategies concerning reproduction, particularly over the mode and frequency of mating, potentially leading to an evolutionary arms race between males and females. In one example, males may benefit from multiple matings, while multiple matings may harm or endanger females, due to the anatomical differences of that species. Sexual conflict underlies the evolutionary distinction between male and female.
Sexual cannibalism is when an animal, usually the female, cannibalizes its mate prior to, during, or after copulation. It is a trait observed in many arachnid orders, several insect and crustacean clades, gastropods, and some snake species. Several hypotheses to explain this seemingly paradoxical behavior have been proposed. The adaptive foraging hypothesis, aggressive spillover hypothesis and mistaken identity hypothesis are among the proposed hypotheses to explain how sexual cannibalism evolved. This behavior is believed to have evolved as a manifestation of sexual conflict, occurring when the reproductive interests of males and females differ. In many species that exhibit sexual cannibalism, the female consumes the male upon detection. Females of cannibalistic species are generally hostile and unwilling to mate; thus many males of these species have developed adaptive behaviors to counteract female aggression.
A courtship display is a set of display behaviors in which an animal, usually a male, attempts to attract a mate; the mate exercises choice, so sexual selection acts on the display. These behaviors often include ritualized movement ("dances"), vocalizations, mechanical sound production, or displays of beauty, strength, or agonistic ability.
Female copulatory vocalizations, also called female copulation calls or coital vocalizations, are produced by female primates, including human females, and female non-primates. Copulatory vocalizations usually occur during copulation and are hence related to sexual activity. Vocalizations that occur before intercourse, for the purpose of attracting mates, are known as mating calls.
Formally, a nuptial gift is a material presentation to a recipient by a donor during or in relation to sexual intercourse that is not simply gametes in order to improve the reproductive fitness of the donor. Often, such a gift will improve the fitness of the recipient as well. This definition implies neutral gifts, costly gifts and beneficial gifts regarding the fitness of the recipient.
Most mammals are viviparous, giving birth to live young. However, the five species of monotreme, the platypuses and the echidnas, lay eggs. The monotremes have a sex determination system different from that of most other mammals. In particular, the sex chromosomes of a platypus are more like those of a chicken than those of a therian mammal.
Sexual selection in birds concerns how birds have evolved a variety of mating behaviors, with the peacock tail being perhaps the most famous example of sexual selection and the Fisherian runaway. Commonly occurring sexual dimorphisms such as size and color differences are energetically costly attributes that signal competitive breeding situations. Many types of avian sexual selection have been identified; intersexual selection, also known as female choice; and intrasexual competition, where individuals of the more abundant sex compete with each other for the privilege to mate. Sexually selected traits often evolve to become more pronounced in competitive breeding situations until the trait begins to limit the individual's fitness. Conflicts between an individual fitness and signaling adaptations ensure that sexually selected ornaments such as plumage coloration and courtship behavior are "honest" traits. Signals must be costly to ensure that only good-quality individuals can present these exaggerated sexual ornaments and behaviors.
Sexual selection in scaled reptiles studies how sexual selection manifests in snakes and lizards, which constitute the order Squamata of reptiles. Each of the over three thousand snakes use different tactics in acquiring mates. Ritual combat between males for the females they want to mate with includes topping, a behavior exhibited by most viperids in which one male will twist around the vertically elevated fore body of its opponent and forcing it downward. It is common for neck biting to occur while the snakes are entwined.
Sexual selection in amphibians involves sexual selection processes in amphibians, including frogs, salamanders and newts. Prolonged breeders, the majority of frog species, have breeding seasons at regular intervals where male-male competition occurs with males arriving at the waters edge first in large number and producing a wide range of vocalizations, with variations in depth of calls the speed of calls and other complex behaviours to attract mates. The fittest males will have the deepest croaks and the best territories, with females making their mate choices at least partly based on the males depth of croaking. This has led to sexual dimorphism, with females being larger than males in 90% of species, males in 10% and males fighting for groups of females.
Inbreeding avoidance, or the inbreeding avoidance hypothesis, is a concept in evolutionary biology that refers to the prevention of the deleterious effects of inbreeding. Animals only rarely exhibit inbreeding avoidance. The inbreeding avoidance hypothesis posits that certain mechanisms develop within a species, or within a given population of a species, as a result of assortative mating and natural and sexual selection, in order to prevent breeding among related individuals. Although inbreeding may impose certain evolutionary costs, inbreeding avoidance, which limits the number of potential mates for a given individual, can inflict opportunity costs. Therefore, a balance exists between inbreeding and inbreeding avoidance. This balance determines whether inbreeding mechanisms develop and the specific nature of such mechanisms.
In behavioral ecology, polyandry is a class of mating system where one female mates with several males in a breeding season. Polyandry is often compared to the polygyny system based on the cost and benefits incurred by members of each sex. Polygyny is where one male mates with several females in a breeding season . A common example of polyandrous mating can be found in the field cricket of the invertebrate order Orthoptera. Polyandrous behavior is also prominent in many other insect species, including the red flour beetle, the adzuki bean weevil, and the species of spider Stegodyphus lineatus. Polyandry also occurs in some primates such as marmosets, mammal groups, the marsupial genus' Antechinus and bandicoots, around 1% of all bird species, such as jacanas and dunnocks, insects such as honeybees, and fish such as pipefish.
Syrian hamster behavior refers to the ethology of the Syrian hamster.
Extended female sexuality is where the female of a species mates despite being infertile. In most species, the female only engages in copulation when she is fertile. However, extended sexuality has been documented in Old World primates, pair bonded birds and some insects. Extended sexuality is most prominent in human females who exhibit no change in copulation rate across the ovarian cycle.
In zoology, copulation is animal sexual behavior in which a male introduces sperm into the female's body, especially directly into her reproductive tract. This is an aspect of mating. Many aquatic animals use external fertilization, whereas internal fertilization may have developed from a need to maintain gametes in a liquid medium in the Late Ordovician epoch. Internal fertilization with many vertebrates occurs via cloacal copulation, known as cloacal kiss, while most mammals copulate vaginally, and many basal vertebrates reproduce sexually with external fertilization.