Nonhuman Sexual Conflict
Nonhuman sexual conflict refers to any type of conflict that arises between members of the animal kingdom as the result of male and female reproductive interests being at odds with one another.
In much of the animal kingdom, mating is not a cooperative process. Instead, it is marked by conflict and arms races between males and females to achieve their opposing reproductive goals (Parker 1979). Such conflict can be manifested in many forms, but they fall into two broad categories: precopulatory conflict and postcopulatory conflict (Parker 2006). The following sections provide brief overviews of the most commonly studied forms of conflict in nonhuman species under these two categories.
In general, precopulatory conflict is centered around identification of potential mates before any DNA has been exchanged (Parker 2006). Typically, such conflict arises from discrepancies in mate value between partners. For the majority of species, there are differences in choosiness between males and females when selecting an appropriate mate. This is mainly due to the fact that eggs tend to be larger, more complex gametes than sperm which makes them more energetically costly to produce (Bateman 1948). Additionally, females of most (but not all) species have a larger minimal parental investment in offspring in forms that include gestation, lactation, and incubation (Trivers 1972). Under such circumstances, females typically have lower reproductive potential than males because they are restrained by a limited number of eggs and heavy parental investment. Therefore, they tend to be choosier when seeking a mate as a poor mate choice can be more likely to lead to reproductive failure relative to males (Parker 1979). Males, on the other hand, benefit the most from a reproductive strategy that involves indiscriminate mate choice in an attempt to produce as many offspring as possible (Trivers 1972).
This asymmetry in choosiness between the sexes has been observed in a variety of species. In some deer species, for example, a routine mating practice is lekking, which refers to males displaying themselves in groups to females and then the females inspecting them and choosing the male of best quality as a mate (Massei and Bowyer 1999). Does prefer bucks with the largest antlers, as this trait is not only indicative of success in aggressive encounters with other males, but it also is an indicator of good genes. Specifically, a study on white-tailed deer found that bucks with the biggest antlers also had better immune function and greater resistance to parasites (Ditchkoff et al. 2001). Choosing bucks with this characteristic will lead to those better genes being passed on to the female’s offspring.
Another consequence of greater female choosiness is greater variance in reproductive success between the sexes (Bateman 1948). Because females only choose the highest quality males as mates, there should be at least some males that are not deemed high enough in quality as mates and therefore do not reproduce at all. Hence, males tend to have greater variance in reproductive success than females. However, one reaction to both the higher male propensity to fail to find a mate and the high degree of selectivity in females for males is to engage in sexual coercion. Although this is a known reason for sexual coercion, it is also practiced by reproductively successful males in an attempt to further increase their reproductive success. Such a strategy is favorable to male reproductive interests at the expense of the female’s ability to choose the best quality mate (Clutton-Brock and Parker 1995).
This form of sexual conflict is common in nonhuman species, and it is studied more heavily in some than others. For example, in many species of waterfowl, such as ducks, males engage in forced copulations with females that are typically already mated to a regular partner (McKinney and Evarts 1998). Additionally, many of the males engaging in sexual coercion have a regular partner as well. Male ducks tend to force copulation upon females that are not currently accompanied by their regular male partner, as male partners tend to be able to successfully defend against intruding males. However, in cases where a female is left alone and is forced into copulation by a strange male, if her regular partner arrives during the act of sexual coercion, he will fight off the intruder but will also force his mate to copulate with him immediately after to increase paternity certainty (McKinney and Evarts 1998).
Sexual coercion among ducks has been such a common part of the species’ ancestral history that female genitalia has evolved to decrease the chances of successful forced copulation and therefore give the female an advantage in sexual conflict. According to Brennan et al. (2009), the duck penis has a corkscrew shape that spirals in a counterclockwise direction, while the female oviduct spirals in a clockwise direction. This morphology makes it difficult for males to penetrate the oviduct and therefore achieve fertilization during forced copulation.
Precopulatory conflict can also occur among hermaphroditic species. In this case, conflict is usually focused on which member of the mating pair will take on the male and female role in terms of parental investment. Usually, both members prefer the male role as this allows for the highest reproductive potential along with the lowest amount of parental investment (Michiels 1998). Therefore, hermaphroditic species have developed antagonistic ways to approach copulation to compete for the opportunity of paternity. In one such species, flatworms, a mating pair will struggle with one another in an attempt to be the first to inseminate via a stabbing motion with its penis. The individual who stabs first tends to father more offspring in more individuals, therefore increasing reproductive success over other individuals that are not as successful in the stabbing battles (Michiels and Newman 1998).
In cases where females may not be able to choose their reproductive partners as a consequence of forceful insemination, there are still ways in which they can prevail in sexual conflict. The activities leading up to and including intercourse are only part of the story, as arms races between males and females are just as prevalent after the completion of copulation. Sexual conflict at this point in time usually revolves around which male will achieve paternity.
Sperm competition refers to a situation in which sperm from at least two different males are present in a female’s reproductive tract simultaneously and thus compete with one another for the opportunity to fertilize ova. Sperm competition could arise as a consequence of sexual coercion such as in the case of ducks as described earlier. However, in many nonhuman species, females actively promote sperm competition by mating with multiple males during a brief period (such as a mating season) (Parker 1970). It has been suggested that such a strategy still allows for female choice, in that only the most adept sperm (and presumably that also translates to the male with the best genes) are successful in such competition.
For instance, chimpanzees engage in promiscuous mating such that most members of a community mate with multiple members of the opposite sex. Under these circumstances, there is a high risk of sperm competition (Harcourt et al. 1981, 1995). Even though, in terms of precopulatory sexual conflict, the most socially dominant ranking males secure the highest degree of sexual access to females, their rates of paternity do not match their copulation frequency. The males with the highest rates of paternity are those that are somewhat younger than the most dominant males, and this is likely due to the superior quality of sperm from younger males (Wroblewski et al. 2009). Sperm production is vulnerable to senescence, and in older primate males declines in semen volume, count, motility, and morphology are commonly found (Kidd et al. 2001). Therefore, it is likely that female chimpanzees are optimizing their chances of reproductive success in sexual conflict in precopulatory contexts by allowing copulations with high-ranking males and in postcopulatory contexts by allowing copulations with younger males. Use of both strategies allows ultimately for the best genes to be passed to the next generation.
Not only does the quality of sperm itself help with the outcome of sperm competition, but also penile ornaments play a role. In many species, especially various insects, the penis has spines or some other form of weaponry that actually cause physical damage to the female during copulation. This has developed to discourage the female from seeking additional males and thus produces an advantage in intersexual conflict (Eberhard 1985). In addition to spermatozoa, other chemical components in seminal fluids contribute to sexual conflict. Specifically, substances in seminal fluids have been known to decrease the female’s desire to mate with another male and/or mimic her own body’s hormones to trigger oviposition as has been observed in grasshoppers, katydids, and fruit flies (Eberhard and Cordero 1995).
Even after accounting for sperm competition, there are still ways to manipulate the outcome of sexual conflict. Female animals sometimes induce abortions in the event that the expected offspring would have poor survival prospects. Mice are known to selectively abort their pregnancies (known as the Bruce effect) when their mate is replaced by a novel male, as unfamiliar males typically commit infanticide when exposed to unrelated offspring (Gangrade and Dominic 1984). Such a strategy saves the female from wasted effort in offspring that are unlikely to survive and prepares them for estrus so they can bear the offspring of the new male. The Bruce effect has even been observed from exposure to just the scent of urine from an unfamiliar male (Gangrade and Dominic 1984). Alternatively, some species use infanticide as a method of inducing estrus rather than spontaneous abortion. For example, when a new male lion takes over a harem of lionesses, he systematically kills all the lion cubs in the harem so as not to invest in unrelated offspring; this also induces estrus in the female lions so that the new male can increase his own reproductive success despite the females’ wasted parental effort (Pusey and Packer 1994).
This was just a brief summary of the types of sexual conflict that occur in nonhuman species, and some form of sexual conflict has been observed in virtually all species that have been studied. This type of conflict exists at the precopulatory level in the form of mate value discrepancy or sexual coercion, or at the postcopulatory level in the form of cryptic female choice and sperm competition. For both males and females, they have evolved adaptations to skew the outcome of this conflict in their favor at the expense of their partner. In many nonhuman species, the individuals with the most reproductive success are the ones that are best able to manipulate their partner.
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