Cryptic female choice refers to the ability of females to non-randomly favor the sperm of certain males for fertilization, even after mating has occurred. This process stands apart from pre-copulatory mate choice, where females might select a mate based on observable traits like elaborate displays or physical size. Unlike these upfront decisions, cryptic female choice operates internally within the female’s reproductive tract, remaining hidden from direct observation. It represents a form of post-copulatory sexual selection, allowing females to exert control over paternity beyond the initial act of mating.
Mechanisms of Post-Copulatory Selection
Females employ physiological and chemical methods within their reproductive systems to influence which male’s sperm ultimately fertilizes their eggs. One direct mechanism is sperm ejection, where a female physically expels sperm from less preferred males after copulation, reducing the number of sperm available from those mates.
Females also utilize specialized organs, such as spermathecae, for sperm storage. These internal structures can hold sperm from multiple mating partners. Females may then selectively release or utilize sperm from preferred males.
The internal environment of the female reproductive tract can also create chemical and physiological biases that affect sperm viability and motility. This involves adjusting the pH levels, deploying immune responses, or releasing specific chemical signals that either promote or hinder the movement and survival of sperm from different males.
Females can additionally influence the amount of ejaculate a male transfers during mating. Some females might terminate copulation prematurely if the male is not favored, limiting the quantity of sperm received. Conversely, they might allow longer copulation times with preferred males, increasing successful sperm transfer.
Evolutionary Drivers of Cryptic Choice
The evolution of cryptic female choice is shaped by pressures that maximize offspring fitness. The “good genes” hypothesis suggests that females use cryptic choice to select sperm from males carrying genes that enhance offspring survival. This improves the genetic makeup of their progeny.
Genetic compatibility is another driver, where females may favor sperm that is genetically complementary to their own eggs. This helps avoid inbreeding depression and can lead to more viable offspring by reducing the likelihood of harmful genetic combinations.
Fertility assurance also plays a role, as mating with multiple males and selecting among their sperm ensures viable sperm are available for fertilization. This strategy minimizes the risk of reproductive failure if a single mate proves infertile or produces low-quality sperm, increasing the pool of potential fertilizing sperm.
Sexual conflict represents an evolutionary dynamic between males and females. Males evolve strategies to maximize their paternity. In response, females develop cryptic choice mechanisms to counteract these male strategies, maintaining their control over fertilization outcomes.
Examples Across the Animal Kingdom
Cryptic female choice is observed across diverse animal groups, highlighting its widespread evolutionary significance. In insects, for instance, female fruit flies (Drosophila melanogaster) can store sperm from multiple males in their spermathecae and selectively use sperm based on male genotype or even sperm length. Studies have shown that females may eject sperm from less preferred males or bias the usage of stored sperm, influencing paternity shares.
Birds also exhibit this phenomenon, with domestic fowl (Gallus gallus domesticus) providing a well-studied example. Female chickens have been observed to preferentially eject sperm from subordinate males or those that are genetically related to them. This behavior allows females to bias paternity towards dominant or genetically dissimilar males, even after multiple matings have occurred.
Spiders and other arthropods frequently demonstrate sophisticated cryptic choice mechanisms. Many female spiders possess multiple sperm storage organs, enabling them to store sperm from different males separately. This anatomical feature allows for selective fertilization of eggs by sperm from preferred males, as seen in certain orb-weaving spiders where females can control which sperm are released for fertilization from specific storage sites.
Fish species with internal fertilization also show evidence of cryptic female choice. For example, in some live-bearing fish, females can influence paternity by selectively retaining or discarding sperm from different males. This internal control allows females to bias fertilization outcomes, even in species where males may attempt to force copulations, demonstrating the pervasive nature of this post-mating selection.