For many species, mating is not the final step in reproduction. It marks the beginning of a silent competition occurring entirely within the female’s body after insemination, known as postcopulatory sexual selection. This internal battle of physiological and biochemical interactions determines which male will father the next generation and has driven the evolution of complex reproductive behaviors and structures.
The Foundation of Sperm Competition
At the heart of postcopulatory selection is sperm competition. This process results from females mating with more than one male in a single reproductive cycle, a behavior known as polyandry. When a female is polyandrous, sperm from different males must compete inside her reproductive tract to fertilize her eggs, a pressure that has led to an array of male tactics.
The dynamics of this contest can be compared to a raffle or a race. In some cases, the male who produces the most sperm has the highest chance of fathering offspring. In other scenarios, victory goes to the male whose sperm are the fastest or most resilient. The prevalence of polyandry across diverse species like house mice and nursery web spiders means sperm competition is a powerful selective force, shaping male anatomy, physiology, and behavior in countless ways.
Male Offensive and Defensive Tactics
In the face of sperm competition, males have evolved a wide range of strategies that can be categorized as either defensive or offensive. Defensive tactics are proactive measures aimed at preventing a female from mating with subsequent rivals, ensuring the male’s own sperm have no competitors. One method is mate guarding, where a male remains close to a female after copulation, physically blocking other males from approaching.
A more invasive defensive strategy is the use of a copulatory plug. After mating, some male animals deposit a gelatinous or waxy secretion into the female’s reproductive tract. This plug acts as a physical barrier, making it difficult for other males to inseminate her. These plugs are found in a variety of species, from garter snakes to certain insects and spiders, and can vary in their effectiveness and duration.
Offensive tactics are deployed when a female has already mated with a rival and are designed to remove or neutralize the sperm of previous males. For example, the male damselfly’s penis is equipped with specialized bristles and horns used to physically scrub out any sperm stored in the female’s reproductive tract from prior matings before he deposits his own.
Beyond physical removal, males can also use proteins in their seminal fluid. In fruit flies (Drosophila melanogaster), seminal fluid contains substances that can incapacitate or kill the sperm of rival males. These same proteins can also influence the female’s physiology, sometimes reducing her receptivity to future matings, serving a dual offensive and defensive role.
Cryptic Female Choice
Females are not passive arenas for male competition; they have evolved their own mechanisms to influence paternity after mating. This suite of female-driven processes is known as cryptic female choice because the selection happens internally and is not outwardly visible. It allows a female to bias fertilization towards certain males over others, even after accepting sperm from multiple partners.
One form of cryptic female choice is the selective ejection of sperm. Females of some species, including domestic fowl, can voluntarily expel the ejaculate of a recent mate, often retaining the sperm of more preferred males from earlier matings. This behavior demonstrates that a successful copulation does not guarantee paternity.
In many insects and other animals, females possess specialized organs called spermathecae for storing sperm, sometimes for extended periods. This ability allows for another layer of choice, as a female can selectively use the stored sperm from a specific male to fertilize her eggs, potentially based on genetic compatibility or male quality.
The mechanisms of choice can also be biochemical. In some species, including humans, the fluid surrounding the egg may release chemoattractants that preferentially guide sperm from certain males over others. This molecular-level interaction shows that the process of choice can be subtle, occurring down to the level of gametes.
The Resulting Evolutionary Arms Race
The interplay between male competitive tactics and female choice mechanisms creates a co-evolutionary arms race. As males evolve more effective strategies for securing paternity, females face selective pressure to evolve counter-strategies to maintain control over fertilization. This reciprocal process ensures that neither sex gains a permanent upper hand.
This ongoing conflict is a major driver of rapid evolutionary change in reproductive traits. The pressure to out-compete rivals has led to the diversity and complexity seen in animal genitalia. Many shapes and structures of male reproductive organs are adaptations for displacing rival sperm or stimulating the female to bias paternity.
Similarly, the biochemical makeup of ejaculates is in a constant state of flux. Males evolve new seminal fluid proteins to manipulate female physiology or attack rival sperm, while females evolve chemical defenses in their reproductive tracts to neutralize these effects. This has resulted in the rapid evolution of reproductive proteins, making them some of the most divergent molecules between closely related species.
This hidden battle is a potent engine of biodiversity. The relentless pressure for males to succeed and for females to control reproductive outcomes drives the evolution of novel traits. This cycle of adaptation and counter-adaptation contributes to the divergence of populations and can eventually lead to the formation of new species.