Ejaculation is the forceful expulsion of seminal fluid from the male reproductive tract. This action is the final physical step in internal fertilization, ensuring the delivery of male gametes—sperm—to the female reproductive tract. While delivery systems vary widely across the animal kingdom, the core purpose of transferring genetic material remains universal among sexually reproducing males. The physiological machinery driving this event is a complex coordination of nerves and muscles, operating outside of conscious control.
The Physiological Mechanism
The physical act of ejaculation is a reflex governed by the autonomic nervous system, which coordinates involuntary actions. This entire process occurs in two distinct, synchronized phases: emission and expulsion. The emission phase begins with the sympathetic nervous system stimulating smooth muscles surrounding the accessory reproductive glands and the vas deferens. This coordinated muscle contraction pushes the spermatozoa and the seminal plasma, a fluid rich in nutrients and enzymes, into the urethra.
Seminal plasma is a mixture of secretions from organs such as the seminal vesicles and the prostate gland in mammals. This fluid contains high concentrations of substances like fructose, which provides energy for sperm motility, and buffering agents that protect the sperm from the acidic environment of the female tract. Once the ejaculate components have been mixed in the posterior urethra, the expulsion phase begins, marked by a shift in neural control.
Expulsion is a somatic reflex involving rhythmic, strong contractions of striated muscles, notably the bulbocavernosus muscle in mammals. These contractions generate the necessary pressure to forcefully eject the semen from the urethra. Throughout this process, the sympathetic nervous system ensures the closure of the bladder neck, which prevents the seminal fluid from traveling backward into the bladder (retrograde ejaculation).
Evolutionary Drivers of Ejaculation
Variations in the volume, speed, and composition of the ejaculate are largely driven by the evolutionary pressure known as sperm competition. This competition arises when females mate with multiple partners, causing the sperm from different males to compete internally to fertilize the limited number of eggs. Males have evolved strategies to maximize the probability that their sperm will succeed in this internal “raffle” for paternity. One common adaptation is the proportional increase in relative testis size and ejaculate volume in species where female promiscuity is high.
For instance, males in highly promiscuous species often produce a large volume of ejaculate containing a huge number of sperm, effectively increasing their “tickets” in the competition. Conversely, some species have evolved a trade-off between sperm quantity and quality, where a lower number of sperm may be longer or faster to outmaneuver rivals.
The seminal fluid itself can also be weaponized, with some species depositing a gelatinous secretion known as a copulatory or mating plug. This plug, formed from coagulating seminal fluid proteins, hardens within the female reproductive tract, physically blocking subsequent mating attempts by rival males. In some rodents, the presence of this plug also promotes the transport of the first male’s sperm, ensuring a competitive advantage. Males of some species can also adjust the amount of sperm they ejaculate based on the perceived risk of competition, allocating more resources when they sense a rival has been present.
Diversity in Reproductive Morphology
The structures responsible for delivering the ejaculate, known as intromittent organs, exhibit great diversity across the animal kingdom, reflecting different reproductive strategies. Most mammalian males possess a penis, which is either musculocavernous (relying on blood flow for erection) or fibroelastic (stiffening due to an S-shaped bend that straightens upon erection, as seen in bulls and boars). Many mammals, including most primates, rodents, and carnivores, also possess a bone within the penis called the baculum or os penis, which provides structural rigidity for copulation.
Insects utilize a structure called the aedeagus, which can be highly complex and species-specific, often featuring spines, hooks, or elaborate shapes that interact precisely with the female’s internal anatomy. Reptiles, such as snakes and lizards, do not have a single penis but instead use a pair of hemipenes, which are often covered in spines or ridges to anchor the male during copulation. Only one hemipenis is typically inserted into the female’s cloaca at a time.
Many male birds lack an external intromittent organ entirely, performing a swift “cloacal kiss” where the male and female cloacal openings briefly touch to transfer semen. The few species that do possess a phallus, such as the Argentine lake duck, often display extreme adaptations, including a long, corkscrew-shaped organ that evolved in response to intense sexual conflict and female counter-adaptations. These anatomical variations highlight how evolutionary pressures drive the divergence of the physical mechanism of ejaculation to ensure reproductive success.