Copulation (zoology)
Copulation in zoology refers to the various reproductive strategies animals employ to ensure that sperm and eggs are brought together for fertilization. This process is predominant in terrestrial organisms and is characterized by the use of intromittent organs, commonly known as penises, where males deposit sperm into the female reproductive system. In aquatic environments, external fertilization often occurs, with sperm and eggs released into the water, as seen in amphibians during amplexus. Some species exhibit unique mating behaviors, such as hypodermic injection in certain invertebrates, where sperm is directly injected into body tissues.
Different animals have evolved varied copulatory organs and techniques; for instance, sharks utilize modified pelvic fins called claspers, while birds and reptiles may rely on cloacal contact for sperm transfer. Copulation duration and frequency can vary greatly across species; some animals may copulate excessively during specific mating periods, while others engage in prolonged sessions to enhance reproductive success. However, copulation can also pose risks, particularly for males, as seen in honeybees and certain spider species, where mating can lead to death. Research continues to explore these complex behaviors, providing deeper insights into the diverse reproductive strategies employed by animals.
Copulation (zoology)
Animals have many strategies to ensure eggs and sperm are in close enough proximity for fertilization to take place. Copulation is a widespread process seen in many aquatic phyla and is the rule in terrestrial phyla. In most forms of copulation, the male reproductive system has an intromittent organ, often called a penis, which deposits sperm into the female reproductive system. Once there, sperm can travel the short distance to the eggs.
In some organisms, pseudocopulation is seen. For example, male bees pollinating flowers may be tricked by certain flowers masquerading as a female mate. The bee approaches the false mate, unknowingly pollinating the flower. In hermaphroditic oligochaetes, such as earthworms (Lumbricus terrestris), two worms align in opposite directions so that their genital pores are applied to the openings of the seminal receptacles of their partners. Semen released by the genital pores flows into the seminal receptacles, where it is stored. Fertilization, however, is external. The worms build cocoons where they lay their eggs and then deposit the stored sperm. Amphibian amplexus is also a form of pseudocopulation. Here, the male frog clasps the female in such a way that their cloacae are in close proximity. Sperm are not deposited in the female’s cloaca, however. Instead, sperm and eggs are released into the aquatic environment for external fertilization.
Copulatory Organs
True copulation takes many forms. In several invertebrates, such as a few flatworms (some Acoela, Rhabdocoela, and Polycladida) and the bedbugs (Cimicidae), hypodermic injection is sometimes seen. In this form of copulation, the female has no external gonopore, and the intromittent organ punctures the epidermis and deposits sperm in the underlying body tissue. This sperm must then migrate through the intercellular spaces to the female reproductive organs for fertilization of the eggs to occur. In most organisms, however, the male does not have to pierce the female’s epidermis, but instead, deposits sperm in an already-present opening of the female’s reproductive system.
In birds and some reptiles (such as the tuatara, Sphenodon punctatus), the male does not have a true intromittent organ. Instead, the male must manipulate the female during mounting so that their cloacae are pressed against each other. During this “cloacal kiss,” the male ejaculates sperm into the female’s cloaca. In some bird species, a false penis is present in the male. These organs are not connected to the ducts of the male reproductive system and thus do not serve as intromittent organs. There is speculation that they may provide necessary stimulation to the female during copulation.
In some fish, fins are modified for semen delivery. In guppies and their allies (Poeciliidae), gonopodia, modified anal fins, are used for insemination. Each gonopodium is a hollow, tubelike structure formed from the paired anal fins of the male. When mating, the male inserts his gonopodium directly into the female’s gonopore. Usually, not all the sperm is used to fertilize this batch of eggs, and the rest is stored in the oviduct walls for future fertilization. Other fish (such as the Coodeidae) have the anal fins modified into andropodia, which are cup-shaped structures that direct the flow of semen into the female without the andropodia actually entering the female’s gonopore. Sharks (Elasmobranchii) have modified pelvic fins called claspers, which the male directs into the female’s cloaca for insemination. Each shark has two claspers, and, depending on the species, either the one closer to the female or both are inserted for copulation.
Males of mammals, some reptiles, and many arthropods also have intromittent organs that deposit sperm directly into the female reproductive tract. In these copulations, by either female behavior or male manipulation, the opening of the female reproductive tract must be exposed. In many organisms, the male mounts a squatting or otherwise stationary female. Male snakes and lizards (Squamata) have two intromittent organs called hemipenes. Males and females line up side by side, and the male uses the hemipenis closer to the female to inseminate her. Many arthropods often go through intricate body contortions to bring the male’s penis in proper position for mating. This may be the common rear-mounting pattern but can also be face-to-face or tail-to-tail. In many animal species, insertion of the penis is followed by one or more thrusting movements that lead to ejaculation.
Copulatory Behaviors
Among animals, both the lengths of time per copulation and the frequencies of copulation vary widely. When a female lion comes into heat, the male will remain near her, copulating up to one hundred times a day for up to ten days. Each copulation, however, lasts for just a few seconds. Other animals may copulate only once, but the copulation may be prolonged. Canid females do not usually remain stationary for mating. To remedy this, once the male mounts, his penis becomes further engorged, and this effectively locks him to the female long enough to ejaculate even if she tries to get away. Other animals, including some species of domestic cats, primates, rodents, and bats, have hooks or barbs on their penises that may also help to lock them to a female for prolonged copulation. In some animals, prolonged copulation can last several hours. This may be a mechanism to prevent other males from fertilizing the same female. Females can also play a role in prolonging copulation. In some water mites (Arrenerus sp.), the female gonopore can be opened or closed by means of chitinous plates. The smaller male inserts his intromittent organ into the female’s gonopore, which then closes, trapping the male. Although sperm transfer is thought to occur in the first few minutes, the female may swim off, dragging the male with her for several hours.
Copulation can be dangerous to males. In the domestic honeybee (Apis mellifera), a swarm of drones pursues the unmated queen. In-air copulation occurs as a drone inserts his endophallus into the queen’s sting chamber. After ejaculation, a small part of the drone’s phallus remains inside the queen, and the drone falls to the ground and soon dies. Several more drones mate and die until the queen’s spermatheca is filled. Male spiders must be very careful when copulating. If a male does not leave the female’s web immediately after depositing his sperm, the female may envenomate and then eat him. The female praying mantis (Stagmomantis carolina) has also been known to begin feeding on the heads of males with which they are copulating. Luckily, the headless male can continue to deposit sperm.
The copulation behaviors in many species remain poorly understood, but new research methods continue to improve scientific understanding of these processes in understudied populations. For example, the Tortrix viridana, often called the green oak tortrix, European oak leafroller, or the green oak moth, was studied in the early 2020s using micro-CT scans of copulating pairs to construct a three-dimensional model. Confocal laser scanning and scanning electron microscopy were also used to better understand the internal interaction between the male and female during copulation.
Principal Terms
Amplexus: A form of pseudocopulation seen in amphibians, where the male mounts and grasps the female so that their cloacae are aligned, and eggs and sperm are released into the water in close proximity and at the same time
Cloaca: A common opening for the reproductive, urinary, and digestive systems
Heat: That part of the estrous cycle when the female is receptive to male copulatory behavior
Semen: Fluid produced by the male reproductive system that contains the sperm
Bibliography
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