Sexual reproduction involves sperm fertilizing eggs, producing zygotes that become embryos. There are many ways ofachieving fertilization. Plants do it the help of pollen-carrying animals and wind (or water) currents, or sometimes just by dumping self-pollen on the receptive surface in the same flower.
Animals exhibit an astounding array of getting sperm and egg together. The seemingly simplest technique is broad-cast spawning; male and female cast their spawn into the water currents and hope for the best (in some ways, rather similar towind-pollinated plants). More spatially focused, salmon spawn next to each other, casting sperm and eggs into the water near a nest. There is no physical sexual contact. In such cases, fertilization is external, in the environment.
There are many ways of achieving internal fertilization, inside a female. Various insects and salamanders use no-contact sex, often making females do the work: males deposit bags of sperm on the outside of their bodies or on the substrate, and females come by to pick them up. Most centipede females pick up sperm bags deposited by males, but in one lineage the males put sperm into a silken net held by the female, who then takes them in.
Sexual contact of some sort comes in a fantastic array of arrangements, and it can be fun to imagine how they all happened to evolve. Here is a sampling:
We all know how males of humans and other mammals deliver sperm. A male’s intromittent (“to place inside”) organ (a.k.a., a penis) places sperm inside a female’s reproductive tract. Some other animals also deliver sperm this way, although the intromittent structures often have different names. Sharks have a pair of claspers near the genital opening that are inserted in a female to guide sperm delivery. Various invertebrates transfer sperm using their legs or mandibles; octopuses use a special arm to insert sperm to a female mantle cavity. Some nematode worms have cuticular spines that are inserted and guide sperm into the female’s genital opening. Male bedbugs deposit sperm directly through the female body wall (never mind the niceties of genital openings), where there is a special internal organ from which the sperm migrate to the ovaries; and there the eggs are fertilized as they are released.
Most reptiles have intromittent organs, in some cases (turtles) a penis but in lizards and snakes there are two ‘hemipenises’, although only one is used at a time. Most birds mate by pressing the genital opening of a male to that of a female. But a few birds, such as ostriches, emus, and ducks, have intromittent organs (penises). When extended, some these are longer than the body of the male.
Dragonflies and damselflies have an apparently unique arrangement. Males store their sperm in one of the first abdominal segments; using a pair of clasping appendages at the end of the abdomen, they grab a ready female behind her head. The female then circles her own abdomen under the grabbing male to bring her genital opening in contact with the segment where sperm were stored. Males have an intromittent organ there, which transfers his sperm (and in some cases removes the sperm of previous matings!).
If all of that is not sufficiently astonishing, consider the fishes; there are more kinds of fishes than all the other vertebrates collectively. And they have some surprising ways of delivering sperm; here are a few examples:
–in some African cichlids, a female approaches a male, who releases sperm that she sucks up to fertilize the eggs she holds in her mouth.
–certain armored catfish go one step farther, females sucking up sperm directly from males’ genital openings and passing them very quickly and viably through the digestive tract. She releases them onto a bunch of eggs that she holds on her pelvic fins.
–male guppies, swordtails, and mollies possess an intromittent organ called a gonopodium. It’s a modified anal fin, long enough to be waved around to attract attention and inserted into a female genital opening to deliver sperm.
–the priapium fishes of southeast Asia bear their genital parts(along with the end of the digestive tract) under the chin (both male and female). The male intromittent organ or priapium (named for Priapus, the ancient Greek god of fertility) is derived from the pelvic girdle and fins. It is bony and muscular and faces rear-ward; it bears a serrated hook, presumably for holding on to a female while sperm are inserted. Reportedly, these little fish mate head-to-head but at an angle, because the priapium is asymmetrical, tending to lean either left or right.
–European bitterling females insert their eggs into a mussel body cavity, using the mussel’s exhalant siphon. Males put their sperm through the mussel’s inhalant siphon, and fertilization takes place inside the mussel. Little bitterlings are brooded by the mussel.
–seahorse and pipefish males hold their sperm in an abdominal pouch. Females do the approaching: they deposit their eggs in the pouch, where they are fertilized