External Fertilization
External fertilization is beneficial as the mother does not have to carry her developing offspring. She lays her brood at one time, this could be hundreds to millions of eggs. This is also dangerous because the
developing offspring are constantly at risk to predators.
Gametes are released in different locations.
Released into the water they are very vulnerable.
Buried in holes or hidden in other places, on land.
No guarding but the risks are very high.
Released into a nest, caver, or on a plant where they can be guarded.
Released onto a parent where they attach to their bodies externally.
Eggs that are collected after spawning and carried internally, until they hatch.
Guarding, Requires More From Parents
Internal Fertilization
the gametes meet inside the female's body. This is called internal
fertilization. All land dwellers need to fertilize this way, but that is not
actually true, ducks are known to mate in the water. Some times the
female duck in drowned during mating. It's a safe environment fo
fertilization. In some species, the females can store the sperm until the ideal conditions are present. e.g. the availability of food.
In some species the males directly insert sperm into the female by using important external sex organs. In other species, the males release sperm by abutting against the female, e.g. chickens In another indirect type of internal fertilization, salamanders and scorpions deposit jelly-like sperm packets into the environment rather than into/onto the female. She then straddles these packets to allow sperm to get inside to her eggs.
Male Sex Organs
Mostly the sex organ is the penis, The penis enters into a female's orifice and deposit sperm. The penis swells with blood making it sturdier and easier to insert into the female.
Some lizards and snakes have a pair of hemipenes instead of a single penis. The hemipenis turns inside out like a finger on a glove to extend outside the body. Some types of spiders and squids deposit their sperm outside the body and use their antennaes and tentacles to pick it up and place it inside the female.
Mostly the sex organ is the penis, The penis enters into a female's orifice and deposit sperm. The penis swells with blood making it sturdier and easier to insert into the female.
Some lizards and snakes have a pair of hemipenes instead of a single penis. The hemipenis turns inside out like a finger on a glove to extend outside the body. Some types of spiders and squids deposit their sperm outside the body and use their antennaes and tentacles to pick it up and place it inside the female.
Female Sex Organs.
Mostly the female primary orifice is either a vagina or a cloaca. The vagina in mammals are used specifically for reproduction. The cloaca is found in birds, reptiles, amphibians, and monotremes. It is used for reproduction as well as excretion of waste. In some birds, both the male and female each have a cloaca, which they press against each other for transfer of sperm.
Mostly the female primary orifice is either a vagina or a cloaca. The vagina in mammals are used specifically for reproduction. The cloaca is found in birds, reptiles, amphibians, and monotremes. It is used for reproduction as well as excretion of waste. In some birds, both the male and female each have a cloaca, which they press against each other for transfer of sperm.
Copulation
When these accessory organs are joined it is called copulationor coitus. There is usually a puzzle-piece fit between a male's and a female's sex organs. The penis is inserted into the vagina and the sperm are ejaculated inside.
The puzzle-piece exception is the banana slug which has a penis that is sometimes the entire length of his body. If he cannot find a mate large enough to accommodate it, then she will chew off the end.
Even worse are bedbugs; they fertilize the female by stabbing a hole right in her abdomen. The sperm are injected directly into the body cavity. No correct fit needed.
Since copulation is not always easy, both females and males may try to escape before completion. To increase success, many species have built-in ways to prevent early separation.
In many reptiles and fish the male may have hooks or barbs on or near his penis (or hemipenis) that attach to the female during copulation. Sharks have been known to severely bite females to hold them during copulation (and these are not even vampire sharks). Dog penises also swell during copulation and are difficult to remove until completion.
When these accessory organs are joined it is called copulationor coitus. There is usually a puzzle-piece fit between a male's and a female's sex organs. The penis is inserted into the vagina and the sperm are ejaculated inside.
The puzzle-piece exception is the banana slug which has a penis that is sometimes the entire length of his body. If he cannot find a mate large enough to accommodate it, then she will chew off the end.
Even worse are bedbugs; they fertilize the female by stabbing a hole right in her abdomen. The sperm are injected directly into the body cavity. No correct fit needed.
Since copulation is not always easy, both females and males may try to escape before completion. To increase success, many species have built-in ways to prevent early separation.
In many reptiles and fish the male may have hooks or barbs on or near his penis (or hemipenis) that attach to the female during copulation. Sharks have been known to severely bite females to hold them during copulation (and these are not even vampire sharks). Dog penises also swell during copulation and are difficult to remove until completion.
Special Copulation Situations
There are also some animals that reproduce via parthenogenesis. Their offspring develop from an unfertilized egg (essentially, a virgin birth). For example, whiptail lizards are all female. However, they still like to perform copulation to stimulate egg development. They climb on each other and pretend they are copulating although no coitus actually takes place.
Honeybees also sometimes practice parthenogenesis. The queen bee can produce male honeybees (called drones) without fertilization. Female bees are all diploid and are produced through fertilization. Only the males are produced without fertilization.
Hermaphrodites also have to come together to trade gametes even though each possesses male and female parts. Sometimes there is a cross fertilization and both partners are fertilized. Other times, one partner takes the male role and the other takes the female role (depending on who remembered their wig).
Flatworms have a special way to determine who the "female" is. They duel with their erect penises until one gets stabbed. The loser is now the "female," but she shouldn’t worry. She is getting inseminated with genes which are stronger than her own. There are actually plenty of miraculous virgin births in the world if you look around.
Sperm Meets Egg
There are also some animals that reproduce via parthenogenesis. Their offspring develop from an unfertilized egg (essentially, a virgin birth). For example, whiptail lizards are all female. However, they still like to perform copulation to stimulate egg development. They climb on each other and pretend they are copulating although no coitus actually takes place.
Honeybees also sometimes practice parthenogenesis. The queen bee can produce male honeybees (called drones) without fertilization. Female bees are all diploid and are produced through fertilization. Only the males are produced without fertilization.
Hermaphrodites also have to come together to trade gametes even though each possesses male and female parts. Sometimes there is a cross fertilization and both partners are fertilized. Other times, one partner takes the male role and the other takes the female role (depending on who remembered their wig).
Flatworms have a special way to determine who the "female" is. They duel with their erect penises until one gets stabbed. The loser is now the "female," but she shouldn’t worry. She is getting inseminated with genes which are stronger than her own. There are actually plenty of miraculous virgin births in the world if you look around.
Sperm Meets Egg
Once a sperm is released it sometimes has a long swim before it actually reaches the egg. Human sperm ejaculations can have up to 300 million sperm, and there can be only one winner. For the lucky first one there, there is a sequence of basic events that generally must happen during fertilization.
1) Sperm must get through any protective layer that the egg has.
2) The egg and sperm must pass the "same-species" test. This is extra important for external fertilization
1) Sperm must get through any protective layer that the egg has.
2) The egg and sperm must pass the "same-species" test. This is extra important for external fertilization
since many types of sperm and eggs could be in the same area.
3) There are also some defensive moves to prevent other sperm from entering, called blocks to polyspermy.
4) The nuclei fuse to create a diploid zygote.
Detailed Fertilization Process
3) There are also some defensive moves to prevent other sperm from entering, called blocks to polyspermy.
4) The nuclei fuse to create a diploid zygote.
Detailed Fertilization Process
Sea Urchins
The process of fertilization has been well studied in sea urchins. When a sperm reaches the egg, the acrosome releases its enzymes and they eat through the outer egg layer. Once through, the head of the sperm elongates like a skinny finger pointing towards the egg. Next, a special species-specific protein on the acrosome tries to match with a receptor on the egg. If they match, then the sperm is engulfed into the egg for nuclei fusion.
Blocks of Polyspermy
The process of fertilization has been well studied in sea urchins. When a sperm reaches the egg, the acrosome releases its enzymes and they eat through the outer egg layer. Once through, the head of the sperm elongates like a skinny finger pointing towards the egg. Next, a special species-specific protein on the acrosome tries to match with a receptor on the egg. If they match, then the sperm is engulfed into the egg for nuclei fusion.
Blocks of Polyspermy
Polyspermy means more than one sperm, and a block to polyspermy prevents the egg from being fertilized by more than one sperm. If two get in, it will become a triploid cell and will not be viable. In sea urchins, there is a fast electrical block to polyspermy that occurs immediately when the sperm enters the egg.
Sea urchins and humans also have a slow block that occurs a minute later. A release of calcium causes a physical barrier to appear or the zona pellucida to disappear. Both blocks prevent more sperm from entering.
Similar to a polyspermy block, males of some species have developed "otherspermy" blocks. These tactics prevent other suitors from copulating after they have finished. In mice, the excess sperm develops into a hard crusty plug that physically blocks other males from copulating. Trust us, it is as icky as it sounds. Some male insects even try to thwart previous suitors. They have special penises that are shaped to scoop any other sperm out of the female to better their own chances.
Sea urchins and humans also have a slow block that occurs a minute later. A release of calcium causes a physical barrier to appear or the zona pellucida to disappear. Both blocks prevent more sperm from entering.
Similar to a polyspermy block, males of some species have developed "otherspermy" blocks. These tactics prevent other suitors from copulating after they have finished. In mice, the excess sperm develops into a hard crusty plug that physically blocks other males from copulating. Trust us, it is as icky as it sounds. Some male insects even try to thwart previous suitors. They have special penises that are shaped to scoop any other sperm out of the female to better their own chances.
Multiple Offspring
In animals with external fertilization, multiple eggs are released, which can be
fertilized by one or several different males. In internal fertilization, there can be multiple offspring if multiple eggs were fertilized. These offspring have the same father or they can have different fathers if the mother mated with more than one male during estrus. For example, a litter of puppies could be fathered by one or more fathers.
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