Most organisms want to produce as many offspring as possible, and make sure that these offspring have the best chances of survival. In this struggle, males and females of various species have evolved various strategies to optimise the chances of their genes passing on. The competition is with the opposite gender, as well as other adults. However, it is not always so binary. At times a species may be made up entirely of females, and at other times the animal might be a male and female at once. This makes for some complicated relationships.
Parthenogenesis is a process of reproduction where the embryo grows directly from an unfertilised egg. It is a form of asexual reproduction, where the species is entirely made up of females only.
Perhaps the most intensely studied, and consequently the most well known example of a species that reproduces exclusively through parthenogenesis is the New Mexico whiptail lizard, or the Cnemidophorus neomexicanus. There are fifteen species in the genus Cnemidophorus that reproduce through parthenogenesis. The populations of the lizards are made up entirely of females, with very little genetic diversity within a particular lineage. Even though the species are entirely made up of females, the animals continue to exhibit sexual behavior. The function is to stimulate the right hormones to optimise the process of reproduction. To do this, some females take on the role of males, and mount the females.
A few species of crabs, spiders and snails are also known to reproduce through parthenogenesis. There are some creatures that normally reproduce sexually, such as komodo dragons, boas or pythons, but have found to reproduce to parthenogenesis. Such cases are considered accidental parthenogenesis. There are some labs around the world experimenting in artificially inducing parthenogenesis in mammals. Some experiments involving rabbits and rats have been successful. However, parthenogenesis in the wild has not been observed with any mammalian species.
Not all organisms are sexually differentiated. Some creatures have two sets of functional sex organs, one male and one female. These kinds of creatures are known as hermaphrodites. Some creatures may also change their genders depending on the conditions. These creatures are known as sequential hermaphrodites. There are two ways hermaphrodites can mate. Bilateral sperm transfer means that both the creatures exchange the sperm required to reproduce. In unilateral sperm transfer, one creature inseminates the other. The process of reproduction can get violent.
Banana slugs have disproportionately large male sex organs. One of the species, Ariolimax dolichophallus, literally translates to “banana slug long penis”. Despite the slugs having both sex organs, researchers noticed that an unusually large number of the animals were missing the penis. This is because after mating, the banana slugs have a tendency of biting off the penis of their partners. This is to reduce the competition, and a strategy to maximise the number of progeny for a single individual.
Some species of marine flatworms engage in a behavior known as penis fencing. Both the animals compete to be the one to inseminate the other. The organs used are sharp two headed probes, which attempt at a traumatic insemination. This is known as intradermal hypodermic insemination. Essentially, the creatures attempt to stab their partners and inject the sperm directly into the circulatory system. A mating pair can either transfer the sperm bilaterally, or unilaterally, even in the same species.
Though there are species of sexually differentiated snails, many of them are hermaphrodites. They hermaphrodite species have pointed structures of calcium and chitin known as “love darts”. These are for sexual selection, with larger love darts increasing the chances of finding a mate. During the mating, the snails anchor themselves to their partners using the love darts. This keeps them together while the mating occurs. However, there is chance that the internal organs of the snails can be damaged because of the darts. The mating can involve either bilateral or unilateral exchange of sperm.
The clown fish are perhaps the most well known example of sequential hermaphrodites. All the clownfish are born males. They live in schools with many males and a single dominant female. The female lays eggs first, which is then fertilised by biggest male. When the dominant female dies, the largest male feeds voraciously, and turns into a female. Then the cycle continues.
A number of species show parasitic mating, where the male is permanently attached to the female, with providing sperm being the only function. These kind of species show extreme sexual dimorphism, where the males are generally much smaller than the female. Unless you know exactly where to look, the male might not even be visible. At other times, the males are absorbed into the body of the females.
The most well known example of this is the Anglerfish, some species are also known as Deep Sea Devils. The males in these species have only one function in life – to find a female to mate with. They cannot hunt food or even swallow, and all their sense organs are only suitable for finding a female. If they cannot find a mate, they die. Once the males find a female, they bite into her skin, and release an enzyme. This enzyme helps them digest the outer skin of the female, and their own mouths. After this process, the male is permanently fused to the side of the female, sharing a common circulatory system which provides the nutrients. Encounters with the same species in the deep sea is relatively rare, which is why these species have adopted this particular strategy for reproduction. In some species, the body of the male deteriorates with only the gonads left functional. Some Anglerfish species have only one parasitic partner, while females in other species may host up to eight parasitic males at once.
The Osedax, also known by the very cool name of zombie worms were discovered as recently as 2002. They get their name because the feed on the bones of whale carcasses. There are entire harems of tiny males enclosed within a gelatinous tubes of the females. Each female can harbour hundreds of males. The reproduction is continuous, giving rise to a large number of species in the genus. The Osedax might have been using this strategy for millions of years, considering the marks associated with the genus found on the whales of bones have also been found on plesiosaurs skeletons.
The Bonellia viridis or the green spoonworm is another example of parasitic dwarfism. The marine worms start off life as primitive organisms without a gender. If a larvae touches the sea floor, it turns into a female and releases a toxin known as bonellin. The larvae that subsequently come into contact with the toxin, all turn into males. The males then consumed by the female through a feeding tube. They spend the remainder of their lives inside the genital sac, providing the sperm as required.
The detachable penis
The argonaut, or the paper nautilus is a kind of octopus that swims to jet propulsion and has a shell. There is extreme sexual dimorphism seen in these animals, with the females having bodies as large as 10 cm, and shells as large as 30 cm. The males do not grow bigger than 2 cm. The males have a special arm, known as the hectocotylus, which contains the sperm. It detaches from the body of the male, swims by itself to the female, and deposits itself into her pallial sac, or an egg chamber. It resides there and provides the sperm needed to fertilise the eggs. A single female can host a hectocotylus from multiple males. After dispatching the “arm”, the male argonaut dies. However, the female can continue to reproduce multiple times.
A species of sea slug known as the Chromodoris reticulata, has inversed the strategy adopted by the banana slugs. The species is made up of hermaphrodites, and after mating, they discard their own penises. These creatures cannot mate for another 24 hours, after which, they have a new penis. Coiled up within the body, are two stand by penises to take the place of the ditched organ. Researchers believe the sea slugs do this, to prevent accidentally inseminating a mate with the sperm of a rival. There are backwards pointing spines on the penis, that can trap the sperm of a rival. There is a potential for these sperm to fertilise the egg on a subsequent mating, which is why the particular species has found it viable to cast off its penis.
Nephilengys malabarensis, also known as the orb-web spider is a species that displays sexual cannibalism. The female eats up the male after mating. During the mating, the male spider deposits its palp, or spider penis, into the genital opening of the female. The palp continues to inseminate the female even after being detached. It also acts as a plug, preventing other rival males from impregnating the female spider. If it survives the attempt to be eaten, the spider continues to fend off rivals by fighting them. With its palp gone, the “eunuch males” are lighter, faster and more energetic than their rivals, who still have their palps with them. Studies have demonstrated that the eunuch spiders stand a better chance at winning a fight against rivals.