Male or female? Sex identity among animals is less cut and dry than you might think.
As the story goes, Hermaphroditus was the son of the Greek gods Hermes and Aphrodite. An exceptionally handsome boy, he encountered the nymph Salmacis in her pool, and for some reason declined her romantic overtures. She jumped him and beseeched the gods to never let them part. The gods, being a literal bunch, merged their bodies into one being, both male and female.
The name “hermaphrodite” is now used to describe organisms that have the gonads, or reproductive organs, of both sexes. However, nature is, as usual, a bit more imaginative than we are, so this condition comes in a variety of flavors.
First, there are species where hermaphroditism is the natural state. Most aquatic invertebrates are simultaneous hermaphrodites, meaning they have both male and female sexual organs. Although a lot of them can self-fertilize, they prefer to find a mate because sexual reproduction tends to produce more genetically diverse (and therefore stronger) offspring — and, presumably, because they get lonely.
A pair of hermaphroditic sea slugs getting hot and heavy. Each slug inserts its translucent penis (p) and penile stylets (s) into the body of the other. Photo credit: Johanna Werminghausen/Wikipedia (CC BY-SA 3.0)
But when both partners can take on the role of either sex, how do they decide who’s who? “Penis fencing” is one solution. Sea slugs and marine flatworms wield their barbed and hooked penile appendages in a fight to insemination, rather than to the death. It’s a heated dispute because females get the short end of the stick, having to use far more energy to bear offspring than males need to produce sperm. One species of sea slugs, in particular, likes to engage in rough play, or “cephalo-traumatic secretion transfer.” The winner of the duel injects his seminal fluid directly into the loser’s forehead, perhaps in order to manipulate her nervous system into rejecting subsequent mating attempts.
En garde! Two flatworms (Pseudobiceros bedfordi) prepare for penis fencing. Photo credit: Nichol Michiels/ Wikipedia (CC BY 2.5)
There are also hermaphroditic species that only have one set of gonads at a time, but switch between them throughout their lives. The switch occurs whenever the balance between reproductive costs and benefits shifts in favor of the other sex. In clownfish groups, the largest individual becomes the female because she will be able to lay the most eggs; the second largest becomes her mate, while the rest are basically eunuch servants. If the female dies, the male switches sexes to take her place, and the next largest non-breeder steps up as consort.
The lucky couple. Photo credit: Jenny from Taipei/Wikipedia (CC BY 2.0)
Hermaphroditism can also occur in species that typically stay one sex throughout their lives. Although the systems for sex determination vary slightly across different species, they generally work on the basis of each sex arising from a certain combination of sex chromosomes. If there’s a problem with cell division at any point during the formation of the egg and sperm, or after they have fused and begun developing into a new organism, some of the cells may receive the male set of sex chromosomes while the others receive the female set. These cells continue to grow and divide, resulting in a “bilateral gynandromorph,” whose body is split right down the middle, half male and half female.
This Common Blue butterfly has male coloring on the left and female coloring on the right. Photo credit: Burkhard Hinnersmann/Wikipedia (CC BY-SA 3.0)
Bilateral gynandromorphs look particularly striking when the species exhibits high sexual dimorphism — meaning the sexes have strong physical differences. In butterflies, as with many other insect species, the sex chromosomes carry genes for body color and size, which explains the dramatic appearance of butterfly gynandromorphs.
Scientists were a bit more surprised when they stumbled upon cases of bilaterally gynandromorphic birds. They had previously believed that hormones control the sexual differentiation of birds, as in mammals. If the same hormones circulate through the entire body, how could these three chickens at the University of Edinburgh end up with such a clear delineation between male and female? The only explanation is that each cell responds to particular hormones according to its individual set of sex chromosomes, like adding blue to primers of either red or yellow; depending on the primer color (or sex of the cell), the effect will be vastly different.
Sexual differentiation in mammals, on the other hand, depends almost entirely on hormonal cues. That’s why we’ll probably never see bilateral gynandromorphy in humans, even if someone has exactly half XX cells and half XY. The development of human secondary sexual characteristics, like breasts and different amounts of body hair, depends much more on which cells receive hormones like testosterone and estrogen than on the sex chromosomes present in each individual cell.
All these examples of gender-bending animals provide a refreshing reminder that mankind’s obsession with classification and labeling can hinder our understanding of the world. Nature doesn’t adhere to our silly notions of sexual identity; as long as the parts get the job done—the job being making babies—they will exist out there in whatever combinations can be dreamed up.
Facebook comments