Dr Karl explains sex-reversal in the animal world
To begin with, consider five bird species commonly found in Queensland: the crested pigeon, rainbow lorikeet, scaly-breasted lorikeet, laughing kookaburra and – of course, my favourite – the magpie.
Scientists examined some 500 carcasses (sorry, dead bodies) of these birds and found that, depending
on the species, some 3–6 per cent showed ‘sex reversal’.
What is sex reversal, you ask? Well, it’s a seeming mismatch between an animal’s sex chromosomes, and the sexual characteristics of its outward appearance and gonads (testes or ovaries). For example, a backyard rooster with its bright comb on top of its head looks very different from a female chicken.
So, back to the study of the 500 dead Queensland birds. Of the 3–6 per cent of sex-reversed Australian birds, most were female according to their chromosomes but had male gonads. The tiny remainder were male according to their chromosomes but had female gonads.
One genetically male kookaburra not only had an oviduct, but that oviduct was distended, indicating the kookaburra had recently laid an egg.
What makes this possible? Well, in humans it isn’t just the XX/XY genes carried on the chromosomes that determine gonads and outward appearances. In humans, a person with XY chromosomes usually has male external characteristics.
But male development depends on the SRY gene carried on the Y chromosome. If a person does have XY chromosomes, but lacks the SRY gene, they will externally look like a human female.
Not all animals have the same sex chromosomes as humans. And sometimes an animal’s sex can be determined by environmental factors such as temperature.
It can get even more complicated.
In some species, different cells in the same animal can sometimes have different sex chromosomes. This can lead to ‘gynandromorphs’ – organisms that show both male and female external characteristics.
How’s that for mixing things up!
