Gynandromorphs are animals with male characteristics in one half of their body and female characteristics in the other unlike hermaphrodites which also develop genitals of both sexes but show generally no other changes. Birds, insects and crustaceans are currently the only types of animals where gynandromorphism has been discovered and it is an extremely rare phenomenon.
In insects and birds, this is likely to be the result of an egg fertilised by two sperm, one with male, one with female chromosomes and on cell division forms an animal that is literally half male, half female. This is not just displayed in their genitalia but their plumage, markings, wing and body shape, and even their brains.
For birds another explanation exists with the case of a chimera where two embryos start to develop but fuse into one.
In crustaceans, however, the cause for gynandromorphism is unknown. The sex in crustaceans is hormonally determined and can vary due to environmental chemicals such as pesticides but is also subject to temperature. This means that a change in the environment of the crustacean could lead to the formation of a gynandromorph.
In May 2005, a gynandromorph blue crab was found which showed the male characteristics of a blue claw tip and pointed underside “apron” on one side and the female red claw tip and rounded underside.
The crab’s special genetic makeup helped researchers better understand the sexual development and breeding behaviours of blue crabs.
In September 2020, a group of researchers came across an unusual looking bird while they were out capturing and banding birds. It was a rose-breasted grosbeak, but it showed the tell-tale signs of a male bird on the right side and female plumage on the left side. The bird was in its non-breeding plumage so the differences will be even more visible come spring when the colours become more vibrant. Annie Lindsay, the scientist who made the discovery had only come across another gynandromorph once before which was 15 years ago.
It is unknown whether these very special birds behave more like males or females as they are extremely rare, so animal behaviour studies are extremely difficult. For this reason, we also don’t know yet whether gynandromorph birds are able to reproduce. After being able to observe a gynandromorph cardinal for 40 days between 2008 and 2010, it was documented that the bird didn’t have a mate and was also unable to sing but it wasn’t subjected to any antagonistic behaviour from other birds.
In Pennsylvania, another gynandromorph cardinal was spotted in 2019 which, again, showed male red feathers on his right side and tan coloured ones on his left. Depending on the reason for the gynandromorphism in this bird, it might also have a half male, half female brain. In this case it is unlikely that the bird would be able to sing (a solely male trait). Another interesting theory arises, as a pure male cardinal tried to court the gynandromorph bird. Gynandromorphs are usually infertile but there could be exceptions. As this particular individual’s left side is female and only the left ovary in birds is functional, offspring might be possible. We will have to wait and see.
A rare gynandromorph zebra finch was also able to shed light on whether sex differences are purely the result of hormone secretions during neonatal life or whether sex chromosome genes acting within cells could also contribute to these differences in cell function. After testing it became apparent that sexual differentiation is controlled by both.
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