It is a lottery whether your offspring end up being right-handed or left-handed, but there is no such ambiguity when it comes to some species of opossums. For them it is a question of sex – the boys are right-handed while the girls end up as ciotógs.
There seems no logic to the balanced share-out in these small animals that live in the forests of Australia, Tasmaina, Indonesia and New Guinea. In humans the right-handers outnumber southpaws by nine-to-one.
Being "handed" one way or the other is nothing new at all however, with many animals showing a clear preference for one over the other whether they are sporting hands, hooves or paws. It is also known that distinct sex differences in handedness in some placental species – those that mature in the female's womb before birth – seem to favour left-handed males and right-handed females.
A research team from St Petersburg State University led by Dr Yegor Malashichev decided to look at preferences in marsupials, animals that are born relatively undeveloped and then mature in a pouch, such as kangaroos and koalas.
They looked at "sugar gliders", Petaurus breviceps, and the grey short-tailed opossum Monodelphis domestica, finding that the split between right and left was strongly sex-oriented. The greys showed a strong preference to the right, with sugar gliders a mix between right and left.
There was no question with the females of both species however will a near universal preference for the left. The preference became more pronounced as the tasks used to separate the ciotógs from the non-ciotógs became more challenging.
The researchers put the strong sex-based preference down to how the animals' brains are wired. "Marsupials do not have a corpus callosum, which connects the two halves of the mammalian brain together," Dr Malashichev said.
The reversed preference in males versus females was down to how their brains wired themselves in the absence of the corpus callosum, he said.
Happily for the leftie females they don't have to struggle with right-handed tin openers or scissors that just don't work when used with the left hand. Not much need for such things if you are living up a tree.
