Depending on your point of view, it is one of life’s great questions.
How does a tortoise that has flipped onto its back, get up again?
It’s not a rhetorical question, and it goes beyond being a metaphorical or metaphysical query, or a subject for drunken debate.
For a tortoise it is a deadly serious matter; being able to right itself counts as one of life’s epic struggles, a potential matter of life and death.
Now scientists have investigated this struggle in detail, determining if and how tortoises have evolved to do it.
Dr Ana Golubović at the University of Belgrade, Serbia and colleagues studied the slow-motion thrashings of an inverted chelonian – in particular the Hermann's tortoise, to see how the shape of its shell impacts its ability to rise again.
Armoured animals can easily lose their balance and fall on their back, where they are vulnerable to exposure, starvation and predation.
Male tortoises actively attempt to flip rivals onto their backs
Tortoises are particularly susceptible, being unable to flip themselves by twisting their bodies inside their shells.
While researchers have long thought that the height and length of a tortoise’s shell may impact righting ability, no one had tested the effect of shell geometry on live animals.
Hermann’s tortoises are medium-sized tortoises that live in the Mediterranean. Female are generally larger than males.
Dr Golubović and her colleagues analysed 118 Hermann's tortoises (54 females and 64 males), placing each on its back and then measuring how much time they spent furiously waving their heads, legs and tail in a bid to recover. They then compared this performance with the geometry of the tortoise’s shell.
They report the results in Zoologischer Anzeiger - A Journal of Comparative Zoology.
The analysis needed to be far more comprehensive than first supposed; taking into account various shell measurements, as well as the body temperatures of the animals. Tortoises are cold-blooded ectotherms, so those with lower body temperatures at any given time might have struggled to have the energy to flip themselves, a variable that had to be eliminated.
The scientists found, perhaps unsurprisingly, that tortoises with more curvaceous shells, rather than flat shells, were more able to right themselves.
However, they also discovered a significant relationship between carapace size and righting performance; with bigger tortoises struggling to right themselves more than smaller tortoises, a phenomenon far more pronounced in males than females.
That suggests that tortoises face a trade off.
Generally speaking, bigger animals fare better than smaller ones. But if Hermann’s tortoises grow too big, they run the risk of being stranded upside down.
Female tortoises grow larger than males, perhaps because the bigger they are, the more viable offspring they can produce – a benefit that outweighs the risk of not being able to get up when looking at the world the wrong way.
Male tortoises face a different dilemma.
Smaller males are more agile, and their greater mobility may allow them to find and mate with more females.
But male tortoises also like to fight each other, by actively attempting to flip their rivals onto their backs.
Here, large size is beneficial, as larger tortoises are likely to win fights.
But if they lose, the new study suggests, larger males may literally be left flat on their backs.