Thanatosis, or apparent death, is when an animal pretends to be dead! Lizards do it, opossums do it (‘playing possum’) and butterflies do it! The idea is to fool the pursuer into thinking you are dead, and hope that they will then leave you alone! Female butterflies sometimes do it to avoid the unwanted attentions of courting males, as described here.
In other words, thanatosis can be a rather drastic mate-refusal tactic. It is adopted by some satyrine butterflies, like the speckled wood, Pararge aegeria, where the females reject mating by ‘playing possum’, i.e. feigning death (Cannon, 2019). They just close their wings and pretend to be dead. I was happy to come across this behaviour whilst walking along a path that runs along the base of the walls surrounding Scarborough castle (below).
There were lots of male Speckled woods (Pararge aegeria) perching and patrolling the paths (actually the other side of the castle shown in the above photo) in what must have been a series of territories. The paths provide ideal sites for these butterflies. They can bask on the gravel; perch on the vegetation and have a great line of sight along the path.
At this time of year (late September) the Speckled Woods tend to be darker than those which appear earlier in the year.
Note the V-shaped posture of the wings (below), called reflectance basking: this wing-opening behaviour is thought to be a way that butterflies use their wings as mirrors to reflect the solar radiation striking the wings onto the body. But see Heinrich (19990) for a critique of this theory!
One particular male had commandeered an excellent site containing a large quantity of ivy, a late flowering plant (below).
Otherwise, it was very similar to other territories along the path.
In the following sequence, the male approaches the female from behind. She then turns so she is at right angles to the male, and starts to lean over slightly!
The female then flipped over into an upside down position (below). Thanatosis is traditionally thought to be a last resort, when other mate-rejection behaviours, such as abdomen raising or wing fluttering/closing, have failed to stop the attentions of a male. None of these behaviours occurred in this interaction.
The male stood above the female for a short while (below).
The male then flew off and left the female adopting the upside down position.
This behaviour usually only occurs with extreme male persistence and is the final part of a mate-rejection behavioural sequence. (Shreeve et al., 2000). The female had probably already mated – satyrine butterflies are essentially monandrous – and did not want to mate again.
When a refusing female plays possum it will remain quiescent after the male leaves before resuming its activities (Shreeve et al., 2000). This one stayed in this position for some time. I gave up before she did!
The male moved to a perch about 1m away.
Male and female Speckled Woods are very similar and I am making some hopefully (!) intelligent guesses about this behaviour. Would welcome any comments.
Cannon, R. J. (2019). Courtship and Mating in Butterflies. CABI. (https://books.google.co.uk/books/about/Courtship_and_Mating_in_Butterflies.html?id=XwHyxgEACAAJ&redir_esc=y)
Heinrich, B. (1990). Is ‘reflectance ’basking real?. Journal of Experimental Biology, 154(1), 31-43.
Shreeve, T.G., Dennis, R.L. and Wakeham-Dawson, A. (2000) Phylogenetic, habitat, and behavioural aspects of possum behaviour in European Lepidoptera. Journal of Research on the Lepidoptera, 39, 80–85.