Dots in spots: butterfly eyespots II. Tinbergen and the Grayling.

In a classic study, the pioneering Dutch ethologist, Niko Tinbergen, and his students famously studied the courtship behaviour of the Grayling butterfly, Hipparchia (=Satyrus) semele, at Hulshorst – a sandy site now located in the De Hoge Veluwe National Park, in the province of Gelderland, in The Netherlands. These studies were carried out for four consecutive seasons I believe, in the summers of 1938 through 1942.

Tinbergen and his students were studying what was then called ‘releasing mechanisms’, stereotypical responses to stimuli. They used differently coloured, sized and shaped butterfly models at the end of a rod and line to investigate what most attracted the males. Shape was shown to be of little consequence, but size mattered: larger than normal models were followed more often. Movement was especially important, with a dancing or tumbling motion eliciting the greatest response. 

The Grayling is a so-called ‘lateral basker’ – holding its body at right-angles to the sun to warm up – and rarely opening its wings other than during courtship attempts.

Perhaps the most famous discovery from this period, was the characteristic ‘bowing’ movement which the male Grayling butterfly performs directly in front of the female. I do not have photographs of this behaviour in Graylings (yet!), but I have photographed it in the Rock Grayling. Tinbergen described it as follows: the male ‘captures the female’s antennae between his forewings and brings them into contact with the scent glands on the top of his forewings’. In Tinbergen’s terminology, this courtship bowing behaviour acts as ‘the releaser’ which is  necessary to ‘secure the female’s cooperation in coition.’ We now know that this movement brings together the androconial organs – which produce a male pheromone – with the chemo-receptors on the female’s antennae, stimulating her and perhaps allowing her to assess the male’s quality and decide whether she wishes to mate with him.

Tinbergen famously called the Grayling, ‘bark with wings’ and it is remarkable how well camouflaged they are, especially when resting on trees (see above and below).

Tinbergen also observed the so-called ‘startle’ response in Graylings, when the forewings are suddenly lifted, i.e. flicked upwards, to reveal the apical eyespot, or ocellus (below). This is a defensive move, probably to startle, frighten or at least distract, would-be predators. Sometimes both apical and anal eyespots are revealed by this motion (see photos are the start and end of this blog). The forewings are lowered, and the spot is hidden when the butterfly feels that it is not threatened and settles down.

The sudden appearance of the eye-like eyespot – with its white pupil (below) – breaks up the camouflage and presumably has a starling effect for this reason. Tinbergen observed that smaller animals, like flies and wasps, are driven away or warded off by wing fluttering. Females also use wing fluttering to ward off males (see below).

Interestingly, Tinbergen (1972 reference) remarked that the ‘eye spots in the female are also much blacker and their centres more striking than those of the males’. This was very prescient, because these butterflies are quite variable, and such differences are not always obvious to the naked eye. Indeed, it has only very recently been determined scientifically that there are such subtle differences in the sizes of the eyespots – including the white pupil, black spot and halo – between the sexes (Dapporto et al., 2018). Females have disproportionately larger wing eyespots than males, perhaps because, being larger – and hence more desirable to a predator! – they need to produce a bigger startle response in order to deter their attacker!

The following three photographs, taken rather late in the season (14th September) in NE Spain, show a courtship attempt by a male, which was refused by the female. She had probably already mated. In the first photo (below) the larger female is seen clearly in focus, whilst the male is approaching from the bottom left.

In the second image (below) the male has moved directly behind the female. She is fluttering her wings, which I interpret as her being  unreceptive; the Grayling way of saying ‘no’!

In the third image (below), the female has flown off and landed on some heather flowers. The male continued to pursue her however, and can be seen just behind her on the right.

It is not always easy to capture courtship behaviour; you have to be there are the right time, early in the season, and have a bit of luck. It is much easier to photograph stationary butterflies, like males sitting on their territories and flicking their wings. Males also expose their forewing eyespots when they settle down, and in my experience they then sit there with their wings folded, apparently unconcerned by a large primate creeping up on them with a camera!

The sort of experiments carried out by Niko Tinbergen in the heathlands, sand dunes, and woodlands of what is now Hoge Veluwe National Park in the centre of Holland, are probably not the sort of research which would get funded today. Yet there is so much we do not know about the habits and behaviour of butterfly species such as this, many of which are threatened and declining in number. I think the example of Tinbergen and his young students, studying these butterflies as the war went on around them (The Netherlands were invaded by Germany in May 1940), is a rather heroic effort. Particularly, as Tinbergen was picked up on 9 September 1940 – whilst he was staying at Hulhorst with his family – and interred in a prison camp by the Nazis.

Links

http://www.zonacharrua.com/butterflies/Britain%20-%20Hipparchia%20semele.htm

https://www.ukbutterflies.co.uk/species.php?species=semele

References

Dapporto, L., Hardy, P. B., & Dennis, R. L. (2018). Evidence for adaptive constraints on size of marginal wing spots in the grayling butterfly, Hipparchia semele. Biological Journal of the Linnean Society, 126(1), 131-145.

Tinbergen N (1941) Ethologische Beobachtungen am Samtfalter, Satyrus semele L. Journal of Fuer Ornithologie 89(3):133–144.

Tinbergen N., Meeuse B.J.D., Boerema L.K. & Varossieau W. (1972): The courtship of the Grayling Eumenis (= Satyrus) semele. (L.) (1942). In Tinbergen N. (ed.): The Animal in its World. Allen & Unwin, London, pp. 197–249.