Less Attractive

But More Successful in Attracting High Quality Males

Sexual dimorphism in which males and females differ from each other morphologically is widespread among animals and occurs in many groups, e.g., to name but a few: insects, spiders, fishes, birds and even mammals. It puzzled already Charles Darwin why, for instance, some male moths and male beetles frequently had much larger and seemingly better developed antennae (which during Darwin’s time had not been identified as the sensors for odours, but were seen merely as touch receptors).  He and many other researchers felt that the larger development of certain structures in male animals served as ‘advertisements of prowess and vitality’ and indicated to a female the presence of a superior sex partner. That enlargements and greater conspicuousness of structures increased the chances of being recognized (and preyed upon) could, of course, have been a handicap, which is why Zahavi suggested females chose males because they had survived and reached sexual maturity despite being more vulnerable and in greater danger of being attacked than cryptic ones.

What, according to Australian researchers Mark Elgar et al. had not fully been considered till 2018 in the discussion of sexual dimorphisms and attractiveness (at least with regard to insects in which female individuals release some odoriferous chemicals known as pheromones) was the role sensors play. Large and often plumose antennae in insects contain receptors that detect the presence of molecules in the air, i.e. chemicals released by plants, possible food items and, of course, females in case of moths and many beetles. Some of these sensors or so incredibly sensitive that only a few key molecules need to be present for the males to respond to. In case of some moths, a male can smell a female 10 km away. To maintain the sensitivity of sense organs, whether they be mechano-, photo- or chemoreceptors and to process the information received by them is energetically expensive as my former colleague Dr Simon Laughlin at Cambridge University has shown. Consequently, as has been argued, those males with the most highly sensitive sensors are more ‘valuable’ than males, whose sensors only respond to the most obvious and strongest stimulation. But how to eliminate the latter and favour the former?

Maybe some female moths and beetles that attract their males with pheromones have found a way. If a female sends out a strong pheromone signal, the latter will disperse widely and reach a huge number of possible male partners, including those that have rather insensitive “noses”, in other words do not exactly possess a highly developed sensory system. But they are not the males the females want to have as “fathers for their babies”; the females want those males that are alert to the slightest of stimulation and what better way to get their attention than to emit only a fraction of the pheromone that is so successful in reaching all kinds of males near and far? This is apparently a strategy that works, because young females which because of their age can afford to be choosy, use it, while older females that have not obtained a partner increase the amount of pheromone they emit.

Whether this idea of “less being more” can be applied to vertebrates as well has not been tested (yet). However, if we take an unbiased, objective look at our own species, aren’t we observing that it is those who are beautiful and attractive as young individuals that need less make-up, lipstick, and other beauty-enhancing stuff than physically less fortunate females, who want to become more noticeable to men through exaggerations? And isn’t the use of perfume, wrinkle-hiding cream, eye-catching jewellery increasing among older females? Perhaps there is indeed a parallel to female moths and beetles. But what about the males that the females attract? There, too, could be a parallel: the less sharply observant and somewhat superficial males do not see beyond the make-up on the skin, the red colour of the lips and the artificially enhanced signals of the female. It takes “sensitivity” and “smartness” (as in case of the male moths and beetles), for males to identify a quality female. And yet, it seems enhanced female signalling is there to stay  –  in moths, beetles and humans as well.

© Dr V.B. Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com, 2021. Unauthorized use and/or duplication of this material without express and written permission from this site’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to V.B Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com with appropriate and specific direction to the original content.

The Shapes that Animals’ Pupils Have

Not Just Little Black Holes

English is a language of ‘homonyms’ as our English, French and Russian teacher Bruno Mannewitz used to say. Many English words sound the same but mean totally different things. Mean and mean, current and current, bat and bat, bark and bark and, of course pupil and pupil come to mind. (On the other hand words that sound the same but are spelt differently like soar and sore, you and ewe, cell and sell, etc. are homophones and equally numerous). This essay, however, is about ‘pupils’, i.e., gaps in the eyes’ iris through which light passes on its way to the retina. That the size of our pupil changes and turns into a tiny circular “black hole” when the ciliary muscle (an involuntary,  smooth and not striated muscle) contracts, yes ‘contracts’ is known! But in human eyes only the pupil’s diameter changes upon an exposure to light;  in some animals, the shape of the pupil changes as well.

Among animals one can find pupils of a variety of sizes and shapes, and it has repeatedly been tried to correlate the way a pupil looks with the way an animal lives and behaves. A pupil like that of the domestic cat, which appears as a vertical slit under illumination, has been linked with small ambush predators that require good distance estimations. It can also be encountered in many birds, reptiles and sharks; not all of them, however, being small if we think of crocodiles and, for example, the lemon shark. Large predators such as tigers and lions, which like the domestic cat may hunt during the day as well as during the night, possess circular pupils like humans. Such pupils will dilate, i.e. increase in diameter at night and contract during the day, giving the predator a superior sensitivity to the dim light available at night and a better acuity, i.e. resolving power during daylight hours.

Pupils that look like horizontal slits are easily observable in sheep and goats, animals in other words that are grazers and in constant danger of being attacked by a predator. The horizontal pupil  provides an excellent field of view of the horizontal surroundings, the area in front and around the animal, but not above it (attacks are not likely to come from there). That, however, is different for creatures of the ocean that do face dangers from above and this could explain the weird often ‘W-shaped’  pupils seen in many squid or the horse-shoe or crescent-shaped pupils of sting-rays and related rhinobatid guitar fish. Given sunny flickers of light from above and dimmer more stable light conditions from below, such unusually shaped pupils are thought to even out the effects of light distortions, excluding unwanted light and shadows and providing the animal with a large visual field.

Perhaps some of the weirdest pupils can be found among some geckos that possess beaded pupils with vertically arranged wider and narrower regions in between. How to relate that to their lifestyle and behaviour is hard to understand and something else, too, is: which has been puzzling me since childhood when I kept Hyla arborea tree frogs and common toads. The latter had beautifully golden horizontal pupils, often considered to be typical of prey animals (but toads are not preyed upon by many species), while the former, when sitting on a horizontal surface, had vertical pupils, commonly seen as a sign of a small ambush predator. However, tree frogs do not always sit horizontally on a leaf but frequently cling vertically to a surface (like they did when resting on the glass walls of my terrarium). Will their vertical pupils then not be oriented horizontally?  And grazing animals like goats with normally horizontal pupils, aren’t the latter vertical when the animal grazes with its head down? Martin Banks et al. in a 2015 article in “Science Advances” have shown that when goats and other grazing animals put their heads down, their eyes rotate to maintain the pupils’ horizontal alignment!  But did the eyes of my tree frogs also rotate?  Alas, it’s too long ago; I can’t remember.

© Dr V.B. Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com, 2022. Unauthorized use and/or duplication of this material without express and written permission from this site’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to V.B Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com with appropriate and specific direction to the original content.

Riding a Cicada

Sounds More than Odd, but Epipomponia nawai Babies Do

As a child during strolls along a railway track with my grandfather, we sometimes stopped to see if we could spot the caterpillars that had eaten holes into some of the leaves of the plants we encountered. One of the most exciting finds at that time, for me anyway, were the huge caterpillars of the Sphinx ligustri moth. I loved to collect various butterfly caterpillars and managed to rear some to adulthood. I was reminded of these childhood exploits when in late August on the balcony just outside my flat on the 15th floor in South Korea, I picked up a seemingly distressed cicada that was unable to fly away. Being curious why it had behaved in this funny way I took a closer look and  -lo and behold-  I found a white insect larva “riding on its back”, actually clinging to its abdomen! My childhood experience told me that this looked like a caterpillar and was not a maggot of a fly or the larva of a parasitic wasp. What was it?

To my great surprise, examining the still fully alive albeit weak cicada and its “rider” under a magnifying glass, I had to conclude that the “thing” was indeed a caterpillar. But with very, very few exceptions like the famous ant-larvae consuming lycaenid butterfly species and the weird insectivorous fly-catching predatory caterpillars of the Hawaiian geometrid moth genus Eupithecia (that I reported on in the essay on “Non-conformists”), caterpillars feed on plants and certainly not on those highly mobile and active cicadas. Or do they?

The mystery was solved when I located some publications that described a species of moth, known as Epipomponia nawai, whose larvae when leaving their eggs, somehow manage to ‘board’ a cicada and then hang on for dear life, sucking body fluids of their host and/or feeding on its cuticle. How exactly the baby caterpillar finds its host and how it then grows and matures  – all on the outside of its cicada host- until it is ready to pupate, are unsurprisingly aspects of this moth’s life cycle that are still not fully known. The specimen that I “looked after” pupated two days later, when the cicada lay in its death throes and could hardly move at all anymore. The then about 10 mm long, white caterpillar constructed a fluffy silky pupal case and I hoped to see the adult soon. Alas, the pupa did not develop, and I had to be satisfied with some information available from two or three publications on this species of parasitic moth.

There are apparently no more than 32 known species of parasitic moths worldwide, all belonging to the Epipyropidae. Although present on all continents except Antarctica, they are nowhere very common and opportunities to carry out behavioural and physiological studies are very limited. One of the best and most detailed observations on the species is that of the Japanese entomologist R. Ohgushi from 1953. He describes several cicada host species and shows that sometimes more than one caterpillar ride on a single host. He also examined the darkly coloured adults of the moth that emerged from the pupae 10-15 days later and credits them with wing spans of 15 to 20 mm. Those available to him died a few days after they had left the pupa and laid some sticky eggs on grass blades or tree bark. It has become known since Ohgushi’s study that the eggs of this moth can also develop without being fertilized. However, how the feeble and miniscule baby caterpillars find and climb on their cicada hosts remains a total mystery.

In Europe a related ‘parasitic moth species’ of the same family, known as Ommatissopyrops lusitanicus, has been described from Spain and Portugal. It parasitises the datepalm pest bug Ommatissus binotatus. My hunch is that there may well be some more hitherto undescribed species of parasitic moths that have gone unnoticed so far. Perhaps people spending their summer vacation in the Mediterranean can take a look at cicadas and plant hoppers and discover a new species. I think that that is entirely possible.

© Dr V.B. Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com, 2021. Unauthorized use and/or duplication of this material without express and written permission from this site’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to V.B Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com with appropriate and specific direction to the original content.