It Takes Energy to Stay Alive 

And to explain that to the students

I always felt teaching energetics to our undergraduate biology students was no easy task. And yet all life depends on energy. So, trying to avoid as much as possible entropy, enthalpy, the law of the conservation of energy and the laws of thermodynamics with its not exactly simple equations, I started my physiology energetics lecture with a thought-provoking example. Imagine, I said, you want to compare the amounts of the required energy to heat up and bring to the boil a cup of water and a bucket of water. You need less energy to heat up the smaller amount, but it also cools down much faster compared with the water in the bucket which had needed a much greater amount of energy to reach boiling point. If you decided to re-heat the water and bring it back to 100°C every 30 minutes, which would have required more input of energy over a 10 or a 100 hour period: the cup or the bucket? An important variable to be considered is of course the temperature down to which the boiled water would be allowed to cool before being re-heated. But all that is calculable and can be expressed in mathematical equations.

Students all know that the reverse reaction of photosynthesis is the one that provides us and other animals with energy by ‘burning fuel’, i.e. the food ingested, and that some of the energy is for growth and work, but some is converted to heat; in fact, ultimately all is dissipated as heat. Daily rates of minimum standard heat production in animals are related to body temperatures, so that small 42°C warm birds have the highest rate followed by mammals with body temperatures of 35-37°C and ectothermic animals like reptiles, amphibians and fishes. Body sizes and weights are further complicating factors, because the slopes of the relationship between rate of minimum energy expenditure and body weight are nearly the same for ectotherm (cold-blooded) and endotherm (warm-blooded) animals. The slopes are not 1.0, which would indicate a direct proportion: the slopes are approximately 0.75 and that indicates that a doubling in body weight does not double minimum energy expenditure. What is, however, interesting is that when the minimum metabolic rate is expressed per gram of body weight, one notes that energy expenditure rates shoot up exponentially as body weight decreases: small animals need more energy per gram body weight than larger ones. This explains why small warm-blooded animals, e.g. mice, shrews and humming birds need to ingest food more frequently than bigger species and, of course, fish, amphibians and reptiles with their lower resting metabolic rates.

Although resting metabolic rate and maximum longevity have been regarded as not always being ideal to explain ageing, there is nevertheless an obvious relationship between an animal’s body size and longevity (humans being long-lived but not terribly huge and heavy are a bit of an exception). In species of bigger animals the latter generally enjoy longer possible life spans than species that contain smaller individuals, cf. mice, dogs, horses, elephants and whales. Small animals have a greater surface area than bigger ones and tend to lose heat more readily to the environment than the latter, but this alone apparently does not explain the slope of 0.75, mentioned above and an even lower one of 0.67 if body surface area and body weight are correlated to each other. To explain this discrepancy some researchers suggested entering time as a 4th dimension into the equation. Since longevity in mammals is related to weight, total metabolic capacity has to be subject to the time that an organism spends being alive.

Humans appear to be a special case as some studies have shown that shorter rather than taller people have a greater life expectancy! However, it needs to be pointed out that socioeconomic status, relative weight, regular exercise, gender and health practice styles can influence the outcome. It has, for example been suggested on the basis of Japanese and Dutch studies, the latter involving 7800 men and women, that the taller Dutch (but not Japanese) women could expect to live longer than shorter individuals, but that did not apply to men. With such uncertainties abound, I think it’s comforting to be just average.

© Dr V.B. Meyer-Rochow and, 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 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, 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 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, 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 with appropriate and specific direction to the original content.