You Don’t See Them, but They Sure Are There

The Ears that Fish Have

That fish can hear was documented nearly 100 years ago by Nobel Prize winner Karl v. Frisch, famous for his work on honeybee communication. Freshwater fish of the carp family, like most bony fishes, not only possess swim bladders that allow them to stay buoyant at different depths, but unlike others also possess small bony connections between the fish’s inner ear and the swim bladder. The gas in the latter gets compressed in response to sound pressure in the water, starts vibrating, and then transmits the signal via the aforementioned small bones, known as Weberian ossicles, to sensitive hair cells of the inner ear. Sound amplitude (loudness) and frequency (pitch) are important, but even in fish with the best hearing, sounds above 6,000 Hz would be ultrasound to them and in swim bladder-lessfish like the fast mackerel and the tuna but even in the more sluggish Antarctic icefishes only lower frequencies can be detected.

Sounds are longitudinally transmitted waves, whose frequencies and amplitudes may vary. Because of the water’s greater density than that of air, sounds are propagated 4.8 times faster in water than in air. In a broad sense sounds are generated by movements or vibrations and in water can be the results of an animal’s vocalizations or activities, of sounds created by ice-floes or logs rubbing or bumping against each other, breaking waves, anthropogenically-produced noise like explosions and disturbances created by ships. In order to sense the sounds, fish use ear stones, i.e. so-called otoliths. Bathed in endolymphatic fluid and resting on a pad of receptor cells with sensory hairs, the two major otoliths are located just below the brain in two bony sacs of the inner ear known as the saccule and the utricle. The otoliths are part of the bony vestibule’s two regions, i.e. the cochlear portion (for hearing) and the vestibular portion with its semicircular canals (for balance and angular change). Thus, otoliths can be said to be involved in the detection of gravity and linear accelerations and serve as a structure of hearing in fish, so well explained in a recent review by Dr. Tanja Schulz-Mirbach of Munich.

Otoliths are hard, durable structures that consist primarily of calcium carbonate (CaCO3) in the form of aragonite.They remain largely unchanged during the digestion in the stomach and gut of a predator. They are thus an excellent structure to estimate a fish’s age, because their size increases by periodically laid down alternating opaque and translucent bands that consist of CaCO3 and collagen fibres. As daily increments are regularly added, researchers can correlate the number of layers with the fish’s body length and use the tabulated data to identify the fish’s age. What makes the study complicated is that the otoliths, not being translucent enough to count the layers, need to be sectioned. Furthermore, although the shapes of the otoliths are species-specific, they can vary in individuals of the same species, depending on the fish’s developmental stage and if the fish was actively swimming or passively drifting.

In Antarctic icefish my Polish colleague Ryszard Traczyk and I have recently concluded that the more spherical otoliths of larval specimens and the longish otolith shapes of the adults are the results of the inertia and friction experienced by the otoliths in their endolymphatic fluid when the fish swim: larvae swim less than adult icefish and the latter swim less than mackerel (which possess the most elongate otoliths). It is entirely possible that oscillations of the dense otoliths generate shearing forces that deflect the sensory hairs of the cells they are resting on, when the oscillations are due to disturbances in the water made by nearby prey or the approach of a predator. Responses to such disturbances in the “acoustico-lateralis” vicinity of the fish would then not only be sensed by the lateral line system, but picked up by the fish’s otoliths too and sent to the brain via the 8th cranial nerve, often referred to as the vestibulocochlear nerve. So, do fish make some noise and can they hear? Actually only a few produce sounds, but all bony fish can hear. However, there’s certainly no need to whisper when you sit in front of your aquarium and watch your colourful aquatic beauties in their 3-dimensional world.

© 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.

Adolf Hitler and the Scrotum

A Discussion about Testicles

Some colleague in Australia wanted to know from me once what my view was on Adolf Hitler’s condition of ”having had only one ball” (= one testicle). I knew nothing about this, but later learned that many people believed that to be true. And so it was (in a way), for according to a report by Prof. Peter Fleischmann in 2015, based on the medical records of Hitler’s health, the man had suffered from an undescended right testicle. Medically this does not mean he had only one testis; it only means that just one was externally visible. Surgically, of course, it is sometimes necessary to remove one (or even both) and historically eunuchs, men in other words who for whatever reason were bereaved of their testicles, come to mind. I remember having read once that some tribal people practiced the ritual removal of one testicle, but despite a thorough investigation involving internet search engines, I could not locate the source of that information and wonder if a reader can perhaps help and find evidence in support of that claim. However, what is well documented is that in most men the left testicle is bigger than the right one and that the Greeks of the antiquity (wrongly) believed that sons were “made” by the right and girls by the left testicle.

Anyway, I want to devote this blog to the structure that males cannot be without: their testes (also known as testicles). If men can be fertile with just one testicle externally visible (whether the other is missing altogether or simply undescended as in Hitler’s case), why do these structures in most but not all mammalian males, dangle on the outside of the body in their little scrotum bag? Amongst bats, insectivores and rodents it is common to find that the testes migrate from the inside of the body into a scrotum during the mating season; afterwards they move back to their hidden location inside the body. But elephants and other mammals known as Afrotheres, which also include elephant shrews, tenrecs, aardvarks, hyraxes, sea cows and several extinct groups, all possess testes that never  – not even during the mating season –  become externally visible and constantly remain hidden and protected inside the male’s body. In whales the testes are permanently internal and that would probably make sense if one considers hydrodynamic drag and having to swim with a dangling scrotum.

A widely accepted theory, for which good experimental evidence exists, postulates that higher internal body temperatures are damaging to the spermatozoa. But this does not square with the examples of species given above that don’t have an externally placed scrotum with testicles in them. Besides, if temperature is so deleterious to sperms, why do all bird species have their testicles inside their bodies, even a bird’s body temperature is usually higher than that of a mammal? The discovery of “testicle descent genes” in the Afrotheres species has recently been announced. This suggests that during evolution these genes either mutated or were inactivated and consequently prevented that the testes descended in Afrotheres. But did their spermatozoa suffer or become damaged? It doesn’t look like that and it seems that protecting the sperm-making structures from physical damage was of greater concern than preventing effects that the higher internal body temperature might possibly cause. Could, therefore, the evolutionary older testicular descendence have had a different function?

That is possible, because in species that exhibit a seasonal testicular “out-and-inward” migration, the appearance of a voluminous scrotum could be a sexual signal, especially if combined with an attractive colour. A permanent display of maleness through externally visible testicles could have been the reason why in most mammalian species the testes are not retained inside their bodies. In humans, following ejaculation, testes are sometimes temporarily retracted into the body cavity: a case of an atavism? An echo from the past? Or a sign of what is still to come?

© 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.

Star-Nose Mole and Desman

They are weird, but cute is something different

In this blog I want to introduce two rather extraordinary species of mammals, of which I had the luck to observe at least one: the Pyrenean desman (Galemys pyrenaicus). In 1981 when I was working in the CNRS Laboratoire souterrain de Moulis in France, I inquired what that long aquarium which I saw in one of the rooms was used for. And I was told of the research on the rare aquatic mole-relative, known as the desman, by a former priest, who had earlier been on a mission in India, fallen in love with a nun, left the priesthood, married her and had returned to France, where he was now carrying out observations on the biology of the enigmatic “desman des Pyrénées”. I met Monsieur Richard, was invited to his home to meet his wife and assisted him in his work on the desman on one evening.

What a strange animal, was my initial thought, when I saw this hyperactive mammal of less than fist-size for the first time! It was bobbing up and down on the water surface like a cork, stuffing a little worm it was holding in its short front paws into its tiny mouth, only to frantically diving about 30 cm to the gravelled bottom of the aquarium again to stick its long and incredibly mobile and prehensile nose between stones and pebbles to detect prey, e.g. insect larvae or small worms that it could then grab and take to the surface to eat or to climb (with the aid of some sharp claws) onto a small stone platform to rest or to scratch and dry its wet fur. I stopped the time the peculiar animal stayed under water and marvelled at (for its body size) huge webbed hind feet. The animal had small eyes, but exceedingly long whiskers (called vibrissae) around its nose and a very long and thin, scale-covered tail, reminiscent of that of a rat (to which the desman is not at all related as the rat is a rodent and the desman belongs to the moles and thus insectivores). It seemed to have a buoyancy problem and easily floated to the surface, which made its dives seem very laborious, requiring a lot of energy; something that must also have been a problem with the very cold mountain streams it preferred as its habitat. Already not exactly common 40 years ago, it has apparently become much rarer still today.

If this aquatic mole-relative wasn’t weird enough, then lets look at another species of mole, one I’ve never seen and know only from reading about its incredible sensory adaptations: the star-nosed mole (Condylura cristata), sometimes also referred to as ‘star-muzzled mole’. This species, like the more common Talpa europaea is virtually blind and spends all its life undergroud in an environment of moist soil in the northeastern region of North America. What makes this species so unique are the grotesque 22 fleshy appendages around the nose, which contain sense organs that respond to the minutest vibrations and serve as what has been termed a “tactile eye”.  In spite of its location on the nose, the appendages do not contain olfactory receptors and do not function like tentacles to grab prey, for there are no muscle fibres in them. Arranged in two groups of 11 on either side, the fleshy appendages, each with its own directional sensitivity, represent specialized mechanoreceptors that contain superfast conducting myelinated axons. Neuronal responses to the faintest stimulation occur with a latency of an average of 11.6 milliseconds. To decide if something is edible requires 25 msec for this mole, but in humans, by comparison, the process has been reported to take 600 msec.

According to Vanderbilt University’s Drs. K. Catania and J.H. Kaas, a little more than half of the brain of the star-nosed mole is dedicated to analyse signals that arrive from the star-like appendages. Each hemisphere of the cerebral cortex possesses clearly visible 11 stripes representing the 11 appendages of the nose-star’s opposite (contralateral) side. Now do these strange two mole species that this blog has been about have something in common? Yes, they do, for the star-nosed mole also loves water and swims well and, as Kenneth Catania in the year 2000 has written  “Mechanosensory organs of moles, shrew-moles, and desmans: a survey of the family Talpidae with comments on the function and evolution of Eimer’s organ”, they also share the organ responsible for the amazing tactile sensitivity: named after Swiss born Theodor Eimer, who first described the organ in 1871.

© 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.