Confounded Dolphin Teeth: More From the Chronicles of the Demise of My Fossil Collection

Many of the fossils I painstakingly collected over the years have entered into a curious limbo, resting now in gallon-sized plastic baggies that are distinguished only by general category of fossil (teeth, shells, etc.) and where found.  These fossils await a final disposition though I do know they won’t be kept.  (My previous post described what a poor job I did of curating the bulk of the collection.)  So far, a few fossils have resisted the deconstruction process as I’ve filled these large bags.  The pufferfish fossils highlighted in the previous post are one example.  A second group of fossils has managed to distract me and avoid the fate of those fossils in the big baggies.  This is a small clutch of tiny dolphin teeth collected along the shoreline of the Calvert Cliffs where the Calvert Formation is exposed (the portion of that formation yielding these teeth dates from possibly a bit less than 17 to about 14 million years ago).  I initially set them aside for two reasons:  I find them beautiful in their simplicity and, until now, I had never attempted to try and assign them to Miocene dolphin genus and species.  (Clearly, decommissioning my fossil collection will take forever if this is going to be my usual pattern of behavior.)

[Later edit:  Is it possible there are bony fish teeth mixed in with these?  I don't think so, but that would definitely confound this exercise even more.]

This post is focused on this effort in taxonomy and (to end any suspense) why it was a failure, despite the apparent diversity in teeth shown above.  I will acknowledge at the outset that much of that "diversity" is likely due to how worn these teeth are.  Nevertheless, there's a bit of a story about dolphin teeth to be told.

Some initial groundwork with terminology and relationships is necessary.  The cetaceans (commonly slapped with the general label whales) are divided into two main groups.  The odontocetes or toothed whales are a taxon which also includes dolphins and porpoises.  The other large category of cetaceans are the mysticetes or baleen whales.  Another point of clarification.  The small cetacean teeth I collected from the Calvert Cliffs shoreline are most likely from dolphins and should not be ascribed to porpoises:  the latter, extant from perhaps the mid Miocene onward, were denizens of the Pacific, and, reportedly, their fossils are particularly scarce prior to the Pleistocene.  (For more on this issue concerning porpoises, see text by Joy Pierce Herrington, in Fossil Marine Mammals (Digital Version), Volume IV of IV - Part 2, North Carolina Fossil Club, edited by Richard Chandler, 2017, pages 6, 118.)

Early on in this taxonomic endeavor, I was encouraged by learning of the recent publication of a superb description of the various toothed whales of the Calvert Cliffs written by paleontologists Stephen J. Godfrey and Olivier Lambert.  (Miocene Toothed Whales (Odontoceti) from Calvert Cliffs, Atlantic Coastal Plain, USA, Chapter 2 in The Geology and Vertebrate Paleontology of Calvert Cliffs, Maryland, USA, Volume 2, Turtles and Toothed Whales, edited by Stephen J. Godfrey, Smithsonian Contributions to Paleobiology, Number 107, 2023.)  It is an expansive treatise on these odontocetes, including discussions and descriptions of 16 dolphin species whose fossils have been found in material shed from the portion of the Calvert Formation along which I collected my dolphin teeth.  (This count is based on Figure 2.73 which lists the named odontocetes by stratigraphy.  I have assumed, possibly in error, that Shattuck-Zones 8 to 13 are responsible for my teeth.)

Yet, this volume, for all of its many merits, turns out to be a thin reed (at least for me) upon which to attempt any extensive taxonomic attribution of the dolphin teeth I have on hand.  I went so far as to create a spreadsheet listing each of the dolphin species described by Godfrey and Lambert, excerpting any text about, or pictures featuring, teeth.  A few of the authors’ descriptions and images prompted one or two rash and wild guesses as to identity, but I ultimately concluded not to suggest any.  Discretion is by far the better part of valor in this instance.  Beyond the Godfrey and Lambert volume, I searched extensively elsewhere for some sort of single guide to fossil dolphin teeth, something similar to the many volumes devoted to fossil shark teeth, and came up empty.  Clearly, I hoped someone had done the heavy lifting in creating such a guide.

There are several key reasons why my taxonomic exercise failed.  I have already mentioned the evident wear and tear on my specimens.  But, in reality, at the heart of my difficulties is the profound impact of evolution on the earliest harbingers of the cetacea that made their way from the land back to the water.  Dolphins, like all of the other toothed whales, evolved from mammals that had typically mammalian teeth with highly varied morphology.  Consider the differences among the teeth in your own mouth.  Over time, as the stem odontocetes evolved to become fully aquatic, their teeth changed markedly.  I was quite taken by a paragraph on these evolutionary changes to their teeth written by paleontologist Steve Brusatte in his terrific book The Rise and Reign of the Mammals:  A New History From the Shadow of the Dinosaurs to Us (2022).

Gone are all the complex cusps and ridges; gone is the lineup of incisors, canines, premolars, and molars; gone is the replacement of baby teeth with adult teeth; gone is the ability to chew.  Instead, all the teeth are conical pegs, which simply cut meat off fish or other whales, which the odontocete then swallows.  Some odontocetes barely use their teeth in feeding and lazily swallow their prey whole.  (p. 280)

I drew three key conclusions from Brusatte’s overview of the changes that occurred.

First, the end result of the process is, in his words, “conical pegs.”  Yes, the conical or peg-like aspect to all of these teeth renders them relatively indistinguishable without close study.  This is unlike fossil shark teeth which come in a plethora of different shapes and sizes which are key to sorting them by genus and species, a task relatively easy for some of them.  (By the way, the teeth of porpoises are not conical but, rather, spatulate or cupped at their end.)

That said, there is in fact some diversity in the dolphin teeth that could be found along the Calvert Cliffs.  At the most extreme, there are the teeth from the three species of the Squalodontidae family that are present here.  The squalodons or, as they are sometimes called, “shark toothed dolphins,” have very distinctive teeth.    Consider those depicted below which appeared in the Godfrey and Lambert volume (figure 2.2, scale bar is 10mm).

Certainly, a couple of these are nothing like the Calvert Formation dolphin teeth in my collection.

Godfrey and Lambert do identify a few potentially distinguishing features of the teeth of several other dolphin species, ranging from tiny cusplets to keels running down the teeth.  Nevertheless, given the state of my dolphin teeth and the inherent "sameness" of nearly all such teeth, I don’t feel I have the necessary guidance I need to go further.  Frankly, it's true that, by and large, Miocene odontocete teeth (other than those from squalodons) are reasonably and justifiably covered by Brusatte’s descriptive phrase “conical pegs.”  Further, isolated odontocete teeth (excepting those of squalodons) are seldom appropriately diagnostic as to genus or species.  This specific point is made quite convincingly by a paleontologist on the discussion site The Fossil Forum in an exchange of messages regarding a search for ID guides to odontocete teeth.  He posted on June 11, 2013:  "I have one piece of advice for you:  Prepare for failure and disappointment.  The general attitude amongst marine mammal researchers is that, with the exception of really distinctive species, isolated odontocete teeth are generally not identifiable even to the genus level, and often not even to the family level. . . .  Odontocete teeth do not preserve the same breadth of diagnostic information that shark teeth have."

A second point to be made from Brusatte’s description is that it suggests that teeth in the jaws of odontocetes are homodont, that is, they do not really vary in morphology by location on the maxilla or mandible – top, bottom, front, or back of the jaws, the teeth are all similar, which they generally are.  Godfrey and Lambert label the teeth of odontocetes (other than squalodons) as “roughly homodont.”  This works to reinforce the sameness of the fossil dolphin teeth we might find along the shoreline.

(As an aside, at this point it strikes me that, were dolphins cartilaginous like sharks, that is, not having a bony skeleton, the work of paleontologists like Godfrey and Lambert would be incredibly more difficult.  Having the remains of bony skeletons to aid in determining genus and species makes a critical difference.)

Finally, Brusatte noted that, in the course of their lives, odontocetes have a single set of teeth.  The impact of this on the relative abundance of odontocete teeth fossils is significant.  I’ll acknowledge that some Miocene dolphins and extant dolphins are very much polydonts, that is, they have many teeth, some species a couple of hundred or more.  Nevertheless, when one considers that an individual shark might in its life span shed tens of thousands of teeth, it’s no wonder shark teeth litter the shoreline along the Calvert Cliffs, while dolphin teeth don’t.

All of this adds up to what I found to be insurmountable hurdles for assigning the teeth I have to genus, much less species, and helps explain the "missing" guide to fossil odontocete teeth.

In conclusion, although I won't be assigning my dolphin teeth to genus and species and, as a result, they will remain unknown, I will assign them to the “keep” category.  Beauty, simplicity, and relative scarcity win out.  So, a bunch of teeth that I can only assign to a collecting location and the broad category of “dolphin teeth,” nevertheless avoids the bardo to which I’ve condemned many fossils from my collection, some of which could be identified.  Quite ironic!

Regarding A Fossil Collection: Post #2Cetacean Middle Ear Fossils, Part 1

This is the second in a series of posts about a recently acquired collection of fossils.  Many of these fossils were collected at Plum Point, Maryland, along the Calvert Cliffs on the western shore of the Chesapeake Bay.  They possibly came from the Plum Point Member of the Calvert Formation which would make them roughly 17 to 14 million years old, an age range straddling the line between the lower and middle Miocene Epoch.  The first post described the collection in very general terms, bemoaning the original collector’s abysmal job of organizing and labeling the fossils, and explored the location where most of them were collected:  Plum Point.  I admit that I paid an inordinate amount of space in that post to the derivation of the name “Plum Point.”

Cetaceans are one of my intellectual “basins of attraction,” something to which I am inevitably drawn.  I find the story of their evolutionary trek from land back to the sea and the challenges that posed irresistible.  So, it’s been quite easy to devote time, energy, and blog space to this “new” collection because most of its readily identifiable specimens are two kinds of cetacean fossils – tympanic bullae and periotic bones, both of which are part of these mammals’ middle ears (more on these below).  Identifying these fossils as bones from cetacean middle ears is quite easy, understanding their function is not.

As an initial step in organizing the Plum Point collection, I transferred to a separate case (pictured below) all of these cetacean fossils that were clearly identified by the original collector as coming from Plum Point.

My objective with this present post is to put these fossils into several broader contexts:  (1) cetacean fossils from the Calvert Cliffs, (2) evolution of cetaceans in general, and (3) cetacean hearing with a focus on the middle ear.  I conclude with a brief display of several tympanic bullae from this collection.  A separate post will explore the Plum Point periotic bones and report on my effort to use these bones to identify cetacean genus and species.  They are considered by some to be diagnostically useful in this regard.

It may be helpful to make three points at the outset:

• the term cetacean includes whales, dolphins, and porpoises, although these animals are frequently referred to collectively as whales;
• fossils of porpoises are not found at the Calvert Cliffs; and
• cetaceans are divided into members of the Odontoceti, the toothed cetaceans which includes dolphins and porpoises, and members of the Mysticeti which have baleen.

Cetaceans in the Maryland Miocene

It’s certainly not surprising that this Plum Point collection is awash with cetacean fossils.  The Chesapeake Group formations (ranging from the upper Oligocene, more than 23 million years ago, to the upper Pliocene, fewer than 3 million years ago) that are exposed along the Atlantic Coastal Plain are considered to “contain one of the world’s richest and most diverse assemblages of fossil cetaceans.”  (Michael D. Gottfried, et al., Miocene Cetaceans of the Chesapeake Group, Proceedings of the San Diego Society of Natural History, Number 29, 1994, p. 232.)  Of the Miocene formations in the Chesapeake Group, the Calvert Formation, which produces fossils found at Plum Point, is reportedly the one with the “highest vertebrate diversity” of all.  (Gottfried, p. 233.)  Of the 24 established cetacean genera listed by Gottfried, et al. as found in the Miocene formations of the Chesapeake Group, fully 19 occur in the Calvert Formation.   Relevant to the Plum Point collection, at least 11 of the cetacean genera from the Calvert Formation are genera of dolphin.  The small size of many of the cetacean fossils in this collection suggest they are from dolphin genera. 

Evolution of Cetaceans

Where do these Miocene cetaceans fit into the broad sweep of cetacean evolution?  The evolutionary transitional changes needed to live in the water occurred before the Miocene Epoch (23 to 5 million years ago).  Fossils considered to be cetacean first appear very early in the Eocene Epoch which ran from 56 to 34 million years ago.  The evolutionary arc of these mammals from land to water is well documented in the fossil record:  in the early Eocene, from a common ancestor arose the taxon that would lead to the modern hippopotamus and a separate taxon from which the stem cetaceans, and ultimately the crown cetaceans, would evolve.  In a beautiful figure (no other adjective for it) showing the phylogenic relationships among the cetacean taxa that evolved from that common ancestor, Michael R. McGowen, et al., delineate the appearance over time of the distinguishing attributes for cetaceans.  (Figure 1, Molecular Evolutionary Tracks Macroevolutionary Transitions in Cetacea, Trends in Ecology & Evolution, Volume 29, Number 6, 2014.)  Given copyright concerns, I thought it inappropriate to reproduce that figure here.

Fossils of the earliest primitive cetaceans are likely more than 50 million years old.  The transition of the early pioneers into the water to the status as fully marine animals took place in what is a paleontological and geological instant, perhaps as few as 10 million years.  The figure in McGowen, et al., shows that the morphological and behavioral changes incumbent upon an initial semiaquatic existence came quickly, beginning with the acquisition of heavy limb bones, loss of some hair, and initiation of underwater nursing.  The steady acquisition of marine and loss of terrestrial attributes meant that, by some 40 million years ago, early cetaceans had become obligate aquatic, that is, they could no longer survive on land.  Paleontologist Nick Pyenson divides the overall evolutionary span of cetaceans into two unequal parts.  As already described, the first (some 10 million years or so) saw key transitional changes necessary for life in the water.  Among these transitional modifications were "shell-shaped ear bones being repurposed for underwater hearing."  The second, longer phase (40 million years) was a time for innovations that allowed cetaceans to diversify markedly:  the development of baleen (Mysticeti) and of echolocation (Odontoceti).  Both of these innovations appeared during the Oligocene Epoch (34 to 23 million years ago) which preceded the Miocene.  (The quoted text is from Pyenson, Spying on Whales:  The Past, Present, and Future of Earth's Most Awesome Creatures, 2018, p. 36; a fuller discussion of these two periods can be found in Nicholas D. Pyenson, The Ecological Rise of Whales Chronicled by the Fossil Record, Current Biology, Volume 27, June 5, 2017.)  

Cetacean Hearing with a Focus on the Middle Ear

Moving into the water posed myriad challenges, one of the most critical was how to hear in this medium so different from air.  Mammalian ears designed for hearing on land are clearly malformed for hearing under water.  For one thing, the acoustical impedance of water and that of the organism's soft tissues are similar, meaning that when such ears are under water, sound will enter the inner ear from multiple avenues, not just through the outer ear and ear canal, and middle ear.  Locating the source and direction of sounds becomes very tricky.  In order to function effectively in water, cetacean ears changed and that transformation happened relatively quickly.  McGowen's Figure 1 shows that almost “immediately” the stem cetaceans developed dense ear bones which were important for hearing under water.  By perhaps as early as some 50 million years ago, stem taxa had evolved a fat pad attached to the lower jaw to aid in hearing (more on that below).  By no later than 40 million years ago outer ears were gone.

To make some sense of these Plum Point cetacean fossils (both kinds coming from the middle ear), I went in search of basic research material on the morphology of the cetacean middle ear.  In a post over a decade ago, I lamented the lack of literature accessible to the layperson on cetacean hearing.  As I prepared the present post, I was dismayed to discover that there were in fact some quite useful sources available back then.  Nevertheless, despite locating these and other sources, my grasp of the basics of cetacean hearing remains tenuous.  I welcome any corrections of errors of fact or interpretation in the following discussion.  In a subsection at the end of this discussion, I discuss some of the principal sources I used.

As the cetacean ear evolved to deal with the new medium through which sound had to travel, I’ve already noted that the bones of the ear became heavier.  The outer ear, of no utility, faded into obscurity.  The air-filled middle and fluid-filled inner ears were encompassed in what is known as the tympano-periotic complex consisting of two bones:  the tympanic bulla and periotic bone.  Pictured below is a periotic bone (top) and a tympanic bulla (bottom) from my Plum Point collection.

To help me orient the overall relationship between the periotic bone and bulla which house the complex, I prepared this drawing of the complex exterior based on a photograph of a modern neonate bottlenose dolphin.

The tympano-periotic complex is largely isolated acoustically from the skull (completely for odontocetes, less so for mysticetes).  In odontocetes, sound waves travel from the cetacean’s thin lower jaw through a fat pad to the tympanic bulla.  For mysticetes, the process through which sound is received is still "unknown," though at least for some genera fat is likely to play a role.  (Maya Yamato and Nicholas D. Pyenson, Early Development and Orientation of the Acoustical Funnel Provides Insights into the Evolution of Sound Reception Pathways in Cetaceans, PLOS One, Volume 10, Number 3, March 11, 2015.)

The bulla is a distinctively shaped bone, much like an elongated cup or scoop, with a greatly thickened medial lip called the involucrum and a much thinner outer lip named the tympanic plate.  Though all mammals have a bulla of sorts, it’s the involucrum that sets the cetacean bulla apart.  As Pyenson observes, the involucrum is a critical feature used by paleontologists to distinguish the earliest cetaceans from other ancient animals:  it's an attribute that "makes them whales and not something else."  (Spying on Whales, p. 29.)   The involucrum and tympanic plates are labelled in the picture below.

The isolation of the bulla from the other bones of the skull and the thick-thin configuration of the bulla lip are instrumental in allowing the bulla to vibrate in response to incoming sound waves (and to forestall conduction of sound to the inner ear through any other bones).  The tympanic plate has a bony connection to an array of ossicles (small bones), beginning with the malleus which in turn connects to the smaller incus and then to the still smaller stapes.  These small bones transmit the sound vibrations to the fluid-filled inner ear canal and to the cochlea.  That process strengthens the sound.  The prominent knob in roughly the top center of the periotic bone (see photograph above) contains the circular passages of the cochlea in which sound waves are converted to electrochemical impulses which are then transmitted to the brain via the vestibulo-cochlear nerve.  The periotic bone has various openings (some of which are visible in the photograph above); through one, the cochlear nerve connects to the brain.  Here is a schematic outline of the key elements in the cetacean middle ear as it directs sound waves to the inner ear.

Some of the Principal Sources for the Preceding Discussion of Cetacean Hearing

Among the available valuable resources on cetacean hearing in general is the first of two extensive posts written by paleontologist Robert Boessenecker for his blog The Coastal Paleontologist:  Perspectives on Marine Vertebrate Paleontology.  This post, titled Bobby’s Guide to Whale & Dolphin Earbones 1:  Introduction, is dated December 3, 2022, and explores the morphology of mammalian ears, contrasting those of land mammals and those of the cetaceans.  (I intend to use the second, Bobby’s Guide to Whale & Dolphin Earbones 2:  Identifying Toothed Whale Periotics, which appeared January 15, 2023, in a later post.)

One of the clearest expositions of the process of cetacean hearing can be found in an article by Sirpa Nummela et al., who used CT scans to delineate how the features of the middle ear of a killer whale (an odontocete) function and how they relate to each other.  (The Anatomy of the Killer Whale Middle Ear (Orcinus orca), Hearing Research, Volume 133, Issues 1-2, July 1999, residing behind a paywall.)

Later, Sirpa Nummela and a different group of colleagues produced a very informative article on the evolution of different ear structures and their role in cetacean hearing.  (Sound Transmission in Archaic and Modern Whales:  Adaptations for Underwater Hearing, Hearing Research, Volume 290, 2007.) 

A wealth of information and relevant images (one of which formed the basis for the drawing above of the complex) can be found in J.G. Mead and R.E. Fordyce’s The Therian Skull:  A Lexicon with Emphasis on the Odontocetes (Smithsonian Contributions to Zoology, Number  627, 2009).

Helpful, and the underlying source of the schematic drawing above of the middle ear, is A Model of the Odontocete Middle Ear by Simo Hemilä, et al. (Hearing Research, Volume 133, 1999, resides behind a paywall).

Selection of Tympanic Bullae from the Plum Point Collection

Here are four additional specimens from the collection.  They are unquestionably quite well worn and their small size certainly suggests dolphin origin.

From my earliest fossil collecting, I found tympanic bullae singularly attractive, capped as they are by the wave-like flow of the involucrum. It's the feature most likely to survive over millions of years, retaining the visible poetry of its sculpted form.

An Infinity of Wonder and Beauty - Review of An Immense World

 One summer night in his bungalow on the Maine coast, as Vincent Dethier lay in bed, he became aware of a presence in the darkness.  It was hinted at by only the slightest sounds, and perhaps a stirring in the air.  This signaled, he surmised, that a brown bat had made the bungalow its summer home and was now engaged in an aerial battle with a moth.  The bat hunted, probing the darkness with ultrasound, listening to and deciphering the returning echoes, while the moth, registering the sound, might abruptly shift course to evade the predator.  To Dethier, an entomologist, the experience was profound:

To realize that a whole world of life and death is being enacted before a person’s eyes and he cannot see it, and around his ears, and he cannot hear it is to diminish whatever feelings of superiority and arrogance one may have.  It is to feel humble in the knowledge that there are other worlds and other perceptions.  It is to appreciate that we are surrounded with an infinity of wonder and beauty.  (The Ecology of a Summer House, 1984, p. 39-40.)

That knowledge of other animal worlds, captured in the concept of Umwelt, is the focus and core of science writer Ed Yong’s masterful new book, An Immense World:  How Animal Senses Reveal The Hidden Realms Around Us (2022).

As Yong writes, zoologist Jakob von Uexküll first used the word Umwelt in 1909 to define that part of an animal’s environment it “can sense and experience — its perceptual world.”  (p. 5)  Yong explores what science can tell us about the worlds that different animals perceive, and the senses they employ to map, navigate, and survive in those worlds.  Put aside the notion of just the traditional five senses being the portals through which animals understand their worlds, or a similarly limited number of stimuli that animals have evolved to register, respond to, and employ.  The overarching world in which we and all other animals live contains each of those perceptual worlds, but no animal perceives them all.  Different groups of animals are at home in one or more of these worlds in markedly different ways and degrees.  Our challenge is to move beyond the limitations of how we humans engage in the world and stretch our understanding to catch a glimmer of those perceptual worlds alien to us, in order to better understand the one world in which we and all other animals live.

There is an urgency, Yong writes, to gaining that understanding and acting on it before human-generated environmental changes – such as light pollution and noise pollution – render entire perceptual worlds barely habitable or, indeed, uninhabitable for their denizens.

Our journey in the book is through several different kinds of perceptual worlds accessible through specific senses.  I came away with a set of critical guideposts that helped me appreciate, though, perhaps, not understand, the worlds Yong was exploring for me.  He writes, “The first step to understanding another animal’s Umwelt is to understand what it uses its senses for.” (p. 61)  Essential to that effort is an appreciation that the world in which we all live is full of “invisible currents of information that flow around us, and which animals can detect with the right sensory equipment."  (p. 173)  Further, we must recognize that all animals are engaged in an evolutionary balancing act – strengthening any particular sense in any particular way carries a cost.  Writ large, an animal’s resources are finite and, so, what is gained to enhance survival is often compensated for by something lost.  Further, given that the overarching world is dynamic, change is the watchword and every animal group’s Umwelt can remain static only at its peril.  Finally, we humans must recognize that our particular Umwelt prejudices us in favor of those senses at which we excel – sight in particular – and limits our ability to understand, intuitively or otherwise, another animal’s perceptual world.  There is no hierarchy of superior and inferior Umwelten, just different ones.  A heady brew of insights, indeed.

Yong devotes chapters of the book to the sensing of smell and taste, light, color, pain, heat, contact and flow, surface vibrations, sound, echoes, electrical fields, and magnetic fields.  The reality of the senses across the animal kingdom is staggering.  Consider just a single aspect:  where the organs or cells that work on the traditional ones might appear.  Eyes are not just for heads, but adorn the inner edges of a scallop’s shells and are on each of a starfish’s five arms; ears can be found on the joints of certain insects, the abdomens of others, the mouths of still others and on antennae; smell is helped by the tongues of snakes and lizards; some insects taste with their feet and legs.  Clearly, this affects how those senses work and what they perceive.

The book is replete with amazing examples of the ways animals understand and live in their Umwelten.  Consider the tiny treehopper which uses its abdomen to vibrate the surface of plants to generate myriad sounds, some deep throated and others shrill, to communicate (the young to tell mom they perceive a threat, adults to call a group together, adults to find mates).  These sounds are generally inaudible to us but not to others in the taxon.  When insects that vibrate plant surfaces to communicate gather together, the cacophony must be impressive.  Or consider the whiskers of harbor seals that stand up from the animal’s nose and eyebrows.  These are sensitive sensory organs that are touched by the wakes left by objects moving through water.  This hydrodynamic touch sense is fine tuned to follow a wake with great precision.  Harbor seals in the wild will lie in wait, their whiskers able to signal not only when a fish swims past but its size as well.  As Yong observes, our own sense of touch is tied to the present, but the seal’s whiskers which respond to the touch of the wake capture the recent past.

One sense that resonated (pun intended) with me in particular was echolocation as practiced by bats and dolphins.  Echolocation differs from the other senses because it’s adding to the environment and using the response to that added energy to define a perceptual world.  For the bat, this sense poses a host of significant challenges – outgoing sounds need to be distinguished from incoming echoes, it must protect its ears from the deafening volume of the ultrasonic calls it emits, the interplay of call and response occurs while bat and prey are on the move – and bats have evolved different mechanisms and behaviors to cope with the complexity of those challenges.

Actually, air is a rather poor medium for echolocation because sound loses energy quickly in it and so this sense can define the contours of a perceptual world only over a short distance.  In contrast, water is an excellent medium for sound conduction and dolphins take full advantage of that.  Indeed, sound retains enough energy as it travels through water that dolphins use echolocation to probe inside of objects.  Animals’ skeletons because perceptible to them as do fish swim bladders, allowing a dolphin to distinguish among potential prey.

At the same time, evolution has fueled responses to the powerful echolocation sense.  Although most insects are deaf, some, including half of moth species, have ultrasonic hearing, an ability that appeared in the insect world after bats came on the scene, perhaps some 65 million years ago.  Thus, a moth might well hear a bat’s outgoing call and take evasive action before the echo reaches the predator’s ears.  Other moths have evolved the ability to make ultrasonic clicks, potentially confusing bats.  Still others evolved long, elaborate tails, incredibly beautiful to the human eye, but misleading to the echolocating bat who may well interpret returning echoes as describing a much larger prey.  The attacker may come away with a mouth full of moth tail while the insect survives to carry on its kind.

Yong’s prose is graceful, carrying the reader along easily in this journey of exploration.  That he has mastered an immense world of information and anecdote is abundantly clear.  I must acknowledge that the cumulative effect of being exposed to these multiple perceptual worlds can be overwhelming, leaving a reader (well, this reader) mostly intuiting what is there, and not understanding it in any meaningful way.  That may be unavoidable in a work of this depth and breadth.  Despite that, I suspect I am now more open to the incredible diversity of the perceptual worlds around me whether I can understand them or not.  (On a very immediate level, this has added a new dimension to my appreciation of the two cats currently sharing our house.)