Fins to Feet

Chiroptera: The Bats

My principal experience with bats comes from summer nights at my Grandmother’s house in South India as a child. Swarms of fruit bats would circle our villa, swooping down on the surrounding vegetation to – we supposed – forage for fruits and nectar. We caught sight of one long-snouted bat drinking from the open lips of a banana flower. I wouldn’t hazard to guess what Taxon I was looking at, since India is home to over a hundred species of bats and I am far from being an expert. Several years have passed and the city I presently live in is home to the largest urban bat colony on the planet. I plan to make a visit to it in the near future – and host at least a few of the resultant photographs here!

The bat holds a unique position among the mammals. They are the only mammals capable of powered flight. They comprise a whopping 20% of all known mammal species. They spend most of their lives in an upside down posture. They rank among the most widely distributed mammals on the planet. However, the most remarkable aspect of bat biology, for me at least, is echolocation – the biological sonar that bats use to navigate and hunt by nightfall. The idea of “Seeing” the world through rebounding sound waves is fantastically alien to our own sensory experience – and I reckon that it is well beyond the limits of human cognition to ever truly understand what it is “like” to experience the world as a bat. Nevertheless, understanding the physical and physiological underpinnings of bat sonar will help us appreciate what a beautiful evolutionary innovation it really is.

A large part of the business of echolocation rests on a simple fact: It is possible to determine the distance between two points by measuring the time it takes for a sound wave to travel from one point to the other and back. The calculation involved in figuring out the distance between two points from a given time delay (that is, the time that passes between sound production and echo) and a known value of sound velocity is trivial provided that neither point is moving.

But consider a bat’s situation, weaving through a cluttered environment at a considerable speed, sound waves bouncing off several objects of varying size, closeness and texture; think of the variety and complexity of the variables involved – and yet bats can accomplish incredible feats of aerial agility in pitch black conditions. Lazzaro Spallanzani, an 18^th century bishop and experimentalist, was surprised to discover that blinded bats could fly confidently around his study without disturbing the wires he had suspended from the ceiling as obstacles. He also discovered that blocking off their ear canals with closed brass tubes drastically diminished their ability to avoid the wires. Bats produce high frequency sounds that lie outside the range of audible frequencies for the human ear – and ultrasound was unknown to 18^th century science. So Spallanzani could only go so far as to say that object perception in bats (or echolocating bats anyway) was related to hearing. The true nature of bat echolocation was only uncovered in the 1940s.

A mental soundscape

The sound source for bat echolocation is expired air. The stroke of the wing and the contraction of the thoracic muscles produces a forceful exhalation. Air rushes past the larynx out through the open mouth or nostrils. Bats have evolved a fairly grotesque complement of noses with various flaps and folds to modify the emergent sound in various ways. Bat calls are ultrasonic and very loud . So loud, in fact, that many bat species find it necessary to close their ears at the moment of sound generation in order to avoid being deafened by their own calls. Some bats are known to produce vocalizations of around 130 db (louder that a rock concert), the very loudest sounds produced in all the animal kingdom. The sound energy is emitted as a directional cone. The call rate changes depending on how close to a target the bat is – from 10-15 pulses per second during normal flight to a continuous buzz just before snagging a winged insect.

Now bat calls are far more structurally complex than one might expect. Calls can sweep through a wide range of frequencies (FM or Frequency Modulated) or hold a single frequency over an extended period of time. CF and FM calls are used in different contexts on account of their different frequency-time profiles. They can also have multiple harmonics. Different species use CF and FM calls for different purposes.

FM call from Pipistrellus pipistrellus

Echo: The sound bounces off a target – say, a particularly unfortunate moth or a tree looming ahead. This reflected sound, or echo, can be orders of magnitude less intense than the emitted call – because of the dissipation of sound energy when it travels through air and when it strikes an object – and the bat ear has evolved to be appropriately sensitive to these quieter echoes. The bat receives the echo response and processes the information in its auditory cortex. Bats do not have an especially high brain-to-body ratio (they lie somewhere between primitive insectivores and other mammals on this scale). But they do have a series of specialized neural pathways and auditory nuclei that act to measure the time delay between echo and call. Bats process various pieces of time-delay and echo frequency information to help create an echo-image of the world. There is evidence to show that, apart from telling the distances to objects, bats can make amazing determinations of size, shape, movement and surface structure from the properties of the received echo.

Constant Frequency calls are used in open spaces because they have a greater operational range. This is because bat ears are most sensitive to the frequencies in their CF calls. Bats also make use of the Doppler effect to detect motion with their CF calls. The Doppler Effect refers to the phenomenon where the frequency of a sound changes depending on the relative motion of the observer and the sound source. The classic example used to illustrate the Doppler Effect is this: a vehicle approaching you produces a sound with a higher pitch (i.e. frequency) than it does when it moves away from you or when it is stationary. The beats of a insect’s wings produces fluctuations in echo intensity that a bat can detect. Thanks to the Doppler effect, the sound returned from a moving target also has a broader range of frequencies than the original CF call. The bat brain can use this information to compute general direction and distance to moving prey. In cases where the frequency of an echo is actually raised above of the audible range for a bat by the doppler effect, they merely reduce the frequency of the call itself (Doppler shit compensation).

Frequency modulated calls can be used in more cluttered environments where it is necessary to clearly distinguish prey from background noise. The broad sweep of frequencies used in FM results in a complex echo structure (a higher resolution echo image) and allows for more precise timing of delay, but it has a smaller physical range. Changes in the spectrum of frequencies of this echo image could indicate a change in distance between the prey and the background. And thus, the bat is able to detect moving prey even in forested areas.

This description is intended to show what a remarkable affair echo-imaging really is. It has allowed bats become “independent of sunlight as a medium for perceiving their world” (“The Biology of Bats”, Gerhard Neuweiler, pg 141). But echolocation has its drawbacks: it involves a serious expenditure of energy and is limited in range.

Bat flight

Bat wings are structurally very different from bird wings. For one, all the digits in the bird forelimb are fused. The 2^nd to 5^th digits of the bat forelimb are greatly elongated and unfused. The wing consists of a membrane of skin stretched out between the digits and between the fifth digit and the sides of the body.  The rigid wings of bird are a better suited for generating lift, but bat wings provide a greater degree of maneuverability on account of how adjustable and flexible they are. The adjustments are performed by the fine action of several separate muscle groups. Bats do however, as a general rule, have slower flight speeds – the fastest known bat clocks in at about 55.92 mph, while the fastest bird can manage level flight speeds of over 150 mph. Bats can brake very effectively by spreading out their hind limbs mid-flight, opening up the uropatagium – a membrane of skin that joins the legs and often encloses the tail – like a drag parachute.

The outspread digits of a bat are light-weight and highly bendable – this is the part of the wing that actually flaps during flight (rather than the entire forelimb, as in birds). Bats need about 8-15 wingbeats per second to stay airborne. The shape of the wing varies depending on the species – fast-flying bats have short, narrow wings, while large bats that eat fruit or pick prey off the ground have large, broad wings.

Bat anatomy and physiology is clearly adapted for life on the wing. The skeleton is light and fragile. The heart is large and muscular – accounting for more of the animal’s mass as a percentage than any other mammalian heart – to provide the rapid circulation required for powered flight. The delicate wing membrane can heal after sustaining tears and wounds. The wing is also provided with sensory receptors that can assess the flow of air over the membrane.

Bats are awkward animals on the ground, however. Their knees are bent backwards and outwards and they lack grasping hand claws. They crawl along surfaces like spiders. Bats have evolved a kind of locking mechanism where the muscles and ligaments of the leg are linked up in such a way that, in a relaxed posture, the sharp claws of the foot are clenched together. While it takes energy for us to close our hands, a bat needs to make an effort to open its foot. This allows bats to hang upside down from the ceiling of a cave without expending any energy!

Bat evolution

Unfortunately, flight-adapted bat bones are thin and do not fossilize easily – and thus the bat fossil record gives scant clues as to the early evolutionary history of the chiroptera. We face similar problems with understanding the early history of the Pterosaurs, a group of flying reptiles that are often mistaken for dinosaurs. Onychonycteris, the very oldest known Chiropteran genus, had longer hindlimbs, more clawed digits and shorter forelimbs than the modern bat. It is unclear whether or not Onychonycteris was capable of echolocation. The first bats appeared in the Eocene (as far as we can reliably tell, anyway), about 40-55 million years ago and, for the most part, appear to be fully differentiated, with a complex auditory apparatus and a nearly modern wing profile.

We observe two major taxonomical divisions in modern bats – the Megachiroptera and the Microchiroptera (megabats and microbats). Megabats have long snouts, big eyes, a claw on the second finger and are primarily found in the tropics and subtropics. Apart from bats of the genus Rousette (they generate ultrasonic calls by clicking their tongues), the megabats are incapable of echolocation. Microbats have small eyes, short snouts with strange noses and are capable of echolocation.

The most recent shared ancestor of microbats and megabats was most likely capable of echolocation. The evolution of flight (powered or otherwise) probably preceded the evolution of echolocation. There are no examples of echolocating ground-based insectivores. There are, however, examples of cave-dwelling birds that have developed a relatively crude form of echolocation. Bats may have developed the ability to echolocate to navigate through caves. Bats invaded a hitherto unoccupied nocturnal ecological niche when they took to the air and the associated selection pressures may well have driven the development of echolocation. Echolocation and flight may have evolved concurrently.

It is suggested that the ability to echolocate was secondarily lost in megabats The microbats retained it and, indeed, it is remarkable to see how closely the echolocative systems of different bat orders separated by many millions of years of evolution resemble one another.

At any rate, bats are a highly successful and well-researched group of mammals – I only wish we had a more robust fossil record to seal the deal on their evolutionary history.

Evolution of the horse

NOTE: This piece is under construction!

An armed man on a rearing horse – this is the very image of martial valor. And it’s been done and redone by a great many artists over the centuries. The soft, glowing colors, stately battle gear and upraised armaments do set quite a scene, but the subject in these sorts of compositions – for me at least – has always been the horse itself, with muscles strung taut, nostrils flaring and hooves ready for the charge. Although the age of cavalry charges and horse-drawn plows has long since past, the horse remains a potent cultural and historical icon – more than 40,000 books have been written on the subject, from Xenophon to Michelangelo – and it might be worth investigating how this remarkable animal came to be. Happily, fossil horses are abundant and we can answer many questions about horse prehistory with some measure of certainty.

The horse fossil record is often seized upon by writers of elementary textbooks as a classic example of how paleontology can help inform our understanding of long-term evolution. Horse evolution, in these instances, is usually portrayed as a sort of evolutionary ‘procession’, with the humble Hyracotherium (invariably described as being “about the size of a fox terrier”) shunted off to the rear and the high-shouldered and gallant Equus leading the parade. Long term readers of this blog will know that evolution is a tremendously bushy affair, and that these sorts of neat, linear progressions are simplistic. Nonetheless, it does capture certain important trends we shall discuss in the succeeding paragraphs.

How do horses fit in, taxonomically speaking, with respect to other hoofed animals?

All hoofed mammals fit into the cladistic group, Ungulata. Hooves are, essentially, the modified tips of toes – and differences in the structure of the foot can be used to divvy up the ungulata into two broad categories: even toed ungulates and odd toed ungulates. Animals in the first category sport two major weight-bearing toes – the third and fourth toes of each leg. This group includes camels, goats, cattle, deer, pigs and a number of other hoofed animals. Odd toed ungulates, on the other hand, support themselves, for the most part, on one toe per foot – the third toe. This group includes horses, tapirs and rhinos.

There is evidence to show that, among the ungulata, horses share a more recent common ancestor with tapirs and rhinos. For one, the odd-toed ungulates all have an extended caecum (an outpocketing of the large intestine that is found in greatly reduced form in humans) that helps ferment and digest the cellulose in grass. They also share striking similarities in the anatomy of the teeth and the ankle bones.

Equus is the only surviving genus of the family equidae, and it includes 3 species of asses, 3 species of zebra and the horse.

Hyracotherium to Equus

Hyracotherium is the earliest known fossil horse. This unassuming animal was about 20 centimeters high at the shoulder and was probably a browser, seeking out its fill amidst the leaf-shrubbery rather than out on a grassy plain, as modern horses do. It lived around 50 million years ago, just 15 million years shy of the extinction of the dinosaurs (the Eocene). It spread throughout the Northern Hemisphere and was, by most counts, an evolutionary success story. But it differs from the modern horse in a number of key respects:

1) The modern horse dwarfs the dog-sized Hyracotherium.

2) Hyracotherium has four toes, whereas the horse sports a single sturdy toe/hoof on each foot.

3) the low crowned teeth of Hyracotherium imply a diet of soft leaves. shoots, nuts and fruits. Modern horses, however, are adapted to high-fibre grasses

4) the brain (specifically the frontal cortex) of the modern horse is considerably larger than that of Hyracotherium.

Why did horses get bigger?

Well, for starters, the fossil record does not tell us a story of uniform progression towards larger body size. For example, some of the distant descendants of Hyracotherium, like the Pliocene horse Nannipus, were even smaller than the earliest horses. The body size of Equids remained roughly constant for several million years before large horses appeared on the scene.

So what selective pressures might have led to an increase in body-size over time? The typical explanation has to do with large body size being a line of defense against predators on the open plains. It may also have to do with the shift in diet from high-quality forage to nutrient-poor high-fibre grass (which was roughly concurrent with the spread of open grasslands throughout the world).

In terms of energy derived per unit bulk, horses cannot process food as efficiently as even-toed ungulates that chew their cud and have a modified stomach with four chambers. The fermentation process that digests cellulose (with the aid of symbiotic bacteria) in the caecum of the horse is almost exactly mirrored in the proverbial “four stomachs” of cows and goats. While they cannot match even-toed ungulates for energy efficiency per unit mass, they can push a greater amount of material through their digestive system in a given amount of time. They are also specially adapted to subsisting on low-quality grasses which their even-toed counterparts could not survive on for long.

Larger animals are able to conserve energy better (on account of their greater ability to retain heat compared to smaller animasl) and this might have been driving force towards greater size. Body size may have also contributed to an increase in running speed.

Hooves and legs

As horses became more adapted for life on seas of rolling grass, they underwent a number of crucial anatomical changes. The length of the bones of the foot increased (a trend we saw in earlier posts on cats and theropod dinosaurs) and the number of hooves decreased, with the third digit becoming more pronounced: these are both adaptations for a cursorial lifestyle. The arrangement of tendons in the lower leg and the connections between the leg bones work to store elastic energy and reapply it with each stride (the so-called “springing step”). Horses have even evolved a way to expend less energy standing up than sitting down.

Grasses are hardy plants and have evolved various means of protecting themselves from plant predators. They inflict heavy wear and damage upon the teeth of herbivores. And, as a consequence, herbivores like the horse, have evolved high crowned teeth, covered in cementum with folds of enamel (hypsodonty) to deal with the tough food. Many of premolars changed to molars. There is also a long gap between the incisors and the premolars that is absent in the very earliest horses, like Hyracotherium. This adds distance between the nose and the eyes, allowing the horse to keep an eye out for predators whilst grazing. Rather fortitiously, it also provides space for the insertion of a bit, an important part of horse riding.

The Horse brain also increased in relative size over the course of the last 50 million years, although the precise reasons for this change are uncertain. It may reflect a major increase in intelligence (a notoriously difficult concept to define in animals to begin with) or may be related to the increasing complexity of the sensory apparatus of the horse.