The Horse and the 22nd Sense.

You know, there are things in history that make one wonder whether the famous philosophers of ancient times weren’t subject to those same little interruptions that molest more modern thinkers. Take Aristotle for instance; he got up to five senses, and stopped! We’ll never know whether it was the dinner bell or a particularly good debate in the forum, that stopped him there but, since then, scientists reckon they’ve found a further 22!

So, recent talk of ‘a’ sixth sense as a result of animal behavior during the approach of the Tsunami can be ignored as journalistic drivel. But the actual behavior of those animals is another thing entirely, and perhaps deserves more thorough treatment than being passed off as a brief human interest story centred on an Elephant plucking two children from the path of the wave and running with them into the forest.

First, let’s deal with that ‘sixth’ sense, to which science has given the rather unwieldy name of ‘proprioception’. If, while he was rushing to catch the debate, Aristotle had continued pondering the question of senses, he might well have wondered what physical mechanism was responsible for keeping him informed about the angle of his limbs during his hurried dash. Each articulation – the scientific word for joint – is designed to operate through a certain angle only; over-articulation is limited by little stops on the bones and by series of check ligaments, but these bone stops can be worn away gradually by rigorous exercise (most particularly when young horses are subjected to high stress exercise by being brought into training too early) – and check ligaments can be torn under conditions of tiredness or high physical stress. It is the job of proprioceptors within each articulation to report excessive angles, just as skin reports the degree of heat or cold. As soon as such a message is received by the brain the order is given for the body to slow and to stop before serious damage occurs.

Why, you might ask, is understanding this ‘sixth’ sense of particular importance to those who ride or exercise horses? And in answer one simply has to look at what can happen in horse-racing when over-articulations occur at high speed on the final bend as tiredness sets in – a broken leg, followed by euthanasia behind a canvas screen set up on the racetrack – and a young horse dead well before its time. Is there a second when the horse attempts to pull up, only to be driven on by the wind-milling arms of the jockey and the touch of his whip? Most likely yes, but the jockey’s thoughts are all on the winning post, not on the signals his mount is receiving from articulations being subjected to the enormous stress of the full escape-speed gallop. The same could be said of the show-jumper, whose landing at extreme fetlock angles after a big jump exposes it to stresses beyond normal design tolerances – for no free horse in its paddock, or wild horse on the moors will jump over such an obstacle, preferring, very reasonably, to go around rather than over. And, in case the racing or show-jumping aficionados feel unfairly singled out we can also mention the lower limb stresses caused by barrel-racing or the sliding stop. With no rider to spur them on, horses whose proprioceptors warn of impending damage pull up safely, suffering no more than pulled check ligaments, just as happens to the human athlete. So when we add our own weight to that of the horse we ride, plus raising the centre of gravity far higher than that of the horse alone we take upon ourselves the duty to listen and be aware for, if the horse is injured during our use as a result of being spurred beyond its safe limits and the reports of its senses, the responsibility is ours.

So much for the 6th sense. So what now do we say about those others, from 7 to 22? How do we explain the ability of animals to, apparently, foresee events we ourselves are blind to? Why was it that so few animals were caught in the Tsunami? Clearly there must be some sense at work, and just as clearly the notion that this is based on precognition is extremely doubtful; if animals had the power of precognition, then catching them or doing anything to them against their will would be extremely difficult!

Since the Asian quake and Tsunami that followed various scientists have proffered theories that could explain the remarkable survival of animals, none of which are new, and an interesting few of which have been given mention in connection with horses in the past.

The first on the list is ultrasound, a range of frequencies above that of human hearing, but to which horses are very sensitive. Ultrasound ‘guns’ or emitters have been fired at race horses with quite disastrous effects, so there is no question that this is part of horse’s sensory range. Both Earthquakes and Tsunami have been proven to produce ultrasonic range sound, so there is little doubt that horses and many other animals would hear the shockwave – the question is would they know what it was, and if not why should they try to escape? Interestingly there is a process, called exaptation, by which complex biological systems evolve in which an existing genetically based behaviour pattern that has evolved for one purpose can become modified so that it serves a second, different purpose. Over a period of time the modification is subject to improvement by way of random mutation and natural selection until the animal is genetically programmed to escape in response to the stimuli – in this case the ultrasonic wave carried through the ground and radiated into the air.

Let’s continue with another stimulus associated with the ear, but in this case the inner ear; that of balance or ‘tilt’. Tests carried out on human subjects showed that a normal person is sensitive to a tilt of 0.1 radian (radian = 57.3 degrees – 0.1 radian = 5.73 degrees of tilt) Unfortunately there are no records of tests on non-human subjects, but it has certainly been established that a number of organisms have far more sensitive vestibular systems in the inner ear than humans possess. Measurement of tilt produced by earthquakes in China and Japan shows it to be just a few micro-radians, but we might reasonably hypothesise that some animals – most likely those that live underground – would be sensitive to these small amounts of tilt. Whether horses might be so sensitive remains untested.

Hygroreception: the ability to sense changes in humidity. Whilst spiders and insects possess specialised hair-like hygrosensensitive sensilla with which to sense humidity and temperature, in vertebrates this is believed to be a part of olfaction and allows an animal to sense such things as buried water. Feral Horses living in the Australian outback have been filmed digging down to reach underground water – to do which they clearly must first be able to sense exactly where to dig. Earthquakes are associated with changes in ground water levels, which in dry areas of the world would be particularly noticeable – but in sub-tropical or tropical areas, where there are high humidity levels in both ground and air it is difficult to imagine how an animal would distinguish such an increase from that preceding a storm. However some pre-seismic animal behavior observed is very similar to pre-storm behavior – so, keeping the idea of exaptation firmly in mind, it has to be possible that in areas subject to frequent seismic activity a seismic hygroception behavior pattern could evolve. Of course we might also guess that there is a better chance of this happening in those animals that possess the most sensitive olfactory equipment – and as the horse has what could very easily be termed a sensationally well developed olfactory array – having not only so great an area of olfactory membrane that it could cover the entire body surface of the horse, but also the veromonasal organ or Organ of Jacobsen it is difficult to think of a more likely candidate.

Lastly we should look at the ability to sense variations in electrical potential and magnetic field strength. And although there may be little evidence, even of an anecdotal nature, for the other senses we have already discussed in the horse there is far more in this area. Horses are far more sensitive to small electrical currents than we are, as anyone who has had any wide experience of electrical fences will be aware. Electric fences are perhaps the best and safest method of controlling horses kept at grass and preventing those ugly little injuries caused by wire cuts, but each time power has to be taken across a gateway a choice has to be made; whether to run the live wire underground through an insulated cable or above using a pair of lightweight insulated ‘masts’. The underground method is generally the cheapest and fastest, but there is a potential problem in that traffic through the gateway over a period of time can create slight wear – and a small escape of power to earth. This can easily result in a gateway through which horses are quite unwilling to pass, most particularly during damp conditions. A couple of times over the last 10 or so years when this problem has arisen I’ve tried removing my shoes in an attempt to feel what the horses feel – always in vain! Yet logic suggests that such a sensitivity must have some purpose, must confer some evolutionary advantage, or why should it become fixed? The White Horse Ethology Project home farm in Northland, Aotearoa is situated on clay soil, and, in combination with the relatively heavy rainfall produces a feature known locally as Toma holes. These occur where underground streams that excavate a passage through the clay come close enough to the surface for the roof of the tunnel to fall in. these holes are particularly dangerous as they can be anything up to a couple of metres deep, steep sided and filled with water. Indeed cattle do at times fall in and, if not found reasonably quickly, either drown or die of exhaustion. Over the years we’ve pulled a couple of very young foals out too – but not once has an adult horse fallen in – leading me to speculate why. Closer observation showed that the adults in family groups appeared to use fan-shaped formations of dung piles to mark both entrance and exits and piles at frequent intervals, from every meter to one per three or so meters, depending on the complexity of that specific part of the route, to mark safe routes through areas of ground in which Tomas feature. Obviously in order for them to be able to mark a route in this way they must be able to sense exactly where that safe route lies. As with the escape to earth of a little leakage from an underground electric fence cable the presence of underground water would result in an alteration in the potential difference, and given the acid nature of clay soils might very well be registered as a very small change in voltage. Equally, electrical discharges associated with seismic activity have been recorded as being strong enough to produce optically visible lights. It does not necessitate too much of an intuitive leap to suggest that such a change would cause animals to move rapidly away from such a stimulus. These voltage changes also precede such things as dust storms and powerful wind events, and have been measured to produce differences as high as 80volts in the forefront of these events. It takes little imagination to see how dangerous weather of this nature can be for animals such as horses, and how potentially useful a survival skill sensitivity to electrical charge would be. Perhaps the real question is shouldn’t we be surprised if animals had not evolved to possess such sensitivity?

And so we come to what is possibly the most complicated of all – the hypothetical ability to sense magnetic field strength variation. Much of the scientific discussion in this area is requires highly specialised knowledge, without which the jargon employed might as well be Martian for most of us! What we are able to say with certainty is that large magnitude increases in field strength have been measured and appear as early as three hours prior to a seismic event. Moreover Bees and some migratory and nocturnal animals have been proven beyond reasonable doubt to utilise magnetic field information to guide movement or travel and to regulate their internal circadian clocks. The way in which this sense operates is one of those beauties of biological design that cause some to speculate that “intelligent design” must surely lie behind evolution. So, even at the risk of ‘blinding with science’ let’s take a look anyway!

Alongside the growth of data on magnetoreceptive organisms has come the discovery that the biophysical response of those organisms sensitive to such fields is due to tiny ferrimagnetic crystals of the mineral magnetite – Fe3O4 for those whose chemistry has passed the test of time! These crystals are formed biochemically and are arranged in linear chains in magnetite-based receptors located in the front of the head. Studies have further shown that the ophthalmic branch of the trigeminal nerve is responsible for acting as the main conduit carrying this information to the brain. I’m not going to go any further with this jargon heavy passage – but the intricate beauty surely justifies the short foray into the scientific realm! Who needs science fiction when science reality is so astounding?

We may not know for certain all those animals in which evolution has refined this particular sense, anecdotally it seems extremely likely that cats, and perhaps horses also, do. And once again the good old electric fence offers a possible insight. My observation sessions to study equine social behavior in the harem groups are frequently accompanied by either or both the farm cats, and at the gateway to one of the range areas I have been lucky enough to witness a curious piece of feline behavior. Next to the metal gate is a wooden stile, on which is located a circuit breaker that allows power to the fence to be interrupted. The supply cable rises from ground level to the switch, which is at mid-thigh height, while the cable carrying power on beneath the ground under the gate drops back down so that the two form a triangle with the ground. Each of the electric pulses that run through the fence cause a magnetic field to be produced and then to decay prior to the next pulse when again a field is generated. Each time one of the cats passes through this triangle they very visibly follow the line of the cables with their eyes first – as if to assure themselves of precisely where to pass through so that no part of them comes in close contact, and for all the world they seem to be actually ‘seeing’ the magnetic field. Of course this makes good sense bearing in mind that it is the optic branch that carries such signals to the brain. Cats, being nocturnal animals might be expected to possess this sense, but it should be remembered that horses are also, at the very least partially, nocturnal too. Plus, evolution has outfitted the horse with a particularly large head, and though the primary purpose is to act as a counterbalance aid to pull the spine straight and allow power to flow through from the powerful quarters it has to be acknowledged that such a head also serves as a very large and efficient receptacle for sensory equipment arrays! Specifically the trigeminal nerve is well represented – so it would not be at all strange if it were to be found that magnetite crystals were produced in the optic branch. Certain equines are also unmistakably migratory, such as the Zebras, and it might well be that Equus Caballus too had migratory behavior patterns back in those times before the plains of Eurasia and North America were tamed by fence and land title.

In the final analysis, if, by such consideration of what it might mean to be an animal in possession of such sensory riches we are humbled by the notion that while we are intellectually supreme in the animal kingdom we may have paid in the coin of restricted sensory ability the end result must be positive. After the world has experienced such a lesson from nature, in which negatives abound, it is surely good to find any beneficial strand – no matter how tenuous.