Why I Am a Christian

Sunday, 30 March 2014

The Behaviour of the Koala. 4. Communication

     Even a solitary, antisocial animal needs to be able to communicate, if only to say: "Get off my turf!" and the three methods of doing so are vocalisations (sounds), facial expressions, and scent. So let us start with the first one.

                                                    Bellowing
     As I reported in another blog, a naturalist was once sent into the aptly named Haunted Hills of Gippsland to investigate the noises attributed to "bunyips". They turned out to be the calls of male koalas in the mating season. Not many city slickers know that these little teddy bears go into rut. The bellow is an awe-inspiring sound, audible up to 50 metres away, sometimes as far as 100 metres. At times the whole Sanctuary fairly reverberated with the sound.
     Typically, a male wakes up, points his head to the sky, and inhales deeply, producing a long, tremulous, "snoring" sound. Suddenly, the air is expelled with a sound like a loud belch, his diaphragm contracts sharply, his head is thrown back even further, and another sequence of "snores" and "belches" follows. A bellow is most likely to last about 15 seconds, but one actually continued for 2½ minutes.
     A subjective description such as that is all very well, but it is useful to put it on a more objective basis, and for that we need a sonogram. This involves, first taping the call, and then playing the tape through a machine which records the frequency of sound against time. These sonagrms were taken from my original scientific paper. The horizontal axis displays the time in seconds, and the vertical the frequency in kiloHerz, or thousand cycles per second in the range most accessible to the human ear.
     The first section is the snore, which you can see as a series of pulses separated by 0.02 second to (at the extreme) 0.2 seconds. When the taped is slowed down, they can be distinctly heard as tiny coughs. Presumably, the represent the vocal chords snapping shut. The belch, on the other hand, is much louder, briefer, and more forceful, the pulses being too close together for the human ear to distinguish.
     During my study I performed a great deal of statistical examination, but the figures I am about to quote are relevant only to those years. As will be explained in a later chapter, things are different now. The period of most frequent bellowing occurs at the same time most young are conceived. For the first six months of the year only one every two hours was given. This started to rise in July or August, reaching a peak in October at between four and nine per hour, and starting to tail off by December. Furthermore, in the first six months 37.5% of bellows were weak, compared to only 8.8% for September to December. A weak bellow may be just half hearted, but it is not uncommon to see a male, especially at the start of the season, straining to the utmost, yet only a hollow, rattling sound comes out. Presumably male hormones are required to induce bellowing, and even more are needed to enlarge the vocal organs sufficiently to produce the complete sound.
     Bellowing begins more or less abruptly at the age of three, reaching its peak a year or two later, and after that gradually declines. For example, at the height of the 1973 season, first-season males bellowed an average of once every 4.0 hours, compared to once every 2.8 hours for those in their second and third season, and once every 4½ hours for the rest. They were much more likely to give a weak bellow, and normally did not start bellowing until September, being still immature before then. One of them, however, kept it up until February, by which time he had the field very much to himself.
     Bellows could be divided into three groups: spontaneous (45.7%), replies (34.8%), and provoked (19.5%). Spontaneous bellows are those made without any obvious external cause, although a bout of three or three may be given at once. They were three times are likely to be weak as a reply or a provoked bellow, which suggests that a reply is a special case of provocation, but both are different from spontaneous bellows.
     Replies are given immediately after the bellow of another male, often setting off a long series of responses, and the percentages of responses did not change with age. Moreover, even "spontaneous" ones were more likely to be given within a short period of another male's bellow, as if the second male had been "primed" by the first.
     A number of other situations would elicit a bellow, most of them concerned in some way with aggression. The more severe the fight the more likely was a bellow to follow. When males fought, it was generally the victor who bellowed, but a male would often bellow even if he had lost a fight with a female. Bellows also, but less commonly, were given at the mere sound of nearby aggressive interactions, or after an attempt at copulation. In all cases, a bellow was more likely to be provoked if there had been bellowing going on beforehand.
     The best hypothesis that fits all these data is that bellows are produced when the bellowing drive reaches a certain threshold. After a few bellows the drive falls abruptly and then slowly rises. However, certain stimuli, such as the sound of another male's bellow, aggression, or the sounds of aggression, can boost the drive higher so that it released earlier, and if the drive is strong enough when the "boost" occurs, bellowing will be instantaneous, and so classified as a reply, or as provoked.
      The average layman would probably call it a "mating call", but ethologists tend to avoid such terms, because they imply function and motivation. It is better to record the situations in which a call is made, and deduce function from that. Looked at this way, the bellow is neither a mating call nor a territorial display. However, it certainly acts as a challenge when given during a lull in a battle, and it may arouse interest in a female on heat, buy mostly the only effect is to elicit a response in kind. Probably, in the bush it chiefly serves to indicate the male's position, size, and virility. What the hearer does will then depend on his/her own sex, size, and familiarity with the area. I do, however, remember playing a recording of a bellow to a certain male, who trembled violently whenever he heard it.
     Only 6% of bellows were made by females, and only a quarter of those were as strong as full scale male call. It is part of oestrous behaviour ie when she is on heat, but most female bellows were given during aggression, and the female frequently faced her opponent instead of pointing her nose in the air.
     One last point: when I first heard this sound, I immediately labelled it a "rumble", and if you go through my notes you will see annotations such as rm1 or rm2, indicating the intensity of the rumble. However, when it came to writing my thesis, it suddenly occurred to me that "bellow" would be a much better term. And now I am pleased to discover that the term I invented has now passed into general usage, even by people who have never read my original research.

Other Vocalisations
      I shall spare you the sonagrams of the other koala vocalisations. Suffice it is to say that they fit onto two continua: of unstructured sounds, and those with relatively pure notes, and harmonics. (A harmonic is a secondary tone vibrating at some multiple of the frequency of the primary tone. You can see this quite clearly in the bellow sonagrams above, particularly the lower one.)
     The unstructured sounds include the squeak, the typical vocalisation of the young which, in older individuals, develops into a squawk, as an indication of mild aggression. Adult males hardly ever use it.
     The grunt is a low, guttural sound made with the mouth closed, and is a sign of mild displeasure. Harsh grunts, on the other hand, are coarse and guttural, the sound of a wild beast, and visitors are usually amazed to hear them issue from the throat of a koala. They are most commonly heard during fights between males.
     The last three vocalisations are the province of females. A snarl is a rasping sound which can be imitated by raising the back of the tongue and vigorously expelling the air. A wail, on the other hand, is a long, drawn-out, more or less pure sound with several harmonics, and may possess a plaintive quality. It tends to merge into a scream, which is louder and even more prolonged. Generally speaking, the scream is a sign of acute fear, the snarl denotes anger, and the wail, mingled anger and fear.
     Sounds intermediate between a bellow and some other call are also occasionally heard from extremely aggressive females. Often she will be snarling loudly, and then suddenly take a deep breath, which then sounds like the "snoring" part of the bellow.
(a) lip raised, ears forward     (b) lips forward, ears relaxed
(c) lips back, ears back                                                             

Facial Expressions
     Facial expressions are also important in communicating emotion. Generally speaking, mammals which live in groups, like men and dogs, have very mobile faces, while solitary species such as koalas, possess only a few general expressions. Nevertheless, they do follow a pattern. As a general rule in mammals, the more aggressive the individual is, the more its facial features are pushed forward, whereas fear pulls them back. You can easily see this in dogs. If a dog's ears and lips are pulled right back, he is really scared of you, and would much prefer to run if it weren't (say) cornered. On the other hand, if its ears are pointing forward and its lips form nothing more than a circle under its nose, then back off! It's about to attack.
     Similarly, the female depicted above in (a) is angry, and probably snarling, for the snarl is typically given with the lips raised. The female in (b) is aggressive, and is probably giving one of those half-bellows, while the female in (c) is screaming in fear.

Scent-Marking
     One of the draw-backs of studying the behaviour of mammals is that, for the most part, we haven't a clue what they are smelling, their sense of smell being several orders of magnitude  greater than ours. For instance, most males can tell by a female's odour whether she is in the fertile stage of her cycle, and ready to mate. They can also gain as much information from another individual's odour about its age, sex, and physical condition as they can by sight. We must also assume that Lone Pine Sanctuary is saturated with the smell of so many individuals in close proximity, but how that affects them we cannot tell.
     This fellow, for example, had just been introduced to a new pen, and apparently was motivated to impress it with his own olfactory brand. It is during the breeding season that males begin scent-marking. Each has a gland in the centre of his chest which secretes a smelly, oily liquid. When marking, the male grasps the bole of the tree (the gentleman at left was remiss in that regard), flattens his chest against it, and rubs it up and down as much as half a dozen times. There is much individual variation, but the peak of scent-marking is reached at age three or four, the same as for bellowing, and it can be shown to be induced by the same factors which elicit bellowing. Sometimes, in fact, bellowing and marking were performed at the same time. It was, however, less than one twentieth as common as bellowing, and is often quite spontaneous, but more likely to occur if the male has just been introduced to new surroundings.


Now, go back to
Part 1. Background
Part 2. Basics
Part 3. Bringing Up Baby
or forward to
Part 5. Sex
Part 6. Fighting
Part 7. Comments and References