Sunday, 30 March 2014

The Behaviour of the Koala. 3. Bringing Up Baby

     One of the most peculiar myths, which may or may not still be current, is that these gormless marsupials spank their children. I don't know where it started, or how, but it was certainly reported as a fact by Ambose Pratt in his 1937 book, The Call of the Koala. Once a visitor to Lone Pine told me, in all seriousness, how she and her husband had once heard heard a loud crying or wailing in the bush, looked up, and saw two adult koalas - Mummy Bear and Daddy Bear, no doubt - chastising Little Baby Bear. The little offender would be turned over one parent's knee and, after that parent had finished paddling its posterior, it threw the child to the other parent for a repeat performance. She recounted it with such visible sincerity, that I would have been tempted to believe it, if it weren't obvious nonsense, and it became a lesson to me for the next time I heard some other improbable tale related convincingly. It is not just that I never observed such a thing myself. It is that, first of all, nothing a baby koala could do could possibly merit punishment and, secondly, the animals do not possess the fixed motor patterns which would enable them to do so. So, with that in mind, let's look at what really happens with bringing up baby.
     A female koala has only one baby at a time. On rare occasions, twins have been recorded, but there is rarely enough room in the pouch for two. The situation we are familiar with in kangaroos, where the pouch opens forwards, is actually unusual, and an adaptation to their vertical posture. In most marsupials, the pouch opens backwards, which means that the newborn has only a short distance to climb to reach the pouch. Koalas are in the same boat, and since their posture is also vertical, the sphincter muscle of the pouch opening closes very tightly, like a drawstring, to prevent the occupant from falling out.
     To my great regret, I was never able to witness the actual birth of a koala. However, I have been informed by staff members who had been so fortunate, that it is just the same as in other marsupials: a tiny neonate, the size of a jelly bean and more embryo than baby, comes out and makes its brief climb into the pouch and attaches to a teat. Once a crew from Film Australia, or whatever it was called then, came and asked me to identify a female expected to give birth within the time frame of their visit. They then set up vigil with camera and lights in front of a bemused looking little lady perched in the fork of a tree. Alas! The baby managed to pop out sometime - I think it was at night - when they weren't watching.
     Gestation (pregnancy) varies from 34 to 36 days. Once the mother has mated, her pouch starts to visibly develop. However, that does not mean that she has conceived; it happens whether the mating was fertile or not. You only know that birth has occurred because the pouch sphincter is now very tight. If not, the female comes back into heat and mates again.
     Another six months or so passes before a tiny head or arm protrudes, and the baby first emerges completely at seven months, by which time it weighs about 230 gm (8 oz.). At this point, let me correct another misconception. Nearly every text you read will tell you that the young first emerge at six months. Not so. I kept track of a small cohort of young whose ages were known to within a couple of days. At age 4½ months one poked its almost naked head out of the pouch. This was regarded as unusual, and it died soon afterwards. Apart from that, the youngest to protrude a head or arm were 173 and 174 days old respectively. When two emerged at 182 and 196 days respectively, they were only partly furred, and the Sanctuary staff considered it to be unusual. Indeed, the younger one was so curled up that it needed human help to return to the pouch. In the wild it would have died. The oldest fully furred young seen to completely emerge were aged 209, 209, 220, and 267 days respectively. So the evidence is clear: the correct age of emergence is seven months, not six.
     I might add that I once wrote to the editor of a reference book about this. He, quite rightly, referred my letter to the author of the "koala" section. The latter then admitted that he had been remiss in not checking the older papers. I find this rather disturbing - and not just because I miss out of the kudos I feel I deserve. You must understand that zoologists obtain their background information in a manner analogous to web searching. On the web you follow link after link in a chain. Likewise, if a zoologist is thinking of investigating a particular species or issue, his first act would be to look up a good comprehensive reference book or article. At the end of this will be a list of primary references which the author had quoted. He will then read these primary references, which will themselves contain further references, and so on. However, if people do not read the "older papers" to start with, then they will not appear in the references, and they will be forgotten to science.
     Anyway, the youngster is now seven months old, and has come out of the pouch for the first time. For the next six or seven weeks it moves in and out of the pouch, until it is just too big. After that, it is carried on its mother's back. It always lies belly down, holding onto her fur with all four legs. It never rides jockey style. As I mentioned before, the young were usually removed from the mother at about 12 months, which would appear to be about the normal age of independence. Removal before that age causes distress.
     At first the cubs gain an average of 285 gm (10 oz) a month, but in the 14th month this suddenly drops to 130 gm (4½ oz), with much variation. For the first born cubs, the magic date occurs at the end of the year, after which sunlight, heat, and rainfall all decline. This must affect the food they receive, and it shows. The first three to reach the age of 700 days (23 months) averaged 3.7 kg (8¼ lb); the last, four months later, was only 2.8 kg (6¼ lb). The two youngest both grew slowly and died at the age of 14 months. In other words, those born later in the season have a reduced chance of survival - which is why the season has to come to an end.
     What is quite remarkable is that a mother does not even clean out her pouch like other marsupials. The baby finally emerges covered in its own tiny droppings, which also line the pouch, but afterwards it always appears clean, so presumably they defecate outside. Yet this lack of hygiene has its dangers. A couple of babies have fouled their pouches and died.
Baby suckling
     At first the baby sleeps in the pouch. When it gets too big, although it rides on its mother's back, it sleeps curled up in her lap, facing her belly, in exactly the same posture as an adult sleeping in a tree. Suckling occurs with the baby either sitting in its mother's lap or draped over her knee, and lasts from anywhere from a few minutes up to half an hour. The oldest cub seen to take milk was 13 months old. If her limbs are in the way when the baby wants to return to the lap, it grasps her fur, puts its head in between the offending limbs, and moves its head from side to side until either its mother raises her arm or the baby is forced to give up. Exactly the same movements are made when it wants to return to the pouch or take milk. I might add that the movements of the youngster obviously cause discomfort to the mother, but she always remains extremely tolerant.
     For that matter, all adults are very tolerant of youngsters. Remember, in the wild they tend to be widely dispersed, but if a male happens to encounter a baby while wandering around his domain, there is a good likelihood that he is the father, so he might as well treat it gently. Even after being weaned, juveniles will climb upon the back of any passing adult, so it is not uncommon to see a young, but very tolerant adult staggering under the weight of an overgrown juvenile. But what, however, can be made of the juvenile who gave up riding after being weaned at 12 months but, still too young to breed, regained the habit in the next two breeding seasons, making a considerable nuisance of himself?
     I've already mentioned how a baby koala learns to eat leaves. The youngest one seen to do so was aged just 217 days, the first time it emerged from the pouch. But the normal digestive enzymes of mammals is insufficient to digest the cellulose in leaves. For that, the koala has a very large caecum - the equivalent of our tiny appendix - attached to its bowels, full of micro-organisms, which are able to do the job for it. Koalas, in other words, are what is known as "hind gut fermenters". Therefore, when the baby is between about five and eight months old the mother produces a special, very sloppy type of faeces, which the baby then devours. It is presumed this contains the micro-organisms necessary to equip its own caecum. I saw the phenomenon only four times, but back in 1937 David Fleay watched a single cub take ten such feeds over a period of 23 days.
     As the baby grows older it gradually makes longer and longer excursions from its mother, exploring the branches and leaves within a foot or two of her, but always keeping a close eye on her. Frequently, the child vacillates very noticeably between the attractions of the environment and a desire to return to its parent.
     But the amazing thing is: I saw no evidence that the cubs recognize their own mothers. I suppose in their solitary life in the wild, any female and cub in the same tree are bound to be related. When the young are nine or ten months old, the nursing females are put together in the same pen, after which a baby can be found on any female whatsoever, and if brought to the main pen, they will happily ride on the back of any adult which, as I have already mentioned, are just as tolerant as the natural mother. If it wants a feed, the baby will try to suck from any female, and will not be repulsed. I once watched a cub, almost independent, walk nearly two metres to climb onto a female's back, but just as the latter was setting off, another cub approached and crawled under her belly. The female sat on the floor with one cub on her back, while the either fed for 27 minutes before disengaging. Indeed, a female was once found with two cubs in her pouch; her own, aged 217 days, and another probably a month older. Another female had a strange cub in her pouch, while her own, smaller baby lay dead on the floor. No doubt the pouch had been too small for both.
     If a baby is removed from the back of an adult, it squeaks loudly, frequently loses control of its bladder, and seeks desperately to get back. I soon learnt that if I placed one of them on the floor and moved my feet a bit, the cub would, as sure as fate, pounce on one of them and eagerly climb my leg. If two cubs were placed on the floor, they would try to climb onto each other's backs. Older cubs are not so desperate. They will calmly look around and walk to the nearest adult without squeaking, and those older still merely wait for an adult to pass by.
     Furthermore, mothers do not appear to recognize their own offspring, but will accept any cub. Again, this is no doubt a consequence of their normally solitary lifestyle, where any cub in the vicinity would almost certainly be her own. However, they do have a "lost child response". Mothers temporarily without a baby (who is most likely asleep on the back of some other female) can be seen wandering around uneasily, as if looking for something. If a cub - any cub - is placed on her back, she will usually settle down.
     I used to go around making babies squeak or playing a tape of a squeak to watch the adults' reactions. Although the results were not completely conclusive, a distinct tendency emerged. A mother without a cub on her back would immediately approach the source of the sound and, rising up, place her hands on my legs. In fact, much to my consternation, one not-so-tame female recently introduced from the bush actually climbed up my legs! She generally settles down if the baby is given to her, or if the squeaking ceases. Mothers with babies still on their backs are not so concerned. The "lost child response" is much weaker for males, non-nursing females, or mothers with pouch young, but there are exceptions. The first time I played the tape, a couple of males became very agitated and headed for the source of the noise. Indeed, it was rather funny to see how spellbound one of those males was when the majority of mothers, whose young were still in the pouch, couldn't care less.

Now you may go back to
Part 1. Background
Part 2. Basics
or forward to
Part 4. Communication
Part 5. Sex
Part 6. Fighting
Part 7. Comments and References