Year of the Gorilla 5: The dark side of gorillas

Gorillas. Peaceful, placid vegetarians, wanting only to be left alone for a photo op with David Attenborough. It is quite impossible that these gentle giants could have a nasty streak.

Yeah. Right.

Actually, all the great apes have a violent streak. Humans of course are the most creative in this respect, but all our close relatives can be killers too. Chimpanzees go in for tribal warfare, orang-utans go for sexual assault and many wild bonobos are missing fingers from fights.

With gorillas, violence takes the form of infanticide. This is not just an occasional accident, but a deliberately pursued policy of attacks on infants, especially by solitary silverbacks, with the goal of persuading females to join new groups.

The rationale behind the attacks seems to go like this:

1. A female with an infant is unlikely to leave the group she is in unless the silverback dies.

2. Silverbacks are hard to kill, especially if they have a group of breeding females to help them.

3. If a female loses an infant, she is likely to abandon the silverback who failed to protect her baby.

4. Even if the male whose group she joins is the one who killed her baby in the first place

Even if a female without a baby joins a new male, if she gives birth soon after joining, there is a good chance that the silverback of the group may kill her baby, despite what are usually vigorous attempts at defence.

This kind of behaviour is widespread in many animals with a reproductive system of a single (or small number) of males and a larger group of females, especially when raising a newborn to independence takes a large part of an adults life. By killing unrelated infants, the male increases the chance of his own offspring being raised before he is replaced in his turn.

So far at least, infanticide has mostly been observed in Mountain gorillas, where infanticide causes about 37% of infant deaths (total deaths in the first year of life run at about 34%). Putting these two numbers together, it suggests that 1 in 10 baby Mountain gorillas are killed by their own kind before they are one year old.

In Western gorillas, infanticide (so far) seems to be less common. This is probably because, as mentioned in a previous post, Western gorillas seem to live in extended clans of related male family groups, and any unattached males are likely to be related to any infants they encounter.

The heavy poaching that Mountain gorillas have experienced has probably made matters worse. Silverbacks are the first to be targeted by poachers, and the result has been a breakdown of gorilla society where unrelated males, fleeing poachers, have moved into areas where they are unknown and any infants are not related to them.

All this has implications for gorilla conservation, especially the release of male orphans into the wild. Release of human-raised or captive bred animals is problematic in any case, because of the risk of exposure of wild animals to human diseases caught in captivity and the difficulty in teaching true independence to an animal which has not had the opportunity to learn how to survive from its mother. Releasing even successfully rehabilitated males into a social environment where they do not know the males in the wild groups they encounter might well do more harm than good, resulting in increased attacks on infants by the new males and a negative impact on the wild numbers.

In future posts, I will discuss how orphaned gorillas are cared for and what can be done to give them at least near-normal lives.

Picture - Salome and Komale, Bristol Zoo gorillas

Bristol Festival of Nature and other news

This morning I went down to the Bristol Festival Of Nature down by the harbour side. This has been going on for several years now and is basically a shop front for a huge variety of societies and conservation organizations, especially those in the Bristol Area. The zoo was represented of course, both by volunteers looking after some invertebrates, and representatives of the Education department – Bristol majors on education and has at the last I heard over 10,000 pupil visits per year. For full details of the programme – which continues tomorrow – see here: www.festivalofnature.org

There are loads of activities for children, and important local groups like the Avon Wildlife Trust and Bristol Naturalists, as well as national groups like the RSPB, The Hawk and Owl Trust, and the Wildfowl and Wetlands Trust.

On other news, as part of the publicity and fund raising for the Year of the Gorilla, one of our Sunday volunteers (the day I work) is doing a sponsored sky dive. All I can say is that she does not look the type to jump out of a plane at 10,000 feet (especially wearing a gorilla costume) If you would like to sponsor her, go here: www.everyclick.com/emmaoneill

I will be posting next week on the Bath & West show, which was a great success - have agood weekend everyone - I have to go paint the hall.

Land of the Dodo 10: The other pigeons

Aside from the Dodo and the Rodrigues Solitaire, there were at least four and possibly five endemic species of pigeon inhabiting the Mascarenes, of which only one, the Mauritius Pink Pigeon (shown at top of post), barely survives. In addition the Madagascar Turtle Dove was present and either survived or was reintroduced from Madagascar.



The main causes of the extinctions were rats, monkeys, and most seriously hunting. Judging from the behaviour of the surviving Pink Pigeons, the birds had practically no anti-predator responses and would not learn caution, or at least were wiped out too quickly to acquire any.

The pigeons can be divided into two genera, the first, Nesoenas with affinities with Africa included the Madagascar Turtle Dove and the Pink Pigeons (two species on Mauritius and Reunion, of which the Mauritius form survives), the second, Alectroenas (Blue Pigeons), with Asian affinities and species on Mauritius, Rodrigues, and probably Reunion. Why the Blue pigeons died out is not altogether clear – Alectroenas pigeons are still widespread on the Comoros Islands and the Seychelles despite human presence. Possibly habitat destruction overwhelmed them too quickly to adapt. The loss of the Mauritius Blue Pigeon, known as the Pigeon Hollandais because its colours resembled the Dutch flag, is a great loss to the islands – a picture of the Seychelles Blue pigeon is shown here:

As well as losses, several species of pigeon have been introduced to the islands – the ubiquitous feral pigeon of course, as well as Spotted Dove and Laughing Dove (both from Africa) and the Zebra Dove of South East Asia. These do not seem to compete directly with Pink Pigeons but they are potential reservoirs of disease.

Today the Pink Pigeon is the subject of intensive effort to rehabilitate the species. Bristol Zoo has bred them in the past, but it has proven extremely difficult to breed outside of Mauritius, and there are currently 49 in European zoos (including four at Bristol), with another 43 in the USA. In the islands breeding and protection of wild nests has boosted the population, and as of December 2008 the wild population stood at 393 birds. This sounds very small, but is a considerable improvement on the situation in 1986, when only 12 birds could be found at the only site they survived in.

Aside from predators, the main threat appears to be habitat degradation. Pink Pigeons are adapted to feed both on the ground at at the end of long branches, and deforestation and introduced plants do not provide the correct food. Fortunately they take readily to bird tables, so supplemental feeding is easy to provide. Indeed, there was an attempt to release birds in one of the botanic gardens on Mauritus. The birds survived, and even bred, but were wiped out by small boys with catapults, for who they made easy targets.

Despite the setback, new populations are being created. Most recently a new sub-population using translocated birds was established at the lower Black River Gorges, and the first fledgling left the nest in September 2008.

As the Reunion Pink Pigeon was closely related to the Mauritius form, which was its ancestor, it might be possible in the future to reintroduce them to Reunion. Unfortunately, at this point politics intervenes. Whereas Mauritius, with Rodrigues, was a British colony and is now independent, Reunion is a French Overseas Department, so there is less connection between conservation on Mauritius and Reunion than there should be.

Return of the Crane

Back in 1981, I was on a bird watching holiday in Norfolk in the east of England. It was the last day, and we had gone into the Norfolk Broads to see what we could add to our list for the week. It was a very foggy day, but we stopped to see what we could find in a field, soon locating a few Golden Plover. At this point, we heard an unfamiliar trumpeting sound coming from the low clouds, and Brian Bland, who was leading the group, immediately said “Those are cranes!” Naturally, we were surprised by this, as Eurasian Cranes are scarce passage migrants in the UK, but we looked towards the approaching sound and made out three cranes which came and landed only a few hundred metres from us. They were not adults however, but a pair with a just fledged chick – the first to be bred in the UK since the 17th century.

It still remains the greatest birding experiences of my life.

Eurasian (they are usually called Common) Cranes are indeed one of the worlds commonest species, with an estimated world population of perhaps 275,000 birds and a range from western Europe to eastern Siberia, but in Britain a combination of drainage of wetlands for farmland and hunting for food (they were a centrepiece of medieval banquets) resulted in their extinction as breeding birds, only showing up when migrants wintering in France and Spain passed through on their way north to Scandinavia and Siberia. A few years before my sighting, a young pair had taken to hanging round Norfolk, and when they apparently vanished in the spring there was much speculation – which happily proved to be correct.

Unfortunately, there is comparatively little breeding habitat available at the original site, and the population since then has grown very slowly, now numbering perhaps 20 or so individuals, despite other wild birds having been attracted to the site to join the founders. For that reason, it has been decided to attempt to establish another population using captive raised birds from wild laid eggs. The first site for the attempt has now been announced as the Somerset levels not far from Bristol.

The project is being coordinated by the Wildfowl and Wetlands Trust at Slimbridge, and when I went there today some of the previous year’s juveniles and subadults were on show. Some of these, plus ones raised in the next year or two, will be transferred to a predator free soft release pen on the levels, in which it is hoped they will eventually breed and establish themselves in the area. Roger Tippett, the president of the Severn Counties Foreign & British Bird Society (see link), gave a very interesting talk on his work as a volunteer at Slimbridge in the Crane School they run each year. Roger will be at the SCFBBS stand at the Royal Bath & West Show this Saturday (as will I) – come and have a chat if you are visiting!

For more information on Cranes, see the WWT trust release here: http://www.wwt.org.uk/text/464/eurasian_cranes.html

Gorilla society

“Gorillas live in family groups of several females and offspring led by an adult male, known as a silverback from his distinctive hair” – that, at any rate, is the standard description you will see outside any gorilla enclosure in any zoo in the world. In reality, the actual situation is rather more complex.

For one thing, as I described in my previous post, gorillas are actually two species, the Western and the Eastern (including Mountain) gorillas. Unfortunately, until recently practically all knowledge came from studies of Mountain gorillas, and it is becoming clear that the Western Lowland gorillas do things differently.

Firstly, although it is true that all gorillas are born in a group, males often do not stay there. On reaching maturity, males will leave and live by themselves until they manage to acquire females to start a new group. Unsuccessful males will live their entire lives as solitary as orang-utans.

In some cases multiple male groups have been seen in both species, but this seems mainly to occur when a silverback dies and there are several sub-adult males among the survivors. Silverback gorillas are such powerful animals that for an adult male there are essentially no natural predators other than humans, so there is no need to band together for mutual defence.

In Mountain gorillas at least 40% of males do not leave, instead remaining with the silverback and joining a small core group of silverbacks in a much larger band. The largest of these groups so far observed had no less than seven silverbacks and the group was over 60 strong.

In a few cases groups with more than one silverback have been seen in Western gorillas, but these are very rare and in most cases seem to involve a father and son situation.

Females usually leave their birth group at around nine years of age, when they reach reproductive age. They may join a lone silverback to start a new group, or they may join an existing group. Even when they join a new group, they may not remain there, especially if they lose an infant.

Recent work on habituated Western gorillas shows an interesting difference between them and Mountain gorillas. Whereas Mountain gorillas are fiercely protective of their females and invariably attack any unknown males they encounter (sometimes with fatal results), Western gorillas seem to associate much more peacefully, even allowing females to sit next to a silverback from another band and then return to their own family. Examination of DNA shows a possible reason – it appears that the silverbacks in these cases are closely related and probably grew up in the same band. It appears that the dispersal of males from Western Gorilla bands may be an illusion – they remain close to the birth groups and in touch with their erstwhile companions. This raises an interesting possibility – that the true nature of Western gorilla society is not the single band but a network of family groups – a sort of “suburbia” if you like,
If this is the case it has implications for how Western Gorillas are looked after in zoos. Although the focus of all management is on creating a single breeding group, it might be better to aim for much larger facilities holding several groups (preferably led by related silverbacks) with facilities for holding non-breeding males either by themselves or in bachelor groups in at least visual contact with their former families