Safety in Numbers

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While on a short safari a few years ago, I spent a day or two in the Ngorongoro crater. It’s an extraordinary place, a giant caldera formed by an extinct volcano, one of nature’s own animal reserves. The guide pointed out a bat eared fox as it passed our truck. ‘He’s off to lunch,’ said the guide, ‘He’ll be tucking into a flamingo shortly.’  Some time later we reached the lake where thousands of the birds congregate, eat and mate and there on the shore was the fox munching away at a flamingo. The flamingos could be seen, and presumably smelt, over a huge distance so the fox was pretty much guaranteed a meal every day and it immediately begged the question as to why these birds advertise themselves as lunch boxes.

 

It is certainly true that had the flamingos dispersed themselves around the crater in their delightful pink plumage, they would still have been a clear target as prey. And it is equally true that the food they enjoyed could be found nowhere else but in the lake. So there are good reasons why thousands of them flock to the same spot. But finding safety from hungry predators is surely not one of them. Where, then, does this notion that there is ‘safety in numbers’ come from?

 

There are a host of examples where predators are given easy access to food because the ‘food’ has made itself obvious by moving around en-masse. One of the best examples has been shown by David Attenborough in his ‘Blue Planet’ series and later in his series on ‘Natures Great Events’. Most years, millions of sardines migrate from southern Africa up the Mozambique coast. Sharks and dolphins, as well as seabirds such as cormorants and gannets, swoop on them and gorge themselves on the giant shoals. The fish form ‘bait balls’ that spin like dancing dervishes but, nonetheless, predators seem able to feed on them at will. The bryde’s whale apparently scoops up 10,000 fish in one mouthful. Researchers are not clear why the fish travel north from South Africa but, for reasons of their own, they do. What is clear is that travelling in giant shoals results in certainty that fish and birds who enjoy sardines will find them.  Dolphins and sharks have excellent senses of smell, hearing and sight. A shoal of a million sardines would be noticed many miles away. So why would they behave in a way that guarantees the attention of predators?

 

There are many similar examples on land, witness the hours of TV footage showing lions and cheetahs tracking wildebeest or zebra and picking off one for the family meal. It is almost as simple for the lion to find an army of wildebeest on the plains of Africa as it is for the bat eared fox to find a flamingo on a local lake though they do, it is true, have much less success. Like the flamingo, one would have thought that, whatever the reason wildebeest and sardines behave like this, it isn’t to ensure they are not noticed by those who may wish to eat them. It is odd then that researchers suggest that the reason is precisely because there is ‘safety in numbers’.

 

One reason put forward is that the changing shape and colour of sardine shoals as they twist and turn into ‘bait balls’ is confusing. A predator is not quite sure what it’s aiming at and may well miss. This does seem feasible. There are, however, two immediate problems with it. Firstly, the predators’ confusion is only a tendency. David Attenborough talks us through one such attack in which dolphins’ corralling techniques take the sardines closer to the surface enabling them to pick off the entire shoal one by one (which greatly assists predator birds). The confusion, if any, clearly did not last. And for the bryde’s whale it may well make the job of consuming 10,000 fish in one swoop even easier. The second problem is that, if the fish had not shoaled in the first place, they would not have needed to invent defensive tricks. If they had dispersed themselves throughout the entire Atlantic Ocean would they not have been more difficult to find? Attenborough implies this by explaining how difficult it is for predators to find small shoals in an entire ocean. How much more so if sardines swam individually?

 

W D Hamilton, one the twentieth centuries great biologists, theorised that there was a geometry to a ‘selfish herd’. This would entail each member of a herd of, let us say, wildebeest positioning itself such that, when one of them gets eaten, it won’t be him. Normally this would mean not being placed at the periphery but manoeuvring oneself into the centre. This is very plausible but is still an explanation only for defensive tactics in a herd, not the reason for herding.

 

It seems that, whereas these researchers keep attempting to find an explanation and purpose for herding, they may have missed the point. Perhaps there is no purpose, as such, to herds or shoals or flocks, just reasons. Wildebeest herd and flamingos flock because that’s where their food is and mates are found. Sardines migrate in shoals because they must all get to another part of the ocean for an as yet unknown reason. These animals have all found a way of surviving otherwise they would not be here to be discussed. But that survival is not attained by living in groups. On the contrary, living in groups endangers lives because, for predators, groups act rather like the neon signs advertising eateries on American highways.

 

What we find instead is that, for reasons involving food and procreation, mass grouping is just inevitable. So, once in a group, the best a creature can do is to develop behaviours and other defences such as colouration which mitigate against being eaten. The prime defensive behaviour for a species like the sardine is to produce young in the millions. This is no use for any particular individual but does ensure that a few survive to enable the species to continue. Indeed, this particular evolutionary development provides strong evidence of precisely a lack of safety in numbers. If a million get eaten the least a creature can do is to ensure a million and two are born. Species who didn’t are no longer here. For individuals, they are lumbered with the inevitability of travelling en-masse and end up with swirling ‘bait balls’ and ‘selfish herd’ geometric manoeuvres. And no doubt there are many other tactics that attempt to ensure that, if they must live in herds, animals can reduce the chance of ending up as lunch.

 

There is one other ‘explanation’ which is the most common given by researchers for the purpose of herding and relates specifically to the notion of ‘safety in numbers’. The theory goes that if a fish is in a shoal of ten it has a one in ten chance of being eaten. If it is in a shoal of 1,000 it has only a one in a thousand chance. Therefore, it is far safer in a large shoal.

 

This explanation creates more questions than answers. First of all, why would the predator stop at one? After the shark has eaten its first fish it will presumably go on to eat dozens, and so will the other hundred sharks (not to mention the lone whale). Second, it may well get through the shoal of ten very quickly but it would have to find it first, a much tougher job than discovering a million. Third, surely it is not being suggested that fish can count. It will notice a large number compared to a smaller and, it appears, is attracted to the larger. But what is the thought process here? ‘The larger shoal must know something because so many are doing the same thing so I will too.’ Or perhaps, ‘If I join a large shoal I can hide. I can see others being eaten and that proves I am still alive.’

 

We have to be careful here to avoid anthropomorphising. We must not get into picture book scenarios of fish with scarves and satchels or mice with pretty hats and flouncy dresses. They do not think or act like us and we need to be sure that the argument does not depend to the least degree on such notions. However, there has to be a thought process of sorts. It is not sophisticated but we can perhaps determine at what level it might operate no matter how crude that might be. To begin with, we know that all living creatures can perceive the difference between life and death. Otherwise they would not be able to choose between the two, would not tend to avoid death in order to remain alive. We know they do that because they are still here.

 

Perhaps their survival is related to a perception of safety rather than safety itself. If wildebeest spread themselves evenly throughout the African plains would more or fewer get eaten? Would they feel too exposed on their own and, if so, what is it about the herd that makes them feel safer. There is a valid argument to suggest that, given the herding instinct, a few who were clever enough to wander off on their own would become distinctly safer because predators would be concentrating on the herd and would not think of looking elsewhere. As far as I know this evolutionary tendency has not arisen.

 

Experts might say this tendency has not arisen because it doesn’t work. If it was a better strategy for survival it would have occurred. But again I think this misses the point. Wildebeest herd primarily to eat and mate. That’s why they crowd together in the places they do. Being eaten is a hazard but not one that takes precedence over finding food and creating offspring. A few with the clever notion of wandering away from the crowd to avoid a cruel death may be disadvantaged by poorer food and the lack of others with the same intelligent thought. So we are back to the notion that they herd because it feels safer. And perhaps the greatest paradox of all is that they feel safer because they see death. The bigger the herd the more likely predators will attack and the more likely some members of the herd will be killed. And what greater confirmation is there of your own survival than to see that others have died? If your mate a few yards away is brought down by a lion how clever do you feel that it wasn’t you? This idea brings together the notion of a calculated chance of survival (one in a thousand is better than one in ten) with Hamilton’s notion of the selfish herd. It is possible that creatures in herds, flocks and shoals perceive safety and risk partly or even largely through that kind of thought process and, like all perceptions relating to risk whether human or animal, it is irrelevant whether or not that perception is accurate. Behaviour seems always to be governed through perception, not precise calculations.

 

What about predators themselves? They group in order to develop a killing team but what is there response when they themselves are under attack. Is there safety in numbers? Quite the reverse. Big cats avoid attack precisely by concealing themselves as individuals. Groups are the equivalent of flags in the ground. Far better to be hidden entirely on one’s own. And if sardines did this just imagine how many there might be? Maybe that’s why they don’t. They would simply have been too successful.