2.1 Purposeful behaviour
The behaviours touched on in Case Studies 1 and 2 seem to be aimed towards some specific goal – reaching a safe environment, the recovery of prey, and so on. We can call this sort of behaviour goal-directed. It can be found everywhere in nature. However, we should be careful: the term can be used to cover a whole range of different cases. Consider the following three case studies.
Case Study 3: Tool-using primates
Most non-human anthropoid primates (monkeys, apes) construct and use tools. Chimpanzees, for instance, use sticks to break open termite nests, pick the locks of their cages, and push away dangerous or unpleasant objects that they would rather not touch; they use leaves to clean themselves and food items as bait. If one is available, they will use a stout rod to prise apart the bars of their cage so that they can put their heads out for a better view. A screwdriver dropped into a chimp’s cage may be used as a spear, hammer, probe, mill, toothpick or for any other purpose the chimp can put it to. Orangutans, however, prefer to hide the screwdriver and then ‘barter’ it for a food reward from their keepers. Gorillas will first try to eat the screwdriver and then ignore it thereafter. However, gorillas in the wild do use sticks as weapons and, in captivity, can be taught to store water in containers.
Both chimps and orangutans can make strategic use of tools to accomplish goals such as obtaining food, often devoting a lot of time and experiment to the problem. They will use sticks to reach out for food outside their cages, searching for longer sticks or even joining sticks together, if necessary, until they have an instrument long enough to rake the food towards them. If food is suspended above them they will make a stable stack of objects, or balance a pole, and then climb it to reach the goal.
Case Study 4: Shellfish-eating birds
The oystercatcher (Haematopus ostralegus) is a shorebird that feeds mainly on mussels and other shellfish foraged from the shoreline. An obvious problem for feeding behaviour of this kind is opening the shell to get at the edible parts inside. Studies of oystercatchers have revealed that they break open shells by two means:
- The shells of mussels washed up by the tide onto dry land tend to be tightly closed. In such cases the bird will move the shell to a dry place where the sand is hard enough to provide support, turn it over so its thinner and more fragile underside is upwards, and then hammer it open with its bill.
- Mussel shells fished out of shallow water are generally slightly open. The oystercatchers prise the shells of these specimens open by inserting their bills into the crack and cutting the abductor muscle which holds the shell closed.
Careful observations have suggested that an individual oystercatcher specialises in one or other of the above techniques. This appears to be behaviour learned from the bird’s parents.
Many species of gull open shells by dropping them on a hard surface from a height of several metres. Herring gulls (Larus argentatus) are particularly good at selecting suitably hard dropping zones, such as rocks, pavements and car parks, and will generally choose the right altitude from which to drop the shellfish, depending on the size of the target. If the shell fails to break, they will pick it up and try again. Crows will drop whelks only onto certain rocks, choosing their target from the point of view of size, flatness and distance from the waterline (so that edible fragments don’t fall into the water); they prefer to drop from a low altitude, so that the edible parts are not widely scattered, even if this means they have to repeat the drop many times.
Case Study 5: Plume-tracking lobsters
The American lobster (Homerus americanus) inhabits rocky, shallow-water coastal habitats that provide adequate food and shelter from predators. They hunt for food at night, their diet including fish, crabs, clams, mussels, sea urchins and sometimes other lobsters. Their nocturnal behaviour, along with the fact that they only have rudimentary eyesight, means that lobsters have to rely on their highly developed sense of smell to detect prey, mates and predators. The strategy they use is known as plume tracking.
Lobsters use structures called antennules, as well as odour-sensitive hairs on their bodies, to detect water currents – known as plumes – bearing the smell of prey, and to aim themselves towards the source. It is now known that they use a principle called chemotaxis, in which they extract directional information from the composition of the plume itself to do this. How this is achieved, or precisely what information is being used, is not well understood. Somehow the lobster remains in the centre of the plume and orients itself towards the source by constantly monitoring changes in the make-up of the plume as it moves. Some researchers believe the downstroke of the anntenule, which is very fast, captures a high-resolution picture of the plume’s structure, while the slower upstroke measures and analyses that structure.
On the face of it, these all seem to be examples of goal-directed behaviour. But let’s consider how they compare.
Given all these cases involve trying to achieve a goal, what differences do you think there are between them? Try to think about the goals themselves as well as the behaviour that leads towards them. What part do you think reasoning might play in these activities?
I think that there is a clear hierarchy of complexity here, moving from very simple to extremely intricate behaviour. To take the cases in reverse order:
- In Case Study 5, the lobster is sensing its environment, doing some internal processing, recognising a stimulus of interest and then adjusting its responses accordingly, to support its primary purposes of feeding, hiding or mating.
- The behaviour of the gulls in Case Study 4 is more clearly recognisable as goal-directed and appears more sophisticated. The primary goal is still simple – to feed – but in this case, the birds’ behaviour is underpinned by refined sensory recognition systems (estimating the position and geometry of the shell, identifying the right type of rock, judging the texture of the sand) and flexible behaviour (choosing the right height for the drop, or dropping the shell again, if necessary).
- Chimpanzees are our nearest relatives genetically, so in Case Study 3 we find behaviour that we could have no qualms about labelling as ‘intelligent’. The apes may have a simple goal – a banana – but in other cases it can be more complex and abstract, such as wanting to have a more interesting view, freedom from the cage, and so on. Even more impressively, the chimp can work out how to reach its goal through a series of stages, or subgoals.
Although these three cases are, of course, very different, it’s difficult to see where one could draw a clear line between goal-directed behaviour that is ‘intelligent’ and that which isn’t. The chimps’ mentality seems to resemble our own. They have abstract goals, they form plans, they appear to have an excellent grasp of causality: for example, they know, after a bit of thought and experiment, that certain actions will lead to given results (if I use the rod in this way, then the bars will bend). However, be aware of a temptation towards anthropomorphism here. It’s tempting to label gorillas as less ‘intelligent’ than chimpanzees, as they show little interest in, or desire to use, tools. But gorillas are herbivores, living in the wild on a simple and abundant diet of nettles and leaves. They would have little use for tools and, presumably, have not evolved to recognise or use them.
But what about the gulls and the lobsters? It would seem quite reasonable to call the lobster’s plume tracking goal-directed, even though it is still a fairly straightforward form of stimulus and response that biologists call taxis. Of course, it’s hard to imagine that lobsters (which are related to spiders) have any awareness of pursuing an aim, but their behaviour is quite intricately purposeful nevertheless. The gulls and crows are even more problematic. At one time, it was common to dismiss this type of behaviour as ‘instinctive’ (as if that explained it away), but repeated experiments have shown that it is learned. What we can clearly see in these cases is the pursuit of basic goals (feeding, etc.) through complex, flexible behaviour.
It’s important to reiterate the main point here: none of these creatures possesses a developed language, as humans do, and none of them, with the possible exception of the chimps, could be said to be reasoning about their goals. Chimpanzees can be taught restricted versions of human language, so goal-directed behaviour can occur, and be effective, even in the absence of logic and symbols. And it surely seems reasonable to see goal-directed behaviour as lying along a spectrum of complexity and sophistication, from the purely mechanical reactions we see in plants through to the complexity of human goals and plans.