Using biological techniques to search the web better

Richard Cross describes how new research is revealing how biological foraging strategies can be adapted to help users search out and find information more effectively on the web.


Foraging for food and information

Peter Pirolli , a pioneer in adopting foraging theories to understand how people find information on the web has been recognised by the CHI (Computer Human Interface Academy) as a leader and shaper in the human-computer interaction field.

Biologists created the foraging theory in the 70s through the work of Charnov who set about explaining aspects of animal behaviour. Specifically they explored how predators choose between an easy to catch, but low energy food value prey and one of bigger energy return but more difficult to catch such as an adult animal.

Another of the key questions explored was when to move to another region richer in food.

It appears that animals choose strategies that maximise the benefit per cost unit. In other words they try those strategies in which a minimum consumption of energy gets the maximum energy in the form of food.

By contrast the optimal strategy to change foraging area within a region is by doing that when the return obtained in a particular area is lower than the average of the entire region.

From the wild to the web

Over ten years ago, Peter Pirolli and Stuart Card, (fellow luminary in the area) at PARC, systematically compared humans seeking information on computers and the Web to animals foraging in the wild for food.

Revisiting his original background in Anthropology and Psychology, Pirolli reframed the conventional approach to information retrieval based on insights from animal behaviour and our primitive instincts.

Pirolli and his colleagues combined these insights with rigorous research involving eye tracking devices, empirical observation and theories of cognition. Applying the theory gave rise to a number of catchy concepts that have played a key role in website design, such as the notion that information is patchy, that information items have different values and costs, that information gatherers will tend to organize their information diets in a certain way. According to Pirolli, web behaviour has many parallels to that of our pre-historic hunter-gatherer ancestors.

Tracking the Information Scent

A seminal concept developed was the notion of “information scent.” Information scent is some perceptual indication to the user that taking a particular route through an information space is, on the average, likely to cause the user to find information in which the user has a high degree of interest.

Pirolli was also instrumental in combining Scent-based Navigation and Information Foraging theory with conceptual and problem-solving theories of cognition developed by computer scientist John Anderson.

Scent-following versus scent-finding

In one study watching the recordings of eye-movements recorded by an eye-tracker Pirolli discovered that there are at least two modes of visual search activity. The first called scent-following seemed to be very directional, as if the eye focus were following cues up a gradient towards a maximum reward. The activity was, according to Pirolli reminiscent of an organism following stimulus gradient (scent) towards a reward. The other mode of visual search observed was a non-directional scent finding activity, aimed at finding directional cues. This activity was seen as reminiscent of organisms that have been alerted to a scent such as a puff of pheromones, but must acquire additional cues to identify the direction of the reward. (For an introducing to pheromones in the context of human communications see The perfect mobile group communications system: adopt nature’s oldest signalling system.

Pirolli and Card concluded that human information foragers—like animals --have some way of evaluating the likelihood of finding target information in a given area. This led them to the idea that associated concepts "rub off" on one another, leaving detectable traces, just as a watering hole frequented by woolly mammoths will smell of woolly mammoths. A hunter-gatherer seeking mammoths is likely to be drawn to the watering hole, if only to look for spoor. Information foragers do the same.

The results of his research showed that the measure of information scent is able to generate accurate predictions to user-website interaction. The notion of Information scent was able to predict links people will click on a web page and shown to be able to predict when people will leave a site. It was shown that when participants left a site, the average information scent of the site was diminishing. A key finding was that the information scent of the web page right before the participants left the site was much lower than that of the site that they switched to. - It is possible that, from experience, people have built up an expectation of the information scent value of various web sites. When the information scent value of a particular web site dropped to a value that is below the expected information scent value of other sites, people may decide to abandon the current site and switch their ‘grazing’ to another site.

Information foraging theory has already provided Web developers with new tools.

A concept extended from Pirolli’s work is the notion of collaborative filtering.

Collaborative filtering enables groups foraging for information to act like a group hunting for food when objects included in their diet are distributed widely and thinly in their environment. By providing a history of use to a digital object, a single user can benefit from the foraging of others as the interaction history of other foragers leave tracks or traces in the virtual environment. - The interaction history of others, attached to an object can come from passive sources, such as access logs, or active sources, such as online papers that allow users to leave comments.

Other tools include ways to spread activation by using labels with strong information scent so that paths are more attractive and lead to richer patches of information.

By efficiently constructing user profiles developers already know their users' information diet and increase the profitability of items in their diets by decreasing the amount of energy expended when foraging for desirable items. Allowing users to take advantage of the paths created by others through collaborative filtering, demonstrated, leads to greater user satisfaction and greatly reduces the cost associated with foraging.


The analogy between food and information has had a major impact on Web designers. But as Pirolli warns there has to be caution -those who seek to provide information--can actively manipulate their environment. With information foraging, there's no guarantee that it will be always benefit the forager. As Card, his colleague points out: "The vendor's interests may not correspond with the searchers. They may camouflage information to hide it or mimic something that they think you want. “

A physicist who has spent many years with the Bushmen of the Kalahari Desert, Louis Liebenberg, once wrote "the art of tracking may have been the origin of science". By integrating lessons from nature into web behaviour, real life surfer Peter Pirolli’s achievements cannot be underestimated in both approach and impact on understanding how we collaborate in the ‘virtual’ world.


1. Information Foraging in Information Access Environments by Pirolli, P. and Card, S. K. (1995).

2. SNIF-ACT: A model of information foraging on the World Wide Web by Pirolli, P. and Fu, W.-T. (2003).

3. The Use of Proximal Information Scent to Forage for Distal Content on the World Wide Web by Pirolli, P. (2004).

About the author


Richard Cross is Managing Partner of MCHGlobal and can be contacted at

Richard specialises in strategic organisational change and corporate transformation. A behavioural scientist by background, as director of MCHglobal he has consulted in over 30 countries with corporate and governmental clients in helping them achieve peak performance.

On the editorial board of Inside Knowledge since inception, Richard perspective on the impact of the Knowledge Economy will feature in the forthcoming book 14 Champions of the New Order. Richard's core interests are in helping organisational adapt to the organic and dynamic business environment of today where intangible dimensions and 'soft' variables such as interpretations, intentions, and relationships yield concrete results.


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