Olfactory navigation is a hypothesis that proposes the usage of the sense of smell by pigeons, in particular the mail pigeon, in navigation and homing. There are two principal versions. Papi's mosaic model proposes that pigeons construct a map from the distribution of environmental odours, within a radius of 70–100 kilometres. Wallraff's gradient theory overcomes the problem of distance limitation by proposing the existence of long-range, stable atmospheric odour gradients. However, the evidence to suggest that pigeons use an ‘olfactory map’ in order to home is not conclusive.
Background Homing can be defined as the ability to return to a set point from potentially anywhere on the Earth's surface, including destinations that are unfamiliar. There are two criteria needed to coordinate this task, a compass sense (a sense of direction) and a map sense (a sense of location). It is the ability to return from unfamiliar locations that posed the question of what sensory cues are used to determine locational information as well as directional information. It has been proposed that the compass sense can be derived from a number of perspectives. Magnetic orientation as a mechanism for directional sense was first put forward in the 19th century. Equally, the sun could be used as a compass in order to navigate home. In 1972, however, Papi and his contemporaries reported that anosmic pigeons (Columbia livia) were severely impaired in orientation and homing performance. On the basis of their results, the hypothesis of ‘olfactory navigation’ was proposed.
Olfactory map Two models for olfactory navigation have been proposed, Papi's ‘mosaic’ model and Wallraff's ‘gradient’ model. Papi's mosaic hypothesis advocates that pigeons construct a map from the distribution of environmental odours, within a radius of 70-100 kilometres. From this information, it is possible to derive the ‘home’ direction when encountering these odours at a release site. An example of associated wind-borne scents would be pine forests, coastlines and pollution from cities. It is argued that pigeons first learn to associate specific odours with particular locations during exercise and training flights. This model has the advantage that it requires the bird only to detect the presence or absence of a range of odours. Therefore, homing is viable only if the release sites are within a proximity that can provide reliable wind-borne cues, although Papi (1990), argues the utilisation of olfactory information obtained during the outward journey. Wallraff's gradient theory overcomes the problem of distance limitation via different means. It proposes the existence of long-range, stable atmospheric odour gradients. The foundation for this navigational map is a spatial representation in which two or more environmental odours have a particular intensity. Odour gradient differs along dissimilar directional axes and, therefore, the pigeon can compare the intensity of the scent at a particular location to its concentration at the home loft. This mechanism in principle could operate over vast distances, but would require the detection and interpretation of minute differences in odour concentration. However, a more poignant question is the existence of predictable odour gradients. Meteorologists deny that odour gradients, as required by this hypothesis, exist in nature.
Empirical evidence
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