Testing ideas about the function of scent marks in territories from spatial patterns

Abstract

Strictly speaking, it is difficult to ascribe function to a signal without some direct indication of the benefits that accrue to the signaller. In this ideal world, the signal and the consequent responses of the receiver would be detected and measured and the way in which the responses benefit the signaller would be clarified. This sort of investigation has rarely been achieved in any signalling modality and is particularly difficult in studies of olfactory signals. But, in the case of scent marks, the long-term nature of the signals allows some inference about this signal–response sequence (review in Gosling & Roberts 2001). Scent marks are placed in the environment, often in the absence of a potential receiver and only detected some time later, often when the signaller is no longer present. When signallers are relatively sedentary, scent marks accumulate and their spatial pattern may reveal something about the intended receiver and thus something about function. For example, when a territorial male antelope places more marks at a boundary where the threat of intrusion by competitors is greatest (e.g. oribi, Ourebia ourebi, Brashares & Arcese 1999a) then we may conclude that the function of the marks is linked to intrasexual competition and, in this case, to territory defence. However, this sort of inference about the function of scent marks can be made only under limited circumstances. Other important factors can affect the pattern of marks and may need to be considered for more general explanations. Most important is that the production and maintenance of scent marks involves costs such as the cost of producing the marking substances, and the considerable time taken to establish and replenish marks. These costs introduce economic constraints to the range of spatial patterns that are possible when scent marking territories (Gosling 1981; Roberts & Lowen 1997;