Functional Ecology 2005 19, 1017–1024 © 2005 British Ecological Society 1017 Blackwell Publishing, Ltd. Mate location tactics in garter snakes: effects of rival males, interrupted trails and non-pheromonal cues R. SHINE,†* J. K. WEBB,* A. LANE* and R. T. MASON‡ *School of Biological Sciences A08, University of Sydney, NSW 2006, Australia, and ‡Department of Zoology, Oregon State University, Cordley Hall 3029, Corvallis, Oregon 97331–2914, USA Summary 1. The ability to follow substrate-deposited scent trails using sophisticated vomerona- sal abilities is a key feature of snake biology. However, previous research on this topic has derived mostly from a highly artificial test situation: captive snakes following continuous trails over structurally simple homogeneous substrates, in the absence of any other cues (e.g. visual or olfactory) either from the target of that search or from other snakes. 2. Our field experimental trials with Red-Sided Garter Snakes (Thamnophis sirtalis parietalis) at a communal den in Manitoba explored wider issues in mate location. 3. Males followed a female’s trail less successfully if she had recently been followed by rival males, suggesting that trailing males obscured the cues left by the female. 4. Males that lost contact with a female’s trail exhibited stereotyped, spatially exten- sive and prolonged circling behaviour centred on the last point at which the trail was detectable; in nature, this behaviour would maximize the male’s ability to relocate a discontinuous trail. 5. Trail-following males switched from chemical to visual cues as soon as they were close to a female, enhancing speed but decreasing the accuracy of mate location (because males could distinguish between females and other males based on either visual or airborne olfactory cues). 6. Incorporating additional cues and broader spatial scales can clarify aspects of trail-following behaviour not evident from studies in small, highly simplified labora- tory situations. Key-words: Chemoreception, olfaction, sexual selection, vision, vomeronasal Functional Ecology (2005) 19, 1017–1024 doi: 10.1111/j.1365-2435.2005.01063.x Introduction Organisms display an extraordinary diversity in their degree of reliance upon specific sensory modalities. The evolutionary success of some lineages appears to be intimately linked to the development of sophisti- cated sensory abilities, with spectacular examples of adaptive convergence in such traits. For example, echolocation plays a major role in the foraging biology of bats and cetaceans (Holland, Waters & Rayner 2004; Madsen et al . 2005), and colour vision is a basic precondition for the spectacular sexual dichromatism seen in many fishes, passerine birds and diurnal lizards (Endler 1983; Eaton & Lanyon 2003). An even more widespread sensory modality, however, is the ability to detect minute traces of chemical cues in the environ- ment, thereby identifying the presence and location of potential prey, predators or mates. Although many kinds of animals display sensitive chemoreceptive systems, one of the most extreme examples involves snakes. Whenever they are active, individuals of most (perhaps all) snake species frequently extrude their tongues to obtain chemical information, which is then analysed in the Jacobsen’s Organ on the roof of the mouth (Gove 1979). This highly sophisticated vomer- onasal ability is widely viewed as a key factor in the adaptive radiation of snakes (Ford & Schofield 1984; Mason 1992; Greene 1997; Greene & Mason 1998). Detailed laboratory studies have confirmed that this system allows snakes to accurately follow substrate- deposited chemical trails left by conspecifics or prey animals. Accordingly, snakes use such substrate- deposited trails for many functions: for example, to locate sexual partners (Mason 1992), to avoid con- specifics (Shine et al . 2004) or predators (Shine et al . 2003a), to find overwintering dens (Gregory, Macartney & Larsen 1987), to locate hidden prey (Clark 2004), and †Author to whom correspondence should be addressed. E- mail: rics@bio.usyd.edu.au