Behav Ecol Sociobiol (2004) 56:257–262 DOI 10.1007/s00265-004-0778-5 ORIGINAL ARTICLE G. P. Brown · R. Shine Effects of reproduction on the antipredator tactics of snakes (Tropidonophis mairii, Colubridae) Received: 12 November 2003 / Revised: 11 February 2004 / Accepted: 18 February 2004 / Published online: 31 March 2004  Springer-Verlag 2004 Abstract Does an animal’s reproductive state influence the distance at which it flees from an approaching pred- ator? We predicted that reproduction would increase ap- proach distances in pregnant females (because they are burdened with eggs and thus less able to escape rapidly), but reduce them in males (because of lower vigilance due to males focusing on mate-searching rather than predator detection). Field data on approach distances of keelback snakes (Tropidonophis mairii, Natricinae, Colubridae) supported both of these predictions. We walked the same 1.3-km transect along a dam wall in tropical Australia on 135 nights, and recorded the distances at which snakes fled from our approach. Locomotor speeds were measured for a subset of these animals. Variations in approach dis- tance due to season, weather conditions, prior capture history and snake body size were minor, but reproduction strongly affected approach distances for snakes of both sexes. Gravid females were slower than non-gravid fe- males, and fled at greater distances. Reproductive status did not affect locomotor speeds of males, but males that were reproductive (i.e., contained sperm) permitted closer approach than did non-reproductive adult males. Repro- duction thus affected approach distances in snakes of both sexes, but in opposite directions and for different reasons. Keywords Approach distance · Cost of reproduction · Locomotor ability · Reptile Introduction Predation is a significant source of mortality for many species of animals, and has acted as a potent selective force on a diverse array of morphological and behavioral traits (e.g., Vermeij 1982; Endler 1986). An animal faced with an approaching predator must make a series of “de- cisions” as to which antipredator tactics (e.g., crypsis, flight, display) to adopt, and how closely the predator should be allowed to approach before those tactics are implemented. Thus, one of the most important antipre- dator decisions concerns approach distance (Lima and Dill 1990; Magnhagen 1991). Fleeing when the predator is so far away that it poses no real threat entails costs in wasted time and energy, whereas delaying flight for too long can increase the animal’s vulnerability if the pred- ator does indeed launch an attack. Optimality models thus suggest that animals should adjust approach distances relative to their own locomotor abilities: a faster animal can safely allow closer approach (Lima and Dill 1990; Dickman et al. 1991). Similarly, approach distances may vary if some individuals are engaged in activities (e.g., social interactions, mate searching) that decrease anti- predator vigilance. Studies on reptiles provide empirical support for this prediction. For example, increased body temperatures (and thus locomotor speeds) decrease approach distances in lizards (e.g., Smith 1997; Cooper 2000). Some of the most dramatic temporal shifts in antipredator tactics relate to reproductive activities. Although reproduction may influence antipredator tactics in both sexes, the causal pathways appear to differ. In squamate (snake and lizard) species, in which gravid (pregnant) females are slowed- down by the burden of the clutch (Shine 1980; Seigel et al. 1987), such animals tend to reduce foraging activities and remain close to cover (Bauwens and Thoen 1981; Brodie 1989; Schwarzkopf and Shine 1992). This shift has been interpreted as an adaptive response to the low- ered mobility (and hence, predator-escape ability) of such heavily burdened females. In contrast, a shift in the an- tipredator behavior of male snakes during the reproduc- tive season, as evidenced by high mortality rates at this time, has been attributed to a much simpler and more direct mechanism: that reproductive males focus on mate- Communicated by: P. Weatherhead G. P. Brown · R. Shine ( ) ) Biological Sciences A08, University of Sydney, NSW 2006, Australia e-mail: rics@bio.usyd.edu.au Tel.: +61-2-93513772 Fax: +61-2-93515609