Behav Ecol Sociobiol (2004) 55:461–468 DOI 10.1007/s00265-003-0727-8 ORIGINAL ARTICLE Lars B. Pettersson · Indar W. Ramnarine · S. Anette Becher · Rajindra Mahabir · Anne E. Magurran Sex ratio dynamics and fluctuating selection pressures in natural populations of the Trinidadian guppy, Poecilia reticulata Received: 5 June 2003 / Revised: 20 August 2003 / Accepted: 28 October 2003 / Published online: 10 December 2003  Springer-Verlag 2003 Abstract In many species, population sex ratios have far- reaching consequences for a wide variety of population- level and behavioural processes and can directly influence sexual selection through differential effects on male and female mating behaviour. Although sex ratios are often treated as more or less stable population characteristics, recent theoretical evidence suggests that sex ratios fluctuate under many conditions, and that the amplitude of these fluctuations can be considerable. Few studies have attempted to quantify this variation in systems with prominent, sex ratio-dependent sexual conflict. One of the species with the greatest potential to integrate these factors in the wild is the Trinidadian guppy, Poecilia reticulata. In this study, we quantified natural sex ratio variation both as detailed longitudinal studies of focal guppy populations and as snapshot estimates across a range of freshwater habitats. In line with theoretical predictions, we expected to detect significant sex ratio variation over time. We also investigated the association between juvenile and adult sex ratios to quantify a possible compensatory feedback implied in standard models of sex ratio evolution. Our results confirm that population-level sex ratios in wild guppy populations have a range of dynamic features, with all four focal populations showing significant variation in sex ratio over time. The survey showed that juveniles were generally close to equal (50:50) sex ratios whereas 7 out of 11 adult sex ratios differed significantly from equality. We found no evidence that a surplus of juveniles of the locally rarer sex had been produced. The results indicate that sex ratios and hence the balance between sexual selection and sexual coercion is normally fluctuating in nature, despite juvenile ratios being close to equality. Keywords Fish · Population dynamics · Sex ratio · Sexual selection Introduction The sex ratio has emerged as a central concept in modern evolutionary ecology. Not only does it provide key links between sexual selection, population dynamics and evo- lutionary trajectories (e.g., Charnov 1982; Caswell and Weeks 1986; Magurran 2001; Pen and Weissing 2002), but it can also be the focus of adaptive allocation strategies where individuals adjust their relative invest- ment in males and females to match current and expected fitness payoffs (e.g., Trivers and Willard 1973; Clutton- Brock 1986; Frank 1990; West and Sheldon 2002). These two aspects of the sex ratio, that focusing on population- level processes and that focusing on offspring sex allocation, have a central property in common. They are essentially dynamic processes (Caswell and Weeks 1986; Byholm et al. 2002). The basis for this is that when one sex dominates numerically, parents biasing their brood towards the rarer sex will experience a fitness advantage, something that can take place even when production costs differ (Fisher 1958; Trivers and Willard 1973). This feedback process is expected to lead to convergence on stable sex ratios or at least to move the sex ratio in the direction of a stable situation (Fisher 1958; Caswell and Weeks 1986; Ranta et al. 2000). However, while tradi- tional Fisherian (Fisher 1958) models of sex ratio dynamics focus on panmictic, homogenous populations where convergence is clearly predicted, a new generation of models (e.g., Caswell and Weeks 1986; Lindström and Kokko 1998; Ranta et al. 2000) has stressed that sex ratio Communicated by J. Krause L. B. Pettersson ( ) ) Department of Ecology, Lund University, Ecology Building, SE-223 62 Lund, Sweden e-mail: lars.pettersson@zooekol.lu.se L. B. Pettersson · S. A. Becher · A. E. Magurran Gatty Marine Laboratory, School of Biology, University of St Andrews, St Andrews, Fife, KY16 8LB, Scotland, UK I. W. Ramnarine · R. Mahabir Department of Life Sciences, University of the West Indies, St. Augustine, Trinidad & Tobago