173 Stickleback Fights: Why Do Winners Win? Inﬂuence of Metabolic and Morphometric Parameters Helga Guderley 1, * Patrice Couture 1,2 1 De ´partement de Biologie, Universite ´ Laval, Que ´bec, Que ´bec G1K 7P4, Canada; 2 Institut National de Recherche Scientiﬁque, Eau, Terre, et Environnement, Universite ´ du Que ´bec, Que ´bec, Que ´bec G1V 4C7, Canada Accepted 4/14/04; Electronically Published 2/25/05 ABSTRACT Pairs of reproductively mature male three-spined stickleback (Gasterosteus aculeatus) were introduced into unfamiliar aquaria and observed until one male became dominant. Skin carotenoid content, morphometric indexes, and metabolic ca- pacities of the axial and pectoral muscles were examined to establish whether morphological or physiological parameters differentiated winners and losers. Stickleback that initiated ﬁghts typically won. Quick initiation led to quick victory. Over- all, winners and losers differed in few morphological or met- abolic characteristics, but these properties and the differences between these attributes for losers and winners of speciﬁc ﬁghts were linked with initiation time and ﬁght duration. Morpho- metric indexes of losers were the primary determinants of in- itiation time and ﬁght duration, whereas for winners muscle metabolic capacities were linked to these ﬁght characteristics. The greater the hepatosomatic index (HSI) of losers, the longer the ﬁght initiation times. Similarly, losers with high HSI and carotenoid levels resisted defeat longer. In winners, initiation time decreased as axial muscle phosphofructokinase levels in- creased and citrate synthase levels decreased, whereas the met- abolic capacities of the pectoral muscle were linked with time to achieve victory. When losers had greater HSI values than the winners of a speciﬁc ﬁght, ﬁght initiation was delayed and ﬁghts lasted longer. When losers had higher carotenoid levels than winners, ﬁghts also lasted longer. On the other hand, when losers had more visceral fat (fat body mass over somatic mass) than winners, both initiation time and combat duration were reduced. These results suggest that male stickleback assess their * E-mail: helga.guderley@bio.ulaval.ca. Physiological and Biochemical Zoology 78(2):173–181. 2005.  2005 by The University of Chicago. All rights reserved. 1522-2152/2005/7802-3103$15.00 physiological status and that of their opponents, in particular the HSI, and adjust their combat strategies accordingly. Introduction The interactions between social status, physiology, and genetics are excellent examples of the intricately balanced links between animals and their physical and social environments. In many vertebrates, social status is closely tied to reproductive success. Dominant animals often obtain the best territories, the best feeding opportunities, and the best mates, thereby maximizing their ﬁtness (Berglund 1997). This suggests that, insofar as physiological or morphological characteristics that predispose individuals toward dominance increase reproductive success, they should be retained by natural selection. The capacity to achieve dominance plays a particularly central role in estab- lishing reproductive success in the many ﬁsh species in which territory possession is required for the primary reproductive strategy (Grant 1997). In ﬁsh as in other vertebrates, social status has major be- havioral and physiological impacts. Subordinate ﬁsh reduce feeding, aggression, and spontaneous activity and show the physiological hallmarks of chronic stress, in particular elevated plasma cortisol levels and altered brain monoamine levels (Noakes and Leatherland 1977; Winberg and Nilsson 1993; Øverli et al. 1998, 1999). Subordinate ﬁsh may undergo im- mune suppression (Pickering and Pottinger 1995). Given the sensitivity of tissue energetic reserves, muscle metabolic ca- pacities, and swimming performance to the feeding status of ﬁsh (Love 1980; Martinez et al. 2003), reduced feeding by sub- ordinate ﬁsh will further decrease their physiological status. In ﬁsh, social dominance is typically achieved through be- havioral interactions in which physical attacks and threat dis- plays play central and often reciprocal roles. The longer two individuals interact, the more they rely on threat displays (Haller and Wittenberger 1988). The energetic costs and dan- gers of attacks make this a sound strategy. Evaluation of the capacities of the opponent is likely to inﬂuence the decision to initiate combat (Francis 1988). Male three-spined stickleback (Gasterosteus aculeatus) that initiate aggressive interactions gen- erally win and often continue to win future encounters (Fitz- Gerald and Kedney 1987; Jackson 1991), becoming more ag- gressive with victory (Bakker et al. 1989). These self-enforcing tendencies should select for aggressiveness in male stickleback,