Abstract In some species, more aggressive individuals are more successful in resource competition. High testos- terone level is associated with increased activity and ag- gressive behavior, and this may have a direct effect on metabolic rate and cause an increase in energy expendi- ture. Here, I examined the influence of exogenously ad- ministered testosterone on aggressiveness and body growth in juvenile Psammodromus algirus male lizards. Juvenile males were given testosterone-filled (experi- mental) or empty (control) implants. Testosterone pro- duced an increase in aggressiveness and activity in the experimental males. However, despite being more ag- gressive, experimental males did not acquire larger home ranges than control males. Experimental males also ex- perienced a significant reduction in growth rate over the 2-month period following implantation. Experimental males also were in poorer condition at the completion of the experiment, compared to control males. These results suggest that although an elevated testosterone level may have positive effects on aggressiveness and activity, it also may have negative effects manifested as reduced growth rate and body condition. Keywords Testosterone · Aggressiveness · Body growth · Juvenile · Psammodromus algirus Introduction One of the most common assumptions about territorial animals is that individuals acquire space by winning contests (Maynard Smith and Parker 1976; Krebs 1982; Maynard Smith 1982; review in Stamps 1994; Stamps and Krishnan 1994). That is, individuals are assumed to gain possession of areas where they win fights or chases, whereas animals that lose agonistic interactions leave areas in which they were defeated. Thus, the more dominant individual secures the territory, often exclud- ing subordinates from optimal habitats (Krebs 1971; King 1973). This assumption forms the basis for con- cluding that the degree of individual aggressiveness or relative dominance may determine the size or quality of a territory that an individual can secure. In some species, more aggressive individuals are more successful in resource competition. A previous study on juvenile Psammodromus algirus lizards showed that more aggres- sive individuals had larger home ranges than did less ag- gressive individuals. Moreover, home range size and quality influenced probability of survival: survivors had larger home ranges with a greater degree of vegetative cover, compared to nonsurvivors (Civantos 2000). However, benefits resulting from an increase in ag- gression may be balanced by long-term costs such as in- creased risk of injury, increased exposure to predators, or increased energy expenditure, which may result in lower lifetime fitness of more aggressive males (Marler and Moore 1988, 1989, 1991). One way to test the trade-off between these potential costs and benefits is by using hormones. The importance of testosterone as a primary modulator of aggression has been documented in many animals (Wingfield 1985; Wingfield et al. 1987), including lizards (Fox 1983; Moore 1988; Salvador et al. 1996, 1997). For instance, Marler and Moore (1988) showed that testosterone- implanted males were more aggressive than controls, suggesting a greater degree of success in male–male competition. In several species, more aggressive males with artificially increased testosterone levels are more successful in reproductive competition (Wittenberger 1981). For example, side-blotched lizards (Uta stans- buriana) increased the size of their territories, obtained higher quality home ranges, and increased their status in a dominance hierarchy (Fox 1983). However, increased testosterone levels also may det- rimentally affect growth rate. This may be of particular Communicated by T. Czeschlik E. Civantos ( ✉ ) School of Biosciences and Process Technology, Section of Biology, Vaxjo University, 35195 Vaxjo, Sweden e-mail: emilio.civantos@ibp.vxu.se Fax: +46-470-708756 acta ethol (2002) 4:91–95 DOI 10.1007/s102110100049 ORIGINAL ARTICLE Emilio Civantos Testosterone supplementation in juvenile Psammodromus algirus lizards: consequences for aggressiveness and body growth Received: 2 November 2000 / Received in revised form: 15 June 2001 / Accepted: 17 June 2001 / Published online: 27 July 2001 © Springer-Verlag and ISPA 2001