PHYSIOLOGICAL TRADE-OFFS BETWEEN IMMUNITY AND REPRODUCTION IN THE NORTHERN CRICKET FROG (ACRIS CREPITANS) MALCOLM L. MCCALLUM 1,3 AND STANLEY E. TRAUTH 2 1 College of Arts and Sciences and Education, Texas A&M University-Texarkana, 2600 Robison Rd., Texas 75501, USA 2 Department of Biological Sciences, Arkansas State University, State University, Arkansas 72467, USA ABSTRACT: Investigations of natural history trade-offs between reproduction and immunity are common throughout the literature. Most previous studies of such trade-offs have focused on how resources can be drawn from immune response to fuel reproduction. Our results demonstrate that resources also can be shifted from reproduction to immunity. Immunologically-challenged male northern cricket frogs (Acris crepitans) expressed reduced investment in reproduction. Spermatic cyst diameter, germinal epithelium depth, and gonadosomatic index were smaller in antigen-injected males relative to those injected with a sham (saline injected) and noninjected control animals. Although body size increased in all groups during this study, linear growth and body mass did not appear to be significantly different among these three treatment groups. These results demonstrate indirectly that in A. crepitans immune response may increase metabolic demand for resources and fuel that need from the stores normally used to support male reproduction. We speculate that anything eliciting an immune response in this species may reduce male fertility, so pathogens and toxins at levels that are currently believed to be relatively harmless may impact populations in ways we could not previously predict. Key words: Acris crepitans; Immunology; Reproduction; Spermatogenesis; Trade-offs TRADE-OFF theory (Maynard Smith, 1974) predicts that each individual has a limited resource base to fuel all necessary physiolog- ical processes (Zera and Harshman, 2001). As demands are placed on one physiological system, the other should experience a com- promised supply of resources (Zera and Harshman, 2001). Thus, available resources are distributed so that the organism’s evolu- tionary payoff (Maynard Smith, 1974; May- nard Smith and Price, 1973) is maximized (Zera and Harshman, 2001). Numerous in- vestigations have examined potential trade- offs between reproduction and the immune function indirectly by assessing parasite loads or disease status and relating them to mating success (Hamilton and Zuk, 1989; Hausfater et al., 1990; Liljedal et al., 1999; Zuk et al., 1990). Pathogen or parasite loads may be confounded by ecological or physiological conditions exclusive of an individual’s immu- nological capacity (Beasley et al., in press; Carey et al., 1996). Consequently, recent studies have begun directly examining the relationship between immune response and reproduction (McCallum and McCallum, 2006; Norris and Evans, 2000). Compton and Derting (2002) conducted one of the few studies of trade-offs between reproduction and immune responses of males. They found that both wet and dry mass of the intestines and of the testes were reduced in antigen-exposed mice. Several studies of birds also have found evidence of such trade-offs. Immune response is suppressed during the breeding season in red jungle fowl (Gallus gallus, Zuk and Johnsen, 1998). Black-headed gull (Larus ridibundus) chicks from eggs with high yolk androgen levels have suppressed immune function (Groothuis et al., 2005). The breeding success of female pied flycathcers (Ficedula hypoleuca) declines in response to increased immune activity (Ilmonen et al., 2000). Conversely, the male mealworm beetle (Tenebrio molitor) increases its terminal re- productive effort by enhancing the attractive- ness of its pheromone signal in response to immune insult (Sadd et al., 2006). Norris and Evans (2000) proposed that the following three pieces of evidence are re- quired to demonstrate trade-offs between immunocompetence and reproduction: (1) immunocompetence must compete with life- history components for access to limiting Herpetologica herp-63-03-03.3d 14/7/07 13:52:35 269 Cust # 04-49 3 CORRESPONDENCE: e-mail, malcolm.mccallum@tamut. edu Herpetologica, 63(3), 2007, 269–274 E 2007 by The Herpetologists’ League, Inc. 269