Journal of Comparative Psychology 1999, Vol. 113, No. 4,388-395 Copyright 1999 by the American Psychological Association, Inc. 0735-7036/99/S3.00 A Testosterone-Mediated Trade-Off Between Parental and Sexual Effort in Male Mongolian Gerbils (Meriones unguiculatus} Mertice M. Clark and Bennett G. Galef, Jr. McMaster University Effects of testosterone (T) on parental behavior of male Mongolian gerbils (Meriones unguiculatus) were examined. After undergoing gonadectomy, castrated males were im- planted with empty capsules or capsules containing T; sham-operated males were implanted with empty Silastic capsules. Subsequently, each male was paired with a pregnant female, and after delivery, families were observed 15 min/day for 20 days. Gonadectomized males without T spent more time in contact with, huddling over, and licking pups than did either sham-operated males or gonadectomized males with T. When given a choice between nest sites and displaced pups, females and males with low T preferred pups, whereas intact males and castrated males with T preferred nest sites. The findings are consistent with E. D. Ketterson and V. Nolan's (1994) hypothesis implicating T in mediating trade-offs between parental and sexual effort. In the laboratory, male Mongolian gerbils (Meriones unguiculatus) generally share parental duties with their mates. Like females of their species, male Mongolian gerbils huddle over, lick, and retrieve pups (Elwood, 1975). Although all male gerbils engage in some parental behavior, they vary considerably in the frequency with which they interact with young. This variation in parental behavior of males is correlated with the intrauterine posi- tions (IIJPs) that males occupied as fetuses and their circulating levels of testosterone (T) both in infancy and in adulthood. Males gestated in lUPs between two female fetuses (2F males) have lower circulating levels of T throughout life and as adults spend more time with pups than do males gestated in ITJPs between two male fetuses (2M males; Clark, DeSousa, Vonk, & Galef, 1997; Clark, vom Saal, & Galef, 1992). Adult circulating levels of T also have marked effects on the sexual behavior of male Mongolian gerbils. Like gonad- ectomized male rats (Baum, 1993), gonadectomized male gerbils engage in little sexual behavior, whereas both intact males and males gonadectomized as adults and given Silastic implants of T are sexually active (Ulibarri & Yahr, 1996; Yahr, Newman, & Stephens, 1979). As one also would expect on the basis of the hypothesis that high circulating levels of T in adulthood are correlated with enhanced sexual behavior in male gerbils, 2M male gerbils are more sexually active than 2F male gerbils (Clark, Tucker, & Galef, 1992). Mertice M. Clark and Bennett G. Galef, Jr., Department of Psychology, McMaster University, Hamilton, Ontario, Canada. This research was supported by a grant from the Natural Sciences and Engineering Research Council of Canada. We thank Paul Ramos, Jessica Stevens, Jennifer Vonk, and Elaine Whiskin for technical assistance. Correspondence concerning this article should be addressed to Mertice M. Clark, Department of Psychology, McMaster Univer- sity, Hamilton, Ontario, Canada L8S 4K1. Electronic mail may be sent to mclark@mcmaster.ca. Ketterson and Nolan (1994, in press) proposed that circulating levels of plasma androgens mediate a trade-off between sexual and parental effort in male birds. In many avian species, artificially elevated levels of T increase sexual effort of males but reduce their care of eggs and young (e.g., Hegner & Wingfield, 1987; Oring, Fivizzani, & El Hala- wani, 1989; Saino & Moller, 1995), even transforming normally highly parental, monogamous males into inatten- tive fathers and polygamous mates (Raouf, Parker, Ketter- son, Nolan, & Ziegenfus, 1997). Relationships between circulating levels of T in adult male 2M and 2F Mongolian gerbils and their relative levels of sexual and parental effort (Clark, Tucker, & Galef, 1992; Clark, vom Saal, & Galef, 1992; Clark, Vonk, & Galef, 1998) are consistent with the hypothesis that in these rodents, as in the male birds that Ketterson and Nolan (1994, in press) described, there may be a T-mediated trade-off between sexual and parental effort. However, available evidence of effects of T on parental behavior of male gerbils is entirely correlational. In the present experiments, we experimentally manipulated levels of T in adult male Mongolian gerbils and examined effects of these manipula- tions on their parental behavior. Experiment 1 We used several unobtrusive measures of male parental behavior to measure effects of circulating levels of T on the parental behavior of male gerbils. First, we determined the amount of time that males spent with pups when their mates were either present in or away from the nest. Although time in contact with pups is not necessarily a measure of parental behavior (e.g., males might be motivated to remain in the nest or in contact with their mates and only inadvertently contact young; Clark et al., 1998), it has been used previously either alone (e.g., Grota & Ader, 1969; Storey & Snow, 1987; Wynne-Edwards, 1995) or in combination with more direct measures of parenting (Gubernick & Alberts, 388