ABSTRACT: We predicted that the proportion of suckling attempts rejected and terminated by the mother would be greater for female foals than male foals, based on parent offspring conflict theory and on the assumption that throughout the study, all zebra mothers were in good condition because of captivity. We presumed that an increasing rate of suckling ter- minated or rejected by a mother would indicate a de- creasing effort by the mother to invest in her offspring. We observed foals of captive plains zebras at the Dvůr Králové Zoo, Czech Republic. We found that the prob- ability of successful suckling tended (slope = 0.0016; Z = 1.78; P = 0.074) to increase with increasing age of the female foals, but decreased (slope = -0.0018; Z = -2.51; P = 0.012) with increasing age of the male foals. The proportion of suckling bouts terminated by the mother decreased (slope = -0.0077; Z = -4.27; P < 0.0001) with increasing age of the female foals, but not the male foals (slope = -0.0005; Z = -0.34; P = 0.732). Our results indicate that conflict between mothers and female foals was less than that between mothers and male foals. The observed sex differences in termination and rejection of suckling bouts could be explained by the different behavior of the male and female foals, or by the selective maternal investment. Finally, we revealed no significant effect of herdmates on suckling behavior. Key words: mother-offspring interaction, parental investment theory, plains zebra, sex difference, zoo ©2010 American Society of Animal Science. All rights reserved. J. Anim. Sci. 2010. 88:131–136 doi:10.2527/jas.2009-1982 INTRODUCTION Lactation is a form of maternal investment that is far more demanding than gestation, estrus behavior, or ovulation (Sadleir, 1984). A recent meta-analysis in mammals and a study in feral horses (Equus caballus) revealed no significant relationship between suckling bout duration or suckling frequency and milk or energy intake (Cameron, 1998; Cameron et al., 1999b); there- fore, the termination and rejection of suckling should be used as a more reliable behavioral measure of par- ent-offspring conflict (Trivers, 1974). An increased rate of suckling attempts rejected by a mother and suck- ling bouts terminated by a mother, rather than by a foal, should indicate increased conflict over resources (Green, 1990; Cameron and Linklater, 2000; Cameron et al., 2000, 2003). Mothers of polygynous mammals in good condition (e.g., in captive conditions with ad libitum access to water and food) would be more likely to invest more in sons than daughters, because sons have more variable reproductive success and a high-quality son would leave more offspring than a high-quality daughter (Trivers and Willard, 1973; White et al., 2007). Thus, we first predicted that the proportion of suckling attempts re- jected by the mother and suckling bouts terminated by the mother would be greater for female foals than for male foals. In other equids, conspecifics may alter the level of optimal investment in offspring through aggressive in- teractions (Lloyd and Rasa, 1989; Rutberg and Green- berg, 1990). Because plains zebra mares form social hi- erarchies (Klingel, 1967; Schilder and Boer, 1987), we also predicted that suckling bouts that are terminated by a herdmate other than a given mother or foal (here- Suckling behavior in captive plains zebra (Equus burchellii): Sex differences in foal behavior 1 J. Pluháček,*† 2 J. Bartošová,* and L. Bartoš* *Department of Ethology, Institute of Animal Science, Přátelství 815, 104 00 Praha-Uhříněves, Czech Republic; and †Ostrava Zoo, Michálkovická 194, 710 00 Ostrava, Czech Republic 1 The authors gratefully acknowledge the assistance of the staff of the Zoological Garden at Dvůr Králové, in particular that of Luděk Čulík, Miroslava Doležalová, Aleš Kopecký, Pavel Moucha, Jan Pařík, Barbara Raková, Kristina Tomášová, Miroslav Špráchal, and Dana Holečková. We are grateful to Sarah King for improving the English. We thank Jorge Cassinello, Phyllis C. Lee, Joy Tripo- vich, Jana Kálnová, Michaela Olléová, Susan Lingle, David Saltz, and 2 anonymous reviewers for their helpful comments on an earlier draft of the manuscript. The project was supported by grant num- ber 523/08/P313 from the Czech Science Foundation (Praha, Czech Republic) and by the Ministry of Agriculture of the Czech Republic (Praha, Czech Republic; MZe0002701404). 2 Corresponding author: janpluhacek@seznam.cz Received March 24, 2009. Accepted September 10, 2009. 131 Published December 4, 2014