Great expectations: dominance, breeding success and offspring sex ratios in red deer

Differences in dominance rank among red deer (Cervus elaphus) females (hinds) on Rhum were related to their breeding success as well as to the comparative success of male and female offspring. Males (stags) born to mothers above median rank were more successful than hinds, while hinds born to subordinate mothers were more successful than stags. The ratio of male to female calves produced by dominant mothers was significantly higher than that produced by subordinates. Since dominance rank among hinds is related to their body weight as adults and to their birth weight, these results suggest that the birth sex ratio may be affected by environmental factors operating during a female's early development. Anita. Behav., 1986, 34, 460-471 Great expectations: dominance, breeding success and offspring sex ratios in red deer T. H. C L U T T O N B R O C K , S. D. ALBON & F. E. G U I N N E S S Large Animal Research Group, Department of Zoology, Cambridge, U.K. Abstract. Differences in dominance rank among red deer (Cervus elaphus) females (hinds) on Rhum were related to their breeding success as well as to the comparative success of male and female offspring. Males (stags) born to mothers above median rank were more successful than hinds, while hinds born to subordinate mothers were more successful than stags. The ratio of male to female calves produced by dominant mothers was significantly higher than that produced by subordinates. Since dominance rank among hinds is related to their body weight as adults and to their birth weight, these results suggest that the birth sex ratio may be affected by environmental factors operating during a female's early development. Differences in dominance rank among red deer (Cervus elaphus) females (hinds) on Rhum were related to their breeding success as well as to the comparative success of male and female offspring. Males (stags) born to mothers above median rank were more successful than hinds, while hinds born to subordinate mothers were more successful than stags. The ratio of male to female calves produced by dominant mothers was significantly higher than that produced by subordinates. Since dominance rank among hinds is related to their body weight as adults and to their birth weight, these results suggest that the birth sex ratio may be affected by environmental factors operating during a female's early development. In many polygynous vertebrates where males compete for females in contests involving pushing, the breeding success of males is likely to be more strongly influenced by their body size than that of females (Clutton-Brock et al. 1977, 1982). Since early growth affects adult body size in red deer (Suttie 1980, 1983), sexual selection will favour faster early growth rates in males than females and a greater allocation of parental resources to juvenile males (Clutton-Brock et al. 1981). Recent studies of goats (Capra domestic) and red deer show that male juveniles are permitted to suck more frequently than females (Clutton-Brock et al. 1981; Pickering 1983) and, in the latter species, mothers who have successfully reared sons are almost twice as likely to fail to breed the following year as those that have reared daughters (CluttonBrock et al. 1981). The stronger effects of early growth on male breeding success suggest that sons born to mothers who are unable to invest heavily in their calves may be unlikely to breed successfully, whereas maternal quality may have a lesser effect on daughters (Trivers & Willard 1973). If so, parents might be expected to vary the sex ratio of their offspring, with superior mothers producing an excess of sons and inferior ones an excess of daughters (Trivers & Willard 1973; Clutton-Brock & Albon 1982). In this paper, we examine the effects of dominance rank on the breeding success of red deer (Cervus elaphus) females (hinds); on the comparative breeding success of their male and female offspring; and on the birth sex ratio of their offspring. The paper develops and extends a previous analysis (Clutton-Brock et al. 1984). In the last section we examine the relationship between the dominance ranks of females and their early development. Red deer have substantial advantages for a study of this kind since individuals of both sexes rarely disperse far from their natal area (Clutton-Brock et al. 1982). Females seldom disperse from their natal group, associating with their mothers and other matrilineal relatives throughout their lives. Males (stags) disperse from their mothers' groups to join loosely-knit associations of stags that occupy areas adjacent to the main hind grounds, except in the October rut when stag groups fragment and individuals collect and defend harems of 2-20 females (Clutton-Brock et al. 1982). Male breeding success can be reliably assessed since oestrus is short (Guinness et al. 1971; Clutton-Brock et al. 1979, 1982). Hinds conceive during the October rut and produce a single calf the following June which is usually weaned 5-6 months later (Clutton-Brock et al. 1982). Both sexes have relatively short lifespans (modal values: 11-13 years for females and 9-11 years for males) and our data now span the breeding lifetimes of both sexes.