We examined data on sex-specific differences in neonatal weight, litter size and adult female body weight in 32 populations of polygynous ungulates of 18 different species to test for the existence of a tradeoff between sex-biased maternal care and the total amount of maternal expenditure. This corresponds to an extension of the hypothesis of Byers and Moodie (1990) that sex-biased maternal care is limited by a high level of maternal expenditure. We did not find a W relationship between sex-biased care and two measures of total maternal expenditure. We highlighted high intraspecific variability in sex-biased care and very low intraspecific variability in total maternal expenditure. Even when this between-population variability in sexbiased care was accounted for, no relationship between sex-biased maternal care and maternal expenditure was detected. Apart from difficulties in finding suitable measures for both variables, two other reasons may account for the lack of a relationship between sexbiased maternal care and total maternal expenditure. Firstly, male offspring seem to be more affected than female offspring by harsh environmental conditions. This may lead to the variation observed in the extent of sex-specific differences in birth weight within a single species. If we assume that ~br a given maternal expenditure reproductive costs incurred by mothers are highest during harsh conditions, this could indicate the existence of a trade-off between sex-biased maternal care and maternal expenditure at the intra-specific level thereby supporting the Byers and Moodie hypothesis. Secondly, polygyny is only a poor predictor of sexbiased care and factors such as compensatory growth or extended periods of growth may be expected to modify, predictions for different species. Thus, environmental conditions and relative effects of maternal care on male and female lifetime reproductive success are better predictors of sex-biased care than total maternal expenditure. Key words Sex-biased care -Maternal expenditure • Ungulates • Neonatal dimorphism 2. Calf growth and mortality. J Agric Sci Camb 95:275 284 Braza F, San Jos6 C (1988) An analysis of mother-young behaviour of fallow deer during lactation period. Behav Proc 17:93 106 Braza F, San Jos6 C, Bloom A (1988) Birth measurements, parturition dates and progeny sex ratio of Dama dama in Donana, Spain. J Mammal 69:607 610 Byers JA, Moodie JD (1990) Sex-specific maternal investment in pronghorn, and the question of a limit on differential provisioning in ungulates. Behav Ecol Sociobiol 26:157 164 Calder WA (1984) Size, function and life history. Harvard University Press, Cambridge Clutton-Brock TH (1988) Reproductive success. Studies of individual variation in contrasting breeding systems. Chicago University Press, Chicago Clutton-Brock TH ( 199 l) The evolution of parental care. Princeton University Press, Princeton Clutton-Brock TH (1992) The evolution of sex differences and the consequences of polygyny in mammals. In: Bateson P (ed) The development and integration of behavi...
