Maternal investment in sons and daughters: problems of methodology

Summary The prediction that mothers should invest more in sons than daughters was briefly examined in American bison (Bison bison) by Wolff (1988). He concluded that (a) male calves suckle more than females, (b) cows that had previously produced sons were more likely to be barren, and (c) cows that had male calves became estrous later in the year than other cows. In this paper we present data from two long term studies at different sites to show that Wolff's conclusions are equivocal at best and difficult to reconcile with predictions of the hypothesis because of questionable methods and assumptions. Offprint requests to: W.C.H. Green