**1. Introduction: Sex-bias in mammals mother resource allocation**

Reproduction in mammalian females demands high energetic costs, driving the mobilization of fat deposits, in both gestation and lactation [1]. In evolutionary biology, numerous hypotheses defend a sex-biased allocation of these resources by the pregnant and nursing females, to maximize the reproductive success of their male and female offspring. Some of these theories support their reasoning in the local resource competition [2, 3], local resource enhancement [4], "advantaged daughters" [5], the "safe bet"/reproductive value [6, 7] and the sex-differentiated sources of mortality [8].

The most well-known and tested theory remains the Trivers-Willard hypothesis that predicts that: 1) females in good body condition will allocate her offspring sex ratio towards males; 2) and that mothers in good body condition will also invest more per son than per daughter if males exhibit greater variation in reproductive value when males exhibit greater variation in reproductive value [9] According to this hypothesis, female mammals are able to adjust the sex of their offspring based on their own condition as a form to maximize reproductive success in the next generation. This theory also states that the mother will adapt her milk production

to offspring gender, for example, by increasing milk production or changing its composition when she is nursing an offspring of the gender that has higher chances of producing future descendants. This strategy is particularly beneficial in species whose males compete for mating, like bovine, with dominant bulls leaving abundant offspring and weaker ones having no offspring at all. On the other side, this hypothesis also describes that investment in female offspring will be more profitable when the mothers are in poor condition because the chance of producing competitive male offspring is low. Well-nourished mothers invest more in male offspring, as strong sons will more likely leave more offspring, whereas even weaker daughters will produce more progeny than weak sons [10].

In agreement with this theory it has been shown that, in humans, the milk produced for males is more energy dense in well-nourished mothers [11], while mothers with low socioeconomic status, when nursing daughters produce milk with a higher fat content than when nursing sons [12]. However, evidence for systematic sex-biased favoring males has been equivocal [13–17]. Post-natal, sex-biased nursing care has been investigated as a possible reason for sex-biased milk production in several mammalians, including humans. Several studies reported evidence of sexbiased milk synthesis in different species but drawing definitive conclusions from these studies has been difficult for several reasons [11, 12, 18–30].

This chapter intents to discuss the evidences pro and against the existence of a sex-bias in cows´ milk production, by stressing the putative effects of the calf gender in consecutive lactations while focusing in particular in dairy cows. Albeit non-consensual, its existence would open new insights on the economic impacts of using sex sorted semen to enhance farm productivity.
