Does Mom have a favourite?

        Growing up with siblings can be a challenge for many reasons; competing for resources (in human terms, often food and toys), mother's attention, and perhaps the turmoil of having that pesky younger brother is a challenge in itself. Consequently, many of us growing up with a sibling have had the novel idea that perhaps we, obviously the superior child, were Mom or Dad's favourite. Recently, scientific studies of various class Mammalia animals have lead researchers to believe that some mothers of the animal kingdom do in fact 'have a favourite'. More specifically, studies have indicated that some mothers variate  their investment in offspring (Hinde, 2014).
      How might an animal, like a  cow, give advantage to one offspring over another? Well, it turns out cows have different synthesis capacities for milk based on their offspring's sex, and mother cows tend to favour their female offspring (Hinde, 2014). Moreover, if you're a cow, it pays off to have an older sister. Cows that rear a female offspring first not only produce significantly more milk for that offspring, but the following offspring (Hinde, 2014).
      However, not all mothers favour daughters. In fact, Pronghorns (Antilocapra Americana) tend to favour their male offspring; yet size variation was still present in same-sex litters (Drik, 2013). Why might this be? Variation between offspring in a given litter increased reproductive  success, specifically in a short time period after birth (4-6 days). In fact, litters that had size variation suffered from less sequential mortalities, where a predator would kill one offspring and then come back for seconds (Dirk, 2013).
     Perhaps some of these variations seem downright unfair, but ultimately, it's important to look through an objective lens at these 'favouritisms'.  These bias investments we find in nature serve to promote the overall evolutionary fitness of the species and thus are essential practices of future generations.

If you'd like more information, please take a look at these articles below:

Hinde, K., Carpenter A. J., Clay, J. S. & Bradford, B. J. 2014. Holseins favour heifers, not bulls: biased milk production programmed during pregnancy as a function of fetal sex. PLoS One 9: e86169.

Van Vuren, D. H., Bray, M. P. & Heltzel, J. M. 2013. Differential investment in twin offspring by female pronghorns (Antilocapra americana). Journal of Mammalogy 94: 155-161.