While many human parents will assure their children they do not have a favourite, in the animal kingdom, moms may provide different levels of care for their offspring.
Strong Offspring Get Fed
Spotted hyena moms allow their cubs to duke it out for access to their teats. They will often allow an aggressive pup to edge its sibling out of a meal time and time again until it starves to death. If resources are scarce, she may be better off to allow the dominant sibling to starve the weaker one.
Male and Female Offspring Get Different Milk
Many mammals and primates tend to provision male offspring with milk that has a higher fat content. Females receive a lower-fat and thus lower quality version, but they may receive larger amounts.
In many primates with polygynous mating strategies (one male, several females) and sexual dimorphism (males bigger than females), mothers provide their male offspring with additional food or milk with a fattier composition. They have a much larger cost to being small than females do. A male’s size will have a massive impact on his future social and reproductive success. It makes a lot of sense for a mother to make sure her sons have the best chance to get as large as possible.
Many moms provision their daughters with extra calcium stores from their milk. Females generally have an accelerated rate of skeletal calcification than males do, and so mothers can provide milk that facilitates the process.
Mom Is More Invested In Older Offspring
Many moms exhibit a bet-hedging strategy – they can have one single offspring when times are bad, or the chance at two offspring when times are good. Sometimes, after making this choice, the mother may not have enough resources to feed more than one offspring.
Since mom has already put more effort into an older offspring, it makes more sense, biologically speaking, for her to continue to nurture the older one and let nature take its course with the younger one.
This post is an adapted excerpt from my book “Wild Moms: Motherhood in the Animal Kingdom“