“Mum, who do you love best?” – Parental favouritism in the animal kingdom

Whilst this is a question we tend to think of as being asked by a child with siblings, it turns out, animal parents have favourites too…

For example, there’s a species of budgie that regurgitates food for its young and males will feed in response to chicks begging whereas females will seek out the smallest offspring and prioritise them.  In bearded vultures, it really does pay to be the favourite; it’s common for parents to only feed the first born chick…  But it’s not just size and birth order that parents prioritise.  Sticking with the bird world, let’s have a look at a few more examples.

Eastern bluebird dads favour their sons, protecting them from danger whilst exposing their daughters.  But sons aren’t equal either, the baby which is brightest in colour will garner dads protection more so than his duller brother.  Mums on the other hand didn’t discriminate.  If we turn to coots, we find much the same, with parents preferentially feeding nestlings which have ornamental plumes over their duller nestmates.  But it isn’t always about how attractive the offspring are:

“In birds, female blue tits, for instance, are better parents to the offspring they had with sexy males.  Not only that, but if the male they have mated with has his colouring dulled, the equivalent of being made less attractive, the female will actively reduce her efforts to feed their offspring.”
– Verdolin

So, what’s going on with parents?  Why do they play favourites with their children?

Well, what resources are available is an essential part of understanding this.  If you have five chicks and food is scarce, you could split it all equally and end up with five slightly less healthy chicks, or you could allocate them in an unequal way and thus play favourites with your children.

With chinstrap penguins who have more than one chick, they will make their kids chase them for food.  The chick that wins the race gets the food and the one that lags behind will ultimately die.  It sounds incredibly harsh but if you only have enough food for one chick, you need to make sure it goes to the one who has the highest chance of surviving to adulthood and having their own chicks.  This explains why parents tend to favour the fastest and strongest of their young, but what about those eastern bluebird dads and the female blue tits?

This type of favouritism, based on appearances, is again about trying to ensure that your lineage will continue.  With the blue tits, sexy males mean sexy children who in turn will have more sex and hence more chicks themselves.  By putting more resources into caring for the chicks from the sexy male, the mummy blue tit is worker harder to ensure her young will become healthy, sexy and successful breeders themselves.  For bluebirds, the females prefer to mate with brighter males and thus in the example above, where dad is trying to protect his brightest son, he is attempting to protect the son who has the best chance of mating when he grows up.

You might be wondering though, why are daddy bluebirds so concerned with their sons and not their daughters.  It’s not just that bluebirds want to have daddy son time, it happens in other species as well.  For example, wandering albatrosses feed their sons more than their daughters and I’m sure there are many more examples out there of dads putting their effort into ensuring their sons grow up to be big, strong, sexy adults.  Essentially it comes down to wanting to continue the family line and sexy males (in species where an individual male has more sex than an individual female) will achieve that.

But it’s not even just after birth that parents play favourites, in some species there can be a disproportionate ratio of males and females born, taking the idea of parental preference to an extreme.  One example of this is the red deer which has more sons if mum is in good condition and more daughters is she is afflicted by parasites, ill health, in a lower rank etc.  This means that more dominant females have more sons than their non dominant counterparts, something that we see with macaques as well.

The Trivers-Willard hypothesis suggests an explanation for this favouritism.  The hypothesis is that with conditions are great, females should give birth to and invest in raising sons over daughters.  When conditions are poorer, the reverse should occur.  As with the other types of favouritism we’ve looked at, this is about ensuring that your children have lots of children and your line continues.  To understand why the Trivers-Willard hypothesis might hold, we need to note a few things:

  • If a mother is in great condition, she is more likely to have a child who is in great condition.
  • If a child is in great condition, they have a better chance of surviving to adulthood.
  • If the species is such that males have the potential to produce more offspring than females, then being sexy is important. If you aren’t a sexy male, then you might not attract any females, thus you won’t have any children and essentially the resources that your mother chose to give you are wasted.  Basically, mum would have been better having a daughter.

Essentially, a male in great condition will have significantly more offspring than a female in the same condition and thus is a better investment.  A male in poorer condition will potentially have less offspring than a female in the same condition and so is a bad choice to invest in.

Whilst it may not be of comfort to you if you aren’t the favourite child, at least you know you’re not alone… And just in case you were wondering, its thought that two thirds to three quarters of human parents favour one child over another…

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