Sibling Rivalry at its Worst

Image Copyright Carin Bondar, 'The Nature of Human Nature'

First born offspring have it pretty good.  Mom is generally healthy and well-rested (having no other children to look after during her pregnancy), and the newly emerged micro-human has the exclusive attention of both parents and quite likely some grandparents as well.  The first born doesn’t have to share the resources of his/her parents like second and third children do.  Being a mother of three children, I think about this quite often.  I feel kind of bad for my youngest child, who gets dragged around to preschool, music lessons, play-dates and the other endeavors of his older siblings.  He inevitably gets his toys snatched away or his snacks devoured before he can get to them…and really there’s not a whole lot that can be done to change that.  A major difference between humans and many other animals with respect to sibling competition is this: first born offspring often attain independence before further offspring are born, leaving the parents to care for only one infant at a time.  This is certainly not the case in our species, where offspring remain dependent for too long for mothers to wait for independence between them (our physiology just won’t allow for 18 years between children!).  Instead, sacrifices are made and a lower amount of parental care is provided to all siblings.  So why not just have one offspring and avoid the need to sacrifice any kind of parental commitment?  This could be risky…if all reproductive effort is placed on a single offspring, and (for whatever reason) said offspring fails to reproduce, your biological fitness is doomed.  The ‘bet-hedging’ strategy¹ has been coined to account for uncertain conditions in the future: there are clear advantages to having more offspring but providing a lower amount of parental care to each.

Other (non Homo sapiens) mammals face a similar conundrum when it comes to repeat child rearing when the first offspring is not yet independent.  Fur seal and sea lion females rear a single offspring at a time, and nurse it exclusively for a period of 2-3 years.  However, many females give birth to another offspring during this nursing phase.  A long term study of Galapagos fur seals (Arctocephalus galapagoensis) and sea lions (Zalophus wollebaeki) addressed the potential conflict between siblings competing for their mothers’ resources, and also the conflict between mothers and offspring (i.e. which offspring should she nurse?  In what situations should she choose the juvenile over the newborn and vice versa?²).  Between-sibling conflict was found to be especially strong when resource levels were low, and the older sibling was unable to forage effectively on its own (away from the supplementation of the mother’s milk).  During El Nino years, (resulting in low ocean productivity) a high level of between sibling conflict was documented, which often resulted in the continued nursing of the older juvenile and the death (by starvation) of the newborn.  However, during periods of high resource availability mothers aggressively defended their newborns against the juveniles’ attempts to nurse.  You might find yourself wondering: why should a female continue to reproduce when there is a chance for her newborn to starve or when her juvenile offspring is not ‘ready’ to stop nursing?  The fluctuating conditions of the Galapagos make it impossible for the adult females to predict what is going to happen, making a bet hedging strategy important.  If food resources are plentiful when a new pup is born, it makes sense for the mother should defend the younger offspring from aggressive attacks from the older one (i.e. get off my boob and go forage for yourself, there’s plenty of food out there).    However, if food resources are scarce, the mother is expected to allow the intimidation (and starvation) of the younger sibling since the older one has already received more of her resources and is more likely to survive to adulthood.  Although the death of a pup is a high price to pay for a miscalculation in available resources, the mother still gets one healthy offspring when times are bad and has the potential for more when times are good.

The Galapagos study shows that bet hedging is an effective reproductive strategy when environmental conditions and resource availablity are unpredictable.  Over the course of her reproductive lifetime, a female seal or sea lion can maximize her overall fitness by bet hedging rather than by waiting for favorable conditions.  Humans on the other hand, have taken a lot of the unpredictability out of our ability to attain resources, so our need for bet hedging is reduced.  While an El Nino year might affect the availability of jack tuna in the fresh seafood section of the grocery store, it really has no bearing on whether there will be enough food to go around.  Although at times my youngest child has to fight a little harder for his share of resources, starvation is out of the question.  Taking the nature out of foraging has its advantages.

¹Mock, D.W. and Forbes, S. 1995. The evolution of parental optimism.  Trends in Ecology and Evolution 7:409-413.

²Trillmich, F. and Wolf, J.B.W. 2008. Parent-offspring and sibling conflict in Galapagos fur seals and sea lions.  Behavioral Ecology and Sociobiology 62:363-375.