James Sweeney

Darcy Geddes

 

Infants of some species are born looking very similar to the adults, with the same cryptic coloring that serves as camouflage.  However, some infants have intense, attention grabbing coloration.  These examples are fascinating from an evolutionary perspective because presumably the bright coloring makes the infants, who are already at great risk from predators due to their immaturity, an even easier target.  The puzzle that this article attempts to unravel is how these seemingly disadvantageous natal colorations have been in fact advantageous and selected for.  Through an examination of the social systems and parenting styles of these species, Hrdy then launches into speculation about human infant looks as they relate to dependency and survival.

            The author first discusses brood reducing species of birds.  Brood reducing refers to the practice of laying more eggs than could be taken care of, so that the number of viable chicks must be reduced.  This is an interesting phenomenon because the laying and hatching of an egg is itself a diversion of resources, and so wasting them on eggs which will never survive to reproduce seems to go against evolution.  However, examination of the species reveal that evolution is not so simplistic as calculated merely the cost of parental investment to number of viable offspring.  Usually the brood is reduced through siblicide, the killing of siblings.  Many of these birds practice sequential laying, where they space their egg laying over a period of time, the end result is that some eggs hatch before others.   It is important to remember that while in general it is to the parents advantage for all the offspring to survive, the sibling are often competing for limited parental resources.  The first laid, first hatched siblings have the advantage over their younger siblings because they are stronger.  There are two types of brood reducers.  One Hrdy calls obligatory siblicidal and these are birds where there is no chance for the younger sibling to survive while the older one is alive.  Presumably the practice of laying the extra, younger egg serves as a kind of insurance in case the older one is killed.  However, if the older one survives it always pecks the younger one to death.  Other species of brood reducers are more indirect, with the older more aggressive siblings bullying the younger weaker ones out of food.  In the indirect birds, the brood is only reduced if food is scarce.  If food is plentiful, the entire brood may survive.  The phenomenon of siblicide is a difficult one for most humans to think about, especially given that the parents do not intervene to save the weaker siblings.  Indeed, the siblicidal behavior is at least partially due to the parents behavior, since it is the length of time spend brooding the eggs rather than strictly when the eggs are laid that dictates hatching time.  Thus, in cases where the parent does not help the weaker sibling, there must be some advantage to the parents’ genes to indirectly encourage siblicide. 

            There are several ways that parents can affect this sibling rivalry, in some cases to increase the stronger bird’s advantage and in other species the parents work to equalize the differences.  One of the ways for the parents to affect the balance of power is through the amount of androgens deposited into the yolk when the egg is laid.  Extra testosterone in the eldest works to increase its advantage, while extra testosterone in the younger bird works to equalize the difference.  Another way that parents can affect which sibling survives is when the parent gives more food to one of the offspring, usually the youngest.  

            This leads to an interesting question of how the parent knows which chick is the youngest.  In the species Fulica americana, the American coot, the skin of distressed birds turns reddish and parents feed distressed birds more often (Lyon, et. al.).  The birds are born with their heads a bright red, so the parents tend to feed the youngest (and therefore neediest) the most.  This very noticeable head coloring is most likely the result of runaway evolution.  The chicks who had balder, redder heads attracted more attention and resources, were more likely to survive and someday have their own bald red chicks survive.  In addition, the genes for parents who preferentially feed the bright colored chicks would also be selected for.  Thus, due to ever-increasing positive feedback, the traits became more and more extreme, much like the sexual selection involved in peacock tails. 

            These odd and sometimes flamboyant colorations raise another question: Could these benefits outweigh the cost of announcing one’s location to predators?  For the birds discussed above the risk is not so great since they loose their fancy coloration fairly quickly and the nests are fairly well hidden.  However some primate infants are so brightly colored that they can be seen over a half-mile away.  And since primates tend to have much longer periods of infancy, dependency and weakness, these bright colors are around longer, presumably acting as a target for predators.  They are not brightly colored for the same reason that the American coots are: primates don’t usually have siblings close in age because the dependency of one child prevents ovulation in the mother. 

            In order to answer the riddle of the advantage of bright colors in infant monkeys, Hrdy examines not just the birth mother, but all the females within the social group that may take on some responsibility for child rearing.  In species that display a flamboyant infant coloring, there is often a considerable amount of allo-parenting, which means that adults other than the birth parents share in the child-rearing.  Before we address the link between flamboyant infant colors and allo-parenting, we should mention the evolutionary significance of infant sharing.  It has been demonstrated that allo-parenting increases the survival rate of infants directly by providing protection while the parent is foraging and also indirectly by allowing the mother to find more food which is shared with the child.  Another advantage is that species who participate in infant-sharing tend to grow at a much faster rate, which allows the mother to reproduce more frequently and also shortens the most vulnerable stage of the offspring’s life (Mitani and Watts).    In infant-sharing groups, Hrdy argues, the more flamboyantly colored primates are handled more by the adults.  This forms a positive feedback loop similar to the coots, only including the community as a whole, with age-mates competing for attention rather than siblings competing directly for food. 

            Hrdy finishes by briefly discussing human infants.   Human infants do not have a striking natal coat the way the other species discussed do. She says that this would imply that human infants do not need a natal coat to attract and sustain maternal (or allo-maternal) care.  However, newborns are the most at risk for maternal abandonment or infanticide.  This is unique in the primate world: in other species of primates mothers are the most unconditionally dedicated to the youngest, with abandonment more likely when the child is older.  What has changed for humans that makes our newborns most at risk during their newborn stage.  Hrdy speculates that what changed was a shortening of breast-feeding times because infants began subsisting on foods like porridge.  When breast-feeding stops the body stops producing a hormone called prolactin, which eventually causes ovulation to begin again.  Thus mothers became hyperfertile, able to become pregnant in increasingly shorter intervals.  Hrdy then speculates that human mothers began to be shaped by the same forces that shaped mammals who bore litters and discriminate between offspring, rather than the primate paradigm of total dedication to the one child.  Harried mothers with many infants of different ages would be selected for if they were discriminating about which babies were given resources.  Hrdy sites as evidence the tendency for human mothers' to abandon their newborns if the timing is poor.  She implies that this behavior, although not common in humans, is far more common in humans than in any other primates, and bears resemblance to the tendencies of litter bearing mammals and birds. 


Critical Review

            Some of her more interesting points were the variation in hormones given to different eggs and how that affects dominance.  Another interesting idea how sibling competition for parental attention can shape extreme changes in physical appearance.

            It would have been helpful to expand the discussion of the costs of bright natal coloring, rather than focusing solely on the benefits.   Another thing that needed further explication is the effects of preferential feeding during sever food limitation.

Although this article was interesting, it seemed to skim the surface of some fascinating phenomena.  Perhaps she was trying to focus on only the natal coats, but such a thing can hardly be understood out of context and a more thorough treatment of the evolution of the system would have been interesting.  For instance, Hrdy speculates about the benefits of natal coats in allo-parenting social systems, but she says very little about the environmental differences that favored allo-parenting in the first place.  That is, she makes an excellent argument for the adaptiveness of infant-sharing and attention-grabbing infant colors.  She also mentions the much more prevalent trait of infant camouflage.  This seems to beg the question of what forces must have been different to push the selection of two such opposite strategies.  Another interesting idea regarding the American coot is why cheaters have not evolved.  If runaway selection is favoring the reddest, baldest infants why have infants not been selected for longer periods of neonatal appearance?

            Her discussion of human infants seems very limited, partially because she discusses humans in a later chapter.  However, her main contention seems to be that humans lack a flamboyant natal coat with which to attract either parents or allo-parents.  However it seems that even without a fancy look, human infants DO attract the attention of all adults around them.  It seems simplistic of Hrdy to assume that a flamboyant coloring would be the only, or even the primary cue for adults to care for a child.  In addition, as with all things human, because of our complex brains and consciousness and the huge part learning plays in our existence, it seems important to consider memes, rather than just genes and phenotype.  The only aspect of Hrdy’s discussion of human maternal desertion that seems really scientific and testable is the idea of studying traditional primates during periods of induced hyperfertility to see whether the added infants caused a shift to the more discriminating maternal behavior.