Have you ever wondered why some animals create a nest or den for their offspring, while others carry their babies everywhere they go? These represent two reproductive strategies, or ways of producing and caring for offspring that survive to sexual maturity, called parking and riding respectively.
Parking is considered the “ancestral state” and is the more common strategy among Eutherian mammals, those furry/hairy, warm-blooded, placenta-growing, live-birth giving breastfeeders (hint: we’re a member!), whose clade emerged around 200 million years ago in the late Triassic period. Parkers leave their offspring while they go out to hunt or forage and can be divided into nesting and nonnesting. Nonnesting parkers do not have a centralized nest or den, choosing new locations to park their offspring when they need to get food. This strategy is thought to help avoid parasites that might build up in a nest if it’s used for a long time in the case of slow growers. Nesting parkers tend to give birth to litters of altricial, blind, hairless, helpless offspring after a short gestation. Once her offspring are able to maintain their body temperature, the mother will leave them in the den. Due to her breast milk composition, which is high in protein and slow digesting, she can stay out for long stretches between feedings. If the mother runs into danger while out she can try to evade it without the burden of her infants. When parkers need to move their offspring they may move them by mouth, in the case of oral carriers or their offspring might cling to their fur in the case of nonoral carriers.
“The evolution of infant carrying behaviour may not be possible in some species if the infants are too poorly developed at birth to cling to the carrier. Altricial species might therefore be constrained to be parkers whereas more precocial species would be preadapted to carrying. If this is the case, one would predict that measures of altriciality will vary with carrying behaviour.” (Ross,757)
Riders do not nest and do not park their offspring, rider offspring follow their mother from birth, either on foot, fin or by clinging to her fur. For this reason, rider infants need to be precocial, or highly developed, at birth. Riders’ breast milk is high in carbohydrates and is quickly digested, requiring frequent nursing, so the baby needs to stay close to its mother. If a chimpanzee mother had to run back to a den to feed her baby four times an hour (Hedberg-Nyquist), she’d use up all her precious calories on the commute alone– not to mention the kind of attention her constant coming and going would draw to the den. Riders tend to have singleton pregnancies, have a higher age at weaning and sexual maturity, lengthening the birth interval between offspring and between generations, representing a reproductive cost that is higher than that of parkers. However, Ross’s research suggests that riders have a lower mortality rate than parkers.
Some mammals do a bit riding and parking, for example, a bat will either leave their babies with the group in the roost (bats show cooperative breeding, helping one another care for offspring) or the babies might cling to their mother’s belly while she’s flying. Pangolin infants are left in nests while they are very young and when a bit older ride on mother’s back or tail. The mouse lemur keeps its young in a nest for three weeks, then she will carry them in her mouth while in transit and park them nearby while she’s foraging. Animals who have mixed strategies like these may represent an evolutionary stage between parking and riding (Ross).
“If non-riding is assumed to be the ancestral state, riding was conserved in nearly all lineages once it has evolved. This suggests that, once carrying has evolved it is difficult to lose, possibly because of behavioral and physiological co-adaptations that occur once infants are carried,” (Ross, 765).
All of the apes, including humans, are biologically riders, however, when it comes to our infants we have a problem: human babies can’t cling on to their mothers (or caregiver) effectively due to secondary altricity (physical helplessness). Yet they need to stay close for frequent feeding. Biologically our only option was to carry them (or find someone to help carry them) but technologically, we have more options including the development of an infant carrier to make up for the physical helplessness of our infants. According to Wall-Scheffler’s research on the energetics of load carrying, the latter is the most likely option taken not just by humans but by our first bipedal ancestors.
“We suspect that the energetic drain of carrying an infant would be such that some sort of carrying device would have been required soon after the development of bipedalism and definitely to allow long distance travel, especially that out of Africa and across Asia.” (Wall-Scheffler, et al.)
For a very long time, infant care technology focused on supporting the biological-rider-needs of the infant, finding ways to secure them to the body of their mother or allomother for safety and food. Over time humans in some societies have become cultural parkers, their infants are kept at home, or in a daycare with other infants (a nest if you will) while their parent or parents go out into the world to work (hunt/forage). Infants may be fed by breast (their mother’s or someone else’s), or by bottle containing human, cow, or plant milk. For mothers who do not breastfeed their infants, the intervals between births is shortened allowing them to have more children during their reproductive years– reducing the reproductive costs associated with being a biological rider. This is very helpful in societies which require children’s labor and is something that is only possible in societies with permanent settlements.
Of course, not all human societies have access to or want the kind of technology that allows humans to be cultural parkers. Even in societies in which breastfeeding mothers go out to work while leaving her infant behind, like the Beng of the Ivory Coast, the baby may not be parked but continue to be constantly carried and even breastfed by others.
Parking and riding are reproductive strategies with the benefits of each outweighing the alternative for the species that practice them. While some animals have a mixed strategy it is all about their biology, whereas humans have a mixed strategy depending on the technology and culture where they live. It was technology which helped our ancestors survive as biological riders and technology that helps (some of) us live like cultural parkers.
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Hedberg-Nyquist, Kerstin. “Developmental Care: A breast-feeding perspective.” Research on Early Developmental Care for Preterm Neonates. Ed. Jacques Sizun and Joy V. Browne. John Libbey Eurotext. 2006.
Ross, Caroline. “Park or Ride? Evolution of Infant Carrying in Primates.” International Journal of Primatology 22.5 (2001): 749-71. Springer. Web. 26 Jan. 2015.
Wall-Scheffler, C.m., K. Geiger, and K.l. Steudel-Numbers. “Infant Carrying: The Role of Increased Locomotory Costs in Early Tool Development.” American Journal of Physical Anthropology 133.2 (2007): 841-46. Web.