Transport Response 1/2

It’s like magic. A healthy newborn, inconsolable despite every effort, suddenly stops crying the moment we step outside for a walk. Is it the sky? Fresh air? Or is it the way I’m moving? Could the sensation of being held by a walking human trigger relaxation in a baby?

For altricial mammals, those who are helpless at birth, being picked up and carried by their mother elicits sudden relaxation making them easier to move. For example, when a newborn puppy is picked up by its mother, observers can see that the puppy’s rear legs and tail curl up, while their body becomes passive. Internally, the puppy’s heart rate drops and respiration slows similar to that of a puppy at rest. This relaxation behavior while being carried is called the Transport Response and it helps ensure the survival of the mother and her offspring by making infants easier to move.

“[Kittens, puppies, mouse pups] assume a passive and compact posture with the hind legs drawn up while being carried. This postural regulation has been studied experimentally in laboratory rates as ‘transport response.’” (Esposito)

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Baby Feet

Humans, like all primates, are physiologically riders. Riders carry their young because their breast milk composition is not suited to parking infants for long periods (Ross). Nonhuman primates can carry their babies without the aid of a tool because their infants are small and able to cling to their mothers from birth. Humans find carrying infants more difficult for three reasons: the relative size of our infants,  non-grasping feet, and lack of adult body hair. In this post, we will focus on how the differences between primate and hominin feet, how feet affect carrying, and when bipedalism emerged.

“Birthing larger infants… also introduces the energetic and biomechanical challenge of transporting a relatively large, helpless newborn. This is particularly the case for pretechnological, upright walking hominids, some of which had reduced pedal grasping abilities.” (DeSilva)

Humans are altricial, our infants are relatively helpless. Nonhuman primates are precocious, meaning that they have accelerated physical development. Both human and ape newborns instinctively grasp with hands and feet. When a chimpanzee is born, they are able to grasp their mother’s fur with their hands and feet and cling on effectively. Whereas a human newborn is not as strong or directed in their efforts though they can strongly grasp with their hands (palmar grasp reflex), they can only flex their toes (plantar reflex) due to the morphology, or shape, of their feet which lack an opposable hallux (a big toe that looks more like a thumb). In humans, this grasping instinct is considered part of the primitive reflexes, which fade as humans develop.

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Meet the Family: Hominidae

Humans did not evolve from monkeys.

Humans did share a common ancestor with chimpanzees some six million years ago.

We are in fact all related, using the analogy of a family tree (see also: phylogenetic tree, cladograms, or the interactive tree of life) can help us understand how we all fit together. In fact, our nearest living evolutionary relatives are the Great Apes, in the Family Hominidae. Family refers to the taxonomic unit, which is larger than genus, which is larger than species. Monkeys are primates (order), like apes, but they form a different family than us, Family Cebidae. So while we share some traits with monkeys, like riding (Ross), our common ancestor is considerably further back in time, perhaps twenty-five million years ago (Stephens).

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Big Babies

Humans, like all primates, are physiologically ridersRiders carry their young because their breast milk composition is not suited to parking infants for long periods (Ross). Nonhuman primates can carry their babies without the aid of a tool because their infants are small and able to cling to their mothers from birth. Humans find carrying infants more difficult for three reasons: the relative size of our infants,  non-grasping feet, and lack of adult body hair. In this post, we’re going to focus on why human babies are so large and helpless compared to other primates and when in our evolution the trend for large infants began.

Our evolutionary cousins, non-human primates in the Family Hominidae, have smaller, more precocious babies than humans. Adult gorillas, for example, are significantly larger than adult humans, yenewborn and adult weightst their newborns are about half the size of our newborns. Chimpanzees, which are our closest extant evolutionary relative and have a similar adult body weight to adult humans, have newborns that are around 3% of adult size, while humans have newborns that are around 6% of adult size (DeSilva).

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To Ride Along

All primates are riders, meaning that primate infants ride on the body, usually of their mother, as she goes about her day. However, riding is a strategy that evolved long before primates and is seen in other mammals. Parking, on the other hand, is considered the ancestral state. Parkers, like foxes, for example, leave their young in a nest or burrow while they forage or hunt.  Continue reading