Rats and Mice and Such Small Deer: Can They Help Us Learn About Human Infants?

Over at http://marginalperspectives.blogspot.com there’s a discussion going on about research on other species. Reference has been made to a research article: Graham,Y.P., Heim, C., Goodman,S.H., Miller, A.H., & Nemeroff,C.B. (1999). The effects of neonatal stress on brain development: Implications for psychopathology. Development and Psychopathology, 11, 545-565. (And yes, if that name rings a bell, this is the same Nemeroff who had to leave Emory because of failure to report his income from a drug company--- http://www.emorywheel.com/detail.php?n=27732). At marginalperspectives, people are talking about whether information obtained by observing rats can be generalized to human infants. They’re most concerned about the effects of separation from the mother, as evidenced by rat experiments (I mean experiments using rats as subjects, not experiments performed by rats).

The Graham et al paper reports, among many other things, that undesirable brain changes occur in young rats who are separated from their mothers, even though in one situation somewhat older pups could see the mothers through a transparent burrow. The marginalperspectives group suggest that this means that similar effects occur in human babies who are separated from their mothers, as, for example, in a hospital nursery, or of course in adoption.

The Graham group notes the large developmental differences between the central nervous system maturity of newborn rats and humans, noting that a human at 24 weeks gestational age (still 16 weeks preterm) is equivalent in maturity to a newborn rat pup. This in itself points up for us differences between the species. In addition, though, we need to look at differences in rat and human experience of separation. When rat pups are experimentally removed from their mothers in this kind of study, they are NOT given substitute maternal care. Separation from the mother means separation from care. When human infants are separated from their birthmothers in medical situations or in adoption and fostering, they are given substitute care. The only parallel involving substitute care for rats lies in the situation described by Graham et al, in which additional handling by human beings is beneficial for rat pups’ brain and behavioral development.

The Graham article does not allow easy generalization of these points from rat pups to human infants. To try to do this is to confound the variables of maternal-like care and of the mother herself. We can’t logically jump from the experiences of rat pups in experimental separation to those of human infants who are fostered or adopted, nannied, nursed, baby-sat, or picked up by their daddies or big brothers or sisters.

Let me point out, by the way, that not only are humans developmentally and neurologically different from rats, and that rat pups are more mature than humans at birth-- rat mothers also behave differently from human mothers. As an old-fashioned psychologist, dating back to the days when every psych student worked with pigeons or rats or both as part of their studies, I know a lot about this (including how to pick the little guys up without getting bitten).

Rat mothers have much different jobs from human mothers/caregivers. It doesn’t matter if they’re depressed or anxious, because they needn’t talk or communicate with gestures like eye movement. They don’t need to pick up a baby and help it latch on to the nipple, or to maintain their marital relationships during the trying first months of motherhood. Here’s what they do: if a baby gets out of the nest, they retrieve it and put it back in so it doesn’t get cold. They lick the babies all over, thus stimulating urination and defecation, and you guessed it, they lick that up too. If a baby is very sick or dies, they eat it (and of course they started their maternal care by eating off the amniotic sacs and placenta, occasionally eating a baby while they were at it). When the babies start to grow fur, the mother gets a lot less interested. (By the way, the pups at birth are just pink bare skin, so transparent that you can actually see the milk in their stomachs after they nurse-- a sight that would probably be comforting to worried nursing mothers if humans had it too!)

Studying other species can give us some good ideas about how humans might develop, but until we test the ideas in humans themselves, we can’t safely conclude that the facts about one species tell us the facts about another. Does anyone remember Thalidomide, the tranquilizer that caused severe birth defects in Europe? It was thoroughly tested with animal models. It wasn’t teratogenic in those cases. But it surely was in humans. It was a bad idea to generalize too quickly across species about that drug, and that should tell us something about generalizing across species about social and emotional development.