4.2 Licking/grooming-arched back nursing
Rat mothers perform a number behaviours towards their pups: they build a nest for their pups, keep them in it and occasionally lick them and nurse them. (Rat fathers have a parental role too but it is not essential and the experimental set-up is simplified by his absence.) Licking occurs predominantly at the time when the dam arches her back and nurses her young, allowing a composite behaviour of licking/grooming-arched back nursing to be identified and recorded. If licking/grooming-arched back nursing is recorded for a number of rat mothers, consistent differences emerge. Some mothers perform licking/grooming-arched back nursing at a high frequency, others at an intermediate frequency and still others at a low frequency. Furthermore, these differences between mothers are consistent from one litter to the next. So if a dam has a high frequency of licking/grooming-arched back nursing with her first litter, she will also have a high frequency of licking/grooming-arched back nursing with her second litter. And although there are consistent differences in frequency of licking/grooming-arched back nursing between mothers, the overall amount of time in contact with the pups does not differ between mothers.
Why is this last statement important?
It means that the rat pups in the low-frequency licking/grooming-arched back nursing group are not being neglected; they simply receive a different pattern of maternal care.
The difference in maternal behaviour is correlated with a difference in the behaviour of their offspring. A commonly used and easily administered test of rat behaviour is the Open Field Test. A relatively large (2 m diameter), well-lit, white, circular arena is used. The floor is marked with a grid of lines, so movement can be quantified. A rat (the procedure invariably uses solitary animals) introduced to the arena can move about a lot or a little and can hug the sides or venture into the exposed central area. The test lasts either five or ten minutes. Rats differ in how they behave in an Open Field Test.
Rats raised by mothers exhibiting a high level of licking/grooming-arched back nursing (lg-abn), tested in an Open Field, spent significantly more time in the central area, compared to rats raised by mothers exhibiting a low level of licking/grooming-arched back nursing. The question is, does the difference in nursing cause the difference in Open Field behaviour? Correlations are intriguing but they do not reveal anything about causes. It could be, for instance, that in the present example, there is a single underlying cause, passed from mother to offspring, that affects both maternal behaviour and Open Field behaviour. If this is the case, then maternal behaviour may or may not affect Open Field behaviour. To answer the question about causes posed above, a fostering experiment is needed.
Fostering separates offspring from their biological mother, and allows the influence of the foster mother on the development of the fostered pups to be examined. Francis and colleagues, the authors of the experiment described below, were very careful to minimise the disruption to the litters that fostering can sometimes cause (Francis et al., 1999). Only two pups were fostered into or out of any of the litters, which all comprised 12 pups. All dams received two pups and all dams had two pups removed; a fostering variant called cross-fostering.
Figure 9 shows how the pups were fostered between the dams.
The hypothesis under test is whether rats which experience a high lg-abn frequency as pups differ in their Open Field behaviour from rats which experience a low lg-abn frequency as pups.
Figure 9 illustrates how two pups were removed from each litter and two pups were added to each litter. (L-L° means the pups were fostered to another low lg-abn dam; H-H° means fostered to another high lg-abn dam; L-H means pups born to a low lg-abn dam were fostered to a high lg-abn dam; H-L means pups born to a high lg-abn dam were fostered to a low lg-abn dam). In addition, two pups were removed and replaced in the same litter. (L-Ls means fostered to the same low lg-abn dam, whereas H-Hs means fostered to the same high lg-abn dam.)
What were the two conditions used in this experiment?
One condition was high lg-abn frequency and the other condition was low lg-abn frequency.
If dam behaviour during infancy is the primary determinant of later Open Field behaviour, how would you expect rats, born of low lg-abn frequency dams, but reared by a high lg-abn dams to behave in the Open Field?
You would expect rats, born of low lg-abn frequency dams, but reared by a high lg-abn dams to behave in exactly the same way as rats, born of high lg-abn frequency dams, and reared by high lg-abn dams. Moreover, they would spend more time in the central area of the Open Field than would rats reared by low lg-abn dams.
The results of this experiment are presented in Figure 10.
Describe the results for rats born of low lg-abn frequency dams but reared by high lg-abn dams.
Rats born of low lg-abn frequency dams but reared by high lg-abn dams (L-H bar in Figure 10) showed no difference in their Open Field behaviour compared to rats born of high lg-abn frequency dams and reared by high lg-abn dams (H-H* bars). However, they spent more time in the central area compared to rats born of low lg-abn frequency dams and reared by low lg-abn dams (L-L* bars).
Is this the result expected if maternal behaviour during infancy is the primary determinant of later Open Field behaviour?
Yes it is.
Having shown that maternal behaviour during infancy is a major determinant of later Open Field behaviour, the same authors went on to demonstrate that it is also a major determinant of later maternal behaviour. When female rats born of low lg-abn frequency dams but reared by high lg-abn dams were themselves mothers, their maternal behaviour was the same as their nursing dam. Thus these two characters (frequency of lg-abn; Open Field behaviour) run in families, they are familial, but are environmentally as opposed to genetically determined.
Note that the fact that a disease is familial, does not necessarily mean that it is genetic. We shall return to this subject later in the course.