Studies of Maternal Retrieving in Rats. Iii. Sensory Cues Involved in the Lactating Female's Response To Her Young 1)

Abstract

Twenty-six lactating female rats were observed in a series of tests designed to identify the sensory cues involved in the retrieving of young to the nest. In some instances females were surgically deprived of one or more sensory receptors. In other cases the females were unoperated and the sensory qualities of the young were modified. The types of desensitization employed were enucleation of the eyes, cauterization of the olfactory bulbs, and transection of the sensory nerves serving the anterior portion of the head, including the snout and lips. Modification of sensory qualities of the young was achieved in different instances by spraying living young with oil of lavender, coating young with vasoline or collodion, killing the young just before the test by separation of the brain and spinal cord, and chilling freshly killed young in a refrigerator. In some tests living young were enclosed in glass vials so that they were visually accessible to the female but could not be smelled or touched. In other tests normal young were confined inside a small, wire-mesh cage so that the female could smell them but could not establish physical contact. In addition to normal or artificially modified young, other retrievable objects were offered to the experimental females. These included young mice and rabbits, and pieces of fresh meat cut to the size of a newborn rat. The results of these various procedures led to the conclusion that the retrieving response is ordinarily governed by a combination of sensory cues. When they are available, visual, chemical, tactile and thermal sensations all contribute to the female's tendency to approach, pick up and retrieve her offspring. No single type of sensation is indispensable. It appears that the female's retrieving behavior, like the sexual behavior of the male rat or like the maze-learning performance of both sexes, normally involves multisensory controls. This finding is believed to be related to the fact that the cerebral cortex plays an important role in the male's copulatory behavior, the female's maternal performance and in maze learning. It is probably the intervention of a cortical component which makes the rat's maternal behavior different from analogous reactions in lower vertebrates. In the latter, much of the reproductive activity seems to be controlled by relatively simple, unisensory forms of stimulation. This could be due to the fact that in these lower forms a higher nervous mechanism such as the cortex is lacking.