The effect of thyroidectomy on the intelligence of sheep

Abstract

The procedure followed in attempting to determine the effect of thyroidectomy on the intelligence of sheep has already been described. 1 Four pairs of twin female lambs from three to four weeks old were caused to learn a labyrinth with a single cul de sac and the twin of each pair making the better record was thy-roidectomized by Dr. Simpson. Two months later the operated lambs had become typical cretins.

One hundred two days following thyroidectomy one of the cretins, already definitely lethargic, and her normal twin began relearning the labyrinth. The position of the cul de sac was then reversed and the labyrinth was again learned. Learning was taken to be complete at the end of three trials without error; an error being counted whenever the animal entered the cul de sac or turned back along the true path. The number of steps taken by the animal and the time in seconds were recorded for each trial. The results were as follows:

One hundred forty-seven days following extirpation of the thyroids another cretin, markedly lethargic, was tested. She was given three trials but was unable to escape from the labyrinth which she had learned before being thyroidectomized. The results of these tests were:

The other two cretins did not become noticeably lethargic, although they were dwarfed in stature as compared with their twins. Tests were begun on one of these lambs and her normal twin two hundred six days following thyroidectomy. The results were :

Tests on the remaining pair were begun two hundred seven days after the operation with the following results:

A more difficult problem was then presented to this pair. The position of the cul de sac was alternated during four successive trials per day and twelve trials without error were required for complete learning.