Bodily Electrical Potential Changes Associated with Ovulation and Early Pregnancy in the Chimpanzee. ∗

Abstract

Recently Burr and associates 1 discovered that ovulation in the rabbit is accompanied by large increases in electrical potential. Observations on women, using the apparatus and technique described by Burr, Lane, and Nims, 2 have been reported by Burr and Musselman. 3 Applications of the vacuum tube potentiometer have been reported also by Reboul, Friedgood, and Davis, 4 and by Rock, Reboul, and Wiggers. 5 The present preliminary report offers the results of an attempt to apply the Burr-Lane-Nims technique to the study of ovulation in the chimpanzee.

A sexually mature chimpanzee (No. 4 in the laboratory series), with regular menstrual cycles, was trained to present the backs of her hands for the application of salt-bridge electrodes and to remain passive during the course of a series of potential difference measurements. Measurements were taken daily over a period of several weeks.

The curve of Fig. 1 shows the potential measurements in millivolts through a normal sexual cycle (April 6 to May 6). Each voltage point represents the median of 10 or more determinations, with the exception of days 5,10, and 12. On these days there were 4, 2, and 4 observations respectively. Occasional refusal of the subject to coöperate or temporary difficulties with the apparatus account for the omission of data for certain days. The menstrual history of our subject and previous results of the controlled mating method 6 enabled us to predict that ovulation should occur in this subject on the 16th or 17th day. Accordingly, on days 16, 17, and 18 one or two additional series of observations were made. The sexual status of the subject is represented below the voltage curve. The line following the 4 days of menstruation represents the relative amount of genital swelling. The heavy portion of this line indicates the period of maximal swelling. General description of the sexual cycle of the chimpanzee has been given by Elder and Yerkes. 7

In contrast with the small variations in potential which appear from day to day, the median for the 17th day is higher than any of those before or after that date. The magnitude of this maximum value is not as great as might be expected, but it occurs at the time predicted, and also, if it is indeed a phenomenon associated with ovulation, stands in appropriate relationship to the beginning of detumescence. All of our isolated fertile matings fall within the last 6 days of maximal genital swelling.

Fig. 2 presents similar data for the next cycle of the same female. A maximum value again appears on the 17th day, followed by another peak 4 days later. Ihis second rise m the curve is ot particular interest because of a subsequent diagnosis of pregnancy confirmed by a series of Friedman tests. Because pregnancy was not suspected at the time that the high readings were obtained on days 21-23, the apparatus was completely dismantled for checking, and a period of 22 days elapsed before it was reassembled and put into operation again. We are inclined to interpret the high potentials of the 17th day as concomitant with ovulation, while those of the 21st day may be correlated with fertilization and early pregnancy changes.

These results indicate that the Burr-Lane-Nims technique may prove valuable for the early diagnosis of pregnancy as well as for the detection of ovulation in the chimpanzee.