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University of Wisconsin
Abstract
The problem of the sex ratio of fetal calves was investigated by Jewell in 1921. He left the problem with this status: "In the younger stages there may be a slight difference in the viability of the two sexes, the male being somewhat more susceptible to intrauterine disturbances. However, this difference does not seem to be constant enough to be of real significance and one could not from the data given here conclude with certainty that in the foetal development in cattle there is a greater mortality in the males during any particular developmental stage. A larger collection of individuals within each group would no doubt clarify the situation." Jewell classified one thousand fetuses of slaughter house origin into ten classes of ten centimeters each on the basis of crown-rump length and drew his conclusions from graphs and retabulation of the data.
For the present study Jewell's records have been combined with 1044 additional records of fetal calves collected in Chicago and in Madison, Wisconsin, packing houses, making a total of 2044 records. This procedure is justifiable as Jewell's data do not differ significantly from the more recent data when the two sets are classified into the 10 centimeter groups and tested by the x2 distribution.
The sex ratio (percentage of males among all individuals) varies from 66 per cent of 126 individuals in the youngest group (5 – 10 cm.) to 47 per cent of 196 individuals in the 71–80 cm. group. The change in sex ratio is not characterized by a steady decline but by a sudden drop from the 5-10 cm. class (66 per cent) to the 11–20 cm. class (56 per cent of 294 individuals). This class is followed by a more or less irregular decline to the 81–90 cm. class which has a ratio of 49.5 per cent of 119 individuals. In the class of greatest length (91–100 cm) there were only 32 individuals, 66 per cent of which were males. This high ratio may be due to the heifer calves having been bore before this length is reached.
The change in sex ratio during prenatal life is highly significant statistically (P< .01) and could be interpreted as being due primarily to the high differential mortality in the early stages (5–10 cm. to 11–20 cm.) followed by a more gradual mortality of males at the later stages of fetal life. The sex ratio near term (secondary sex ratio) of approximate equality appears to result from the loss of at least one out of every two males that are found at the earliest stage at which the sexes can be distinguished macroscopically. The stage of high male mortality corresponds roughly to the third month of pregnancy. The second and third month after breeding is indicated to be a common time for cows to return into oestrum. This might be explained by the frequent resorption or abortion of male fetuses. Naturally it is not certain that the findings are entirely typical of cattle on farms, since there are many shy-breeding animals among those killed at slaughter houses.
The percentages of all calves, of all bull calves, and of all heifer calves borne in the right horn of the uterus were not significantly different from 58 in all the stages of fetal life. In other words, prenatal mortality did not affect the calves borne in the right and left horns differentially.
1 Paper from the Department of Genetics, Agricultural Experiment Station, University of Wisconsin, No. 23 5. This work was supported in part by a grant from the Alumni Research Foundation.
Facilities for the study of the fetuses were kindly provided by Armour and Co., Chicago, and Oscar Mayer and Co., Madison, Wis.
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