Experiments to modify the sex ratio in the toad

Abstract

Several series of experiments were made in the spring of 1910 in order to ascertain whether the sex ratio in the toad can be altered by subjecting the eggs to different environmental conditions at or before the fertilization period.

Lots of eggs fertilized in various solutions of alcohol (.13 per cent. to 2 per cent.), as well as those fertilized with sperm from the right or from the left testicle of three different males, all gave practically normal sex ratios as the proportion of females ranged from 46.92 per cent. to 55.64 per cent.

Three batches of eggs from two different females were fertilized out of water and kept in a moist chamber for several hours. Each batch of eggs gave an unusual excess of females, the proportion of females varying from 60.86 to 70.83 per cent.

Mature eggs from another female were subjected to the action of a 2.5 per cent. solution of salt or of cane sugar for ten minutes and then fertilized in tap water. In each case 70 per cent. of females was obtained. In this series of experiments, as well as in the preceding one, it is probable that the eggs lost water during the fertilization period or at least were prevented from absorbing water during this time.

Seven lots of eggs from four different females were fertilized in solutions of hydrochloric or of acetic acid, the strength of the solutions varying from .OI per cent. to .0025 per cent. In every instance the percentage of females obtained was from 10 to 20 per cent. lower than that which is probably normal for the species. Lots of eggs from the same females fertilized in alkaline solutions (NH₄0H or NaOH) of the same strength gave proportions of the sexes within the range of probable normal variations in the sex ratios of different lots of individuals.