TESTICULAR ASYMMETRY AND SEX RATIO IN BIRDS. 



HERBERT FRIEDMAXX, 

 ARNOLD BIOLOGICAL LABORATORY, BROWN FNIVEKSITY. 



It has long been known that in some birds the right testis never 

 attains the size of the left one even at the height of the vernal 

 hypertrophy of the gonads. Newton, in his Dictionary of 

 Birds (p. 784) writes that, "... generally the left testis is bigger 

 than the right, although both are equally functional." Riddle, 1 

 working with pigeons at Cold Spring Harbor, found that the 

 discrepancy in size between the two testes in hybrids increased 

 with the width of the cross involved. That is to say, the left 

 testis was proportionately larger in hybrid birds whose parents 

 were of two different genera than in birds resulting from the 

 mating of two congeneric species. The sex ratio of the offspring 

 was found to be apparently interrelated with the discrepancy in 

 size of the testes. Excess of males was always correlated with 

 proportionately larger left testes, while in birds in which the two 

 testes were equal in size the sex ratio was approximately one, i.e., 

 as many of one sex as of the other. 



If there is anything in this seeming correlation it should be 

 possible to arrive at some idea of the sex ratio in wild species of 

 birds by examining the testes of adult males in breeding condition. 

 Those in which the left testis is much larger than the right should 

 have an excess of males in their total population and this excess 

 should vary directly with the amount of difference between the 

 two testes of the adult male. The importance of the sex ratio in 

 such matters as the courtship habits and territorial relations of 

 birds must be very great and its accurate determination for any 

 species is prerequisite to a proper understanding of the habits of 

 that species. 



With the hope of getting some new light on this subject in 

 wild birds I kept note of the relative size of the testes of all adult 



1 Riddle, Oscar, "Further Observations on the Relative Size of the Right and 

 Left Testis of Pigeons in Health and Disease and as Influenced by Hybridity," 

 The Anatomical Record, XIV., 1918, pp. 333-4. 

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