l64 ECOLOGY AND LIFE HISTORY OF THE COMMON FROG 



that no one has demonstrated in any species of Rana that the sex call 

 attracts the female. 



Sex Recognition 



The methods by which animals recognize the sex of each other are 

 often complicated, and may not be at all obvious until the behaviour 

 has been closely studied. The frog is no exception. Pioneer work on 

 sex recognition in frogs, but not on this species, was carried out by 

 Noble and Farris (1929), using the American Wood Frog, R. sylvatica, 

 considered by systematists to be closely related. Since then, and since 

 my paper, other studies have appeared, for example W. M. S. Russell 

 (1955) oil Xenopus laevis, and Aronson (1944), on American species, 

 so that it is now possible to discuss the question with a reasonably 

 broad view, especially as the more general work on animal behaviour 

 can contribute to a better understanding. 



It is not possible to see all the details in a pond, so that I made a 

 special aquarium for the observations. This had a glass bottom and a 

 mirror back, and, by lighting it from below, there was scarcely any 

 chance that an important action would pass unobserved, for I had 

 only to move my eyes to see, in a second or so, several different aspects 

 of the same pair of frogs, either a direct view, one in the back mirror, 

 or, reflected in another mirror placed on the table under the aquarium, 

 a view from underneath. 



In all the period of observation, both at the ponds and in the tank, 

 only rarely did one male vigorously grab another, and even then the 

 embraces lasted only a few seconds. The males do not invariably give 

 either their sex-warning croak or their ordinary croak when mere 

 contact occurs. On the other hand, both females introduced into the 

 tank were seized within about five minutes. In the description of the 

 behaviour of the spent females (Fig. 46), it will be seen that these 

 females, which resemble the males in figure, are frequently seized 

 vigorously. It is impossible to reconcile these observations with those 

 of Noble and Farris on the Wood Frog. In these animals the male 

 seizes another indiscriminately. If the seized animal is a female, she 

 possesses a stout figure and remains silent, but a male has a slender 

 figure and gives out a short rapidly repeated croak that acts as an 

 indication of his sex. Cummins (1920) and Liu (193 1-2), attached 

 importance to the repulsive action of the male that is seized. Release or 

 retention depends on these differences of behaviour and characteristics. 



