The Biological Basis of Sexual Behaviour in Amphibia: III. The Role of Distance Receptors in the Establishment of the Mating Reflex (Coupling) in Xenopus Laevis (The South African Clawed Toad): The Eyes

The type of animal used, and the technique employed to induce the mating reflex, have been described in a previous paper in this series by one of us (H. A. Shapiro, this Journal).

Pairs of toads (a male and a female in each pair) were injected with 0-5 ml. of an extract of early pregnancy urine (1-3 months’ amenorrhoea) prepared according to the directions of Katzman and Doisy (1932). The mating reflex was exhibited within 10-12 hours later. Coupling pairs were removed and placed in separate containers. Thirty-three such coupling pairs were collected and divided into three groups.

In the first group (fifteen pairs) the eyes of each male partner were removed.

In the second group (thirteen pairs) the eyes of each female partner were removed.

In the third group (five pairs) the eyes of both the male and the female partners were removed.

In order to remove the eyes the animals were anaesthetised by immersion in a mixture of ether and water. The eyes of Xenopus are very prominent and can easily be dissected out. Great care was taken to remove the retina completely. Very little bleeding occurred after the operation. A second injection of the pregnancy urine extract (0-5 ml.) was given intraperitoneally immediately before or after the extirpation of the eyes.

The animals remained in good condition and showed no signs of shock. Haemorrhage was minimal and gave no cause for concern. Attempts at coupling, accompanied by vigorous croaking on the part of the male, were observed within 1-2 hours after the operation, and on two occasions the normal lumbar embrace was adopted by the eyeless males within 40 min. after the injection had been given.

It was noticed that a considerable number of abnormal positions of coupling was adopted. These abnormal positions are seen only very infrequently when both partners are normal. The eyeless male frequently embraced a hindlimb or a forelimb or even the head of the female. Ultimately, however, such positions were relinquished and the normal lumbar embrace adopted in each of the fifteen pairs injected.

Fertilised ova were recovered and reared to the tadpole stage.

The female toads appeared to withstand removal of the eyes as well as did the males. The normal lumbar embrace was adopted without difficulty within 10-12 hours after injection in each of the thirteen pairs injected.

Fertilised ova were recovered and reared to the tadpole stage.

Abnormal positions of coupling as described in group (a) above were again observed. All five pairs used in this experiment, however, coupled in the normal way within 10-12 hours after injection.

Fertilised ova were recovered and reared to an advanced tadpole stage.

The fact that the normal lumbar embrace can be adopted by the eyeless male suggests that the eyes do not play an important role in the orientation of the male during clasping. It is likely that the eyes facilitate the adoption of the normal lumbar embrace, as the adoption of abnormal embraces (e.g. with hind- or forelimbs) was frequently observed in the case of the eyeless males. During such an embrace the male would execute the forward movements characteristic of coupling, even though the lumbar region of the female was not being clasped. The eyes, therefore, act as a refinement which assists in the establishment of the normal embrace. They are not at all, however, essential, as the total time taken did not differ from that for normal pairs. This is further substantiated by the experimental results obtained with the animals in group (c).

The passive role of the female described in the first paper in this series (Shapiro, this Journal) is again emphasised in the experiments in group (b), when eyeless females were paired with normal males. The abnormal positions characteristic of the eyeless males were not observed. The time taken for the establishment of the reflex was normal.

In all cases fertilised eggs were recovered and reared to an advanced tadpole stage. The duration of the experiment was too short to permit of gonadal atrophy after eye removal, but this point is being investigated in a chronic experiment.

1. The absence of the eyes in either the male or the female or in both partners of injected pairs of South African clawed toads (Xenopus laevis) does not interfere with the orientation of the male in the adoption of the normal lumbar embrace during coupling.

2. The more frequent adoption of abnormal embraces (e.g. with the hind- or the forelimbs of the female partner), in the case of the eyeless male, before the adoption of the normal embrace, suggests that the eyes may play an unimportant role in facilitating adoption of the lumbar embrace.

The expenses of this research were defrayed by a grant from the Union Government Research Grant Board.