A study of the neural control of sexual clasping behaviour in Rana angolensis bocage and Bufo regularis reuss with a consideration of self-regulatory hindbrain systems in the Anura.

Abstract

1. During amplexus, Rana angolensis and Bufo regularis males assume a specific clasp posture on the female and maintain the clasp by means of elaborate clasp patterns. The clasp maintenance behaviour of the male is adapted to the environment where amplexus takes place, namely, deep water conditions in R. angolensis and shallow water and terrestrial conditions in B. regularis. 2. The neural mechanisms underlying clasping behaviour have been investigated by subjecting the central nervous system to a series of transections. Standardised levels of transection have been selected and groups of males and females of each species have been transected at these levels and tested for clasping behaviour with a variety of tactile stimulations. 3. In both species, males transected at the posterior end of the medulla oblongata (anterior to spinal nerve 2) displayed a tonic flexure of the forelimbs which appears to have no sexual significance. However, males transected in the anterior medulla (immediately anterior to the roots of cranial nerves V and VII) and isthmus regions initiated and maintained a clasp. The initiatory movements were stereotyped within each species. Transected males of both species were able to distinguish between normal and abnormal clasp objects and only maintained a clasp when the clasp object provided the correct input. Transected B. regularis males were able to make minor adjustments from an abnormal position on the female to the correct clasp position. An elaborate adjustment pattern was shown by transected R. angolensis males. It is concluded that a selective co-ordinates clasp initiation; part of this mechanism is situated in the hindbrain (i.e. cerebellar region and medulla oblongata). 4. The clasp posture and patterns of B. regularis males transected in the anterior medulla, were identical to those of intact males. This suggests that the hindbrain co-ordinates the clasp maintenance behaviour of the male. The clasp patterns and posture of transected R. angolensis males differed slightly from the normal, whereas those of partially transected males were normal. This was taken to suggest that although most of the co-ordination or clasping is carried out by the hindbrain in this species, the mid-and forebrain also play a part. 5. Females of both species, transected in the anterior medulla oblongata, could be stimulated to show clasping behaviour. These patterns were particularly well co-ordinated by B. regularis females. 6. Males of both species transected in the mid-brain (mid-optic region) failed to show clasping behaviour when stimulated appropriately. It is suggested that inhibitory mechanisms for clasping are situated in the posterior mid-brain. 7. The results obtained from Xenopus laevis, B. regularis and R. angolensis by the author have been summarised and compared. It is concluded that the system which co-ordinates clasping behaviour in the Anura is not purely spinal, as has been suggested by previous workers, but involves much of the hindbrain. This system is activated by specific input and is able to function in the absence of the mid- and forebrain. The hypothesis is put forward that (a) the clasp control system functions as a self-regulatory unit without the aid of the higher brain during amplexus; and (b) this self-regulatory hindbrain system is a specialised central mechanism which has evolved in conjunction with the long amplexus period of the Anura.