Reproductive isolation in smooth and palmate newts (Triturus vulgaris vulgaris and T. helveticus helveticus)

Roberts, Julie Marie (1990). Reproductive isolation in smooth and palmate newts (Triturus vulgaris vulgaris and T. helveticus helveticus). PhD thesis The Open University.



This thesis reports a study of reproductive isolation between two closely related sympatric British species of amphibian, the Smooth newt, Triturus vulgaris vulgaris and the Palmate newt, T, helveticus helveticus. Investigation shows that reproductive isolation is not attributable to differences in habitat, the timing of courtship or the failure of spermatophores to adhere to the cloaca of a heterospecific female and it is demonstrated that a proportion of FI hybrid larvae are capable of survival beyond metamorphosis; by elimination, the lack of naturally occurring hybrids is primarily a consequence of sexual isolation.
Techniques for distinguishing both purebred and hybrid larvae and adult newts of both sexes and species based on differences in their external morphology are evaluated. The most suitable distinguishing characters (on which visual discrimination in newts may be based) are described. The variation in the courtship behaviour of T. h. helveticus is described in detail and compared to that of T. v, vulgaris. The similarity in form and transition of male acts suggests that qualitative differences in display and order of acts are not the primary source of information on which female discrimination is based. Quantitative differences in the proportion of acts performed probably serve to emphasise species-specific cues which play a direct role in sexual isolation. À causal analysis is carried out on a relatively recently recognised I. h. helveticus display act, the wiggle. Evidence is presented that the function of this act is to re- attract a female that is becoming unreceptive and it does not appear to contribute to reproductive isolation.
Interspecific mating trials show that female J. h. helveticus are less receptive to heterospecific courtship than conspecific courtship. Male T. V. vulgaris display less vigorously to heterospecific females manipulated by means of a harness to perform positive behaviour than to conspecific females. It is demonstrated that male T. h. helveticus show selectivity towards conspecific females over heterospecific females on the basis of both visual and olfactory cues alone, whereas in male T. v. vulgaris and female T. v, vulgaris, the same preference is only observed on the basis of olfaction. When presented with an artificial fanning tail mimic, females of both species preferred the faster (T. h. helveticus) fan speed.
It is suggested that sexual isolation is the consequence of a dual system in which males fail to recognise, or are less receptive to the olfactory and/or visual cues produced by heterospecific females and consequently do not initiate courtship with them. Where interspecific courtship does occur, the stimulation produced by the male is inadequate to raise the receptivity of the female at a high enough rate for the encounter to progress to spermatophore transfer.

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