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The Isolating Value of Specific Song Patterns in Two Sibling Species of Grasshoppers (Chorthippus Brunneus Thunb. and C. Biguttulus L.)

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The astonishing specific distinctiveness of sounds produced by singing animals has raised the assumption that this diversity has been favoured by special selective factors. It is supposed that the main factor is the survival value attached to reproductive isolation. To check this idea the isolating value of specific song patterns was studied in two closely related species of short-horned grasshoppers (Chorthippus brunneus Thunb. and C. biguttulus L.). Apart from small, but non-overlapping morphological differences, these species are most strikingly distinct in their song patterns. Alhough they are truly sympatric and are not separated by an ecological barrier, in nature they keep apart very well. This is corroborated by the exceedingly small amount of hybridisation that occurs in nature (hybrids being detected easily by their song, which is intermediate between the parental songs). A comparison of the mortality of raised hybrids and the pure species raised under the same conditions did not point to a higher mortality of the former. Obviously, an ethological barrier exists. Observation of the mating behaviour (quite the same in both species) strongly suggests that the song has an important function. The song frequency of male and female is increased and there is a more or less regular alternation of the sounds between male and female. Distinct orientation movements, mainly of the male as a reaction to the sounds of the female, lead to an approach of male and female. After they have met, this exchange of sounds is continued until the male is successful in his jump onto the female. Interspecific copulations are difficult to obtain, even under optimal conditions. They are, however, facilitated considerably if homogamic mates of each partner are placed within hearing distance. Therefore, besides an effect on the orientation, the song seems to have some specific stimulating influence too. It was established that both male and female answered exclusively to the sounds of their conspecific partners. Hence the indicated orienting and releasing effect of the song is lacking between a male and a female of different species, which seems the cause of the reduction of heterogamic copulations. To substantiate this view a quantitative comparison of the behaviour in the homo-and the heterogamic situation was made. Apart from an increased song frequency of both male and female in the homogamic situation, it was found that in the homogamic situation (1) the orientation of the male towards the female was improved, (2) the orientation of the female towards the male was improved, (3) the locomotion of the male was increased, (4) the number of copulation attempts of the male was increased, (5) the willingness of the female to accept the copulation of the male was increased. Once a male had jumped successfully on top of the female, however, no difference in the success of attaching the genetalia was observed in the two situations. An isolation, due to morphological incompatability, does therefore not exist. These five effects form together the ethological barrier. A number of experiments were made to determine whether specific distinctive stimuli, other than those provided by the song, are responsible for these effects. Only in one case was another stimulus found, viz., a specifc distinctive olfactory one, working on the receptiveness of the biguttulus female (additional to a specific song stimulus). No attempt was made to unravel the separate effects of the three different kinds of stimuli connected with the sound-production, viz., the acoustical stimuli, the visual stimuli provided by the characteristic movements of the legs, and the tactile stimuli due to the vibration of the substrate. The conclusion is drawn that the reduction of hybrids in nature is brought about almost wholly by specific song reactions. To determine the isolation between the hybrids and the parental species, a number of backcrosses were raised. No signs of hybrid sterility or hybrid breakdown were found. An ethological barrier of the same kind as those existing between the species was, however, found between the hybrid males and the females of the parental species. The intermediate song of these males was not answered by the pure females. Hybrid females, however, answered both to the song of the hybrid males and of the males of one or both species. It seems a safe statement, that the specific distinctiveness of the song patterns in this particular case not only plays a part in the isolation, but forms almost the only effective isolating mechanism. The implications of this state of affairs regarding to the origin of the species are discussed. It is suggested that the isolating value of the specificity of the song patterns is not developed as a secondary mechanism to perfect an isolation based on other reproductive barriers, but that it has originated early in the history of these species as an independent mechanism.

Affiliations: 1: Rijksmuseum van Natuijke Historie, Leiden, Instituut voor Oecologisch Onderzoek, Arnhem, Holland


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