How species are formed and how species remain separate are crucial questions in evolutionary biology. The offspring of crosses between different animal species are often infertile or die when still in the womb. A mule, for example, cannot reproduce. A sheep-goat hybrid, the result of a cross between a sheep and a goat, is usually stillborn. Such hybrids can also be dysfunctional, for example, because the sounds they make are a mixture of sounds from both parent species.
Remarkably, the offspring resulting from a cross between the ring-necked dove and the vinaceous dove, the two species studied by Den Hartog, are able to function and reproduce. Although the hybrid doves make a different sound to their parents, this sound is still functional.
By coincidence, biologists discovered the existence of a hybrid dove alongside the ring-necked dove and the vinaceous dove in a region of Uganda. The DNA of the hybrid dove has genes from both parental species. In order to mate, a male dove must defend his territory and attract females. This is why he coos. The coos of the ring-necked dove and the vinaceous dove are different and the hybrids have their own coo as well.
Den Hartog investigated the role of cooing in the process of species formation and hybridisation. She wanted to do this by determining the degree to which male doves of different species react to the coos of their own species, to those of the other species and to the coos of the hybrids. To this end she recorded the coos of the different species of males (for examples see fragments below) and subsequently played these recordings in the wild.
The researcher placed loudspeakers in the territory of a male dove to measure the response of the dove to other doves' coos. On hearing a male from his own species cooing in his territory, a male dove will usually approach the intruder and attempt to chase him away, cooing as he does so. This sometimes leads to fights. The male does this to defend his territory; a male dove cannot reproduce unless he has his own territory. However, a male dove can only chase off intruders from his territory if the intruders can 'understand' his coo and recognise the threatening message. Vice versa, the male dove must also be able to recognise the intruders' coos to understand that they are possible rivals. A strange coo can therefore hinder a successful sex life.
As soon as the male doves in the study recognised a coo and noticed the loudspeakers, they started an attack. They recognised the coo as being the coo of a rival.
Hybrids are not choosy
Den Hartog found that the response of the hybrid males to the coos of the hybrid doves was the same as their response to the ring-necked and vinaceous doves. In the populations of ring-necked doves, the males reacted the most to the ring-necked coos, then to the hybrid coo and the least to the coo of the vinaceous dove. The responses of the vinaceous doves were similar. In the area occupied by the hybrid doves, the hybrid coo is just as effective as the coos of both parental species. In addition, the coos of the hybrid males can sound unique but also sound like the coo of one of the parental species. In the latter case it may be possible for them also to hold their own within a population of that parental species.
Den Hartog suspects that it is possible for the hybrid species to continue to survive and multiply. They can do this because the hybrid coos seem to work well in obtaining a territory. There is also quite a lot of variation in the hybrid coos. It would therefore appear that hybrid animals are not always weaker than their parental species.
Hens are more choosy
The role of female doves, or hens, is just as important. Their job is to select the most suitable male. The hens do not always stick to their own species. They can even have a preference for 'strange' males. And doves are certainly not monogamous. A hen will also stray from the nest. She may mate with a bachelor dove and then bring up the baby doves with her faithful male back at the nest.
Paula den Hartog could carry out her research thanks to grants from the NWO division WOTRO Science for Global Development. She started her research in 2003. Before that she was already involved in other projects relating to doves' coos. Her PhD supervisor Carel ten Cate has previously received NWO grants for his research into bird sounds.