New species can develop when cuckoos rely on other birds to raise their chicks, according to a study published in Science.
The theory is that when two species interact closely together, they can drive each other’s evolution and lead to the development of new species. However, until now, evidence for events like this has been scarce.
Now, a team from the University of Cambridge, UK, has found evidence that coevolution followed by speciation occurs in the wild. The team studied the evolution of cuckoos and the host birds they exploit.
Bronze cuckoos lay their eggs in the nests of small birds. When the cuckoo chick hatches, it pushes the host’s eggs out of the nest. The host bird not only loses its eggs, but also spends a few weeks rearing the cuckoo instead of breeding itself. The cuckoo’s trick is that their chicks closely match the appearance of the host’s chicks, enough to fool the parents into accepting the cuckoo.
These interactions drive the cuckoos’ chicks to be similar to different hosts. If the cuckoo chick were to be very different from the hosts’ chicks, they would be rejected. This forces the cuckoos to diverge and separate into different lineages, each mimicking its favoured host. This is the first step to forming a new species.
“This exciting new finding could potentially apply to any pairs of species that are in battle with each other. Just as we’ve seen with the cuckoo, the coevolutionary arms race could cause new species to emerge – and increase biodiversity on our planet,” said Professor Kilner in the University of Cambridge’s Department of Zoology.
Crucially, the differences in the chicks of different bronze cuckoo lineages are still visible in the adults, typically seen in subtle differences in plumage and calls of the adults. This way, males and females that specialise in the same host to recognise each other.
“Cuckoos are very costly to their hosts, so hosts have evolved the ability to recognise and eject cuckoo chicks from their nests,” said Professor Naomi Langmore at the Australian National University, Canberra. “Only the cuckoos that most resemble the host’s own chicks have any chance of escaping detection, so over many generations the cuckoo chicks have evolved to mimic the host chicks.”
Coevolution is most likely to drive evolution when the cuckoos are costly to their hosts. This becomes a race between the host defenses and the cuckoo counter-adaptations. “This finding is significant in evolutionary biology, showing that coevolution between interacting species increases biodiversity by driving speciation,” said Dr Clare Holleley at the Australian National Wildlife Collection within CSIRO, Canberra.
Langmore NE, Grealy A, Noh HJ, Medina I, Skeels A, Grant J, Murray KD, Kilner RM, Holleley CE. Coevolution with hosts underpins speciation in brood-parasitic cuckoos. Science. 2024 May 31;384(6699):1030-1036. doi: 10.1126/science.adj3210