Macho behaviour saved aggressive giant pigeon from extinction
Three hundred years ago, the island of Rodrigues in the middle of the Indian Ocean was a scene of brutal struggles. With the rising sea levels, the island shrank in size, leading Island birds to fight each other fiercely. Because of that, only the most aggressive individuals survived, according to new research by University of Amsterdam scientists.
Armed with “boxing gloves,” two Island birds strike each other, engaged in a fierce fight to claim a vast territory rich in food and mates on this small island, the size of Terschelling, in the middle of the Indian Ocean. Such clashes were common on the Mauritian island of Rodrigues some 300 years ago. The fact that this aggression dominated a bird species closely related to the dodo is remarkable, say researchers from the University of Amsterdam and Maastricht University, who published their findings in the scientific journal iScience.
“It’s absurd to think that aggression played such a central role in the survival of this Island bird,” says Kenneth Rijsdijk, a researcher at the Institute for Biodiversity and Ecosystem Dynamics (IBED), involved in the study. “Rodrigues, a volcanic island formed millions of years ago, was inhabited solely by birds and reptiles. With no predators present, island species typically exhibit naive and peaceful behaviour, living with a sense of trust. This is why three centuries ago, biologists could easily capture the dodo on nearby Mauritius; these animals had no fear. We call this ‘island naivety,’ also seen on islands like the Galapagos. The reason why this Island bird was so aggressive remained a mystery for a long time.”
Mass Grave
Rijsdijk first encountered the tale of the Island birds in 2005 while working with an archaeologist friend on a mass grave of dodos and reptiles on Mauritius. A local resident mentioned that birds with large, bony knuckles at the ends of their wings—essentially weapons—had once lived on a nearby island. “Evolution of weaponry is relatively rare in nature,” Rijsdijk explains. “It’s energetically expensive to produce. We’re familiar with animals like the rhinoceros or deer, each with their own form of weaponry for defending territory and securing mates. But for an island bird, which already expends significant energy laying eggs, to evolve weapons for survival is unusual. What’s more, both male and female Rodrigues solitaires had these ‘boxing gloves.’”
The Island bird Rodrigues solitaire (Pezophaps solitaria) is now extinct. The bird was last seen on the island in 1770, likely wiped out by cats introduced by turtle hunters in the early 18th century. “What’s remarkable,” Rijsdijk continues, “is that the Rodrigues solitaire was well documented in the seventeenth century, which gives us extensive knowledge about this flightless bird, unlike the dodo. We know, for instance, that the Rodrigues solitaire could live up to thirty years, the size of its territory, and that it displayed aggressive behaviour towards others of its kind. However, how this behaviour and the development of their wing knobbles evolved has never been studied,” Rijsdijk adds.
Natural Selection, Giants, and Dwarfs
According to Charles Darwin’s theory of evolution, natural selection ensures that only individuals with traits advantageous for survival in a given environment thrive: the survival of the fittest. Aggression, however, has traditionally not been considered one of those traits. But evolution on islands can take unusual turns, Rijsdijk notes. “Giants” can emerge, such as Galapagos tortoises, or “dwarfs,” like certain extinct dwarf hippos. “Now, aggression and the evolution of weaponised bone structures provide another example of this phenomenon,” says Rijsdijk.
Rising Sea Levels
Rijsdijk, a geoscientist rather than an archaeologist or biologist, primarily studies how landmasses change over thousands or millions of years. He theorised that a dramatic reduction in land area over a short period might have made aggression the best strategy for survival, favouring the Island bird species with knobby wing bones.
To validate this theory, Rijsdijk used a sea level model to analyse Rodrigues Island. “Over the past decade, I worked with colleagues to develop this model, which can now calculate coastline changes over time with metre-scale accuracy,” says Rijsdijk. “We discovered that since the last ice age 18,000 years ago, rising sea levels reduced the island’s size tenfold. This would have caused intense territorial battles among Island birds vying for space on the dwindling land.”
Rijsdijk’s colleague, Jasper Croll, an ecologist and UvA doctoral candidate, combined data from the coastline model with an innovative demographic model that incorporates game theory—complex probability and decision strategies. He found that as an island shrinks, aggressive individuals become dominant, passing on the genes responsible for their bony wing structures.
Toxic Corporate Culture
Rijsdijk anticipates that once the sea level model is made public, it will have widespread applications, including more accurate predictions of sea level rise, the impacts of urbanisation and deforestation, and migration patterns. “Anything related to landscape change can be simulated with this model. But using it to explain behaviours like aggression is surprising,” he says. “The demographic model could also potentially explain how toxic corporate cultures form. If aggressive, boundary-pushing leaders pass on their behaviours, replacing more peaceful colleagues, an irreversible cultural shift can occur. In governments, this could even lead to authoritarian regimes, just as aggression became the norm for these Island birds.”
