Pink pigeons in Mauritius have made a remarkable comeback from the brink of extinction, but are still losing genetic diversity

In the 1980s, there were only about ten pink doves left in the wild. Known to scientists as Nesoenas Mayeri, the species is found only in Mauritius, the Indian Ocean island that was once home to the dodo. Like the dodo, the rosy pigeon became an easy target for cats, rats and other predators introduced by humans, who also cut down almost all of its native forest. However, unlike the dodo, the rosy pigeon has made a remarkable recovery since then.

Fortunately, the Mauritius Wildlife Foundation had already taken 12 birds from the wild in the 1970s and 1980s to establish a captive population. The young of these birds were later released during the 1990s and early 2000s and there are now at least 400 living in the wild. The species has been officially listed twice, from “critically endangered” to “vulnerable”.

However, such a severe population bottleneck can lead to significant “genomic erosion,” where a species becomes genetically less healthy because many animals are closely related. To examine the exact impact, we worked with a team of scientists to sequence the DNA of 175 birds sampled between 1993 and 2010 during the population recovery period. Our results are now published in the journal Conservation Biology. Unfortunately, we found that the species continued to lose genetic diversity even as overall numbers increased during the successful conservation and rescue program. We speculate that the bottleneck must have changed something in the pink dove’s DNA.

Today, the pink pigeons are mainly found in a small forest in the south-west of Mauritius.
carl-jones, Author provided

To understand what caused this ongoing genetic erosion, we looked at data from 1,112 rosy pigeons in European and US zoos. This data was collected over four decades and included the level of breeding success and longevity of each bird along with inbreeding levels calculated using pedigrees. Based on the relationship between these factors, we found that the species had a worryingly high “genetic load.”

The genetic makeup basically consists of many harmful recessive mutations that have the potential to reduce an animal’s ability to reproduce. This could be seen, for example, in a reduced number of eggs hatching, or in the number of young that successfully fledge the nest. Before the pink pigeon bottleneck, the effects of these mutations were masked, as there were plenty of healthy genetic variants to compensate for the harmful ones. However, small populations are much more vulnerable to random fluctuations in genetic makeup, making it possible for harmful mutations to take effect.

Inbreeding can also make such mutations more harmful, if the offspring of two related individuals inherit the same harmful mutation. When that happens, the healthy genetic variant no longer masks the damaging effect of the mutation and the individual may not hatch or fletch the nest. We believe this caused the continued genetic erosion of the rosy pigeon in the wild.

Ultimately, the approximately 480 birds living in the wild are related in some way to the ten that managed to survive in the wild in the 1980s and the 12 that were used to establish the captive-bred population. This has resulted in slow and prolonged inbreeding. In turn, this meant that the pigeons were less successful at hatching eggs and fledging, and they didn’t live as long.

Since only the fittest pigeons were likely to hatch, fledge and reproduce, fewer birds actually contributed to the next generation (and genetic variation was lost), even as their total numbers increased. Consequently, the population continued to lose genetic variation despite growing in size.

points and lines
Pedigree of the captive population of rosy pigeons managed by the European Association of Zoos and Aquariums (EAZA).
Sam Speak and Harriet Whitford (pink pigeon herd book holder, Jersey Zoo), Author provided

What we have learned from this study is that for the rosy pigeon to avoid extinction, we will need to reintroduce the young of birds raised at Jersey Zoo and other EU zoos. These pink pigeons harbor genetic diversity that has since been lost on Mauritius, and reintroducing them would reduce the level of relatedness on the island.

Our study also shows that conservation cannot simply stop after a population appears to have recovered in numbers. We believe that genomic analyzes are needed to truly assess the conservation needs of a species and its potential for recovery. This can not only be done by analyzing population figures. The data generated by the Earth Biogenome Project will be critical in identifying species most at risk of extinction. The project aims to sequence two million species in the next ten years. This data will not only help in our assessment of biodiversity, but should also be used to guide future conservation action.

Leave a Comment