Whoops
In many early mornings, the new day is accompanied by the pleasant "song" of the birds. Such "songs" have been passed down from generation to generation for hundreds of thousands, or even tens of thousands of years. In fact, the song of birds, the chirping, is a trait that develops through learning, and like human language, it is learned from parents and neighbors.
Evolutionary biologists have long believed that traits such as mingxiao, which are acquired through imitation learning, are evolutionary signals that can develop rapidly in a fairly random direction, that it is an uncertain and error-prone process, so that mingling can easily change slowly but inevitably over the years. Moreover, the evolution of features such as chirping is also regarded by biologists as an important process of animal speciation.
However, a new study, after recording the chirping of sunbirds from Sky Island in East Africa, was surprised to find that they had barely changed for more than 500,000 or even million years. The chirping of sunbirds is almost indistinguishable from that of their ancestors who have been around for a long time. The new findings were published in the recent Journal of the Royal Society.
The sunbird of Sky Island
The new study investigates the chirping of the eastern double-collared sunbird in East Africa. The East African double-necked nectar birds, also known as the Sky Island Sunbird, are a class of songbirds that live in the Afromontane Forests of Eastern Africa and are geographically widespread, with different populations occupying discrete, island-like habitats.
Six different lineages of oriental bi-collared sunbirds have been found in the mountains of East Africa (black), many of which have been isolated from each other for hundreds of thousands of years. | Image credit: McEntee et. al.
The East African bi-collared nectar bird is made up of about 5,000 different species and is one of the most diverse birds in Africa and Asia. Many of these birds have curved beaks that are able to sip nectar deeply into the curved flowers. Integrative biologist Professor Rauri Bowie is one of the authors of the new study, and he has a long and strong interest in these sunbirds.
During his doctoral studies, he discovered that the two eastern bilech nectar birds that were thought to be distributed on many mountaintops in East Africa were actually at least 5 species, and possibly even 6 species. Although their feathers also have little difference in color and have a very similar appearance, they have huge genetic differences, indicating that they have experienced a long period of isolation on different summits.
He then wondered if the sunbirds' chirpings would be as unchanged as their feathers. Between 2007 and 2011, he and other researchers visited 15 Sky Islands in East Africa, ideal places to explore the evolution of sunbirds, with stable mountain forests. There, they recorded the chirping of 123 birds from 6 different lineages of eastern bilechs.
Using a statistical technique that distinguishes between gradual and abrupt changes in characteristics such as the chirping of birds, the researchers found that differences in chirping appear to be independent of the length of time an individual population has been isolated, estimated based on genetic differences in DNA. Two long-separated species have nearly identical chirps, while two similar-looking but shorter-separated species may have very different calls.
Ultrasound map of the representative songbirds of the 6 lineages of the Oriental bi-collared sunbird (left) (right), which shows that the songsters showed significant differences 2.7 million years ago, and although the sunbirds of these 6 lineages look almost identical, they represent 5 to 6 different species. | Image credit: McEntee et. al.
They found that sunbirds living on high mountaintops in East Africa were isolated from populations of other similar or similar species for perhaps tens to millions of years.
The lack of a changing environment leads to stagnation
This conclusion surprised the researchers. They have long believed that birds' chirping is easily altered, in part because of some past research on birds in the northern hemisphere. Birds in the Northern Hemisphere have been experiencing changing external environments as glaciers have come and gone over the past tens of thousands of years. Changes in the environment can lead to changes in haliffing, chirping, mating behavior, and so on.
If such a situation occurs in humans, then the human dialect will most likely change. It doesn't take long to tell where a person comes from from a regional difference in language.
New research suggests that this is not the case for birds, and that even very unstable traits, such as learned chirps, can remain unchanged for a long time, showing long-term stagnation.
The researchers say this stagnation may be related to a lack of change in the living environment of these birds. In the tropics, especially in East Africa (from Mount Kenya to Mount Kilimanjaro in Tanzania, south via Malawi to Mozambique), the mountaintop environment there has hardly changed geologically during the same period. As a result, the birds involved in this study did not alter their need for colorful feathers, nor did they alter the dynamics of their complex chirping.
Important social signals
Therefore, what evolutionary biologists should figure out is which forces limit social signals such as chirping and feather color over time, and which forces cause them to change. The evolution of these signals is studied because they are essential to the formation of animal species.
According to the researchers, biologists have recognized that there are two barriers that prevent animals from mating, one is a pre-mating disorder and the other is a post-mating disorder. Pre-mating disorders are signals that prevent one individual from mating with another. For birds, this may be because its song is incorrect, or it looks different from other birds. But if such a bird mates with other birds, they can still produce offspring. Post-mating disorders are actually mechanical reproductive incompatibilities, which cause them to produce no offspring even when mated.
Chirping is considered the most important barrier to isolation before mating and one of the key ways birds distinguish between each other. It is remarkable that a learned trait can sustain stagnation for hundreds to thousands of years. Based on the study, biologists argue that traits like learning chirps and feathers don't necessarily vary in isolated populations, and that they can evolve rhythmically and then stagnate for a long time — perhaps hundreds of thousands of years.
#创作团队:
Author: Sugar Beast
Typography: Wenwen
#参考来源:
https://news.berkeley.edu/2022/01/12/some-birds-sing-the-same-song-for-hundreds-of-thousands-of-years/
https://royalsocietypublishing.org/doi/10.1098/rspb.2021.2062
#图片来源:
Cover image: Alexas_Fotos/Pixabay
Top image: Keith Barnes via Berkeley.edu