How Different is Too Different?
This Peruvian warbling-antbird must walk a fine line between being different enough from its competitors to reproduce successfully, while staying similar enough to be able to recognize and outcompete the same competitors (Image Credit: Hector Bottai, Image Cropped, CC BY-SA 4.0).
Range-wide spatial mapping reveals convergent character displacement
of bird song (2019) Kirschel et al., Proc B, https://dx.doi.org/10.1098/rspb.2019.0443
In nature, many different organisms can be found in a single location, and sometimes those organisms are closely related to one another. When this happens, classical evolutionary theory predicts that these closely related species should differ in some ways, so as to differentiate members of their own species from others and avoid the costs associated with breeding with a mate that will not produce any viable offspring. This is called character displacement, and there are many examples of this in nature where two different species may be very similar when they live in different places (allopatry), but when they live in the same place (sympatry) they will differ in appearance, behavior, or the exact part of the local habitat that they live in (see Niche Partioning below).
A specific form of character displacement, called agonistic character displacement, occurs when traits or behaviors associated with competition differ between closely related species living in the same area. This is thought to reduce the costs of wasting energy on competing with an organism that you don’t really “compete” with. Agonistic character displacement can, however, result in greater similarity of traits when similar species live together, but previous studies in this area have not accounted for other causes of this similarity. Today’s authors wanted to do just that.
What They Did
To understand how traits differ in sympatry and allopatry, the authors used two species of antbird, the Peruvian warbling-antbird (Hypocnemius peruviana) and the yellow-breasted warbling-antbird (H. subflava). These species are an ideal system due to their large range overlap and their innate song determination, meaning that the species’ songs are genetically determined and subject to selection.
The authors conducted extensive field surveys throughout the natural habitats of the two bird species, in addition to compiling sound files from public and private databases. This allowed them to analyze how the song varied in the many locations that these bird species lived, both in sympatry and allopatry. In addition to the song characteristics, the authors surveyed the environment itself to see if that could be the cause of song similarity or difference in these birds.
Did You Know: Niche Partitioning
When faced with intense competition in your habitat, you can either outcompete your competitors or divide up the space that you share with said competitors. This is called niche partitioning, and is similar to character displacement in that species will be able to live in the same area (be sympatric), but they may not use the exact same space/behaviors. This can be seen with partitioning of time (owls flying at night and hawks flying during the day), partitioning of food (benthic sticklebacks feed on the bottom of the lake while limnetic sticklebacks feed on the waters surface), or partitioning of habitat (anolis lizards are perhaps the best examples of this, with specific species found on specific parts of a single tree!)
What They Found
Overall, the song characteristics of sympatric species were more similar than the characteristics of species that lived in different locations. This similarity arose along a gradient, with the songs becoming more and more similar the closer the two species were to the sympatric zone.
Interestingly, the environment was not responsible for driving any of the variation in bird song along the gradient from allopatry to sympatry. This indicates that the similarity in birdsong when these species live in the same area is not due to difference in the forest structure itself.
This study was very thorough in its use of both private and public records, plus the authors collected field data themselves. One issue, however, was the way that they collected their environmental data. The authors used satellite imagery to determine forest structure from the allopatric to the sympatric zone. While it would be very difficult (if not impossible!) to actually survey the forest in person, using satellite data only allows you to make inferences based off of the canopy, and there is a chance that what is under the canopy in one location is quite different than what you see under a similar canopy somewhere else.
The results of this study are the exact opposite of what standard theory would predict, similar species display similar songs when they live in the same area. While this is counter to what standard character displacement theory predicts, it is consistent with the idea that competition can result in similar species showing similar characters so that they can still recognize a competitor in their environment.
The traditional idea of character displacement is that similar species will diverge in their characteristics when they live in the same area, but what this study has shown is that you can find the opposite pattern in some systems, and that this pattern is present at continental scales, highlighting the importance of performing multiple studies using different systems when asking the same question.