What makes an urban bird?

This blog post is provided by Jenny Ouyang and tells the #StoryBehindthePaper for the paper “Changes in the rearing environment cause reorganization of molecular networks associated with DNA methylation”, which was recently published in Journal of Animal Ecology. In their paper they explore the impact of urbanisation on DNA methylation in young birds.

Urbanization is one of the most prevailing forms of habitat change, causing biodiversity loss through local extinction processes. Urban expansion is expected to impact a quarter of all endangered species in the next decade. Nevertheless, individuals vary in their response to these drastic changes, with some unable to occupy these new habitats while others persist and thrive.

This difference in the ability to adapt has promoted the study of phenotypic traits that allow individuals to inhabit urban areas. A key trait that can facilitate adaptation is the degree at which animals respond to stressors. This stress response, which includes elevation of baseline circulating concentrations of glucocorticoids, has a heritable component and exhibits individual variation. Epigenetic mechanisms can alter organism function without changes in the DNA sequence, representing a possible mechanism for the observed response to urban stressors.

Female house wren at an urban site (Caughlin Ranch) with a caterpillar ready to feed its offspring in the nest box. Photo credit: Michael Dale

In a recent paper in the Journal of Animal Ecology, we explored the contribution of DNA methylation towards observed urban phenotypes. When house wren parents bred in nestboxes across a network of urban and rural field sites, we moved their offspring across and within sites to disentangle the contribution of genetic and plastic mechanisms to the glucocorticoid phenotype. This type of experiment is an inter- and intra-environmental cross-foster to analyze the contribution of DNA methylation to early-life phenotypic variation.

Young house wrens were moved within and among urban and rural sites to investigate the contribution

We observed age-related patterns in offspring methylation, indicating developmental effects of the rearing environment. We further discovered different networks of genes were important at hatching compared to fledging. For example, we found several genes involved in auditory response and learning networks were differentially methylated across experimental treatments. Analyses showed that cellular respiration genes were differentially expressed at hatching and behavioral and metabolism genes were differentially expressed at fledgling. Lastly, hyper-methylation of a single gene (CNTNAP2) is associated with decreased glucocorticoid levels and the rearing environment. Urban house wrens appear to be hypermethylated during hatching compared to their rural counterparts. As offspring aged, wrens that stayed in the same environment increased methylation frequencies but wrens that moved between environments did not show a similar increase. These age and environment-related changes in methylation frequencies suggest that the urban phenotype is a result of both genetic and environmental factors.

Adult house wren at a rural riparian habitat. Photo credit: Chris Halsch.

Our findings are suggestive that DNA methylation can shape the physiological phenotype and is empirical evidence for a mechanism by which individuals thrive in changing environments. Together, this work provides an unprecedented empirical system that we have leveraged to explore the influence of both genetics and environment on DNA methylation. DNA methylation may be a mechanism by which individuals adjust to novel environments during their lifespan. Understanding the genetic and environmental basis of local adaptation is important in predicting species’ responses to an urbanizing world.

Author bio

Jenny Ouyang – I am an integrative physiologist at the University of Nevada, Reno. I am interested in how animals physiologically adapt to changing environmental conditions.

Read the paper

Read the full paper here: von Holdt, B. M., Kartzinel, R. Y., van Oers, K., Verhoeven, K. J. F., & Ouyang, J. Q. (2023). Changes in the rearing environment cause reorganization of molecular networks associated with DNA methylation. Journal of Animal Ecology, 00, 1– 17. https://doi.org/10.1111/1365-2656.13878

Long days but little time: Goslings grow faster at high latitudes

This blog post is provided by Michiel Boom and tells the #StoryBehindThePaper for the paper “Postnatal growth rate varies with latitude in range-expanding geese – the role of plasticity and day length”, which was recently published in the Journal of Animal Ecology.

While some species are clearly struggling to adapt to this rapidly changing world, the barnacle goose seems to be prospering. Contrary to other species, this Arctic migrant has not shifted its breeding area to the north, but has expanded it to the south. While the original breeding areas are located in the Russian Arctic, barnacle geese now also breed in the Baltic region and along the North Sea coast. This exceptional situation offers researchers a unique natural experiment that can be used to answer questions about the adaptability of this species, as well as to study general processes that vary with latitude.

Barnacle geese arriving in the Arctic to breed. When the geese arrive, the breeding grounds are often still covered by snow and ice. 

The main reason for the journey to the Arctic, for birds such as the barnacle goose, is reproductive success. Birds fly to the far north to take advantage of the vast amounts of food available there during the summer. Above the Arctic Circle, the birds also experience the benefits of 24 hours of daylight, which means they can forage all day long. The barnacle geese that breed at lower latitudes may have to make do with lower quality food and less daylight for foraging. However, breeding in the Arctic does not only have advantages. A migratory lifestyle also comes with time constraints, and Arctic birds have a relatively short time window to raise their young.

Three newly hatched goslings in a nest in Arctic Russia. Goslings are still a bit wet immediately after hatching hatched, but become fluffy when their down feathers get dry.

In our study, we looked at whether the goslings of barnacle geese breeding at different latitudes differ in growth rate. Growth rate is an important aspect of a bird’s life cycle, which can have an impact on future survival and breeding success. We found that goslings in the Arctic grew faster than goslings at lower latitudes, and found that these differences can largely be explained by differences in daylight experienced by the goslings. Therefore, it seems that the time goslings have to forage determines how fast they grow. Although goslings grow more slowly at lower latitudes, the adult birds of breeding populations at different latitudes do not differ in size. Despite the slower growth rate, the goslings at lower latitudes are able to reach the same size as geese in the Arctic, and thus have to grow longer. The time pressure experienced by Arctic geese seems to force them to grow quickly in order to leave the breeding area before winter sets in. Breeding in the Arctic therefore not only imposes the need to grow quickly, but also offers the opportunity to do so.

Graphical summary of the results: Goslings from the Arctic population (red) grow faster than goslings from the Baltic (yellow) and North Sea population (blue). When the amount of daylight experienced at the different latitudes is taken into account, the differences in growth rate disappear.

In a comparison between species breeding at different latitudes, we find a similar relationship between growth rate and latitude. In fact, the increase in growth rate with latitude that we find in the barnacle goose is comparable in magnitude to the increase in growth rate in several waders and waterfowl species.

The differences in growth rates between the northernmost and southernmost barnacle goose populations in our study were found to be so large, and have arisen in such relatively short time that it is highly unlikely that this is an evolutionary adaptation. Rather, our results indicate that this is a plastic response to the environment. A flexible growth rate can help birds cope with variation in food availability, but can also play an important role in colonizing new breeding areas, which might help in adapting to a changing environment.