Along with the altered landscape, urban organisms also encounter novel (anthropogenic) food resources. And while anthropogenic food can be abundant in cities, it is often lower in quality than natural food. This can lead to a situation in which low-quality but abundant food attracts animals to urban areas, but due to its reduced quality, this food resource is unable to sustain the population. Alternatively, anthropogenic food waste may subsidize natural diets, leading to increased reproduction and a lower death rate during the winter when natural foods are less available. In addition to being lower in quality, anthropogenic foods tend to be higher in sugar and fat and have been shown to contribute to latency in chipmunks (Lyons et al. 2017), and hyperglycemia in raccoons (Schulte-Hostedde et al. 2018).
To examine the relationship between urbanization, anthropogenic food, and cholesterol levels lead author Andrea Townsend and coauthors investigated cholesterol in urban and rural American Crows (Corvus brachyrhynchos). This research has just been published in The Condor where the authors outlined three objectives:
- Determine if cholesterol levels increased as urbanization increased.
- Assess if cholesterol positively or negatively predicts body condition or survival.
- Experimentally test if higher cholesterol levels are driven by access to anthropogenic foods.
To explore the first objective, the authors sampled crows in the university town of Davis, California along with the surrounding more rural area. The authors identified crows nests in each area and then, using the 2011 National Land Cover Database, created a 10 ha buffer around each nest site and used the percentage of impervious surface as an index to quantify urbanization (figure 1a). The authors then collected blood from 140 nestlings in Davis, California and evaluated plasma cholesterol levels. To test if cholesterol levels increased as urbanization increased the authors used a linear mixed effects model with percent impervious surface cover, nestling age, and sex as fixed effects. Townsend et al. found that cholesterol increased as urbanization increased, meaning that crow nestlings in more urban areas had higher cholesterol levels (figure 2a).
To explore their second objective the authors used body measurements and nest monitoring of the same 140 nestlings in Davis, California. First, to assess if cholesterol positively or negatively predicted body condition the authors used a linear mixed effects model with cholesterol, impervious surface, age, and sex as fixed effects and found that there was no effect of cholesterol on nestling body condition. The authors also assessed if cholesterol positively or negatively predicted nestling survival using a binomial distribution (survived/died) generalized linear mixed model with cholesterol, impervious surface, age, and sex as fixed effects. The authors found that there was no effect of cholesterol on fledgling success meaning that high cholesterol levels did not benefit or harm body condition or survival of crow nestlings. However, Townsend et al. did find that urbanization was negatively associated with long-term survival, with survival in the first two and a half years of life declining as impervious surface increased (figure 2b).
To test their third objective, the authors randomly selected 10 nests from Clinton, NY and placed three McDonald’s cheeseburgers (42 mg cholesterol per burger) within 10 meters of the nest for 5-6 days a week for 1-6 weeks. The authors then took blood from the nestlings and tested cholesterol levels. The authors then used a linear mixed effects model with cholesterol as the response and supplementation (yes/no), the number of days supplementation occurred, or the total number of burgers provided as fixed effects and family group as a random effect. While, the authors did not quantify if the burgers were consumed by the adults, cached, or fed to the nestlings they still found that mean cholesterol levels were higher in nestlings that had supplemental burgers placed nearby though this difference was not significant (figure 3).
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Cholesterol is essential for properly functioning physiology, though excessive cholesterol can cause a suite of problems. While anthropogenic foods can be high in cholesterol, this study showed that high cholesterol levels do not decrease nestling survival, even though nestlings that had supplemental burgers placed near their nests had a higher average cholesterol level. Additionally, this research showed that crows in more urban areas had higher levels of cholesterol. Understanding the influence of human foods on wildlife is essential to our understanding of how urbanization alters the physiology and evolution of urban wildlife. Future studies should examine not only cholesterol but also glucose, fat, and sodium levels in urban wildlife.
Read the full manuscript here:
Featured image: “DSC_0159” by RachidH is licensed under CC BY-NC 2.0
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