Urbanization is rapidly growing worldwide, often bringing negative effects on wildlife through loss of habitat and disturbances such as light pollution and noise. Yet some species manage to adapt to cities, either due to their inherent traits, like boldness or being omnivorous, or by adjusting their ecological or behavioral traits to take advantage of urban resources. Among these resources, food plays a particularly important role for reproduction and survival.
Cities offer many human-related food sources, including garbage, garden plants, and food that animals steal from people. Supplementary feeding, where people intentionally feed or put food out for small birds and mammals, is another important food resource. Examples include garden bird feeding or giving nuts to squirrels in parks. These foods usually have high calorific value, and many species depend on and benefit from them. While supplemental feeding has been shown to boost reproductive success, many recent studies have also shown that its effects can be negative and are species- and context-specific. Additionally, most previous research has focused on bird species, and the effects on reproduction of mammals remain unclear.
To bridge this gap, Dr. Yu Takahata from the Research Organization of Open Innovation and Collaboration, Ritsumeikan University, Japan, and Dr. Nobuyuki Kutsukake from the Research Center for Integrative Evolutionary Science, The Graduate University for Advanced Studies, SOKENDAI, Japan, studied the reproductive conditions of female Eurasian red squirrels living in rural and urban areas. Rural squirrels, which receive no intentional supplemental feeding, offer a clear comparison to squirrels in urban areas. "Our results provide evidence that supplemental feeding can foster reproduction and urban adaptation of wild squirrels," explains Dr. Takahata. "This study also shows how easily human activity influences urban wildlife's ecology." Their study was made available online on October 20, 2025, and published in Volume 105, Issue 6 of the journal Mammalian Biology on December 1, 2025.
The study was conducted at nine sites in Obihiro City, Hokkaido, Japan, including four urban parks located in residential areas and five rural forests sites 10 kilometres away from the city central of Obihiro city. The researchers directly observed reproductive conditions of 40 female red squirrels every day during the breeding season (late March to June) from 2020 to 2022. Eurasian red squirrels typically mate from winter (January–February) until early summer (June–July), and can have up to two litters depending on their body condition.
The researchers investigated six aspects of reproductive biology of the squirrels: rate of pregnancy, age at initial pregnancy, maximum age of reproduction, birth date, number of weaned young per litter, and whether they had two litters.
The results showed that the urban females had a 24% higher pregnancy rate and gave birth about a month earlier than rural females. They also reproduced earlier, at 1 year of age, while rural females did so at 2 years of age. Reproduction was also longer in urban population, continuing up to 5 years of age, compared to 4 years in the rural population. None of the rural females raised a second litter, while 40% of urban females did. Urban squirrels weaned an average of three young per litter while rural squirrels weaned one and a half.
"These findings suggest that urban squirrels have better reproductive conditions than rural squirrels," notes Dr. Takahata. "Together with our previous study, which showed that urban females gain higher body mass due to better dietary conditions, this study suggests strong positive effects of supplemental feeding on reproductive conditions of squirrels."
The researchers also discussed other possible urban factors, including higher urban temperatures, artificial light and light pollution, and differences in predation pressure.
Overall, the results indicate that high-quality supplemental food can positively support mammalian population growth and conservation in urban areas. "Our findings will play a vital role in urban planning efforts aimed at improving biodiversity," concludes Dr. Takahata.
