Research: feeding helps urban squirrels give birth

From the city the animal may find attractive food sources, but the sapuska is not always very healthy, and sometimes gathering around it can expose you to diseases and beasts.

At least for urban squirrels, feeding seems to do more good than harm. After all, fed female squirrels have more young, say Japanese researchers Mammalian Biology in the journal.

Research was made on Japan’s northernmost island, Hokkaido, where the winter is reminiscent of a Finnish one with its freezing temperatures and hanky.

The species of squirrel that was studied was also the same one that climbs here as well. It spawns in spring-winter, March–April, and if it is in particularly fertile condition, a second time in June–July.

The lives of squirrels were monitored in four city parks and five nearby forests, in all of which the researchers had marked the squirrels individually for several years. It was therefore possible to keep track of the reproduction of females of different ages.

In forests, squirrels look for their food from nature. In the parks, they were fed all year round with, among other things, tree nuts and seeds and peanuts.

Spring-winter the breeding season started a month earlier in the parks than in the forests. In addition, almost half of the monitored city females gave birth to two litters per year, none of the forest dwellers.

Over time, the difference became more pronounced. In the parks, the females began to reproduce a year younger, became pregnant more often, raised more chicks to weaning age and were able to reproduce for a year longer than in the forests.

Calories are apparently running high in the city. The other quality of the supply weighs less when adding to the family, because the female mammal does not carry food to her young like birds, but nurses them.

Pulsivity helps milk production, and according to previous measurements, feeding increases it.

Others too however, according to the researchers, the factors could have had an effect. Reproduction can also be promoted by the fact that the city offers year-round warmth and sheltered nesting sites.

It is also possible that the urban cranes started their breeding season early because the artificial lighting misled their internal clock into interpreting the winter day as longer than it really is.

Published in Science in Nature 2/2026