The Squirrels of Metro Manila
On a Monday morning outside a house in Forbes Park, Makati, children leave for school, their parents get in their cars to go to work, and a squirrel runs across a wire over their heads. This might be a typical scene in many cities around the world. But in Manila, most people could live their entire lives without ever seeing a squirrel.
This is because there were never any squirrels in Metro Manila, at least until fairly recently. While there are several squirrel species present in the Philippines, they only live in forests, and are mostly present in Mindanao, the southernmost region of the country, far away from Manila.
This mega-urbanized city is the most densely populated in the world, home to both busy business centers filled with skyscrapers, and expansive slums with houses stacked, literally, on top of each other, where people can barely afford food. This is a far cry from the forests that Philippine squirrels live in, and it is not a surprise that people might never see, or even hear about the squirrels of Metro Manila.
But in a few places, squirrels have actually managed to make their homes; in pockets where trees flourish. At the north end of the city, they’ve been spotted in the Ninoy Aquino Wildlife Parks, and on the campus of the University of the Philippines. Down south, they’ve been seen in the Manila American Cemetery, and a few gated communities.
So in a city where there is barely enough space for people, how do animals from far away find space for themselves? And how do they survive long enough to spread?
A squirrel hanging upside down from the trunk of a tree in Forbes Park, Makati.
As it turns out, they are not Philippine squirrels at all. In The Mammals of Luzon Island, a guidebook to local mammals, they have been identified as Callosciurus finlaysonii. Commonly known as Finlayson’s squirrel, or the variable squirrel, they are a species of tropical tree squirrel native to the forests of Thailand, Cambodia, Laos, and Vietnam.
According to the book, they were introduced to the Philippines in the 1960s as pets, with affluent families getting them illegally off the black market. Either by accident or because households tired of caring for them, the squirrels were released, and they first settled in the neighborhood of Forbes Park, and its neighbor, Dasmariñas Village; wealthy subdivisions with streets lined with tall trees.
While being imported explains how C. finlaysonii arrived in Manila, the jump from a few released pets to a stable population is a considerable one, especially since they naturally make their homes in forests, not cities. So what allows these squirrels to actually thrive in an environment so different from their natural habitat?
Tree squirrels are tenacious animals. A study published in 2007 in the Journal of Mammalogy, analyzed population data from several instances of tree squirrels being introduced to new environments. The study noted that 93 percent of the time, in instances where less than 10 squirrels were introduced, the population survived for over 50 years. The study also found that, in best case scenarios, populations of less than 15 squirrels could persist for over 100 years.
An important factor in this, the researchers noted, is the rapid pace at which tree squirrels mature and reproduce. They are capable of reproduction at only 1 year of age, and a pair of squirrels can produce 2-3 litters of up to 8 babies each. And while surviving through the first year of life can be difficult under hostile circumstances, survival rates increase to up to 80 percent afterwards.
And still, the space in the city where they were released is far from hostile. Being the world’s most densely populated city, space is at a premium in Manila, and trees tend to be few and far between. But the villages that were their first homes are affluent enough to have an abundance of them, and these trees can provide food, places to mate, and a way for them to explore and spread.
Adding to this, affluent neighborhoods have more nutrition available to wandering animals, as noted in a paper published in Urban Ecology. Aside from the amount of plant life present in these areas, more well-off households will produce more food waste, which squirrels can thrive on. With all these factors contributing to its survival, Finlayson’s squirrel was able to find a home in Manila.
A specimen of C. finlaysonii finds something to eat high in a tree. Forbes Park, Makati.
As impressive as that is, their story does not stop at them settling in to a new place. From their beginnings in gated communities in the heart of the city, their range—the area in which they can be found—seems to be expanding.
It might seem like Manila, made up mostly of crowded roads and highways, tightly packed slums, and skyscrapers, would be extremely difficult for squirrels to live in. But somehow, as noted in The Mammals of Luzon Island, they have not only been able to survive, but also to spread across Manila—over a distance of 40 km, to La Mesa Dam at the very north, and as far south as Laguna, the province just south of the city.
This is because tree squirrels are extremely courageous travelers. Researchers in Finland tracked movement patterns of urban squirrels. The data they gathered, published in Behavioral Ecology, showed that Sciurus vulgaris, the red squirrel, tended to avoid roads in their day-to-day movement, and would only rarely cross them. However, during dispersal movement—movement from the area an organism is born in to the area it breeds in—a shift in their movement patterns was observed.
In autumn, when S. vulgaris is expected to disperse, they were much less cautious when it came to roads. They did not keep their distance from them, and crossed roads much more frequently. And more than being willing to cross roads, they also seemed to be extremely skilled at it. Over the course of the study, only one squirrel was found to have died after being run over while crossing.
The tree squirrels’ ability to courageously and skillfully move through even urban environments gives us an idea of why C. finlaysonii is able to slip through the dense city outside Forbes Park, and arrive at other places where it can find trees, as far away from each other as they may be.
C. finlaysonii is often seen walking across wires, like this specimen in Dasmariñas Village, Makati
These squirrels are survivors, being able to make a journey from the forests of Thailand to Metro Manila, a city barely able to support native wildlife, let alone a species from far away. But this story—like all things in nature—does not happen in a vacuum. Some consideration also needs to be paid to the possible effects that they, as an introduced species, can have.
For example, C. finlaysonii has been introduced in two cities in Italy, and the forested areas around them. Dr. Sandro Bertolino, an Italian ecologist, studies this. Having examined the widespread damage that these squirrels have caused to trees—to the detriment of local logging industries—he considers them pests, and calls for action to control their population.
And these squirrels, like any other introduced species, can also bring with them foreign microorganisms—bacteria, viruses, parasites—that could make local plants or animals sick. And while these might not factor too much into life in Manila, they could spread to less urbanized areas, where they could cause greater damage.
And yet, even this is not a foregone conclusion. While some introduced species can cause widespread damage, others can slip into the ecosystem with relatively little effect. Ecologists will usually tend to err on the side of caution, assuming that introduced species will eventually cause harm, but there is no way to truly predict how different factors will interact with each other, and the overall effect they will have.
Bertolino also notes, for example, that Finlayson’s squirrel has been observed damaging birds’ nests and taking their eggs. This could have consequences for the bird population of Manila, but while there is evidence for this within the squirrels’ native range, no evidence of this behavior has surfaced in areas where it has been introduced.
With the possibility that these squirrels might become pests, action to control their population might be prudent. But that would require time and resources, when they might have very few negative effects at all.
Squirrels can use wires to find new trees to live in. Dasmariñas Village, Makati.
So even in a place as heavily urbanized as Manila, there are ecological forces at work. And even though forces like the introduction of these squirrels have the potential to influence our lives—damaging important crops, or threatening native species—they can be easy to miss. Still, squirrels will keep running above our heads, on wires, in trees, whether we pay attention or not; and it might be a good idea to look up once in a while. At the very least, it is a rare glimpse of nature in the city.
Photographs by Regina Ira Antonette M. Geli. Used with permission.
This essay received the Merit Award from the 2017 Asian Scientist Writing Prize.
Works Cited
Heaney LR, Balete DS, Rickart EA. 2016. The mammals of Luzon Island: biogeography and natural history of a Philippine fauna. Baltimore: John Hopkins University Press.
Wood DJA, Koprowski JL, Lurz PWW. 2007. Tree Squirrel Introduction: A Theoretical Approach with Population Viability Analysis. Journal of Mammalogy 88:1271–1279.
Pickett STA, Cadenasso M, Grove JM, Nilon CH, Pouyat RV, Zipperer WC, Costanza R. 2008. Urban Ecological Systems: Linking Terrestrial Ecological, Physical, and Socioeconomic Components of Metropolitan Areas. Urban Ecology:99–122.
Fey K, Hämäläinen S, Selonen V. 2015. Roads are no barrier for dispersing red squirrels in an urban environment. Behavioral Ecology 27:741–747.
Bertolino S, Lurz PWW. 2011. Callosciurus squirrels: worldwide introductions, ecological impacts and recommendations to prevent the establishment of new invasive populations. Mammal Review 43:22–33.