Temperate zone residents who visit tropical forests are often surprised to find familiar-looking squirrels amidst the exotic vegetation and strange animals such as monkeys, coatis, agoutis, and anteaters. While some squirrel species are very familiar in North American and European cities and forests, and have been studied extensively , the tropical species are not well-known to scientists. It would also be incorrect to assume that all questions about temperate zone squirrels have been answered.
The tropical red-tailed squirrel is the only squirrel species living on BCI, although two other species of squirrel are found on the nearby mainland. Pygmy squirrels (Microsciurus alfari) used to occur on BCI, but were last seen in the 1960’s. Variegated squirrels (Sciurus variegatoides), which are common in and around towns and pastures in the old Canal Zone, were never reported on BCI once it became an island. The variegated squirrel is larger than the red-tailed squirrel and has a black stripe down the back and gray sides in this part of Panama. The pygmy squirrel is tiny, dark brown, has a very narrow tail and secretive habits. Evidence from their feeding ecology supports the idea that these two species have rather different ecological roles from the red-tailed squirrel. Considering the great variety of squirrel species throughout the entire tropics, squirrels seem to be a highly adaptable group, living at high altitudes and low. Of all the tropical species, however, the species on BCI may be the best-known to scientists.
Tropical squirrels do not readily reveal the details of their lives. Even individuals who have been equipped with radio-collars, and who have been followed by scientists for several weeks, are reluctant to be observed entering or leaving their nests. When a squirrel detects a follower near its nest, the squirrel may sit quietly, waiting for the human to go away before the squirrel will enter its den. This is especially true of mothers with young in the nest. If disturbed by observers at the nest tree, some mothers will panic and immediately move their young to a new location, laboriously hefting their helpless youngsters one at a time down to the ground and over to the new den. Radio-tracking on BCI has revealed that mothers establish several alternate nest sites already prepared for occupancy against just such an emergency. Since the main predators of nestlings (cats, tayras, raccoons, snakes, and monkeys) may find the nestlings by detecting the mother’s activities at the nest entrances, then suspicion and caution when being followed are highly adaptive behaviors.
Red-tailed squirrels are medium-sized tree squirrels with glossy red-brown to yellow-brown fur on the body and a rufous-orange tail usually tipped in black. The bright orange belly is often streaked or spotted with white or sometimes with a large patch of white on the belly. They can be noisy, making soft barking or loud squawking sounds. They may be located by the noises made when they gnaw on hard-seeded fruits. The head and body length is up to 24 cm with a tail about 20 -28 cm. An adult may weigh from 400 – 520 g with adult females averaging 5.5% heavier than males. This squirrel has a wide distribution from Costa Rica to northern South America and shows great variability in color, often being rather dull-colored in parts of its range. However, in central Panama, it is a very distinctive species with vivid coloration that is not easily confused with the coloration of any other species.
Diurnal in habits, these squirrels are most likely to be seen during their period of greatest activity, between about 6:30 am and 11:00 am. They are less active in mid-day, usually taking a siesta, and have a late afternoon activity period. They are most abundant in second growth forest, on ridge-tops, and in forests with abundant Dipteryx and/or palms. On the mainland, where red-tailed squirrels occur in the same general areas with variegated squirrels, the two species show habitat preferences. Red-tailed squirrels prefer humid or wet forest while variegated squirrels are found more often in dry deciduous and disturbed forests. In mixed forests or at forest edges the two species overlap in distribution.
Red-tailed squirrels are readily seen foraging on the ground or perched within 2 m of the ground, feeding; however, they also use the forest canopy and understory trees, traveling extensively on lianas. Sometimes they can be observed feeding in the crowns of tall trees more than 35 m above the ground. This species is very common on BCI and is one of the most likely mammals to be seen. Density estimates on BCI over several years have varied between 1 and 3 squirrels per hectare in some habitats. Females that become established on territories are the longest-lived individuals. One marked female on BCI is known to be at least 10 years old. Subadults are the most vulnerable group of individuals, and are the most likely to disappear during times of food shortages.
They can be trapped only with difficulty in the December to August period, when food is fairly abundant. They are caught more easily in the rest of the year, when their preferred foods are scarce. Live-traps set in vine tangles, on leaning fallen trees, or on logs that are 1 to 2 m off the ground are effective. Their preferred bait is Scheelea palm fruit, which can be frozen for use when it is out of season. Dried corn on the cob is attractive to them, but not as effective as Scheelea. Squirrels on BCI have been marked with numbered metal ear-tags (like pierced earrings) and chain necklaces with colored ceramic beads.
Breeding usually occurs during the period of greatest fruit abundance, which on BCI begins in December and lasts until September. Mating bouts have been observed in all months from November to August. However, a sudden onset of vigorous breeding activity is most obvious at the beginning of the period of fruit abundance, in late December. Mating activity abruptly ends in mid- August in most years. On BCI, the fruiting tree species that seems to trigger squirrel breeding activity is Dipteryx panamensis, which starts to ripen fruit in December. This fruit crop is apparently critical to the breeding of squirrels on BCI, for when poor crops of Dipteryx have occurred, breeding activity has fallen off and the squirrel population has declined.
It has also been noted by William Glanz that in areas where Dipteryx does not occur, breeding may begin at other times of the year and may be related to the abundance of other food sources. For example, in Paraiso (30 km SE of BCI), breeding starts in late March or early April and is correlated with the ripening of Scheelea fruit, while in the highlands of Costa Rica, Giacalone noted that the onset of breeding coincides with the peak of blooming of the highly edible flowers of Quercus (oaks) in March.
The list of foods used by these squirrels includes 58 species among the more than 1400 plant species on BCI. The bulk of the diet is composed of the hard-seeded fruits of Dipteryx panamensis, Scheelea zonensis, and Astrocaryum standleyanum, and the large oily fruits of Gustavia superba. Squirrels also feed on various smaller fruits, flowers, tree bark, tree gums, new leaves, fungi, insects, and frog eggs. Females and subadults have more diversified diets than adult male squirrels, who feed primarily on the four species named above.
When courting, squirrels are easily located and approached, being very noisy and careless during their mating chases. Females are ready to mate only briefly, so males visit around the neighboring territories, socializing and checking on the breeding status of local females. As a female approaches her time for mating, some males may congregate around her for a few days. On the day of estrus she may have attracted a large group of males to her territory. One female may be courted by as many as 10 or more males who compete to remain near the female until she chooses to mate. Males chase and fight each other, chucking and squealing, biting and wrestling, and often falling out of trees. The excitement may last an entire morning or even most of a day. The chase route is within the female’s home range, often just a few trees where the female repeatedly runs up and down, alternately resting and running. Although a dominant male may stay near her for most of the chase, she may, in the end, choose to mate with a non-dominant male.
The males do not participate in care of the young. The gestation period is about 44 days. Females produce one or two litters per year. Two young is the usual litter size. The young are naked and blind at birth. Nests for young are located in tree cavities, often abandoned woodpecker holes. They are lined with the soft, shredded inner bark of lianas and trees. Nests of males are usually outside nests built of leaves and twigs.
The home ranges of females do not overlap, or do so at places where desirable fruiting trees are shared. The sharing is grudging: females have been observed to chase other females at such trees, and resort to time-share strategies to avoid fights. These defended home ranges are termed territories. Territory size, determined by radio-tracking, varies from 2 to 4 h among females. The establishment of a territory that has sufficient resources (fruiting trees, nest sites, good cover) to raise a litter is probably fundamental to the success and longevity of a female. Males, however, have home ranges that overlap with those of both males and females.
Squirrels often make loud, hoarse, rasping alarm calls (chucking) at humans, but behave very differently when a tayra approaches. Tayras feed on the sweet pulp of Dipteryx fruits, but have a reputation as predators. Squirrels quietly fade into the foliage and leave the immediate area if a tayra comes to feed at the same tree. Likewise, experienced squirrels leave fruiting trees when capuchin monkeys approach. Capuchins have been observed to feed on nestling squirrels and to lunge after unwary adult squirrels preoccupied with courtship activities. Large snakes and predatory birds also feed on squirrels.
Many important questions remain unanswered regarding the ecology of this species. The role of red-tailed squirrels as predators and dispersers of seeds is not understood. These squirrels hoard numerous seeds of several large-seeded tree species. They usually tuck seeds into crevices between lianas and tree trunks. While some seeds may survive to germinate, it seems to be a rare event. It is not known whether hoarding by squirrels facilitates or impedes seed survival. Squirrels are known to draw heavily on hoarded seeds (up to 20% of feeding observations) during the lean months, and seem to destroy entire seed crops in some years. Likewise, they are also known to feed heavily on immature seeds of Astrocaryum in some years, possibly eating an entire crop before it fully ripens. How often does it happen that low squirrel numbers and heavy fruit production coincide to make seed germination possible?
The use of hard seeds by red-tailed squirrels has been shown by Glanz to be a preference that is not shared with the larger variegated squirrels. The larger squirrels prefer soft fruits and flowers. This finding is in contrast to the situation among African squirrels, in which increasing species size correlates with increasing hardness of foods taken. No one has investigated the jaw lever system and musculature of red-tailed squirrels which permits them to utilize seeds that are considerably harder than seeds used by the larger gray squirrels in northern forests.
Variations in fruit crops of critical food sources appear to affect the population levels of squirrels. These fluctuations have been under study for several years. Short-term (annual and several-year) fluctuations of 2- to 3-fold seem to correlate well with measured variations in fruit crops. However, some longer-term (25-year) fluctuations of larger magnitude (perhaps 10-fold differences) have been recorded, for which no correlated fruit crop studies are available. The current mammal census and fruit crop monitoring on BCI is aimed at finding causes for long-term population increases and declines.
© Jacalyn Giacalone 1997