DISCUSSION OF SOCIAL BEHAVIOR

social system

TUPAIA The only other description of the social organization of wild treeshrews is in the excellent and detailed study of T. glis on Singapore by Kawamichi and Kawamichi (1979, 1982). Their dense study population of about thirty resident adults of each sex had a social system virtually identical to those I found in the Tupaia species in Sabah. Of sixteen male territories they recorded, fourteen were monogamous, with one included female, and two were polygynous, one with two females and

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one with three. One of the females of this latter triplet was associated with two males, with about half of her territory overlapping each (Kawamichi and Kawamichi 1979). The Kawamichis observed territorial chases 33 times and scent marking 110 times, with males marking significantly more than females. These frequent interactions were likely due to the dense population, while in Sabah the rarity of chases and marking behaviors can be ascribed to the much lower densities and rarefied populations of all species.

Based on their behavior in captivity,Martin (1968) correctly inferred that Tupaia glis (= belangeri) lived in groups no larger than a family unit of two parents and their young. He surmised that the male and female were strongly pair-bonded, because captive pairs slept together in the same nest box, rested in contact, and marked each other. None of these behaviors was seen in the species studied in Sabah; but nonetheless, the concordance of territorial boundaries, and the frequent encounters between co-owners of territories, implies stable bonds between males and their one or more females, though not strict monogamy. Because the T. glis studied by Kawamichi and Kawamichi (1979) exhibited social organization apparently identical to that of T. longipes, T. minor, T. montana, and T. tana in Sabah, it is likely that the laboratory-bred stock of T. glis (of unknown origin) studied byMartin (1968) was not originally fundamentally different from other Tupaia and that the nest-sharing and other cohesive behaviors were artifacts of the close conditions of captivity.

The Kawamichis used the term “solitary ranging pair” for this social system of solitary animals that are monogamous on a common territory that each sex defends against its own gender. This has also been called Type I or “facultative” monogamy (Kleiman 1977). The Kawamichis cited a few other species with this system, including some nocturnal prosimians (Bearder 1987) and pikas (Ochotona spp.; Smith and Ivins 1984), but it is actually found in many mammals, including elephant shrews (Rathbun 1979), maned wolves (Dietz 1984), agoutis (Dubost 1988), and others (Kleiman 1977). There is no fundamental difference between solitary territories where a male overlaps one female and those where he may overlap one, two, or more, as in Tupaia species, except that as the system becomes more skewed (more females sequestered by each male), there are other developments such as sexual dimorphism and skewed reproductive success. This last may be one of the most common social organizations among “solitary” territorial mammals, such as galagos (Charles-Dominique 1977), ocelots (Emmons 1988), spiny rats (Emmons 1982), and probably hundreds of rodents. Little phylogenetic

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significance can be attached to the social organization of Tupaia, because it is a simple system that occurs again and again in many orders throughout the Mammalia. It is more interesting as a reflection of ecological factors.

Obligate, as opposed to facultative, monogamy in vertebrates such as birds (where it is the dominant system) and mammals such as foxes and small primates is generally agreed to be the result of increased offspring survivorship if both parents contribute to their provisioning or care (Kleiman 1977). I believe that in treeshrews and other species with facultative monogamy, where the father has no direct role in caring for the young, the defense of a territory where young remain until they reach adult size is a large indirect contribution to their nutrition. This may be as important to their survival as more direct provisioning in obligate pairs. If a male is able to defend two females, then each of their territories has only half a male feeding on it, which can only be to their advantage.

Arboreal, terrestrial, lowland, and montane species of Tupaia all had versions of the same social organization, but especially in T. gracilis and T. tana variations in the number of female territories on a male territory, or vice versa, and the deviations from exact concordance of male-female boundaries give both sexes possible access to several mates within the system of monogamous pair territories. It would be interesting to test the paternity of Tupaia young, to see how monogamous the social system really is.

PTILOCERCUS From all meager evidence, like Tupaia, pentail treeshrews seem to be monogamous but with a completely different social organization. Unlike Tupaia, pentails have a den-centered existence. An adult pair and their offspring always used a single nest site together and radiated their foraging centrifugally from it. If the pair lives together permanently with their young, they would seem to have cohesive, obligate or Type II monogamy. It is yet unproven that the male stays permanently with the female; and the length of tenure of the young with their parents is unknown. Pentails foraged alone, perhaps in partially exclusive foraging domains within the area used by the group. Group life in pentails therefore is not likely to be related to shared foraging information or security from predators while active, because they avoid each other while foraging. This pentail social organization resembles that described for a tiny emballonurid bat, Rhynchonycteris naso (Bradbury and Vehrencamp 1976), in which adult members of a colony use individual solitary foraging beats, juveniles forage near the colony, and an adult male uses the

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extreme periphery, outside the core area, which he defends. The bats are territorial, and I suspect pentails to be, but there is no evidence for it at this time. This type of system seems likely to arise when a pair or a group is needed to effectively defend a large foraging space or a sleeping site.

A single den tree formed the hub of pentail social life, and perhaps some special safety or other feature of the den site makes it a rare commodity in which a pentail family forms a nucleus. Because pentails are torpid and defenseless by day (Whittow and Gould 1976; Gould 1978, pers. obs.), they may require unusually well protected sites, such as hollow, high canopy branches with small entrance holes (perhaps from within the tree) that predators such as monkeys, martens, weasels, and snakes cannot enter or dismantle. In captivity P. lowii has the odd behavior of liking to sleep in glass jars (Lim 1967; Gould 1978). Gould (1978: 6) reported, “Despite an abundance of snug and quiet nest boxes as many as three pentail shrews crowded themselves into a single jar.” This mystifying behavior (the jars were clear, transparent glass) may indicate a preference for tight, smooth-walled narrow tubes such as branch hollows. Grouping also may have physiological advantages. If the diurnal torpor is an energy-saving mechanism (implying that pentails have an energy problem), then group huddling while inactive, as pentails do in captivity, may conserve heat and energy (Whittow and Gould 1976; Gould 1978).

territory formation and failure

The species of Tupaia differed in the cohesiveness of pairs that shared territories, as defined by the amount of recorded interaction. Tupaia minor and T. montana pairs spent the most time together, including some complete days of activity. These two species had the smallest home ranges (table 7.2) and were thus the most likely to be near each other by chance, but lesser treeshrew pairs often traveled together while insect foraging. The other three Tupaia had only brief contacts, sometimes daily, which consisted of male-initiated visits to females, within solitary foraging itineraries.

An intriguing behavioral question is how pairs of such solo-foraging and independent individuals as T. longipes make their extended boundaries so strongly concordant over as much as 1,000 m of borders. Kawamichi and Kawamichi (1979) suggested that pair members use scent marks to adjust their boundaries to those of their partners, a likely possibility. If true, I surmise that only the male fits his territory over the range

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of “his” female(s), whose own territory is determined only by the landscape and the borders of neighboring females. A male's goal would in this case be to exclude the neighboring males from any overlap with his mate. The sloppy territorial boundaries of T. tana with respect to individuals of the opposite sex seem to imply either weaker pair bonding, tenure of males too short to establish tight pair borders, or inability of males to defend all areas where females encroach other males. Another possibility is of a different, more polyandrous/polygynous social system, in which the male and female territory maps arise independently of each other and are controlled only by within-sex interactions, so territories are placed wherever an individual can muscle in among members of its own sex. Too much coordination of pair territories is present for this last to seem likely. For example, in 1990 T. tana M62 (not then radio-collared) was present in a small area between other males, but when M55 died and F58 and F65 both vanished, he expanded into a territory of an entirely different shape from those of his predecessors, and coincidentally F100, daughter of F58, acquired a territory that tightly overlapped his but included half of her mother's old area and half of F65's (figs. 9.7, 9.8). Together these two set up a new territory with new boundaries. The similarity of many large treeshrew male and female superimposed areas implies joint coordination between pairs (fig. 9.8). A critical feature of T. tana male territories is that they are larger than female territories but not usually large enough to include two females. Males thus encroach slightly on neighboring females, and at odd points, females on male neighbors. The only two likely courtship interactions that I saw involved only primary resident pairs (F109 and M111, F65 and M77). Males also have larger apparent ranges than females because males make sorties into neighboring zones, probably trespassing to investigate neighboring females and/or the presence of other males.

In T. gracilis and T. longipes I witnessed the formation of new territories by females that slowly pushed resident females aside and took over all or part of their space within the territories of resident males (F173, F86). This gradual encroachment is distinct from the all-or-none results of contests between mammals that fight physically for dominance. I suspect that these female-female interactions took place without much influence by the males: the male slender treeshrew visited the areas of each of the two females almost daily, and it seemed that he acquired two females by their spatial rearrangement, without changing his own. This was probably also the situation with T. longipes M64 on the territories of F56 and F86, but his boundaries during their interactions were unknown.

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The spatial arrangement of Tupaia territories is therefore the result of complex interactions of each sex with both the animals of the same sex that surround it on all sides and the territorial boundaries of their own mates. In at least some species, the same-sex interactions sometimes result in polygynous or polyandrous deployment of opposite-sex territories, although the norm for all Tupaia species studied in the field so far appears to be monogamous pairs that largely overlap the same ground.

Despite the presence of clear territorial boundaries, the territoriality of Tupaia species has some flexibility. The Kawamichis saw the breakdown of territoriality at a large fruit tree:

From early February until mid-March, one large fig tree (Ficus dubia) provided an ample supply of ripe fruits. The sweet smell was perceived by us 250 m from the tree. Besides a pair … whose territories included the fig tree, all nine adjacent residents and seven other residents from the surrounding area came to the tree…. In addition, nine non-residents including six vagrants were also counted there…. The owners of the tree chased the visitors of the same sex. By mid-March the figs were eaten or rotten, and the residents subsequently confined themselves to their previous ranges. (1979: 392)

At Danum Valley bait (trap) sites had a similar effect of drawing some Tupaia into the territories of others. Prebaiting probably enhanced this behavior. The ranges of one pair of radio-tagged T. tana also seemed to be extended for a few days by some long daily journeys to a fruiting Polyalthia sumatrana that was signaled by noisy flocks of hornbills and other birds. The treeshrews could have detected the bird activity at this tree from afar. Possibly territory owners do not or cannot invest much energy in defending large fruit sources that vastly exceed their own needs.

ecological aspects of territoriality

The solitary foraging, long daily active period, and large daily movements of treeshrews show that their food resources are scattered, small, and hard to find. Because they use almost all of the time available in a day for foraging activity, there is not much leeway for more. Defense of feeding territories may sequester the food supply and allow treeshrews to feed within a smaller time and space than they would if conspecifics competed on the same feeding grounds. treeshrew territoriality is very likely the simple defense of adequate food resources for reproduction coupled with the sequestering of his mate by a male. That territories of T. glis in Singapore were one-tenth the size of those of the similar T. longipes in Sabah (Kawamichi and Kawamichi 1979), but the social system was identical

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to that in Sabah under tenfold lower population density, is evidence that the basic system of pair territories is robust to opposite resource extremes. In the next chapter we shall see that the T. glis in Singapore and peninsular Malaysia appear to have compressed their territories (or the Sabah Tupaia expanded theirs) such that they achieve similar resource levels as measured by reproductive output.

Unlike most other nonvolant mammals, treeshrews have no sexual dimorphism in body size (see table 2.2). This is consistent with a social system in which long-term monogamous pairs are the norm, effective sex ratio is not highly skewed, and territorial possession is not determined by dramatic physical dominance or fights (Kleiman 1977). The slow territorial displacements we registered do not suggest that territories are contested by deterministic battles in which the strongest or largest animal wins during a single contest. The energetically difficult life of treeshrews may give them little time for activities other than foraging, and absence of sexual dimorphism may also reflect strong energetic constraints on body size. A pair of solitary foragers on a common territory may be the most efficient use of both space and time for cryptic animals for whom sociality carries no feeding or predator escape advantages.

what some other shrews do

The social organizations of true shrews and elephant shrews illustrate the kinds of patterns found in other primitive mammals and give a perspective on those of treeshrews. The true shrews are the most energy limited of all mammals, and energetics overshadows their lives, but the two subfamilies, Soricinae (including most New World and many Old World genera, such as Sorex, Cryptotis, and Blarina) and Crocidurinae (including many Asian and tropical genera, such as Crocidura and Suncus), seem to have different life history characteristics, or strategies (reviewed in Churchfield 1990). Sorex species, where known, form exclusive territories as subadults or young adults, with no overlap between sexes or individuals during the winter, but in the summer breeding season males start to wander over female ranges, and offspring overlap with adults. Males seem to have no positive interactions with females or young apart from copulation. In the larger Blarina brevicauda territoriality in winter is exclusive between all individuals, but in the summer it is exclusive within the sexes, and males and females overlap. Cryptotis parva are gregarious and live in small colonies where adults share the same nest and home range. In several Crocidurines (Suncus and Crocidura species) there

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is apparent obligate monogamy, wherein the male shares the nest with the female and young, helps to build it, and shows some parental care behaviors toward his young. Crocidura russula and Cryptotis parva are not territorial. Elephant shrews of several species show strict, within-sex territoriality, wherein monogamous pairs share contiguous territories (Rathbun 1979), but members of pairs forage alone and interact only occasionally. If a male disappears, a neighboring male can expand to cover two females, but this situation is temporary.

These other small insectivorous mammals therefore have an array of social structures some of which are the same as those of tupaiids. Even such tiny, short-lived mammals as shrews express a wide spectrum of social organization, including solitary, monogamous, and group living species, and spatial organization that ranges from extensive overlap to territoriality between or within sexes. Advanced cognitive powers are not needed for expression of any of the mammalian social systems, which show great variability within families and even within genera. Tupaia species have a social system like that of some prosimians, but so do shorttailed shrews, agoutis, elephant shrews, and spiny rats. The spatial organization of mammals is plastic and changes to accommodate changing ecological conditions.