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Carpenter 1984), gibbons (e.g. Gittins 1984), and some tamarins of the genus Saguinus (Peres, un- published data). Such a strategy of range use may.
Behavioral Ecology and Sociobiology

Behav Ecol Sociobiol (1989) 25:227-233

9 Springer-Verlag 1989

Costs and benefits of territorial defense in wild golden lion tamarins, Leontopithecus rosMia Carlos A. Peres* Department of Wildlife and Range Sciences, University of Florida, Gainesville, FL 3261i, USA

Received November 1, 1988 / Accepted April 30, 1989

Summary. The costs and benefits of territorial defense were examined in a group of five wild golden lion tamarins, Leontopithecus rosalia, at Poco d'Antas Biological reserve, Rio de Janeiro, Brazil. I examined the effects of both interference and exploitative competition between groups of tamarins by comparing their use of space, time budgets and foraging success in different contexts of intergroup interactions and in quadrats shared and not shared by other groups. Tamarins spent more time moving and vocalizing, and less time feeding, foraging, and resting during intergroup encounters than in non-encounter contexts. Irrespective of intergroup distance, the group spent more time in overlapping areas of the range periphery than expected on the basis of quadrat availability. In those areas, however, foraging success per unit of foraging effort was lower than in exclusive areas of the range center. This suggests that access to high payoff central areas depended on costly defense (by both interference and exploitation) of the range periphery. Time and energy invested by the resident group in territorial defense (1) increased the availability of food in the range center, and (2) minimized food loss to neighboring groups in the range periphery. These benefits are likely to justify the costs of defense for an animal which depends on easily-depletable food supplies, such as prey in microhabitats and small concentrations of fruits and nectar.

Introduction

Resources should not be defended unless the benefits of doing so are greater than the costs (Brown 1964, Brown and Orians 1970). A common benefit * Present address: Sub-Department of Veterinary Anatomy, University of Cambridge, Cambridge CB2 1QS, United Kingdom

of territoriality is that by preventing other individuals from using any given food supply, animals can increase the availability of food in their home range (e.g. Gill 1978). Territoriality should thus have consequences on the foraging strategies of both residents and intruders. Forest primates living in small groups are frequently able to behave territorially because of their small spatial requirements (Milton and M a y 1976, Mitani and R o d m a n 1979). In the Neotropics this is the case of Aotus (Wright 1978), Callicebus (Robinson 1979), and all tamarins and marmosets studied to date (e.g. Rylands 1983; Terborgh 1983; Hubrecht 1985; P. Garber; in litt.). In this paper I attempt to examine the costs and benefits of resource defense and how these relate to intergroup feeding competition in golden lion tamarins, Leontopitheeus rosalia. I distinguish the two mechanisms of resource competition: interference and exploitative competition (Park 1954). Interference competition occurs through interactions between competitors decreasing one another's access to food sources. Exploitative competition occurs when competitors, which need not interact directly, deny one another's access to food by directly depleting common resources (e.g. Davies and Houston 1981; Paton and Carpenter 1984). Groups of lion tamarins interact with one another by interference during intergroup encounters, and by exploitation in the overlapping parts of their ranges. Methods

Field work was carried out in Poco d'Antas BiologicalReserve (22~ 30'-33' S, 42~ 15'-19' W), a 5200-ha remnant of the coastal Atlantic forest of southeast Brazil. Systematic observations of a group of tamarins and its neighbors were conducted from April to July, 1984, and from May to August, 1985. These periods were largely restricted to dry seasons (Peres et al. in press), thus providing comparable conditions of plant phenology and climate between both years.

228 Selectively-loggedprimary forests and secondary forests at various topographic levels dominated the vegetation in the study site. More detailed descriptions of habitat types used by tamarins have been presented elsewhere (Peres 1986a, 1986b; Peres et al. in press). A study group of 5 individuals was captured, marked, tattooed, and radio-collared in both years of study. Three of the group-members in 1984 had disappeared and were subsequently replaced by two immigrant males and an offspring female in 1985, maintaining a group size of five. I followed the group for a total of 106 days, but only 55 of those were complete (i.e. continuous dawn-to-dusk sampling). Onset and cutoff of data collection followed tamarins' emergence from or retirance to one of their shelters. Daily activity period averaged 9 h 36 rain (n = 55, sd= 39 min). Behavioral and spatial data consisted of scans (Altmann 1974) and group locations every 15 rain. Behavior patterns were divided into several broad, mutually exclusive categories : move, rest, feed, forage, and other. I examined the foraging success of lion tamarins by considering the two most important components of their diet, i.e., animal prey, such as arthropods and tree frogs, and fruits (Peres et al. in press). Capture of animal prey involved clearly identifiable manipulative foraging patterns. Non-mobile prey items extracted from small, discrete and easily depletable microhabitats (e.g. bromeliads) accounted for 98% of the 317 observed captures (Peres 1986a). This allowed me to quantify the economics of foraging activity by recording during scans the outcome of each foraging bout directed towards animal prey in terms of whether or not tamarins completed a successful capture. Prey capture success is then defined as the percentage of individual foraging bouts (n = 1,068) which resulted in successful captures. Foraging success for fruits is defined simply as the percentage of time spent feeding on fruits, since once a fruiting tree or liana was found it involved little additional search. Quarter-hour locations were plotted on a 1 : 5,000 scale map encompassing all 0.0625-ha quadrats used exclusively by the study group or shared by neighboring groups. I examined the foraging success of tamarins as a function of the straight-line distance between any given foraging location and the home range center (hereafter, centrality distance) in each year. The range center was assumed to be the geometric center of a polygon created by the intersection of 6 lines dividing the home range at 15-degree intervals. A Fortran program was used to calculate centrality distances for each occupation record in a 0.0625-ha quadrat based on its x,y coordinates. The number of quarter-hour occupation records increased from 1,247 in 1984 to 2,408 in 1985, and the number of hours of observations increased from 505 in 1984 to 551 in 1985. Observations were either made at close range (within 3 m) or through a pair of 10 x 40 Zeiss Dialyt binoculars. In order to clarify the variation of tamarins' use of space, I used the terminology adopted by Waser and Wiley (1979), as following: (1) activity field, as defined by the distribution of the study group's time as a function of the total number of sample locations, (2) isolation field, or the relative exclusiveness of the study group's use of space in terms of the time they spent at a given quadrat divided by the time spent by all groups at that quadrat, (3) aggression field, or the agonistic tendencies of the study group in terms of its probability of drawing back from or attacking another group at any given location. Determining the way in which aggression and activity fields interact to produce a pattern of intergroup isolation is a primary concern in this study. Resource competition should correlate positively with activity fields, but negatively with isolation fields. I used comparisons between different contexts of inter-

group interactions to examine the effects of interference competition on tamarins' time budget, use of space, and foraging success. The effects of exploitative competition on these same parameters were examined by comparisons between exclusive and overlapping quadrats in the home range of the study group. Overlapping quadrats are defined as those used by more than one group, irrespective of intergroup distance, or those used during close-range intergroup encounters. Unless otherwise stated, data analyses were conducted primarily according to Siegel (1956). When analysing tamarins' activities, I calculated expected values for the time budget during encounters from the behavioral data obtained when the group was outside encounters, or not in encounter quadrats, or both. This null model assumes that time budgets during encounters and at other times are independent from one another. In addition, I examined cell contributions to overall Chi-square values (Brown 1974),

Results

Intergroup interactions I distinguish two forms of intergroup interactions: face-to-face and long-range encounters (Peres 1986a). Face-to-face encounters are defined as boundary contests while the groups remained in visual contact with one another, usually within 25 m . L o n g - r a n g e e n c o u n t e r s a r e d e f i n e d b y i n t e r g r o u p d i s t a n c e s o f 40 m o r m o r e , a n d w e r e c h a r a c terized by vocal interactions such as long-calls (sensu M c L a n a h a n a n d G r e e n 1977). The frequency of counter-calling increased as groups approached one another. These approaches were always followed by face-to-face encounters c h a r a c t e r i z e d b y h i g h levels o f i n t e r g r o u p a g g r e s sion and frequent long-calls. Long-calls then escalated to a staccato of short whistles or "chatter". These tended to occur when groups were separated b y d i s t a n c e s o f 40 m o r less. A g g r e s s i v e i n t e r a c tions and postures, particularly between adult males, included chases, arch-walk displays (Rathb u n 1979), a n d b r i e f f i g h t s . C h a s e s u s u a l l y i n v o l v e d all g r o u p m e m b e r s a n d r e c i p r o c a l d i s p l a c e ments of neighbors over an imaginary boundary line. E n c o u n t e r s t e r m i n a t e d w h e n t h e g r o u p s " l o s t interest", drifted apart to intergroup distances of o v e r 80 m , a n d t h e level o f v o c a l i n t e r a c t i o n d r o p p e d t o a few, i f a n y , l o n g calls. T a m a r i n s w e r e then operationally defined to be "outside encounters". Encounters occurred on an average of once e v e r y 2.1 a n d 1.6 d a y s i n e a c h y e a r o f s t u d y . T o t a l observation time of tamarins during encounters w a s 25 h 36 m i n ( n = 16) i n 1984, a n d 66 h 57 r a i n (n = 44) i n 1985. M e a n d u r a t i o n o f e n c o u n t e r s w a s 1 h 36 r a i n i n 1984 (sd= 53 m i n ) , a n d I h 18 m i n i n 1985 ( s d = 4 4 m i n ) , b u t e n c o u n t e r s o f u p t o 6 h were recorded. Long-calls, boundary approaches, and face-to-face encounters occurred primarily

229 PERCENT OF LOCATIONS (n=2,408)

PERCENT OF LOCATIONS (n=1,247) 0.08% 0.24% 0.40% 0.56% %

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Fig. 1. Differential use of space and time by the study group in 1984 (left) and 1985 (right). Each 1/16 ha quadrat is assigned to one of five classes of occupation intensity defined as percentages of the total number of quarter-hour locations obtained

early in the morning (Peres in press). Long-range encounters were more evenly distributed throughout the day.

Intergroup spacing In 1984 the home range of the study group was contiguous with that of 6 neighboring groups. This number increased to 7 in 1985. The combined home range overlap of all neighbors, measured in 0.25 ha quadrats, was similar between the two years of study. Neighboring groups used 61.4% and 60.5% of the study group's home range in 1984 (36.2ha) and 1985 (48.7ha), respectively (Fig. 1). Groups of lion tamarins were never observed to change direction of movements to avoid intergroup encounters. I tested whether rates of encounters were different from random by comparing the observed frequency of encounters with that calculated by Waser's (1977) random movement model. This model generates an expected frequency of encounters, Z = 8 (p) (?) (o-)/rc, where

p = density of groups; ?-= average velocity of the group; o"= 2 d + s, where d-- the criterion distance at which an encounter is said to occur, and s = diameter of a group. The observed rates of encounters ranged from 7.3 to 11.3 times greater than their respective estimates based on random movements (1984: Z = 0 . 0 6 5 : 1985: Z=0.055). Thus intergroup encounters did not follow a random pattern; groups met much more often than expected on the assumption that their movements were independent of one another. Spatial responses to longrange signals, the frequent use of resources in overlapping areas, and a tendency to approach boundaries and sites of previous encounters all increased the probability of encountering neighboring groups.

Use of space Tamarins concentrated tain areas of the range years they spent more used by other groups,

much of their time in cerperiphery (Fig. 1). In both time in 1/16-ha quadrats or those within encounter

230

group encounter sites. This occurred irrespective of the presence of other groups. The variation in use of the home range resulted in a doughnutshaped activity field for both years (Fig. 2).

Table 1. Number of 1/16-ha quadrats and quarter-hour group locations in encounter sites and elsewhere in the group's range during both years of study Quadrats

Locations

1984

1985

1984

1985

Time budgets

Encounter sites Elsewhere

48 306

127 360

322 925

996 1412

Total

354

487

1247

2408

Intergroup encounters. The time budgets of the study group in both years were combined and compared between different encounter contexts. There is a significant effect of encounters on the overall time budget (Xz=293.8, df=8, P < 0 . 0 0 1 ) when comparing during and outside encounter time budgets. I examined which activities contributed most to the overall chi-square value by replacing the observed number of records with those expected, considering time in non-encounter contexts as the null state. I assumed that time budgets during either types of encounters were independent from those when animals were not interacting with other groups. Expected values were then directly derived from the time spent outside encounters. Cells were not considered significant if the exchange of their observed values resulted in a decrease in the overall X z to less than the critical X ~ at P = 0 . 0 5 (e.g. Brown 1974; Robinson 1981). The time budgets of the study group associated with different contexts of intergroup encounters are presented in Table 2. The observed numbers of activity records during long-range and face-toface encounters were compared with those expected on the basis of time spent outside encounters. This null time budget was significantly different from that observed in long-range (X 2 = 46.3, df= 4, P < 0.001) and face-to-face encounters (X2 = 325.8, df=4, P