ANT 154B Course notes- Lecture _8

ANT 154B Course notes- Lecture _8 - ANT 154BN Course notes...

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Unformatted text preview: ANT 154BN Course notes Lecture #8: Brains & cognition 27 January 2011 Key terms and concepts are indicated in blue Introduction Ecology of grouping- how do ecological factors act as selection pressures on primate groups? Basic socio-ecology Ecological pressures Female counterstrategies? Female distribution and sociality Male strategies No Group size, Social system Yes factors generally thought to increase group size: Thursday, January 28, 2010 Wrangham 1982 • between group contest feeding competition 4 • predation factors generally thought to decrease group size: • within group scramble feeding competition • infanticide • disease • parasites • Factors affecting group size vs. those affecting population density • for now, we assume population density is equal, and consider factors that push GS up and down Outline 1. Types of feeding competition 2. Effects of competitive regimes 3. Overview of infanticide ANT 154B Lecture #8 course notes page 2 of 9 1. Types of feeding competition What sort of competition? Resource “patchiness” Food-ranges too big to defend Day-journey Annual range Resources defensible? Food-ranges economically defensible N o s Ye Scramble Wildebeest ignore each other Thursday, January 28, 2010 Contest Gibbons defend territories 9 Limited resources -> Competition Contest Resource distribution Resources defensible Fights/threats? Win by... Winners Losers Thursday, January 28, 2010 Scramble Equal No No Efficiency None All 10 Unequal Yes Yes Dominance Territory holders/ dominants Floaters/subordinates Competitive regimes WGS = Within-group scramble WGC = Within-group contest BGC = Between-group contest (BGS = Between-group scramble) to be continues next time... ANT 154B Lecture #8 course notes page 3 of 9 2. Effects of competitive regimes Effects of food competition Type of competition Within-group scramble Between-group contest Within-group contest Effects costs of grouping benefits of grouping benefits of coalitions Strong effects on grouping and social relationships Thursday, January 28, 2010 13 Within-group scramble Logic: individuals in larger groups must travel further to fulfill daily caloric requirements Logic: individuals in larger groups must travel further to fulfill daily caloric requirements Logic: individuals in larger groups must travel further to fulfill daily caloric requirements Logic: individuals in larger groups must travel further to fulfill daily caloric requirements # food patches visited per day group size Thursday, January 28, 2010 16 Thursday, January 28, 2010 17 Thursday, January 28, 2010 18 Scramble competition (a cost of grouping): • predict within species, a positive relationship between day range and group size • cost-of-grouping varies among species,may help explain differences in average group size Cost-of-grouping varies among species 2000 Day-range Gray-cheeked mangabey 1000 Red colobus 0 0 20 40 60 Group size Mean Group Size Thursday, January 28, 2010 20 to be continues next time... ANT 154B Lecture #8 course notes if we want to compare species, we need a comparable measure: RRC provides this page 4 of 9 RRC: an index of scramble competition Janson and Goldsmith • Predicting group size in primates Bigger groups -> longer day-range 327 2200 -i Day range (m) y/x = S (slope) /H = Rof H = daySrange RCsolitary 10 15 20 Group size Group size regression re t dai y a o p e, s Janson & Goldsmithlahing groluppsitzh l1, fromicgroueasizres4thie dixpdcdedy (1995), dataengt,hwth mangabeys e vi eet b the path lengt at eH h m su Figure 1 Calculation of the relative ranging cost (RRC). The slope of the Scramble competition in folivores foraging effo Thursday, January 28, 2010rt of a solitary individual. Thus, RRC measures the increased ranging cost of an additional group member, scaled relative to the daily path length of a solitary animal. Data are from Ctrcoabus alHgrna fWaser, 1977). These data deviate significantly from a linear regression (quadratic term is positive, t — 37.4, p < .01), but we ignored quadratic effects in calculating RRC, as n o n e of the other studies in Table 1 had significant quadratic terms. recent evidence suggests that even in folivores, there is scramble competition (e.g., red colobus) METHODS day of travel would entail lesser fitness costs to an animal that would normally travel 4500 m if solitary than to o n e that moves only 100 m when solitary (see Table 1). Because S a n d H d o not measure foraging efficiency directly but rely instead on related increases in ranging costs, we call their ratio the relative ranging cost (RRC). Beside its relationship to fitness costs, the ratio S/H has the added advantage that any differences in path lengths between studies caused by distinct field methods (e.g., Fossey and Harcourt, 1977) will have little effect on this estimate of food competition because any bias of a given study will appear in both the numerator and denominator, thus canceling o u t If RRC does index food competition, then, for several reasons, we expect that species with high RRCs should show smaller group sizes than species with lower RRCs. First, even though individuals in groups of different sizes may attempt to reach the same net energy intake, they may not be able to do so and dius suffer reduced daily food intake, widi presumed fitness costs in larger groups (e.g., van Schaik and van Noordwijk, 1988). Achieving the same net energy intake in large and small groups should be easier when RRC is small rather than large. Second, even if increased foraging effort perfecdy compensates for reduced foraging efficiency in larger groups, individuals may suffer opportunity costs of less time for other fitness-enhancing activities such as social interaction (Dunbar, 1988) or avoiding predation by resting in sheltered locations (Janson, 1992). These opportunity costs will increase more slowly with group size when RRC is small rather than large. Third, even if no opportunity costs exist, an upper limit to foraging effort must exist (e.g., day length: Janson, 1988a) beyond which compensation for reduced foraging efficiency is impossible. In populations with large RRC, this limit will be We reviewed primate field studies that included ranging data and used every study we could find that gave daily padi lengths for at least two groups of different sizes studied in the same site during the same period (cross-sectional data) or one group with varying sizes over time 4(longitudinal data). Data on groups widi fewer than five days of observations were dropped. We did not exclude studFig. i1 Changes ome-rintake ies n which hin (a) ange quality was known to differ among rate (bites/min) sand oing so groups becau e d (b) might have biased our results agamovespecies widi mmin)d inst ment rates (m/3 arke of red nd Seyf (Piliocolob; b competition between groups (Cheney acolobus arth, 1987)us ecause we are interested in testing tephrosceles) in Kibale National the extent to which deP i rk, U up siz ∗ in all p i creased foraging efficiency may limat groganda. e indicatesramates, eliminating those species in statisticallyasignificant iency which for ging effic diffham, 1between uld con- end might increase with group size (Wrang erence 980) wo start and Intake (see te stitute a serious possible bias in our datxt foromorer, etails) id a. H weve d we d rate exclude studies in which one or more groups were provisioned or had access to human garbage (e.g., Stolz and Saayman, 1970). Group size was taken here as social group size in(bites/min) species widi relatively cohesive groups (even if occasional subgrouping has been observed), but as foraging group size in fission-fusion species such as spider monkeys (Aules). For sped e s widi multilevel social units (Papio hamadryas, Thertrpithecus gdada), we used die social level for which data on daily padi lengths were given (in bodi cases, die band); the results for these species might differ if daily padi lengths for odier social levels were available. To calculate group size, we subtracted infants because they (1) d o not locomote indepen- Movement dendy, (2) have average energetic demands of usually less rate dian one-diird diat of an adult, even including die cost of transport (based on models in Dunbar, 1988), and (3) often (m/3min) are produced in a seasonal peak that could confound die effects of group size and season on dairy padi lengdi. We esti- reached at smaller group sizes than in those with low RRC Until now, nobody has attempted to use within-population regression parameters of ranging to explain primate group sizes, despite some efforts to relate other population-specific ecologic y/xal=costs (slope) of social structure across primate speS to aspects cies (Isbell, 1991; Mitani and Rodman, 1979; Wrangham et al., 1993). Our goal is to discover how much of the variation in mean group size between primate populations may be attributed to variation in this simple within-population measure of die costs of increased ranging (RRC). Of course, many odier factors are likely to affect group size besides food competition as reflected in increased daily path lengths. First, previous results comparing ranging patterns of fruit- versus leaf-eating primates (Clutton-Brock and Harvey, 1977; Isbell, 1991) have shown that increases in foraging effort with group biomass are far less evident across leaf-eating species dian across fruit-eating ones. Thus, we thought it important to e amine wh animalx(slope)ether our measure of food competition could predict group size in both diet types. Second, if social group size is a balance between social net foraging costs and individual predation risk (e.g., Dunbar, 1988; Terborgh and Janson, 1986; van Schaik, 1983), dien group size may also vary in relation to factors that influence predation levels in primates after accounting for the influence of foraging competition. To obtain a more complete understanding of the de- . terminants of primate group size, we include both diet and *Relative ion risk (te Cost possible correlated of predatRanging rrestriality: Crook and Cardan, 1966; body size: Cheney and Wrangham, 1987) along with RRC as predictors of primate group siz21in an expanded e Thursday, ance of for analysis. Our results strongly support the importJanuary 28,- 2010 aging competition and predation risk in determining primate group sizes in fruit-eating species, although not always in leafeating ones. Mean size group Folivores S = RRC * H Frugivores RRC (Relative Ranging Cost) Janson & Goldsmith (1995) 22 small group Food for folivores is limited? start 20 15 10 5 0 large group * end start end Intake Rate (bites/min) Mean day range (m) * ll Patches* all Apatches* * Young eaves* young Lleaves* Mature Leav matureesleaves * * Movement Rate (m/3 min) 2 1. 5 1 0. 5 0 Food tree density (trees/ha) * all patches* Food for folivores is limited? All Patches* young leaves* mature leaves * Young Leaves* Mature Leaves Red colobus, Gillespie & ursday, January 28, 2010 (e.g., red leaf monkeys) rate x=1.50, paired t=−4.01, p<0.0001; mature leaves intake rate: n=16, start rate x=9.99 end rate x=10.31, paired Red colobus monkeys, Kibale, Uganda t=−1.25, p=0.23; mature leaves movement rate: n=14, Thursday, January 28, Chapman 2001end rate x=0.19,2010 t=0.61, p=0.55; start rate x=0.30 paired Fig. 1). 23 As predicted, patch occupancy time was significantly affected by the size of the patch (dbh) and the number of animals feeding in it (R2 =0.145, p=0.037, n=44). Dbh and feeding group size were not correlated (Pearson r=0.002, p=0.989, n=44), suggesting that occupancy time is simultaneously affected by both factors. The effect of this relationship is weak, likely reflecting the fact that other factors, in addition to foraging efficiency, influence patch occupancy. For example, experimental work using desert rodents to test various predictions of optimal foraging theory have found that giving-up time is affected by a number of factors including predation risks/costs and missed opportunity costs (Brown 1988; Kotler and Brown 1988; Brown 1989; Brown et al. 1994). Discussion We found that red colobus monkeys in Kibale deplete food patches when feeding on young leaves, as indicated by decreasing gains (intake rate) despite increasing feeding effort (movement while feeding). Furthermore, patch occupancy time was affected by patch size and feeding group size. This provides evidence of a group size-effect, where larger groups deplete patches more quickly, are forced to visit more patches, and will thereby accrue greater travel costs than smaller groups. These results suggest that red colobus do experience within-group scramble competition, Snaith & Chapman 2005 and that this type of competition may be an important fac24 tor determining group size. Further studies are required to document the intensity of scramble competition by directly examining the effect of group size on travel costs by measuring inter-patch distance and day range, while controlling for variation in food availability. The results of this study, in combination with the evidence outlined in the introduction, suggest that our current understanding of folivore food competition is inadequate. Existing applications of socioecological theory to the variation in primate social behaviour are based on the assumption that within-group scramble competition is either weak or absent among folivores (Clutton-Brock and Harvey 1977; van Schaik and van Hooff 1983; van Schaik 1989; Isbell 1991; Janson and Goldsmith 1995). This assumption is based primarily on studies that found no relationship between group size and day range length. However, based on the accumulating evidence, it is possible that these studies were confounded by habitat variation, and that day range would be related to group size if food availability could be accurately measured and held constant. It may be that folivores avoid or mitigate the costs of scramble competition by adjusting group size to food conditions at broad temporal and spatial scales, or more immediately by fission–fusion behaviour. This hypothesis is supported by the studies presented above that have documented a relationship between red colobus group size and habitat quality, group size and day range, and/or fission–fusion in response to food availability. If folivores experience significant within-group scramble costs, a shift in the interpretation of the competitive regime of Red leaf monkeys, Gunung Palung, Indonesia Thursday, January 28, 2010 Marshall, Boyko, Feilen, Boyko, & Leighton 2009 AJPA to be continues next time... 25 ANT 154B Lecture #8 course notes Between-group contest Logic: When food in large clumps, BGC may become important page 5 of 9 “Intergroup Competition” Model Food distribution Small high quality clumps or uniform distribution Discrete, large high quality clumps Coalitions pay Groups as coalitions against other groups No coalitions against other groups Kin are reliable partners Zero-sum game Female kin groups (female philopatry) No kin groups (female transfer) Kin-bias in alliances larger groups Thursday, January 28, 2010 smaller groups Thursday, January 28, 2010 Philopatry in sex deriving most benefit from coalitions 29 Wrangham 1980 30 Large groups more successful in inter-group encounters High ranking groups have higher quality home ranges +8 Probability of winning +5 -8 +2 0 -2 -4 Core area food quantity and quality index Distance from home range center Group rank puchin monkeys, BCI ursday, January 28, 2010 Black 2008 Crofoot et al. & white colobus, Kibale, Uganda Thursday, January 28, 2010 32 to be continues next time... Harris 2006 33 ANT 154B Lecture #8 course notes Within-group contest: increases the benefits of coalitions example of squirrel monkeys (Saimiri spp.) 4 very similar species, different social relations. found widely through Central and South America we will talk about three of these: • oerstedii- Costa Rica, Panama • sciureus- Brazil, Colombia,, French Guiana, Guyana, Suriname, Venezuela • boliviensis- Brazil, Peru, Bolivia Generalized Saimiri features: • small body size (A) Feeding: Prefer soft fruits, also consume arthropods / small vertebrates page 6 of 9 (B) Flexible groups: Groups coalesce; sexes partly separate (FF-groups; MM-groups) (C) Susceptibility and adaptation to predation Conspicuous foragers (15-75 / group) ~1 raptor attack / group / week Anti predator behaviors Mothers & young huddle, mob Form frequent poly-specific associations (e.g., with Cebus) Births synchronized within groups Highly variable Saimiri features: female bonding and aggression Highly variable Saimiri features: • female bonding & aggression –> Aggression/hr (female) FF coalitions Bolivian squirrel monkey, Peru Red-backed squirrel monkey, Costa Rica Saimiri boliviensis 0.29 Saimiri oerstedi 0.004 Common No Philopatry Bolivian squirrel monkey, Peru Red-backed squirrel monkey, Costa Rica Female Mostly male Saimiri boliviensis Saimiri oerstedi Thursday, January 28, 2010 39 Small patches with few fruits Large patches with many fruits <– these differences map on to food distribution to be continues next time... 40 Thursday, January 28, 2010 ANT 154B Lecture #8 course notes page 7 of 9 Troop-living primates: why females stay with kin Within-group coalitions pay (over food or safety) + Resident – Nepotistic – Dispersal – Egalitarian Bolivian squirrel monkey, Peru Red-backed squirrel monkey, Costa Rica Saimiri boliviensis Large fruit trees Frequent female aggression (70 x S. o.) Saimiri oerstedi Small trees, tiny fruit crops Rare aggression 41 Saimiri sciureus (common squirrel monkey) in Suriname Thursday, January 28, 2010 Saimiri boliviensis 242 BOINS KI, SUGHRU E, SE LVAGGI, QUATRONE, HENRY & CROPP Saimiri oerstedi Saimiri sciureus > 0.33 The typical temporal and spatial presentation of ripe fruits within the Aggression/hr 0.004 0.29 crowns of fruit plants varies across the three squirrel monkey species. Fruit (female) tend to have a few small fruits ripen resources exploited by S. oerstedii each day within small fruit crowns. Over 70% of fruit crowns harvested by S. oerstedii are less than 5 m in diameter and another 20% vary between 5No Common 10 m in diameter. In contrast, fruit species used by S. boliviensis tend to have FF coalitions crowns more than 20 m in diameter, which account for 43% of their fruit feeding time. The crown diameter distribution of the effective fruit patches harvested by S. sciureus (<5 m, N D 289; 5-10 m, N D 343; 10-15 m, N D 333; 15-20 m, N D 201; 20-25 m, N D 7; >25, N D 0) was Female Mostly male relatively Philopatry homogeneous across the ￿rst 4 categories. However, virtually all the effective fruit patch sizes (93%) harvested by S. sciureus were in the smallest size category (<5 m). The percentage of time each species spent harvesting fruit in each effective patch size category is depicted in Fig. 2. The distribution of effective fruit patch sizes frequented by S. boliviensis is signi￿cantly different from both S. oerstedii (Kolmorgorov-Smirnov D D Bolivian squirrel 0:620, p < 0:001) and S. sciureus (D D 0:685, p < 0:001). Saimiri sciureus monkey, Peru and S. oerstedii are also signi￿cantly different (D D 0:158, p < 0:001). The primary fruit resources for S. sciureus are markedly clumped within Saimiri boliviensis a tree crown. Frequently, a distinct raceme or panicle of fruits ripen at the Rare? neither? Intense aggression, but no FF coalitions! Red-backed squirrel monkey, Costa Rica Common squirrel monkey, Suriname Saimiri oerstedi Saimiri sciureus Small patches with few fruits Large patches with many fruits Fig. 2. The estimates of time allocation by squirrel monkeys across distributions of effective fruit patch sizes at the three study sites. Small patches with very high density of fruits Intense WGC -> individualistic competition to be continues next time... ANT 154B Lecture #8 course notes page 8 of 9 Take home messages 1. Basic socio-ecological model: resources (food) determines female distribution, males map on to females. Other ecological factors (e.g., predation, disease) and male strategies (e.g., infanticide) also probably have important influences on group size and social system. 2. The distribution and defensibility of resources importantly influence competitive regimes (i.e., scramble vs contest). 3. All other things being equal, WGS tends to limit group size, BGC tends to increase GS, and WGC increases the benefits of coalitions. Question to ponder In this and previous lectures, you have learned about relationships among diet, body size, and group size; and about how the nature and distribution of resources can influence competitive regimes and dominance behavior. Based on these general relationships, answer the following question. For the purposes of this question, you may assume that territoriality is solely related to defense of food (e.g., not females). If you have no information other than body size for two species, the Micromono (1 kg) and the Macrosimian (50 kg), make predictions about each of the two species’ diet, group size, degree of territoriality, dispersal regime (which sex is philopatric), and dominance behavior. Be sure to briefly explain the logic behind each prediction. 3. Overview of infanticide infanticide = killing of infants, young juveniles by conspecifics • done by both FF and MM, but more common and more readily explicable in males Beware the naturalistic fallacy! Infanticide: the big picture • Widespread (mammals: mostly carnivores, rodents, and primates) • Can be a strong force • First major blow to concept of primate group solidarity (1970’s, Hrdy) • Strongly resisted at first • Not “good for the group” A. Occurrence Rarely seen directly (quick, rare) Observed: ~60; suspected: 2-300 (~40 spp) to be continues next time... ANT 154B Lecture #8 course notes B. Typical patterns 100%: infants killed are suckling 91%: by immigrant or non-group males 75%: killer mates mother after infanticide 45-70%: occur in 1-M groups page 9 of 9 Langurs: the classic case Most infanticide occurs soon after group take-over Youngest infants most at risk Hanuman langurs Red howlers Some estimated infanticide rates # attacks % successful % total inf. mortality Red howlers Blue monkey Gorilla 164 50 50 17 71 47 38 37 to be continues next time... ...
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