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Shellfishing and the Colonization ofSahul: A Multivariate Model Evaluatingthe Dynamic Effects of Prey Utility,Transport Considerations and Life-History on Foraging Patterns and MiddenCompositionBrian F. Coddinga, James F. O’Connella & Douglas W. Birdb

a Department of Anthropology, University of Utah, Salt Lake City,Utah, USAb Department of Anthropology, Stanford University, Stanford,California, USAPublished online: 17 Jul 2014.

To cite this article: Brian F. Codding, James F. O’Connell & Douglas W. Bird (2014) Shellfishing and theColonization of Sahul: A Multivariate Model Evaluating the Dynamic Effects of Prey Utility, TransportConsiderations and Life-History on Foraging Patterns and Midden Composition, The Journal of Islandand Coastal Archaeology, 9:2, 238-252, DOI: 10.1080/15564894.2013.848958

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Journal of Island & Coastal Archaeology, 9:238–252, 2014Copyright © 2014 Taylor & Francis Group, LLCISSN: 1556-4894 print / 1556-1828 onlineDOI: 10.1080/15564894.2013.848958

Shellfishing and theColonization of Sahul:A Multivariate ModelEvaluating the DynamicEffects of Prey Utility,Transport Considerationsand Life-Historyon Foraging Patternsand Midden CompositionBrian F. Codding,1 James F. O’Connell,1 and Douglas W. Bird2

1Department of Anthropology, University of Utah, Salt Lake City, Utah, USA2Department of Anthropology, Stanford University, Stanford, California, USA

ABSTRACT

Archaeological evidenceof shellfishexploitationalong thecoastof Sahul(Pleistocene Australia-New Guinea) points to an apparent paradox.While the continental record as a whole suggests that human popula-tions were very low from initial colonization through early Holocene,coastalandperi-coastal sitesdating to that timearedominatedbysmall,low-ranked, littoral taxa to the near-complete exclusion of large, higherranked, sub-littoral species, precisely the opposite of theory-based expec-tations, if human populations and predation rates were indeed as lowas other data suggest. We present a model of shellfish exploitation com-bining information on species utility, transport considerations, andprey life-history that might account for this apparent mismatch, andthen assess it with ethnographic and archaeological data. Findings sug-gest either that high-ranked taxa were uncommon along the Pleistocene

Received 24 June 2013; accepted 13 September 2013.Address correspondence to Brian F. Codding, Department of Anthropology, University of Utah, 270 S.1400 East, Room 102, Salt Lake City, UT 84112, USA. E-mail: brian.codding@anthro.utah.edu

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coastlines of Sahul, or that abundant and commonly taken high-rankedprey are under-represented in middens relative to their role in humandiets largely as a function of human processing and transport practices.If the latter reading is correct, archaeological evidence of early shellfish-ing may be mainly the product of subsistence activities by children andtheir mothers.

Keywords children’s foraging, colonization of Sahul (Australia/New Guinea), division oflabor, human behavioral ecology, shellfish exploitation

INTRODUCTION

Archaeological evidence of early shellfishcollecting on Sahul (Pleistocene Australia-New Guinea) points to an apparent paradox.While theoretical predictions from foragingtheory lead to the expectation that the ear-liest foragers on the continent would haveencountered abundant large high-rankingshellfish taxa offering relatively high aver-age rates of nutrient return relative to timespent collecting and processing, archaeolog-ical data from coastal shell middens indi-cate that foraging emphasized smaller, muchlower ranked shellfish taxa. There are at leastthree plausible explanations for this pattern:

1. Rapidly changing Pleistocene sea lev-els kept the abundance of large, high-ranked, slow-growing taxa low, leav-ing foragers no option but to pursuesmaller, lower ranked prey (Beaton1995);

2. Early human populations were muchhigher than previously thought, plac-ing sustained collecting pressure onhigh-ranked prey which rapidly re-duced their numbers, leaving onlyan ephemeral archaeological signature;and

3. Transport considerations favored thedifferential discard of inedible compo-nents of relatively high-ranked prey ator near the point of acquisition, leav-ing little or no evidence of their use inbase camp middens. The material signa-ture of lower ranked taxa may then bethe result of foraging by children, whoexperience lower encounter rates withlarge prey due to their slower walkingspeed (Bird et al. 2004), and by their

mothers, who may be more interestedin low-risk than high-energy resourcesfor provisioning young (Codding et al.2011).

Resolving this issue is crucial to understand-ing the process through which Sahul wascolonized and the early foraging decisionsof individuals on a previously unoccupiedlandscape (O’Connell and Allen 2012). Herewe address the problem with a novel modelthat examines prey utility, transport con-siderations and life-history. Combined, thisapproach may better predict both resourceexploitation patterns and their archaeolog-ical consequences. We assess the model inlight of ethnographic and ethnoarchaelogi-cal data, then consider its implications forunderstanding the record of early shellfishexploitation in Sahul.

THE MODEL

Our model is based on well-establishedideas about prey choice, central placeforaging and life-history. These allow us tomake predictions about which resourceswill be more (or less) likely to be targeted,which will be more (or less) likely to berepresented in archaeological depositscreated by central place foragers, and whichwill be more (or less) resilient to predationpressure or environmental change.

The prey choice model (PCM) allowsthe derivation of simple predictions aboutwhich species are more likely to be pur-sued on encounter (Bettinger 2009; Bird andO’Connell 2006; Charnov and Orians 2006).When searching a patch, a forager will en-counter a variety of different resources. If

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foragers are trying to maximize the rate ofnutrient acquisition, then they should pref-erentially pursue resources with higher post-encounter return rates (e/h), measured asthe expected nutrients (e) gained from ac-quiring a resource (typically measured as en-ergy) over the handling costs (h) associatedwith pursuing and processing that resource(typically measured in time). If higher rank-ing resources are sufficiently abundant, thenforagers should only take those resourceson encounter, passing over lower rankingresources; however, as the abundance ofthe highest ranking resource declines, for-agers should begin to take lower rankingresources in rank order. From a forager’sperspective, the question is: “Should I pur-sue this resource, or pass it up and con-tinue searching for other, higher ranked re-sources?” The answer depends on whetherthe post-encounter return rate (e/h) for thatresource is expected to increase the overallreturn rate (E/T) which includes all energyacquired in a patch (E) relative to all in-patchsearch and handling time (T). These modeldynamics allow the derivation of predictionsabout which taxa a forager should prefer-entially pursue and offer a proxy for overallforaging efficiency based on the presence orabsence of lower ranking resources.

Central place foraging models (CPF) al-low the evaluation of the costs and benefitsof field-processing resources or transport-ing them whole from an acquisition localeto a central place (Bettinger 2009; Charnovand Orians 2006; Metcalfe and Barlow 1992;Orians and Pearson 1979). Once a foragerhas acquired a resource, s/he must make de-cisions about how to best transport that re-source to a central place. Assuming that indi-viduals are attempting to maximize the rateat which resources are delivered to a cen-tral place, foragers may be better off cullinglow-utility items (i.e., shell or bone) in thefield and returning to their central placewith only high-utility parts (i.e., edible tis-sue) than making a greater number of tripstransporting both high- and low-utility parts.Assuming constant processing costs acrosstaxa, the net gain in utility as a function ofprocessing (the increase in high utility partsper load) can predict which items are more

likely to be field processed at a given dis-tance from a central place. Given that indi-vidual shellfish have an edible portion (meat)and a non-edible portion (shell), meat:shellratios provide a reliable proxy of the poten-tial benefit gained from culling shell in thefield, thereby increasing the utility of eachload returned home. Those taxa with higherproportions of meat to shell are less likelyto be field processed while those with lowmeat:shell ratios are more likely to be pro-cessed in the field. Because larger taxa gener-ally have lower meat:shell ratios than smallertaxa (e.g., Bird et al. 2002), these predictionslikely hold with the inclusion of processingcosts.

Life-history models (LFM) provide av-enues to investigate variability in the param-eters that govern individual growth, matu-ration, reproduction, and mortality, whichaggregate to produce population level ef-fects. Because different taxa have differentlife-history characteristics, they should re-spond differently to human exploitation,which in turn, will alter a forager’s futureencounter rates. With invertebrate taxa, life-history parameters such as growth and mat-uration rates should provide a reliable proxyof the relative susceptibility of a taxon toover-exploitation. Likely the result of natu-ral selection (Gadgil and Bossert 1970), therate of maturation covaries with a numberof life-history parameters and can be a pre-dictor of population level parameters (Peters1983). As such, rates of maturation for indi-vidual taxa should provide a relative mea-sure of the likelihood or speed at which apopulation could recover from exploitation(Whitaker 2008; Whitaker and Byrd 2012).Due to the effects of biological scaling and al-lometry,bodysizeandpost-encounterreturnrates (at least, for sessile resources; see Birdet al. 2009) should co-vary positively whilebody size, meat:shell ratios, and maturationrates may co-vary negatively (Peters 1983).This could be problematic as high-ranking re-sources may be more likely to be processedin the field, but if particular taxa exist as out-liers in these relationships (i.e., high rank-ing, slowgrowingbuthighmeat:shell ratios),they should provide key proxies to monitorchanges in foraging patterns through time.

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Combined, these models allow us tomake predictions about which speciesshould be preferentially pursued, whichshould be more or less likely to end up inarchaeological sites and which are moreor less likely to be depleted by predation.Following O’Connell (1995), these modelscan be used to derive specific deduc-tive predictions that can be tested withethnographic, ethnoarchaeological andarchaeological data. Because these predic-tions are derived from a general theory ofbehavior, they avoid direct ethnographicanalogy.

Predictions

Imagine a parameter space represented bythree variables: shellfish utility (e/h, on theFigure1:y-axis), transportcosts (operational-ized by meat:shell ratios, Figure 1: x-axis)and susceptibility to over-exploitation (mea-sured here through the proxy of maturationrates). How different shellfish taxa fall outwithin this parameter space should allow usto identify target taxa that will allow us to in-vestigate foraging behavior, midden compo-sition and how they are expected to changeover time.

Figure 1. Schematic predictions of midden composition based on the combined effects of eachtaxon’s post-encounter return rate (e/h), and likelihood of field processing (meat:shellratio). Taxa with low meat:shell ratios (frame a) are less likely to end up in middens astheir shells should be frequently culled in the field (unless the shell itself provides somevalue). Early middens should be dominated by higher ranking taxa with relatively highmeat:shell ratios (frame b). If these become less abundant in the environment due to over-exploitation, then foragers should begin to take lower ranking resources which will cometo dominate later middens (frame c). The rate of transition in midden dominance from b→ c depends on the life-history characteristics of the taxa in question: fast-growing speciesshould be harder to deplete while slow-growing species rapidly decline in abundance. Note:These values are in dimensionless space.

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Taxa with very low meat:shell ratios(Figure 1: frame a) should rarely appear inmiddens due to the low net utility of a loadreturned to a central place with such a largeamount of shell relative to a limited amountof meat. If shellfish rank varies inversely withmeat:shell ratios, then evidence for the ex-ploitation of high-ranking taxa is unlikely tobe recovered from archaeological depositsas their shells should be culled in the fieldeven over short transport distances. This isthe case for many molluscan species fromthis region (Bird and Bliege Bird 2000).

High-ranking taxa with high meat:shellratios (Figure 1: frame b) should always betaken on encounter by foragers and are likelyto always be transported whole to the centralplace. As such, these taxa should dominateearly middens. Those with slow maturationratesshouldbethebestsourcetomonitor theeffectsof resourceover-exploitationthroughtime. Those that mature early are likely to befairly resilient to over-exploitation and maydominate middens throughout the prehis-toric record.

Lower ranked taxa with high meat:shellratios (Figure 1: frame c) should come todominate latter middens if higher rankingresources become less abundant as a resultof over-exploitation. Those that take longer

to mature will be particularly useful to moni-tor human foraging and population pressureover time. Those that are relatively resilientto over-exploitation (rapid growth rates andshort maturation rates) should dominatelater shell middens (and shell mounds)following declines of the highest ranking re-sources (e.g., Whitaker and Byrd, this issue).

RESULTS

Resource-Specific Attributes

Table 1 provides a summary of available dataon shellfish genera post-encounter returnrates (e/h), meat:shell ratios, and age at ma-turity for taxa potentially exploited by theearliest inhabitants of Sahul. Mean values arealso represented in Figure 2 following thelayout of predictions in Figure 1.

The two large bivalves, Tridacna andHippopus, would be excellent candidates tomonitor foraging pressure given their highrank and slow growth rates. However, giventheir low meat:shell ratios, they are unlikelyto be transported to midden deposits pro-portionally to the frequency with which theyare taken. This expectation is supported byethnoarchaeological studies indicating that

Table 1. Proxy data for shellfish utility (e/h), transport cost (meat:shell) and susceptibilityto over-exploitation (age at maturity), ordered by mid-point return rate (e/h).

e/h Meat: shell Age at

Taxon (kcal/hr) ratio maturity (yr)

Tridacna 2,622–13,064 0.170 4–5

Hippopus 1,680–9,120 0.115 6–7

Lambis 3,412–5,106 0.092 2

Trochus 977–3,904 0.208 1–2

Cypraea 2,214 0.278 1

Chiton 446–2,228 1.159 1–2 + ?

Nerita 42–1,106 0.301 1.6

Turbo 520–606 0.248 3–4

Strombus 294–543 0.148 1–2

Asaphis 42–78 0.338 1–2

Data on shellfish returns from Bird and Bliege Bird (2000), Bird et al. (2004), Kennedy (2005), andThomas (2007); on meat:shell ratios from Bird and Bliege Bird (1997), Erlandson (1988); on age atmaturity from Beesley et al. (1998), Munro (1993), Nash (1993), and Yamaguchi (1993).

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Figure 2. Shellfish genera plotted as a function of post-encounter return rate (e/h) and meat:shellratios following predictions in Figure 1. Shading indicates growth rates measured by timetill maturity from short (white) to long (black). Genera plotted by mid-point values ofranges given in Table 1.

the material record of Tridacna and Hip-popus under-represents the frequency withwhich they are actually acquired (see below;Bird and Bliege Bird 2000; Bird et al. 2002).

Unfortunately, within the suite of re-sources available to the earliest colonists ofSahul (and for which data are currently avail-able), there are no species that can be con-sidered high ranking with high meat:shell ra-tios (i.e., falling within parameter space bin Figure 1). Evidence from North Americasuggests that Abalone (Haliotis spp.) may behighly ranked and have high meat:shell ra-tios (Kennedy 2005), but conclusive data onAustral species are lacking. The absence ofspecies within this parameter space suggeststhat efficient tidal foraging in Sahul required

field processing. As a result, we are unlikelyto acquire an unbiased picture of the pre-historic exploitation of these higher rankingtaxa.

With relatively low returns but highmeat:shell ratios, small gastropods such asNerita and small bivalves like Asaphis alsohave rapid maturation rates, suggesting theyare resilient to over-exploitation. As wewould predict, these taxa dominate mod-ern middens within permanent villages inthe Torres Strait Islands, Australia, and West-ernKiribati,Micronesia,wheresustained for-aging pressure does not seem to diminishtheir populations (Bird et al. 2002; Thomas2007). These taxa should be relatively re-silient to over-exploitation, their abundance

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Table 2. Shell midden composition across chronological analytical units at Buang Merabak.Shell genera presented in weight (g).

Taxa 1 2 3 4 5 6

Cypraea 90.6 223.0 16.6 0.0 0.0 13.6

Strombus 34.2 62.8 1.2 0.6 0.0 0.0

Isognomon 15.3 66.1 4.7 0.0 0.0 0.0

Trochus 41.4 153.0 83.8 23.9 12.5 9.3

Barnacle 50.0 136.9 797.9 8.9 0.9 2.0

Limpet 7.8 26.5 33.3 24.2 0.5 0.0

Nerita 52.7 226.0 258.0 1398.4 122.0 265.8

Tectarius 2.5 3.1 11.3 79.2 11.3 49.9

Chiton 22.6 61.0 53.9 267.3 99.7 509.4

Turbo 13.1 71.1 7.9 165.5 58.9 408.9

From Balean (1989) reported in Leavesley and Allen (1998); see also Rosenfeld (1997). Note: Giventheir extremely high meat:shell ratio, chiton likely contributed even more to the early diets than isrepresented by shell weight alone (Leavesley 2004).

is less likely to vary with human foraging andpopulation pressure over time.

The moderately sized gastropod Turboprovides a welcome exception: with rel-atively long times to maturity and highmeat:shell ratios, we should expect thatTurbo will be exploited when the abun-dance of higher ranking resources are low,and should be one of the first taxa withinthis lower ranked set to exhibit over-exploitation. As such, diachronic trends inthe exploitation of Turbo should provide akey proxy to understand foraging pressureon shellfish species, and as such, a proxy forhuman population densities.

With by far the most extreme meat:shellratio, chiton (class Polyplacophora) may alsofall within this category. Larger species ofchiton are likely to have higher return ratesand slower maturation rates, while still main-taining relatively high meat:shell ratios; un-fortunately, precise data are not availablefor Austral species. Based on observationsof Meriam shellfishers by one of the authors(DWB), limited data on chiton exploitationby contemporary foragers may result fromlow encounter rates with larger chiton cou-pled with consistent decisions to pass oversmaller chiton in search of more profitabletaxa.

An Ethnographic Test

Previous ethnoarchaeological investigationwith Meriam Islanders in the Torres Straitexplored the archaeological consequencesof trade-offs that shellfishers face in acquir-ing, handling, and transporting a number ofthetaxa thatPleistoceneforagerswouldhavelikely encountered on colonizing Sahul (Bird1996; Bird and Bliege Bird 1997, 2000; Birdet al. 2002, 2004). Consistent with expec-tations framed by the prey choice model(PCM), Meriam foragers restrict the suite oftaxa they handle on encounter to those shell-fish that will increase overall foraging effi-ciency (E/T) while collecting on the reef flator harvesting within the rocky shore. Forexample, while reef flat collecting, foragersroutinely select exposed tridacnid clams(Tridacna spp. and Hippopus hippopus),Lambis lambis conch, and large Trochusniloticus specimens, all of which signifi-cantly increase overall foraging return ratesif handled on-encounter. Conversely adultshellfishers almost always pass over spec-imens that offer returns for time handlingthat would reduce foraging efficiency: smallStrombus (Conomurex) luhuanus conch,small Trochus, and small bivalves embeddedin the reef. Meriam foragers also pay close

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attention to changes in patch return rateswhile intertidal foraging, and switch fromreef flat collecting to rocky shore harvest-ing (for Asaphis violascens clams and Neritaspp. snails)when an incoming tide depressesthe overall return rate from collecting in themid-littoral below that expected from har-vesting in the near shore.

While observed time allocation andselectivity are consistent with predictionsgenerated by the PCM and patch residencemodels, they are in sharp contrast to thefrequency of different taxa of shellfishfound in either contemporary or prehistoricshellmiddens on the Meriam Islands. Themost important resources for contemporarycollectors (especially large tridacnids andLambis lambis) are very rare in the archae-ological deposits, which are dominated bysmall gastropod shells (especially nerites,small strombids, and small Trochus niloti-cus) (Bird et al. 2002). We can account forthese differences by considering the effectsof differential field processing and age-linkeddifferences in foraging returns. While reefflat collecting, children walk more slowlyand encounter high-ranked resources at alower rate than adults, resulting in loweroverall return rates and a predictablybroader range of selectivity that includesthe smaller gastropods. So while both adultsand children collect the higher ranked shell-fish on-encounter, only children regularlycollect the abundant small strombids andsmall specimens of Trochus niloticus thatare common in the shell middens.

But why are tridacnid and Lambis shell,which make up the bulk of shellfish har-vested, so rare in the deposits? If we con-sider how time spent field processing canincrease the ratio of high to low utilityparts (meat:shell), the central place forag-ing model discussed above generates precisepredictions about which types of molluscsshould be field processed if a forager’s goalis to increase the rate at which shellfish meatcan be transported home. The model well an-ticipates the fact that Meriam foragers almostalways cull the shells of large tridacnid clamsand Lambis conch while on the reef. Theseshellfish offer high post-encounter returnrates and have low meat:shell ratios, and thus

field processing quickly frees up more spacefor more meat, and more time for more for-aging and transport. As such, Meriam adultsand children commonly forage well beyondthe predicted thresholds at which in-bulktransportation (without culling the valves) ofthese large shellfish will increase the homedelivery rate of meat. Conversely, the smallerreef flat shellfish that children often collect,along with rocky shore taxa such as Asaphisand Nerita that both adults and children har-vest, are always transported in-bulk, and for-agers very rarely cross the predicted fieldprocessing thresholds while collecting theseprey. We suspect that Pleistocene foragerswould have faced similar trade-offs whenshellfishing in intertidal zones.

Early Middens in Sahul

Data are available for at least six Pleistocene‘middens’ (coastalandperi-coastal sitesyield-ing marine shell) with dates older than 30kya. Buang Merabak (Leavesley and Allen1998; Rosenfeld 1997) and Matenkupkum(Gosden and Robertson 1991) are locatedon New Ireland, Kilu (Wickler 2001) is onthe northern end of the Solomon Islands,and Noala Cave (Veth et al. 2007), ManduMandu Creek (Morse 1988), and Devil’s Lair(Dortch 2004) are located along the coastof modern-day Western Australia (see Fig-ure 3).1 For more detailed summaries on thedating of each site, see Allen and O’Connell(2003, 2008), O’Connell and Allen (2004,2007, 2012), and O’Connell et al. (2010). Ev-idence for shellfish exploitation at many ofthese sites is extremely sparse; nonetheless,the available data allow us to run an initialtest of these predictions.

Higher ranking species with lowmeat:shell ratios are rare in these early mid-dens. At Buang Merabak, chiton, Turbo, andNerita dominate the Pleistocene assemblage(Leavesley and Allen 1998). At Kilu, Ner-ita dominates assemblages dating before theLate Holocene (Wickler 2001). At Matenkup-kum, Gosden and Robertson (1991) reportlarge Turbo shells, indicating the exploita-tion of lower ranked taxa, but—assumingthat human predation will drive down

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Figure 3. Map of coastal and peri-coastal archaeological sites with evidence of marine shell exploita-tion prior to 30 kya.

shellfish size (e.g., Klein and Steele 2013)—only limited predation pressure on thesetaxa. While Tridacna is absent from the fau-nal assemblages recovered from these earlylevels, Leavesley and Allen (1998) reportthat large Tridacna shells and Turbo op-ercula, originally recorded as white chert,were used to manufacture tools at BuangMerabak. This suggests that these taxa weretaken on encounter, likely for their food andshell, but only transported and depositedwithin midden contexts for purposes relat-ing to the latter. This also suggests that suchhigh-value shell might be secondarily trans-ported away from middens for subsequentuse, a pattern that may further bias such ataxon’s representation in the archaeologicalrecord.

At peri-coastal sites along the shore ofmodern-day Western Australia, shellfish taxaare dominated by different genera specificto the local environment; but even so, thetaxa exploited at these sites share character-istic with those exploited in the Bismarks.At Noala Cave on the Monte Bello Islands,foragers were exploiting a variety of whelks(Terebralia; Veth et al. 2007), which are ex-pected to be low ranked. Perhaps at oddswith findings elsewhere, the earliest layersat Mandu Mandu Creek reveal chiton andbaler (Melo) (Morse 1988). While precisedata on these taxa are not yet available,what we know suggests that they may berelatively high ranked with high meat:shellratios (Figure 1: frame b). If these werelarge chiton and Melo, this evidence for the

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Figure 4. Battleship plot showing the pro-portion of total weight per an-alytical unit for important Pleis-tocene shellfish taxa at Buang Mer-abak. Data from Leavesley and Allen(1998), shown here in Table 2. Basedon radiocarbon dates from Rosen-feld (1997), analytical units 6 (2-sigma range 38,090–34,523 cal BP),5 (24,141–22,505 cal BP) and 4(24,782–23,356 cal BP) date to thePleistocene, while analytical units 3,2, and 1 all date to the Holocene(2-sigma ranges calibrated in Ox-Cal 4.2.2, Bronk Ramsey 2009, usinga marine curve from Reimer et al.2009 and local correction by Petcheyet al. 2004). The two most abundanttaxa in the earliest Pleistocene de-posits (Turbo and chiton) are alsorelatively slow growing. Their over-exploitation through the Pleistoceneis mirrored in the rising dominanceof faster growing Nerita. Discontinu-ity between the Pleistocene-Holocenerecord is likely due to sea-level stabi-lization at distances closer to the site(see Lambeck and Chappell 2001).

exploitation of high-ranking taxa during thisinterval supports the idea that large, high-ranking taxa were present, pursued on en-counter, but only transported to centralplaces when meat:shell ratios were high orwhen the shells had some added value.

This might also be true in the southwest-ern corner of the continent, where small

fragments of abalone (Haliotis) are someof the only taxa represented in the earlylevels at Devil’s Lair (and at Tunnel Cave;Dortch 2004; J. Dortch, personal commu-nication, March 2013). Abalone too likelyfalls within the parameter space shown inFigure 1, frame b, representing high rankand high meat:shell ratios, leading us to ex-pect that it will be transported whole. Theadditional value of abalone shell—for rit-ual, ornamental or other purposes beyondsubsistence—likely increases the probabilitythat it will be transported whole. This is con-sistent with finds at Carpenter’s Gap in theKimberley, where early levels contain pearlshell (O’Connor 1999).

Diachronic Trends at Buang Merabak

Temporal trends from the well-reportedfinds at Buang Merabak can further our un-derstanding of these patterns. As predictedfrom our model, while we should not expecthigh-ranking, low meat:shell taxa to be de-posited at central places, we should be ableto monitor the abundance of Turbo and chi-ton through time as an indicator of foragingpressure.

As evident in Figure 4 and Table 2,the temporal trends at Buang Merabakshow a decline in the proportion of Turbo(τ = 17.6, p = .0361) and a declinein the proportion of chiton (τ = 4.2,p = .1498) through the Pleistocene. Theseare mirrored by an increase in Nerita (τ= 7.0, p = .0905).2 Such patterning hasalso been reported at Kilu (Wickler 2001).While significant, declines in the relativeabundance of Turbo and chiton occur overa long span of time, suggesting that ittook most of the Pleistocene record be-fore human populations were high enoughto place consistent pressure on these taxa.This interpretation is supported by the ra-diocarbon probabilities from Buang Merabakwhich suggest intermittent occupations(Figure 5a) as well as occupational esti-mates from Australia, which suggest verylow populations levels during this time(Figure 5b; Williams 2013). These find-ings suggest that Pleistocene occupations atBuang Merabak were intermittent, perhaps

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Figure 5. Occupational history of Buang Merabak (a, Leavesley 2004, Leavesley and Allen 1998,summed calibrated radiocarbon proababilites from OxCal 4.2.2, Bronk Ramsey 2009,using a marine curve from Reimer et al. 2009 and a local correction by Petchey et al.2004) relative to the estimated total popualtion of Australia (b, assuming a foundingpopulation of 5,000 people, Williams 2013) and estimated flucutations in sea level (c,Lambeck and Chappell 2001).

covarying negatively with the distance to thecoast (see Figure 5a and 5c), and were alwaysat a low densities.

DISCUSSION

This article began with a conundrum: the ar-chaeological record suggests that the earliestcolonizing populations of Sahul dispropor-tionately exploited low-ranked littoral shell-fish species, despite theoretical predictionsto the contrary. While a number of hypothe-ses could be proposed to explain this conun-drum, we suggest that these can be assessedby incorporating models of prey efficiency,transport utility, and life-history to predict

the targets of foraging, the resulting middencomposition and the potential impact of hu-man foraging on shellfish species. In light ofour model, the apparent conundrum actu-ally makes sense: high-ranking shellfish taxaavailable to the early foragers of Sahul hadlow meat:shell ratios (Figure 1: frame a). As itwouldnotmakeeconomicsense to transportshell from these taxa (unless the shell itselfhad some added value; e.g., Leavesley andAllen 1998), we should not expect middensto contain shell from these taxa. But whatthen accounts for the deposition of lowerranking littoral shellfish species?

Given the extremely low estimated pop-ulation levels and intermittent occupation ofsites like Buang Merabak, it seems unlikely

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that the acquisition and deposition of low-ranked taxa was due to over-exploitationof higher ranking taxa. If the abundanceof these larger high-ranked species declinedrapidly, then we might have evidence for alarger colonizing population placing morepressure on resources than traditionally as-sumed. However, given the long period oftime that passed before human populationsreached densities required to overexploitfairly slow-growing taxa like Turbo, we sug-gest that human population densities werequite low, their mobility quite high, and assuch, they placed minimal predation pres-sure on the littoral environment. Combinedwith evidence for the rapid dispersal of peo-ples along the coastlines of Sahul, this im-plies that it takes very little impact to makeadjacent, unoccupied patches more appeal-ing to foraging populations (O’Connell andAllen 2012).

It still could be that these taxa occurredin lowabundancedueto thenaturaleffectsofa fluctuating coastline (Beaton 1995). Whilealternative lines of evidence (such as paleon-tological finds) will be required to test thisfurther, we suspect that habitats (e.g., broadfringing reefs) supporting healthy popula-tions of some of the highest ranked shellfish(such as Tridacna) would have been com-monly encountered when Sahul was first col-onized (Pope and Terrell 2008). But again,that such taxa are high ranked and often as-sociated with low meat:shell ratios leads usto suspect that they would have been impor-tant prey options whose archaeological pres-ence was heavily filtered by field processing.This is consistent with ethnographic obser-vations of tropical reef flat foraging, wherehighly ranked Tridacna are always exploitedon-encounter, but the valves are rarely trans-ported back to a central place (Bird et al.1997, 2002, 2004; Thomas 2007).

If our interpretations of these results arecorrect, it seems that the most reasonableexplanation for the high abundance of low-ranking taxa at early sites may result fromthe differential foraging constraints faced bychildrenandtheirmothers.Becausechildrengenerally walk more slowly than adults andbecause they may have less strength andskill in processing, they are likely to havelower encounter rates with high-ranked prey

and are less able to field process taxa evenwhen it might be desirable to do so (Bird andBliege Bird 2000; Bird et al. 2004; Meehan1982). Further, given the limited availabil-ity of alternative caregivers expected withsmaller populations (Codding et al. 2011),mothers with young children will generallyexperience greater trade-offs between childcare and foraging. Intertidal shellfish forag-ing provides an activity with lower costs ofchild care than many other alternatives (e.g.,sub-tidal foraging, pelagic fishing) and de-spite being low return, shellfish (especiallysmall taxa) are characterized by low acquisi-tion variance, making them ideal resourcesfor provisioning dependents (Codding et al.2011). Indeed, cooperative parent-juvenileunits may do better overall by working to-gether in lower return activities than byworking apart (Hawkes et al. 1995; Kramer2011).

While this seems like the most plausi-ble explanation for the Pleistocene recordof shellfishing on Sahul given the availableevidence, more work is required to truly un-derstand these patterns. Continued archaeo-logical and paleoenvironmental work is nec-essary, but research might benefit most fromcontinued ethnoarchaeologial and actualis-tic experiments. This includes developingsimple measures of utility, processing costs,and meat:shell ratios for key species (e.g.,Halitois, Melo, and chiton) and additionalstudies of well-known taxa (e.g., Turbo, Ner-ita, and Asaphis) to further our understand-ing of variability within species. Additionalwork along these lines should greatly im-prove our interpretations of the archaeolog-ical record and allow us to solve such conun-drums as the record of early shellfishing bythe colonists of Sahul.

ACKNOWLEDGEMENTS

Thanks to Richard Klein for suggestingthe idea for this symposium and collec-tion of articles. We remain indebted tothe Meriam Community for hosting DWB’sethnographic work over many years. Thisarticle has benefited from comments, sug-gestions, and work by Jim Allen, RebeccaBliege Bird, Shannon Arnold-Boomgarden,Joe Dortch, James Holland Jones, Peter

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Veth, Adie Whitaker, and David Zeanah.Three anonymous reviewers provided de-tailed comments that greatly improved thefinal result.

FUNDING

The U.S. National Science Foundation andThe Leakey Foundation provided financialsupport.

END NOTES

1. Early middens (pre–30 kya) for which datawere unobtainable include other sites alongthe Cape Range of northwestern Australia(Przywolnik 2008; Morse 1988; Morse 1993).

2. Pearson’s correlation analyses run in R (R De-velopment Core Team 2013). Figure 4 pro-duced using the battleship.plot function in theplotrix library.

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