Ethology 094\ 162*171 "0888#Þ 0888 Blackwell Wissenschafts!Verlag\ BerlinISSN 9068Ð0502

Department of Psycholo`y\ University of Washin`ton\ Seattle

The Function of Male Dominance in the Eusocial Wasp\

Mischocyttarus mastigophorus "Hymenoptera] Vespidae#

Sean O|Donnell

O|Donnell\ S[ 0888] The function of male dominance in the eusocial wasp Mischocyttarus masti`ophorus"Hymenoptera] Vespidae#[ Ethology 094\ 162*171[

Abstract

Males of a Neotropical eusocial wasp\ Mischocyttarus masti`ophorus\ aredominant over their female nest mates[ Mischocyttarus masti`ophorus males behaveaggressively toward females\ while females rarely bite or chase males[ Aggressiveinteractions between the sexes are behaviorally indistinguishable from dominanceinteractions among females[ Males are long!lived as adults\ and can reside on nestsfor periods of at least one month[ Furthermore\ males comprise a large proportionof post!emergence colony populations throughout much of the colony cycle[ Maleson the nest perform maintenance tasks at low rates\ but contribute little otherlabor to their colonies[ Males do not forage\ but consume a disproportionateamount of the food "nectar and insect prey# collected by workers[ Males in somecolonies direct disproportionate amounts of aggression toward their queens\ whichmay further contribute to males| food procurement[ These data suggest that adultmales represent a considerable energetic and labor cost to their colonies[ I hypo!thesize that the dominance structure of M[ masti`ophorus directs food resources toadult males\ with the function of increasing their longevity[ Increased male lon!gevity may be selectively advantageous in tropical species such as M[ masti`ophorusthat found new colonies throughout much or all of the year[ When females initiatenew nests over much of the year\ individual males| mating opportunities may betemporally distributed\ favoring longer adult lifespans[ Male dominance is pre!dicted to occur in other populations of independent!founding Neotropical Pol!istinae with asynchronous colony foundation[

Sean O|Donnell Department of Psychology\ Box 240414\ University of Wash!ington\ Seattle\ Washington 87084\ USA[ E!mail] sodonnelÝu[washington[edu

Introduction

Dominance hierarchies result from social interactions\ often aggressive innature\ that a}ect the outcome of within!group competition for access to repro!

U[ S[ Copyright Clearance Center Code Statement] 9068!0502:88:0942Ð9162,03[99:9

163 S[ O|Donnell

duction or resources[ Dominance interactions among female nestmates are a preva!lent feature of many insect societies\ occurring both within and between repro!ductive castes "Pardi 0837^ Wilson 0864^ Gree} 0885#[ Relatively little attentionhas been paid to the roles of male eusocial insects in dominance hierarchies[

Males of independent!founding eusocial wasps often leave their natal coloniesquickly\ spending only a few days at the nest before departing permanently to joinmating aggregations "West!Eberhard 0858^ Jeanne 0861^ Tindo et al[ 0886#[ Inother wasp species\ male tenure at natal nests is prolonged "up to several weeks^Litte 0868#[ However\ in all species studied to date\ males are socially subordinateto females or do not participate in dominance interactions "Litte 0868\ 0870^ Starks+ Poe 0886#[ Females frequently chase and bite males\ and males respond tofemales with submissive behaviors that include ~eeing and crouching[

Although male eusocial wasps occasionally perform on!nest tasks at low rates"reviewed by O|Donnell 0884#\ the presence of adult males can represent a substantialcost to colonies[ Males do not forage for materials used by their colonies\ but theyfrequently rely on food collected by their nestmates[ Therefore\ adult male presencepotentially alters their colony|s nutritional balance and task!performance patterns"Suzuki 0875^ O|Donnell 0888a#[ The nutritional costs of maintaining male adults\and their low genetic relatedness to sisters resulting from haplodiploidy\ have led tothe prediction that adult hymenopteran males are unlikely to receive cooperativebehavior in eusocial species "Hamilton 0853^ Starks + Poe 0886#[

Research on independent!founding wasps has focused largely on temperatespecies\ and few tropical species have been studied for the duration of the annualseasonal cycle "West!Eberhard 0858^ Jeanne 0861^ Litte 0870#[ This temperate biascreates a large gap in our knowledge of social insect natural history\ becausethe vast majority of independent!founding wasp species are restricted to tropicallatitudes[ For example\ the largely Neotropical genus Mischocyttarus comprises¼ 199 species of eusocial wasps\ only two of which range north into the UnitedStates "Richards 0867#[ Because wasp colony developmental cycles and socialcomposition can be a}ected by climate "Lorenzi + Turillazzi 0875^ O|Donnell0885#\ studies of tropical species are of great importance to understanding theevolution and ecology of eusocial behavior[ Social structure\ including the rolesplayed by males\ may di}er among temperate and tropical species[

The independent!founding eusocial wasp Mischocyttarus masti`ophorus iscommon at high elevations "above 0349m# in Monteverde\ Costa Rica "O|Donnell+ Joyce 0888a#[ Observations and collections of M[ masti`ophorus colonies"O|Donnell + Joyce 0888b# demonstrated that males often comprise a large pro!portion of the wasps on colonies that have produced adult o}spring "post!emerg!ence colonies#[ Furthermore\ males are present in colonies collected in di}erentmonths and at di}erent seasons of the annual rainfall cycle\ suggesting that adultmales are resident on nests throughout much of colony development[ These colonycomposition data suggest that males may play an important role in M[ mas!ti`ophorus social behavior[

The goal of this study was to determine the role of males in M[ masti`ophorussocial structure[ Rates of dominance interactions and rates of on!nest and o}!nest

164Male Dominance in Eusocial Wasps

task performance were quanti_ed for individually marked wasps in colonies thathad produced adult o}spring "post!emergence colonies#[ These data were used toassess the dominance status of males relative to females\ rates of task performanceby males\ and the e}ects of maleÐfemale interactions on the allocation of food inthe colonies[ I conclude by discussing the possible adaptive signi_cance of maleÐfemale dominance interactions in M[ masti`ophorus and the evolution of maledominance behavior in independent!founding eusocial wasps[

Methods

Colony Composition and Male Longevity

M[ masti`ophorus nests are restricted to areas above 0349m elevation withlow insolation and high inputs of wind!driven moisture "cloud forest^ Clark et al[0887# at Monteverde\ Costa Rica "09>07?N\ 73>38?W#[ I collected 21 post!emerg!ence M[ masti`ophorus colonies at the study site in Monteverde in three _eld tripsbetween May 0883 and Aug[ 0886[ Collections were performed at night to ensurethat foragers had returned to the nests[ All adult wasps were stored in _xative"formalin!acetic acid!ethanol# or ethanol and later counted[ Male M[ masti`ophorusare easily distinguished from females by their elongate\ _lamentous antennae"Richards 0867^ O|Donnell + Joyce 0888a#[ Morphological di}erences betweenqueens and workers are not evident\ and female castes were distinguished bybehavioral di}erences "below#[

Adult wasps in eight colonies were marked for individual recognition withpaint pens between 07 Jul[ and 8 Aug[ 0886[ These colonies were collected on 16Jul[ to 20 Aug[ 0886 and the presence of marked males was noted upon collection[The time elapsed from marking to collection set a minimum boundary on maleadult longevity[

Behavioral Data

In Jul[ 0886 adult wasps present on six post!emergence M[ masti`ophoruscolonies "henceforth labeled A through F# were marked for individual identi_cationwith paint pens^ these colonies were subjects for detailed behavioral observations[Five of the behavioral subject colony nests were located on the eves and rafters ofa building at ¼ 0499m elevation^ the other subject colony nest was built on therain gutter of an outbuilding at 0429m elevation[ Detailed behavioral observationswere conducted between 14 Jul[ and 00 Aug[ 0886[ Behavioral data were collectedby an observer seated on sca}olding or standing on a ladder within 9[4m of theface of a nest[ Observations were conducted for three continuous hours in themorning "between 9799 h and 0039 h local time# and two continuous hours in theafternoon "between 0299 h and 0529 h local time# on two consecutive days per nest\providing a total of 09 h observation time per colony "8 h total at colony E#[Because Mischocyttarus species nests are simple\ open combs\ all individuals atthe nest were visible to the observer[ M[ masti`ophorus colonies rarely exceedpopulations of several dozen adults "O|Donnell + Joyce 0888b#\ so it was possible

165 S[ O|Donnell

to collect behavioral data on the entire colony simultaneously[ All occurrences ofnest maintenance tasks\ social interactions\ ~ying departures and arrivals\ andtransfers of food and building materials collected by the foragers were recorded[For each behavioral act the identity of the wasp"s# and time to the nearest minutewere recorded[ Social interactions that were coded as dominance interactionsincluded chasing\ biting\ displacing and darting "one individual ~ees another uponapproach but is not followed#[ In all cases\ a subordinate individual was clearlyidenti_ed by crouching or ~eeing from the aggressor "see Ito¼ 0874 for descriptionsand illustrations of dominant and subordinate behavior in Mischocyttarus spp[#[For every pair of adults that engaged in social aggression\ the individual that wasthe aggressor most often was de_ned as dominant over the other[ A single femalewas dominant over all female nestmates in each observation colony\ and only thesemost dominant females were observed laying eggs "O|Donnell 0887#[ The mostdominant females were designated as queens\ and the others as workers[ Becausethe observation colonies were in the post!emergence phase when they were located\I could not determine whether workers were daughters or subordinate cofoun!dresses that remained on the nest[

In Jul[ 0886\ I observed dominance interactions on three additional post!emergence colonies using ad libitum sampling "Altmann 0863# for a total of 9[4 hto 0 h per colony^ wasps were not individually marked in the colonies observed[

Results

Presence of Males on Nests

M[ masti`ophorus post!emergence colony populations ranged from two to 017adults "x¹ �12[8 adults^ n�21 colonies#[ Males were present in 09 of 03 post!emergence colonies collected at Monteverde in Apr[ and May 0883 "onset of therainy season in Monteverde# and comprised 14Ð47) of the adult wasps on nestswith males[ Similarly\ males were present in 01 out of 04 post!emergence coloniescollected in Aug[ 0886 "mid rainy season# and comprised 00Ð57) of the adultwasps on nests with males[ Overall\ males accounted for 30) of the total adultwasps in post!emergence colonies "not including colonies without males#[ Colonieswithout males had smaller numbers of females than those with males "KruskalÐWallis test x1 �4[40\ df�0\ p³ 9[94#\ suggesting that males were not producedearly in the colony cycle[ In 0886\ individually marked males were present on neststhat were collected 09 d "four males#\ 08 d "three males#\ and 21 d "one male# afterthe males had been marked[

Both females "f# and males "m# were present in the six behavioral observationsubject colonies "colony A 06 f\ 7m^ colony B 02 f\ 02m^ colony C 03 f\ 18m^colony D 07 f\ 00m^ colony E 09 f\ 8m^ colony F 19 f\ 3m#[ The three coloniesobserved ad libitum in 0886 also comprised both males and females[ Althoughadults were not counted in these colonies\ populations exceeded 19 adults in each[

Male Dominance Behavior

On the three colonies observed ad libitum\ males chased\ bit and displacedfemales\ while females responded to male aggressive behavior with subordinate

166Male Dominance in Eusocial Wasps

behavior\ including ~eeing\ crouching and ~ying from the nest[ Female aggressiontoward males was not observed during the ad libitum observations[

Dominance hierarchies could not be analyzed for linearity or transitivitybecause low!ranking individuals rarely or never engaged in dominance interactionswith each other in the six colonies with marked adults "Tufto et al[ 0887#[ However\males were dominant over females in nearly all cases] in all but four maleÐfemalepairs that engaged in dominance interactions\ the male was dominant over thefemale "n�070 pairs#[ Males were aggressive toward females at higher overall ratesthan females were aggressive toward males in each of the six colonies "binomial testp³ 9[94^ Fig[ 0#[ Males were aggressive toward other males at lower rates\ similarto the rates of femaleÐmale aggression "Fig[ 0#[ In _ve colonies\ males displayedaggression toward the queens[ In four of these colonies the queens received moreattacks from males than any other female^ queens received male aggression atdisproportionately high rates in the same four colonies\ i[e[ "Pr"attacks on femalesdirected at queens##:"Pr"female time on nest accounted for by queens## for colonyA�3[22\ colony B�0[65\ colony C�0[32\ colony D�1[44\ colony E�9[53and for colony F�9[ Queens were never dominant over individual males[

Males frequently attacked returning foragers\ and males took food "nectarand insect prey# from arriving foragers at signi_cantly higher rates than workers"paired t!test\ t�1[47\ p³ 9[94^ Fig[ 1#[ Males did not collect materials for colonyuse when they departed their nests "see below#[ Queens took food loads from

Fi`[ 0] Rates of dominance interactions "mean across six colonies2 SD# among Mischo!cyttarus masti`ophorus adults[ Rates shown for each category of interacting individuals"categorized by sex of aggressor and subordinate# are attacks per individual per hour on thenest[ Rates of male dominance over females were higher than female over male rates in all

colonies

167 S[ O|Donnell

Fi`[ 1] Rates of taking food from foragers by adult Mischocyttarus masti`ophorus wasps ofdi}erent castes and sexes "mean across six colonies 2 SD#[ Rates for each colony werecalculated by dividing the number of food forager arrivals during which all individuals ofeach caste or sex took food by the number of forager arrivals during which they were present

on the nest

arriving foragers at higher average rates than workers in all colonies "Fig[ 1# andmales often subsequently took food from their queens "own pers[ obs[^ rates ofmales taking food from queens were not quanti_ed#[ However\ male aggressiontoward females was not always associated with food solicitation[ Sixty!eight percent of all male aggressive acts toward females "range 38Ð70) across the six subjectcolonies# were directed at individuals that had not returned from a foraging trip inthe previous 04min[

Male Task Performance

Males performed tasks on their nests\ typically at low rates relative to femaleworkers\ i[e[ "male task performance rate "acts per male per h on nest## ] "femaletask performance rate "acts per female per h on nest## for colony A�9[14 ] 9[14\colony B�9 ] 9[95\ colony C�9 ] 9[09\ colony D�9[02 ] 9[27\ colonyE�9[96 ] 9[78 and for colony F�9[02 ] 9[33[ Males fanned their nests "possiblyfor thermoregulation# and removed excess water from the nest surface[ Males neverengaged in nest construction or defended their nests against parasitoids\ whilethese acts were performed by females in all colonies[ Males occasionally fed piecesof insect prey that had been collected by female foragers to larvae\ after the maleshad masticated the prey[

Males frequently left their nests\ with trip duration ranging from 5min toover 2 h[ Typically\ most of the males in a colony left their nests in late morning\and returned in the afternoon[ However\ some males were nearly always presentat each nest[ Males never transferred materials to nest mates after returning to thenest and therefore did not forage for food or building materials for colony use[

168Male Dominance in Eusocial Wasps

Off!Nest Male Aggregation

An aggregation of males formed on the building where the observation col!onies were located[ The aggregation was found in the same spot daily from 07 Jul["when it was discovered# until 18 Jul[ "when colony C was collected\ after whichthe aggregation did not form#[ The aggregation ranged in size from seven to over19 individuals\ and included up to _ve marked males from colony C[ Small numbersof females visited this aggregation "from one to three individuals simultaneously#[Female visitors interacted with the males in the aggregation\ rapidly rubbing theirantennae with the males| antennae\ but no copulation was observed[

Discussion

Male Aggression and Dominance over Females

Male aggression toward females was a pervasive feature of M[ masti`ophorussocial life[ Males were aggressive toward queens\ foragers\ and workers on the nest[The relatively low rate of maleÐmale dominance interactions suggests that maledominance over females is not merely a result of heightened aggressive behavioron the part of males\ and raises the question of the possible adaptive value of maleaggression toward females[

Dominance by male eusocial wasps on their natal nests\ and prolongedinvestment in adult males by females\ are surprising from a kin selection perspec!tive[ If males are produced by a single queen\ then they will be related to siblingworkers by r�9[14 "on average#[ When queen!produced males remain in theirnatal colonies\ the elevated average relatedness among nestmates caused by haplo!diploidy is diminished[ According to kin selection theory\ hymenopteran males areunlikely to be recipients of worker cooperation and investment\ given their rela!tively low genetic relatedness to sibling workers "Hamilton 0853^ Suzuki 0875#[During behavioral observations\ post!emergence M[ masti`ophorus colonies werefunctionally monogynous\ suggesting that the queens had produced the males[However\ most M[ masti`ophorus colonies are founded by groups of females in thestudy population "O|Donnell + Joyce 0888b#\ raising the possibility that sub!ordinate cofoundresses share in o}spring production early in the colony cycle"Reeve + Nonacs 0881#[ The evolution "via kin selection# of extended investmentby cofoundresses in their own adult male o}spring is more likely than investmentin males by sibling workers\ but male M[ masti`ophorus dominated all femalenestmates\ including workers[ Furthermore\ all marked females acted as workers\performing tasks that included foraging "O|Donnell 0888b#[ Because the femalecastes could not be distinguished morphologically\ the precise natures of cofoun!dress and reproductive female "gyne# interactions with males await further inves!tigation[

Costs of Male Dominance

In addition to the e}ects of genetic relatedness\ _tness costs and bene_ts toaltruists must be accounted for in order to model the evolution of cooperation via

179 S[ O|Donnell

kin selection[ Adult male wasps impose obvious nutritional and temporal costs totheir colonies[ Given the high rates of food investment in males\ the prevalence ofmale aggression toward females and prolonged male tenure on nests\ these costsare likely to be great in M[ masti`ophorus[ However\ as in other species of primi!tively eusocial wasps "reviewed in O|Donnell 0884#\ M[ masti`ophorus males con!tributed some labor to their colonies[ Male labor o}sets the cost of their productionand maintenance to some extent "Jeanne 0875#[ However\ given males| low ratesof task performance relative to female workers\ especially their failure to forage\male labor is unlikely to fully explain the evolutionary maintenance of male domi!nance[

Function of Male Dominance

Extended investment in adult M[ masti`ophorus males may be adaptive\ despitethe associated social costs[ The e}ects of dominance interactions on individual lifehistories are often mediated through di}erential access to food resources withinsocial groups "Mock et al[ 0889^ Hunt 0883#[ I hypothesize that M[ masti`ophorusdominance structure has evolved in response to selection for increased malelongevity\ and that male aggression channels the colonies| food intake towardadult males[ Two aspects of male dominance in M[ masti`ophorus suggest that itplays a role in social procurement of nutrients[ First\ males took much of the foodbrought to the colony by foragers[ Second\ males frequently dominated theirqueens and\ in some colonies\ males directed disproportionate amounts ofaggression toward queens[ Queens play a pivotal role in regulating food intake andutilization in primitively eusocial wasp colonies\ including M[ masti`ophorus "Reeve+ Gamboa 0876^ O|Donnell 0884^ O|Donnell 0888b#[ Male dominance over queenscould function to further divert nutrients away from workers and brood[ Similarly\Kojima "0882# noted that males of the Australian eusocial wasp Ropalidia plebianatook prey loads from female nestmates\ including dominant females\ and were{involved in the dominance hierarchy of the colony|^ however\ data on rates ofdominance interactions were not presented[

If dominance over females has evolved to enhance male longevity\ then the_tness costs of maintenance of a population of adult males must be lower than thecosts of rearing additional males to replace those lost to mortality[ Furthermore\the males must gain _tness advantages from remaining on their natal nests asadults[ Given that mortality rates are generally higher for wasps away from neststhan for those on nests "O|Donnell + Jeanne 0884#\ and the fact that M[ mas!ti`ophorus males have access to ready and abundant nutrition at their nests\ thelongevity of males is likely to be enhanced by extended nest tenure[ Male con~ictwith female nestmates over the duration of tenure at nests has been documentedfor other eusocial wasps "Starks + Poe 0886#\ suggesting that it is often in malewasps| best interest to delay permanent departure from nests[

Male M[ masti`ophorus wasps are long!lived as adults[ Although sample sizeswere small\ I found that males resided on natal nests for periods up to severalweeks[ In order to accrue direct _tness bene_ts from increased lifespans\ males

170Male Dominance in Eusocial Wasps

must have access to mates for extended periods[ Male nest residence\ and its e}ectof increasing male longevity\ may be favored by natural selection when there areextended opportunities for mating[ Compared with temperate Mischocyttarus wasppopulations "Litte 0868#\ colony foundation is asynchronous in tropical Mis!chocyttarus species "Jeanne 0861^ Litte 0870#[ I have observed and collected M[masti`ophorus colonies at Monteverde in all stages of colony development\ includ!ing newly founded colonies that had not yet produced adult o}spring\ in the dryseason "Jan[ 0884#\ early wet season "Apr[:May 0883#\ and late wet season"Jul[:Aug[ 0886# "O|Donnell + Joyce 0887b#[

New colony foundation in di}erent seasons indicates that males have oppor!tunities to mate with nesting females during much or all of the year[ When M[masti`ophorus gynes are produced\ they probably leave their natal nests to initiatenew colonies elsewhere[ Males\ in contrast\ need only depart from their neststemporarily to mate with available females[ Males are only rarely present at pre!emergence nests "S[ O|Donnell + F[ J[ Joyce\ unpubl[ data#\ indicating that matingtakes place at o}!nest sites[ Males of other Mischocyttarus species seek matingopportunities while away from their nests "Litte 0868\ 0870#[ Mating behavior of M[masti`ophorus is unknown and copulation has not been observed\ but preliminaryobservations suggest that males form mating aggregations away from their nests[

Male participation in dominance hierarchies has also been observed in M[collarellus in the relatively aseasonal Atlantic lowland forests of Costa Rica "E[Smith\ pers[ comm[#[ Whether M[ collarellus males are consistently dominant overfemale nestmates is unknown\ as is the phenology of colony foundation in thispopulation[ I predict that dominant males will be found in other eusocial wasppopulations where males are long!lived and have temporally dispersed oppor!tunities to mate[

Acknowledgements

Sara Ranger assisted with data collection[ Thanks to Alan and Marlene Pounds and Frank Joycefor logistical assistance\ and to Jane Brockmann\ Susan Bulova\ Robert Jeanne\ Em(�lia Martins\Elizabeth Smith\ Mary Jane West!Eberhard\ and three anonymous reviewers for helpful discussionsand comments[ Elizabeth Smith drew my attention to the occurrence of male dominance in M[ collarellusand shared her unpublished preliminary data on that species[ Frank Joyce allowed me to work underhis research permits from the Costa Rican Ministry of Natural Resources[ Financial support wasprovided by the University of Washington Royalty Research Fund[

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Received] March 17\ 0887

Initial acceptance] June 01\ 0887

Final acceptance] August 09\ 0887 "J[ Brockmann#