Ethology 094\ 292*205 "0888# Þ 0888 Blackwell Wissenschafts!Verlag\ Berlin ISSN 9068Ð0502 Communication Biolo`y Research Group\ Department of Zoolo`y\ University of the Witwatersrand\ Johannesbur` and CNRS URA 556\ Laboratoire d|Etholo`ie Experimentale et Comparee\ Universite Paris Nord\ Villetaneuse Fecundity and the Behavioural Pro_le of Reproductive Workers in the Queenless Ant\ Pachycondyla " Ophthalmopone# berthoudi Matthew F[ Sledge\ Christian Peeters + Robin M[ Crewe Sledge\ M[ F[\ Peeters\ C[ + Crewe\ R[ M[ 0888] Fecundity and the behavioural pro_le of reproductive workers in the queenless ant\ Pachycondyla " Ophthalmopone# berthoudi[ Ethology 094\ 292*205[ Abstract Morphologically specialized queens are absent in Pachycondyla " Ophthal! mopone# berthoudi "ant subfamily Ponerinae#[ Instead\ several of the workers mate and reproduce "gamergates#[ Gamergate proportion in nests commonly varies between nests and di}erent times of the year[ Individual fecundity of gamergates varies according to the number of these individuals in a nest\ and we examined their behaviour in relation to fecundity in nests with di}erent proportions of gamergates[ In nests with high proportions of gamergates\ they exhibited a diversity of behaviours inside the nest and in some cases could not be distinguished behav! iourally from sterile workers[ The fecundity of these gamergates was low and variable[ In nests with low proportions\ gamergates were relatively more fecund\ and did not participate in colony labour[ The behavioural pro_le of gamergates is therefore linked to their reproductive physiology\ which is in~uenced by the number of mated individuals in the nest[ Corresponding author] M[ F[ Sledge\ Dipartimento di Biologia Animale e Genetica\ UniversitaÁ di Firenze\ Via Romana 06\ I!49014\ Florence\ Italy[ E!mail] msledgeÝdbag[uni_[it Introduction Division of colony labour has been proposed as one of the major reasons for the considerable ecological success of social insects "Oster + Wilson 0867^ HoÃlldo! bler + Wilson 0889^ Bourke + Franks 0884#[ The simultaneous performance of di}erent tasks by colony members serves to enhance the e.ciency with which the environment is exploited for colony growth and maintenance[ In many ants this is further enhanced by morphological specialization[ Queens possess enlarged abdomens and ovaries for egg production\ while sterile workers are specialized U[ S[ Copyright Clearance Center Code Statement] 9068!0502:88:0943Ð9292,03[99:9 293 M[ F[ Sledge\ C[ Peeters + R[ M[ Crewe "reduced ovaries and a simpli_ed thorax# for performing tasks related to colony growth and development "foraging\ brood care\ and defence#\ and engage in little\ if any\ reproduction[ In most ants division of labour within the worker caste is based on temporal changes in task performance\ where workers specialize on particular tasks "e[g[ defence or brood care# for prolonged periods of time "Oster + Wilson 0867^ HoÃlldobler + Wilson 0889#[ Recently\ though\ alternative views have been proposed as to how labour is organized in social insect societies "Deneu! bourg et al[ 0876^ Tofts + Franks 0881^ Traniello + Rosengaus 0886^ Franks et al[ 0886#[ There may even be further specialization within the worker caste\ where workers are physically adapted to performing certain behaviours\ and several size classes of workers may occur "HoÃlldobler + Wilson 0889#[ This latter phenomenon is noticeably absent in all but one species of the ant subfamily Ponerinae "Me`a! ponera foetens^ Wilson 0842^ Longhurst + Howse 0868^ Villet 0889a#[ As in other ants\ colony labour in ponerine ants is performed by functional groups of indi! viduals specialized in the di}erent tasks required within the society\ with two to _ve categories of workers usually de_ned "Villet 0889a\b#[ This has been dem! onstrated in several species\ such as Pachycondyla apicalis "Fresneau + Dupuy 0877#\ Pachycondyla caffraria "Agbogba + Howse 0881#\ and Ponera pennsyl! vannica "Pratt et al[ 0883#[ The subfamily Ponerinae is of further interest because of the existence of ¼ 099 species in which the queen caste is absent "Peeters 0880\ 0882#[ Studies on such species are of interest because they approximate an ancestral condition and o}er valuable information on the factors that a}ect social evolution in many other animals[ In these ants\ mated egg!laying workers "gamergates# have replaced queens as the functional reproductives\ with either monogyny "single gamergate# or poly! gyny "multiple gamergates# described in di}erent species[ Division of labour in queenless ants is similar to that of those ants with queens[ Colony labour is performed by functional groups of sterile workers\ and there is a clear pattern of temporal polyethism "Peeters 0886#[ In polygynous queenless ants the in~uence of the level of reproductive activity on the behavioural pro_les of reproductive wor! kers has not previously been investigated[ In monogynous species\ gamergates have a behavioural pro_le distinct from that of sterile workers as they generally perform fewer behaviours than sterile workers and are never active outside the nest "Ware et al[ 0889^ Villet 0880a\b\ 0881#[ Furthermore\ in some species gamergates can be distinguished by the performance of unique behaviours\ e[g[ egg or brood care "Villet 0880b\ 0881#[ In this study we investigated the relationship between gamergate ovarian activity and behaviour in the queenless\ polygynous ponerine ant Pachycondyla " Ophthalmopone# berthoudi[ As a result of di}erences in male success at _nding nests during the limited period of male activity "Jan[ to Mar[#\ nests contain varying proportions of gamergates "2Ð52) of individuals# "Peeters + Crewe 0874#[ Furthermore\ the proportion of gamergates decreases within nests after the mating period due to an increase in the number of sterile workers\ colony _ssion events\ redistribution of individuals to other nests\ or gamergate death[ We have docu! mented that in P[ berthoudi individual gamergate fecundity varies according to the Fecundity and Behaviour in a Queenless Ant 294 number of gamergates in a nest "Sledge et al[ 0885#[ As this reproductive e}ect is expected to in~uence the way in which labour is organized in the colony\ we assessed the e}ects of variation in fecundity on gamergate behavioural pro_les in nests with di}erent proportions of gamergates[ Methods Seven nests of P[ berthoudi were collected over a 1 yr period from Dunstable Farm "29>49? E\ 14>14? S# in the Northern Province of South Africa[ The colonies were maintained in modi_ed Lubbock nests placed in large wooden arenas[ Tem! perature and humidity were maintained at 14Ð29>C and 49Ð69)\ respectively[ Workers were individually colour!coded using Tippexþ correcting ~uid[ Colonies of this ant are polydomous "one to seven nests per colony#\ and thus the terms {colony| and {nest| are used here with distinctly di}erent meanings[ Scan samples "n 49# of instantaneous behaviours performed by each ant were collected from each nest in order to group individuals performing similar behav! iours[ Nest observations spanned either 19 or 15 d[ The results were analysed using the centroid!linkage hierarchical cluster analysis of Systat 4[91 and Statistica 3[4 for Windows[ The groups resulting from the cluster analyses were characterized in the following way] similar behaviours were grouped and their frequencies summed and divided by the number of workers in the colony[ This resulted in a colony! average frequency for each set of behaviours[ The frequency with which a particular group of workers performed a speci_c set of behaviours was then compared with the colony average\ and this was used to determine the best set of behaviours characterizing the group[ This method\ together with cluster analysis\ has illustrated the distinct behavioural pro_les of gamergates in other studies "Fresneau + Dupuy 0877^ Villet 0880a\b\ 0881#[ A similar agglomeration level was chosen delimiting groups in each of the nests[ This choice was arbitrary\ but was made to be as biologically meaningful as possible in that we felt it approximated existing groups within each nest[ As gamergates cannot be recognized visually\ the ovaries of all workers were dissected on completion of behavioural observations to determine reproductive status[ Gamergates are recognized by the presence of sperm in the spermatheca "sperm reservoir# and the presence of developing oocytes in the ovaries[ The following features were recorded in order to assess the fecundity of individual gamergates] 0[ the number of yolk!containing mature "with a chorion and at least the size of a newly laid egg# and immature oocytes^ and 1[ the presence of large\ dark yellow bodies "corpora lutea# at the base of individual ovarioles[ These are formed by the remains of nurse and follicular cells when eggs are laid\ and indicate the level of past egg laying by an individual[ Egg resorption does not occur in P[ berthoudi[ Results Reproductive Status of Workers Table 0 presents the demographic data of the seven study nests and the results of dissections of workers[ Gamergate number ranged from one "0[5) of nest R# 295 M[ F[ Sledge\ C[ Peeters + R[ M[ Crewe Table 0] Nest composition and reproductive status of workers in seven nests of Pachycondyla berthoudi[ Nest O was the only nest that was collected with males Mean "SD# Mean "SD# no[ of Nest Date of Workers Workers No[ of no[ of mature immature code collection collected dissected gamergates oocytes oocytes A 08 Mar[ 0883 24 24 5 9[55 "9[36# 0[55 "9[36# B 06 Mar[ 0883 40 40 02 9[50