The costs and benefits of mating with multiple partners are

Transcription

1 Behavioral Ecology Vol. 9 No. 2: Sex ratio conflicts, mating frequency, and queen fitness in the ant Formica truncorum Iiselotte Sundstrom* and Francis L. W. Ratnieks b Department of Genetics and Ecology, University of Aarhus, DK-8000 Aarhus C, Denmark, and b Department of Animal and Plant Sciences, University of Sheffield, Sheffield S10 2TN, UK We examined the effect of facultative sex allocation by workers on queen fitness in a Furnish population of the ant Formica truncorum. Workers rear female-biased broods in colonies headed by a singly mated queen and male-biased broods in colonies headed by a multiply mated queen. As a result, multiply mated queens have a 37% fitness advantage over singly mated queens. Neither reproductive output nor worker population of colonies varied with queen mating frequency. We suggest that singly mated queens persist in the population because fitness benefits to multiply mated queens via sex allocation are balanced by costs of additional mating*. Alternatively, singly mated queens may persist simply because some queens lack opportunities to mate multiply or because male control sometimes prevents additional matings by queens. Kty words: eusodah'ty, Formica, queen fitness, sex allocation, social insects, worker-queen conflict. [Behav Ecol 9: (1998)] The costs and benefits of mating with multiple partners are major factors in the evolution of animal mating systems (Thornhill and Alcock, 1983). Benefits to males are obvious, because having multiple partners readily translates into more offspring. Benefits to females are not obvious, particularly in species where males provide no parental care or other resources, or where females need to mate with only one male to ensure their fecundity. In social Hymenoptera, queens mate early in adult life. Remating rarely occurs, possibly only under laboratory conditions (Rosengren et al., 1993), and sperm stored in a queen's spermatheca remain viable for the life of the queen, up to 30 years in some ants (Pamilo, 1991). In fact, the life span of a queen may directly depend on the availability and viability of the sperm stored, and several studies have shown that multiply mated queens have more sperm than singly mated ones (Fjerdingstad, 1996; Fortelius 1994; Rdseler, 1973). In ants, queen mating frequency varies from exclusively one (e.g., Soienopsis, in two species the mean effective paternity frequency is 1.0), from one to a few (e.g., Formica; in six species the mean varies between 1.03 and 1.48) to almost always greater than one (e.g., Atta colombica, the mean is 1.96) (Boomsma and Ratnicks, 1996; Fjerdingstad et al., 1997). Several hypotheses have been put forward to explain how a queen's fitness may be affected by her mating frequency (Crozier and Page, 1985; Crozier and Pamilo, 1996). The*e can be classified as: (1) queen fecundity: mating frequency influences the number of sperm stored and hence maximum fecundity. As a result, multiply mated queens may have a longer reproductive life than singly mated queens because they have more sperm; (2) colony performance: mating frequency influences the genetic diversity of the queen's diploid offspring (workers, new queens, and diploid males), which in turn may affect colony survival and efficiency; and (3) facultative sex allocation. The three hypotheses are not mutually exclusive, and they all predict a fitness advantage for multiple mated queens. L. StmdstrOm ts now at the Department of Ecology and Systematic*, Helsinki University, PO Box 17, FIN Helsinki University, Finland. Received 24 February 1997; first revision 5 June 1997; second revision 15 July 1992; accepted 16 July O 1998 International Society for Behavioral Ecology Facultative sex allocation is a strategy in which a colony's workers preferentially rear the sex that is of greater value to them (Boomsma and Grafen, 1991; Ratnieks and Boomsma, 1997). When colonies each have a single queen and queens are inseminated either singly or multiply, workers can enhance their inclusive fitness by preferentially rearing females in colonies headed by a singly mated queen and males in colonies headed by a multiply mated queen (Boomsma and Grafen, 1991). Such facultative sex-ratio biasing also affects the fitness of the mother queen (Ratnieks and Boomsma, 1995). A queen is equally related to her sons and daughters, so from her perspective the value of males will increase when worker control results in female-biased populationwide sex ratios (Crozier and Pamilo, 1996; Trivers and Hare, 1976). Even when workers make errors in assessing queen mating frequency, multiply mated queens more frequently head malebiased colonies than do singly mated queens and therefore have a fitness advantage over singly mated queens (Ratnieks and Boomsma, 1995). Theory suggests that facultative sex allocation by workers can select for multiple mating by queens (Ratnieks and Boomsma, 1995). In a population with mixed single and double mating, the maximum fitness difference due to sex allocation between doubly mated and singly mated queens that have successfully established a colony varies from 3-fold to 1.33-fold, depending on the proportion of doubly mated queens in the population (Ratnieks and Boomsma, 1995). Benefits of multiple mating decrease if workers make errors in assessing queen mating frequency, if some males are workers' sons, if colony sex ratios are less extremely bimodal than the worker optimum or if the populationwide sex ratio is less female-biased than the worker optimum (Boomsma and Grafen, 1991; Ratnieks and Boomsma, 1995, 1997). In this study we quantified the effect on queen fitness caused by facultative sex allocation by workers in response to variation in the mating frequency of their mother queen. We did this by assessing the relative fitness of singly mated and multiply mated Formica truncorum queens using field data from a Burnish population is which facultativs soxrfstio biasing by workers occurs (Sundstrom, 1994). Importantly, some colonies have been followed during the entire period of study and have shown little variation in sex allocation ratios (Sundstrom, 1994,1995b, unpublished data). This suggests that agedependent changes in colony sex ratios are uncommon and

2 Sundstrom and Ratnieks Queen fitness in ants 117 TaMel Average aex ratios and queen fitness (i ± SD) in and and number of adub workers and sexual production in Sex ratio (% females) Queen fitness No. of reproductive* No. of workers ' * if if n Colonies \fearl 2 3 Mean Colonies Year Mean headed by singly-mated queens t : t i : i = t t 0 JO headed trf doubly mated queens t i: 0J t = i : t J2 i: t tOJ i t 035 2J51 2tO t :0J5 8(9) 8(9) 8(11) 8(8) 8(8) 8(10) 1.74 i ± t * i i J2 ± (11) 10 (14) 10 (13) 8(11) 8(9) 8(10) 4274* ± ± ± i ± : t : t : t t t : i t tS * Based on average values, without correction for productivity differences among colonies. b With correction for productivity differences among colonies (Bourke and Franks, 1995). c Numbers in brackets give the total number of live colonies in the year of sampling. that each colony consistently produces either a male-biased or female-biased sex ratio over several years. For the same reason, sperm depletion seems an unlikely cause of male bias because if no new workers are produced the colony will die within a year. Nevertheless, colonies headed by multiply mated queens produced male-biased sex ratios during several consecutive years. METHODS Study rtr g*'^»* gi1 The study population of F. truncorum is located on six adjacent islands in southwest Finland (Sundstrom, 1994, 1995b). Thirty nests were censused between 1989 and The data for have been published (Sundstrom, 1994, 1995b), but those for are new. Because some colonies died or were newly founded during the study, the number of colonies censused varied, but averaged 16 per year. The total number of active colonies averaged 20 per year (Table 1). Hence, on average 80% (range %) of the entire population was censused each year. Colonies are conspicuous and immobile, so the entire population can be censused and individual colonies followed year to year. Genetic studies have shown that the population is largely panmictic and that each nest has a single queen (Sundstrom, 1993). Colony sex allocation ratio, production of new queens and males, and worker population were estimated using mark-recapture methods (Sundstrom, 1995b). Queen mating frequency was determined by analyzing worker genotypes at polymorphic allozyme (Sundstrom, 1993, 1994) and microsatellite DNA loci (Chapuisat, 1996). Following the use of microsatellites, one queen previously classified as single mated (Sundstrom, 1994) was found to be double mated (Gertsch P, personal communication), and is here included in the multiply mated category. The large amount of allozyme and microsatellite variation results in a negligible probability, 0.7%, of misclassifying a multiply mated queen as single mated due to identical paternal genotypes at all loci. Based on year-to-year consistency of nest location and worker genotypes, some queens heading the study nests are known to have lived at least 9 years (Sundstrom L, unpublished data). In F. exsecta the average life span of a queen was estimated at 30 years (Pamilo, 1991). The great longevity of Formica queens in single-queen nests makes it impractical to calculate queen fitness over the entire queen life span. Nevertheless, our data cover 6 study yean over a 7-year period. The colonies that have been followed during the entire study period show little year-to-year variation in sex allocation ratios (Sundstrom, 1994,1995, unpublished data), suggesting that age-dependent changes in sex ratios are of minor importance. Data on colony sex-allocation ratios were collected for 6 years, and data on worker population and number of queens and males produced were collected for 3 of these years. We therefore divided our data into two groups: one where only sex ratios were measured ( ) and one where sex ratios, worker population, and sexual production were measured ( ). No data were collected in Because some colonies died or were newly founded, the identities of the colonies studied in the two 3-year periods are not entirely overlapping. For die analysis of sex ratios and queen fitness we only included colonies for which data were available for all 3 years, or , or both. For the analysis of worker population and sexual production, we used the same set of colonies. However, productivity and worker population data were available only for 1 or 2 years in some of these colonies. This resulted in missing values that were replaced by estimates (see below). In populationwide sex ratios were estimated directly by summing the male and queen production of individual colonies (Sundstrom 1995b). Sexual production were not measured in Instead we used die arithmetic mean of the population sex-allocation ratios in years as an estimate. Calculation of t»wln»iw» fitness In a large panmictic population, the inclusive fitness, W v of a queen can be expressed as W q - N(b mq V a m/m+ b^vj/f), (1) where N is die total allocation to reproduction, b m^ and are the regression relatedness of the queen to the male an female reproductives produced in her colony, V m and V, are the reproductive values of males and females, m (= 1 fi and / are die proportional allocation to male and female reproductives in individual colonies, and Af ( = 1 F) and F are die proportional allocation to male and female reproductives in die population. (For details see Crozier and Pamilo, 1996.)

3 118 Behavioral Ecology Vol. 9 No. 2 Tablet ANOVA of <U F< among colonies in worker populatki I production, aexratio,and queen fitness per year and mating Mates (Af) tear (r) Interaction (Y* M) Years Error term Sex ratio Queen fitness Sex ratio Queen fitness No. of reproductives No. of workers F p < > >.10 M * colony df ,15 F 2J Y* C P >:!S.01 df 2.2 F M* Y* P >:JS c df ,32 2,30 2,30 Significances are based on exact F tests, with degrees of freedom given for both the main effect and the error term. Worker production of males affects h^ and the ratio \fc V m. In queenright colonies both genotype distribution* and relatedness coefficients strongly suggest that worker reproduction does not occur (Sundstrom 199S, Walin L, Sundstr&m L, Seppa P, and Rosengren R, submitted manuscript). Another source of worker reproduction is orphaned nests. During the entire study period seven colonies died. Two produced only males during their last season, whereas the other five had no phase of exclusive male production before death. In the two colonies that produced males only, no change in male genotypes occurred between the penultimate and final years. This strongly suggests that worker reproduction is negligible in orphaned colonies and in the population as a whole. Hence, V, = 2V m, ifc, and h^ - 1, so that b^vf - b ax> V m - 1 (setting V r - 2; cf. Pamilo, 1991). The total production of reproductives did not differ significantly between colonies headed by singly mated or multiply mated queens (see below). Therefore, Equation 1 simplifies to W^ - (HI/M) + (X/F), (2) in which the subscript i refers to a specified queen or her colony. Equation 2 was used to calculate the inclusive fitness of singly and multiply mated queens using the populationwide M and.f values for each year in , or their estimates in , and the m and/values per colony per year for each year. Statistical analyses Parametric tests were used after confirming that all dependent variables WCTC normally distributed (Wilk-Shapiro rankit plot r > 0.9). We used two-way ANOVAs to test the effects of mating frequency and year on queen fitness ( and ), sexual production and number of adult workers ( only). The same colonies were used in different years, hence colonies were entered as randomized blocks within each mating class and year (split-plot design). Because the set of colonies used in and were not completely overlapping, these periods were analyzed separately to avoid too unbalanced a design. For the data the design was still slightly unbalanced, and to overcome this we replaced the missing values (six for sex ratio and queen fitness analyses, nine for colony size and productivity analyses) with the yearly average (Sokal aad Rohlf, 1995). Exact F tests were constructed to test significances. RESULTS Table 1. gives, per mating class, the average sex allocation ratio, queen fitness, number of reproductives produced, and number of adult workers. The average populationwide sex ratios expressed as the proportion of females, F (Af = 1 - F) were 0.67 in 1992, 037 in 1994, and 0.76 in 1995, yielding a mean of 0.67± (SD). The mean value of 0.67 was used as an ***i*m't* for the populationwide sex-allocation ratio in Neither the number of reproductives produced nor the number of adult workers per colony differed significantly between singly mated and multiply mated classes (Table 2). The number of reproductives produced varied significantly between years, whereas the number of adult workers did not (Table 2, Figure 1). A pairwise comparison of worker population between years gave no significant differences between any pair of years for the singly mated class (^^ «1.3, df 9, p >.10), whereas one pair of years, 1992 and 1994, differed significantly for the multiply mated class (t WMX «2.87, df «5, p ".035). However, this difference was not significant following Bonferroni correction for multiple comparisons (/>>.10). Because colony productivity did not differ between the two classes, no productivity correction was needed. Both colony sex ratio and queen fitness differed significantly between mating classes (Table 2, Figure 1), with an average fitness ratio (H^ / Wy of 1.37 (±0.11, SD). Thus the difference in sex allocation ratios between colonies headed by singly mated and multiply mated queens gives a 37% fitness advantage to multiply mated queens. Of the 30 colonies studied 17 were headed by a singly mated queen, 10 by a doubly mated queen, and 3 by a triply mated queen. The paternity contribution of third males was invariably low, and the effective paternity frequency of triply mated queens about two (Walin L, Sundstrom L, Seppa P. Rosengren R, submitted manuscript). During the study seven colonies died three headed by a doubly mated queen and four by a singly mated queen. This suggests that colony mortality does not strongly depend on queen mating frequency (Fisher's Exact test, p «* 1.0). Of four newty founded colonies, two each were headed by singly mated and doubly mated queens. DISCUSSION Possible factors w» Facultative sex allocation by workers results in an average 37% fitness advantage to multiply mated F. truncorum qtleetw compared to singly mated queens. The disadvantage of single mating was not compensated by higher colony productivity or higher colony survival of colonies headed by singly mated queens. If anything, productivity seemed lower in colonies headed by singly mated queens. Hence, the 37% fitness advantage is most likely a minimum estimate, and the multiply mated queens may have additional

4 Sundstrom and Ratnieks Queen fitness in ants 119 Singly mated I I Multiply mated B O ,94-96 tttlt Figure 1 Variation (mean ± 95% Q) among colonies in (A) worker population, (B) sexual production, (C, D) queen fitness per year and mating class. fitness gains through higher colony productivity. Given this substantial advantage, why are singly mated queens a frequent phenomenon? The proportions of singly mated and multiply mated queens in the population may reach an equilibrium where benefits from multiple mating are balanced by costs of multiple mating (Ratnieks and Boomsma, 1995). Importantly, the fitness benefits of multiple mating decline with increasing proportions of multiply mated queens in the population. This is because male-bias colonies become the balancing class and workers in them are selected to produce both males and females (Boomsma and Grafen, 1991; Ratnieks and Boomsma, 1996). For example, the maximum fitness advantage of doubly mated queens drops from 3:1 to 1.33:1 as their proportion increases from 25% to 100% (Ratnieks and Boomsma, 1995). This decline still occurs when workers make errors in assessing queen mating frequency (Ratnieks, 1990). The frequency of doubly mated queens could thus increase from an initially low level until the costs and benefits are equal. Costs of multiple mating may arise through increased risks of predation, injury, or disease transmission during longer nuptial flights. In some Formica ants, queen mortality during nuptial flights apparently is high (O'Neill, 1994). Alternatively, single mating may prevail because some queens are unable find a second mate. In F. truncorum females outnumber males because the populationwide sex allocation ratio is female-biased, and females and males have approximately equal weights (Sundstrom, 1995a). Combined with the short life span of male ants, a relative shortage of males may arise. Thus, females may be unable to remate because males are hard to find. Single mating by queens could be partly a result of male mating tactics (Boomsma, 1996). Sexual selection theory predicts that the more limiting sex will be choosier (Andenson, 1994). Males may reject mated females if virgin females are easily available and males are sperm limited. This is because a second male will likely father only a fraction of a multiply mated queen's daughters. This effect is even stronger in ants with facultative sex allocation because colonies headed by multiply mated queens produce male-biased sex ratios. When a colony's males are the queen's sons, they are unrelated to the queens' mates. Thus, males may benefit from preventing their mates from subsequent matings. Potential male control methods, such as mating plugs or mate guarding, have not been reported in Formica. However, Formica males could exercise some control by pheromonally marking queens during copulation. Second males would then be able to easily recognize virgins from nonvirgins. Factors decreasing die fitness advantage of multiple ""*i"g In the study population the effective paternity is 1.43 and mean sperm bias is 0.33:0.67 (Sundstrom, 1993). Given these values, the theoretical maximum fitness advantage of multiply mated queens over singly mated queens is approximately 75% (Ratnieks and Boomsma, 1995). Several factors might cause the observed fitness advantage of multiply mated queens (37%), to be lower than this. The population-wide sex allocation ratio may be less female biased than die worker optimum, so that die fitness benefits from producing males are lower. Given the observed effective paternity, the worker-optimum populationwide sex-allocation ratio is 67% females (Boomsma and Grafen, 1991), which closely matches the observed average value, 0.67± Unequal sperm use and errors in the assessment of queen mating frequency raise this value only slightly (Ratnieks and Boomsma, 1996). Workers may respond to factors in addition to queen mating frequency when optimizing colony sex ratio. In the study population there is a positive correlation between colony productivity and female allocation, with productivity explaining 23% of colony sex-ratio variation (Sundstrom, 1994, 1995b). Small colonies headed by a singly mated queen produced some males, and large colonies headed by a multiply mated queen produced some females, thus decreasing the fitness differences between singly and multiply mated queens (Boomsma and Grafen, 1991; Ratnieks and Boomsma, 1995). Workers in some colonies may assess queen-mating frequency incorrecdy. Given the observed mating frequencies

5 120 Behavioral Ecology Vol. 9 No. 2 Singly mated Multiply mated " Figure J Yearly colony-specific sex allocation ratios of the colonies included in this study. Note that the set of colonies presented here is not identical to that in Sundstrdm (1994) * Proportion females the singly mated class of colonies is expected to produce exclusively females and the multiply mated class of colonies both males and females (Boomsma and Grafen, 1991; Ratnieks and Boomsma, 1996). Thus, the fitness of singly mated queens is enhanced if the workers wrongly assess their queen as multiply mated and rear a male-biased sexual brood, whereas the fitness of multiply mated queens decreases. A few colonies with singly mated queens produced male-biased broods, but all had low productivity, so both assessment errors and productivity correlations equally well explain male bias in these colonies (Figure 2). Conduaioaa Multiply mated queens have a substantial fitness advantage over singly mated queens, which is approximately half die theoretical maximum. Despite die considerable fitness advantage of multiple mating, singly mated queens remain frequent in the population. Sex allocation is probably only one of several factors-that affect queen fitness, and sax alleaation benefits of multiple mating may be balanced by costs of additional matings. In die absence of productivity or mortality disadvantages to established colonies headed by multiply mated queens, costs of additional matings may maintain variation in mating frequency. If mating frequency is at an equilibrium of equal fitness for single and multiply mated queens then copulation costs must be high to compensate for die 37% fitness advantage via sex allocation to die multiply mated queens. Few data are available on costs of mating in ants, presumably because diese are extremely difficult to quantify. Some testable predictions can, however, be made: (1) If male interests limit opportunities for queens to mate multiply, then mated queens should be less attractive to males seeking copulations. (2) If male abundance affects die ability of queens to mate multiply, then queen mating frequency should be higher in populations where other factors acting in addition to relatedness asymmetry (such as local resource competition among females) increase male bias, i.e., the populationwide sex ratios are more male biased than expected based on die frequency of multiply mated queens in die population alone. (S) Where costs of additional matings can be estimated, we predict diese to be high. We thank Roos Boomsma, Andrew Bourke, Else Fjerdingstad, Laurent KeiteT and Laura Walin for discussions and commerce on previous drafts of the paper. Pia Ceruch, Roger Foreman, and Helena Aberg helped in the field, and the Tvarminne Zoological Station provided working facilities. Pia Certsch kindly kept L.S. up to date on microsatellite screenings of the colonies. The Danish National Science Research Council and Swiss National Science Foundation provided funding (to l_s.) and the research network "Social Evolution" financed

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