Keywords

These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

17.1 Introduction

Standing in a dense and humid rainforest of French Guyana, I spotted a hill that was not overgrown with vegetation and therefore offered a better view through the impenetrable ground vegetation. This was welcome given I tried to spot red-handed tamarins ( Saguinus midas ) in the canopy some 20–30 m above me. However, I should have been warned that there was a reason for the absence of vegetation on that hill, as within minutes, I found myself covered in thousands of angry ants , some of them heavily armed with razor-sharp mandibles, all crawling up my boots and trousers and every single one dedicated to their last drop of hemolymph to fight me off that hill. By setting my foot onto this mature colony of Atta leaf-cutting ants , I had triggered an alarm that spread rapidly. Such colonies grow to the size of a family home, containing up to 8 million workers and surviving in the wild for 20 years or more (Weber 1972). This experience of interacting with a fully grown social insect organism was further elaborated a couple of weeks later when our field station was raided by a colony of Eciton army ants . These ants maintain no permanent nest structures but build temporary bivouacs made by workers for brood rearing, reaching colony sizes of up to 20 million individuals in some African species (Hölldobler and Wilson 1990; Raignier and van Boven 1955). Any form of resistance would have been futile as each worker is armed with pointy mandibles as well as a stinger, and I was therefore defeated once more.

Amazingly, in both of these cases, I was confronted with the offspring of a single reproducing female, known as the queen. These animals are well hidden and protected by their societies, because they are the sole reproductive individuals in the colony and therefore highly valuable, as they cannot be replaced in most species. This cryptic lifestyle of queens is typical for all eusocial ants , bees , wasps , and termites . Yet these queens represent the pinnacle of social evolution, as their reproductive potential is key for their social lifestyle. The life histories of social insect queens are quite variable between species, and as already indicated above, often truly spectacular as their reproductive traits are either unique or have evolved to spectacular extremes. These traits determine colony success, which depends on maintaining a large number of helpers, all originating from fertilized eggs. As queens dominate reproduction in these societies, fathers have adopted a very cryptic lifestyle. In the case of the hymenopteran social insects (ants , bees and wasps)—fathers are only present as stored sperm within the queen (Boomsma and Ratnieks 1996).

In this chapter, I focus on social insect queens and explore what we know about these extraordinary animals’ potential to determine or bias paternity after copulations and in the absence of males, especially whether and if so cryptic female choice is present. The study of the reproductive biology of social insects received scientific attention only recently, but males and their reproductive agendas were studied in more detail, for reviews see for example (Baer 2003, 2005, 2011; Boomsma et al. 2005; Hölldobler and Bartz 1985). As I point out, social insects offer unique opportunities to investigate cryptic female choice (CFC), because the available theoretical framework of inclusive fitness theory allows to develop specific predictions and hypotheses, and newly available knowledge and technologies offer ample opportunities for experimental work to address them.

I start this chapter with a brief introduction to social insects and their reproductive biology. Because the mating biologies can differ substantially between social insect species, this introduction presents a very generalized overview describing reproductive traits and behaviors as found in a majority of species. Such a general overview is important for the following section presenting a theoretical framework of why CFC is expected to be important in social insects and discussing the available empirical support found in the literature. I will also point out research areas and questions that should receive further attention in the future to stimulate further research using social insects to study CFC.

17.2 Social Insects

Eusociality is defined by the presence of a division of labor, cooperative brood care, and overlapping generations within a colony (Wilson 1971). It is widespread in bees and wasps, and present in all known species of ants and termites . Darwin was puzzled by the presence of a worker caste in these species (Rubenstein 2012) that does not reproduce but altruistically helps raising non-own offspring. However, it was not until Hamilton (1964) and Trivers and Hare (1976) formulated the necessary theoretical framework of inclusive fitness and kin selection that a powerful evolutionary explanation became available to explain altruistic helping. In essence, the incentive for helping increases with increasing relatedness between a helper and the individual receiving help, thereby increasing inclusive fitness of the helper. The development of kin selection theory triggered a substantial body of theoretical work to identify conflicts arising through cooperation in these societies (e.g., see West et al. 2002; Bourke and Franks 1995; Queller 2003), including reproductive conflicts among colony members (Ratnieks et al. 2006) and their potential consequences for social evolution. These contributions stimulated a wealth of empirical work to test ideas derived from theory (see, e.g., (Bourke and Franks 1995; Foster and Ratnieks 2000). As a consequence, sociobiology became and remains a vibrant field of research (Wilson 2000). Its output of research is comparable to another field receiving broad scientific attention, being sexual selection, which studies biases in paternity contributions to explain fitness consequences of male–male competition and female choice (Baer 2014). Despite the common interest of both fields in the genetic makeup of offspring, kin selection research developed independently from research conducted on sexual selection (Boomsma 2007). The reasons being that studies on kin selection were more concerned with the consequences of paternity distributions (Baer 2014), whereas sexual selection research such as CFC focused on explaining how paternity distributions are generated and their effects on the evolution of individual life history traits.

17.3 The Reproductive Biology of Social Insect Queens

Social insects are characterized by the presence of extreme levels of reproductive skew, where one or very few females (normally referred to as queens, sometimes as gamergates) monopolize reproduction. In the hymenopteran social insects , queen development is typically initiated during the egg or larval period and is dependent on environmental factors that trigger elevated juvenile hormone levels (Penick et al. 2012), such as the amount of food provided to a larva (Alford 1975). In honeybees, the provisioning of royal jelly to 3 day old larvae initiates modifications in DNA methylation that triggers the relevant developmental pathways for queen development (Maleszka 2008). When virgin queens hatch, they are supported by their sister workers until they reach sexual maturity. They then leave the colony to take part in nuptial flights to choose mates and copulate before founding a new colony. With some known exceptions such as honeybees or swarm-founding epiponine wasps [see (Ratnieks et al. 2006) and references therein], queens do not return to their maternal colony. Instead, they go through a phase of solitary living, which can be of substantial length and can include periods of hibernation, dispersion, or foraging (Alford 1975). In many species, only a single queen initiates a new colony, but multiple foundresses have been reported in some wasps, ants , and termites (Schmid-Hempel and Crozier 1999; Atkinson and Adams 1997). During that time, queens also perform worker tasks such as foraging (Pollock et al. 2012; Hölldobler and Wilson 1990), symbiont cultivation (Fernandez-Marin et al. 2004), brood care, or nest defense (Fig. 17.1). Colony foundation is the most critical time period in the life of a queen and is accompanied by extreme levels of queen mortality (Diehl-Fleig 1995; Baer et al. 2006; Schmid-Hempel 1998). Queens need to maintain their reproductive potential by continuously producing eggs as well as by keeping sperm alive and viable within their spermatheca. Associated sperm storage costs can be substantial and trade off with other life history traits. In the leaf-cutter ant A. colombica , for example, queens that mate more often or store higher numbers of sperm during their nuptial flight have a reduced capacity to up-regulate their immune system during colony foundation, which makes them susceptible to infections (Baer et al. 2006).

Fig. 17.1
figure 1

A leaf-cutter ant queen (Atta colombica) photographed after her nuptial flight, while initiating a new colony. The queen has dug a first initial brood chamber 20–30 cm below the surface and has started to metabolize her flight muscles to lay eggs and maintain a small initial fungus garden. A colony can reach sexual maturity within 5–7 years, containing around 7 million workers and producing next generations of thousands of males and gynes during each mating season. Picture by the author

Full size image

As soon as a first generation of helpers emerges, queens become reproductive organs within a larger “superorganism .” They are then responsible to deliver large numbers of fertilized eggs to build and maintain a colony’s worker force. This requires queens of some species to be spectacularly fertile and able to lay hundreds to thousands of eggs per day. In honeybees, for example, queens can lay up to 2000 eggs per day, the equivalent of their own body weight (Maleszka 2008). Once colonies have reached their mature size, queens produce new generations of sexual offspring. In the hymenopteran social insects , queens control the fertilisation process and therefore, the number of males (unfertilized eggs) and queens or workers (fertilized eggs) produced (Heimpel and de Boer 2008). Reproductive conflicts arise between the queen and her workers over the sex ratio in sexual offspring (Ratnieks et al. 2006; Boomsma 1996; Tsuji 1996), and workers sometimes modify primary sex ratios in their own interest, for example, by eating queen- or male-destined eggs/larvae (Sundström 1994). In some species, workers also kill the queen and replace her with one of their sisters or start to lay their own, unfertilized eggs (Winston 1991; Alford 1975; Foster and Ratnieks 2000).

Social insect queens are therefore characterized by astonishing levels of lifetime fecundity. Contrary to other animals, reproduction and longevity are positively correlated in social insects (Heinze et al. 2013). Honeybee queens are only marginally larger than workers, but can live up to 8 years and produce around 1.7 million fertilized eggs (Baer 2005). In the fungus growing ant A. colombica , queens initially store up to 450 million sperm, allowing them to maintain colonies for decades and consisting of several million workers (Baer et al. 2006; Weber 1972). Army ants seem to hold the current world record, with queens of some species storing up to 1 billion sperm and fertilizing 250 million eggs (Kronauer 2009). Such continuous high levels of female fecundity are truly spectacular, but, as already mentioned, are all achieved during a single round of sperm acquisition early in the life of these animals.

In summary, reproductive queens are found in low frequencies in insect societies, but they are key individuals initiating new colonies and producing most if not all offspring. They evolved a number of spectacular adaptations to achieve astounding levels of fertility, and elaborations in reproductive traits as found between species are key determinants of their eusocial lifestyles.

17.4 Why Social Insect Queens Should Be Choosy

As already pointed out, queens of social ants, bees , and wasps preform only a single round of mate choice and sperm acquisition early in life and never remate once they have started to lay eggs (Boomsma et al. 2005; Baer 2011). In the majority of hymenopteran species, males die during or shortly after copulation and only survive as stored sperm inside their mate. As a consequence, sperm rather than egg number limits the size and longevity of their societies. Termites provide an exception because males survive alongside the queen as kings and continuously remate with them to replenish sperm supplies (Hartke and Baer 2011). However, as for the ants , bees, and wasps, termites are also closed genetic systems where no additional genetic contributions are typically accepted after an initial round of mate choice (Boomsma et al. 2005). Such “marriages for live,” combined with the observation that only a single or a few males sire offspring in many species (Boomsma 2009; Hughes et al. 2008), determine the genetic architecture of the colony before workers are produced. Single or highly skewed paternity distributions as found in many social insects are expected from kin selection , because they maximize relatedness among helpers and thereby increase the incentive of helping (Jaffe et al. 2012). Consequently, a queen’s decision with whom she mates, the number of mating partners she chooses to copulate with, and the amount of sperm she stores from each of her mating partners are of paramount importance defining the success of the later emerging society. There is indeed ample empirical evidence that these mating decisions of queens can have dramatic fitness consequences, see Table 17.1 for some examples. As inferior mating decisions such as inbreeding (Armitage et al. 2010) cannot be corrected later in life, queens are expected to be more than passive ejaculate recipients and extremely choosy in order to identify preferred or high-quality males and/or to discriminate against unwanted males or their ejaculates. Consequently, some of the spectacular reproductive characteristics found in some social insects are expected to represent evolutionary end points that evolved through continuous rounds of (cryptic) female choice.

Table 17.1 Life history and fitness consequences resulting from mate choice decisions of social insect queens
Full size table

As in other animals, female choice of social insect queens can occur both precopulatory as mate choice or postcopulatory as CFC . There is some empirical evidence for mate choice in social insects, for example, based on secondary sexual male traits (Izzo and Tibbetts 2012), or females resisting copulations or sometimes even killing males (Baer 2003). Overall, very few studies investigated precopulatory female choice, but typical indicators for the presence of female choice such as elaborate secondary sexual male traits seem mostly absent, but see (Izzo and Tibbetts 2012) for an exception. This lack of empirical work on precopulatory female choice is partially caused by experimental limitations, because mate choice and copulations in social insects are often difficult to observe in the field or in the laboratory (Baer 2003). However, precopulatory female choice could in fact be less important in species where queens participate in short nuptial flights and are exposed to various environmental risks such as predation/parasitism or adverse climatic conditions (Fig. 17.2) (Hölldobler and Wilson 1990). As a consequence, queens mate quickly and rather indiscriminately with males to collect ejaculates and perform mate choice postcopulatory during the sperm storage process in less dangerous environments. A precondition for postcopulatory CFC is polyandry, i.e., females mating with different males. Analyses of queen copulation frequencies show that polyandry is more widespread in social insects than indicated from paternity analyses in worker offspring (Baer 2011; Boomsma and Ratnieks 1996; Jaffe et al. 2012). This implies the presence of postcopulatory mechanisms that reduce and/or bias paternities (Jaffe et al. 2012). CFC of social insect queens could therefore provide the proximate mechanisms to explain the ultimate mismatch between observed queen mating frequencies and paternity. A first step to test this idea is to look for empirical evidence in the published literature.

Fig. 17.2
figure 2

The reproductive biology of honeybees has been intensively studied, partially because of commercial interests for breeding programs and the development of an artificial insemination technique. Honeybee queens perform one or very few nuptial flight(s) and mate in quick succession with a large number of males (drones). In A. mellifera, only a very small fraction of 3–5 % of sperm initially received by the queen will be stored in the spermatheca, a process lasting up to 40 h. Picture taken from the footage of the theatrical documentary “More than Honey,” for video footage see the movie trailer at http://www.ciber.science.uwa.edu.au/blog/?page_id=121

Full size image

17.5 Evidence for Cryptic Queen Choice

A search using Web of Science in March 2014 using “CFC ” and “social insect” as search parameters resulted in a list of only 12 papers, 7 of which I (co)author. This illustrates that CFC has basically not been investigated in social insects , despite its predicted impact on eusocial living. However, there are a number of studies available that investigated the reproductive biology of social insects and provided some evidence for the presence of cryptic queen choice, although these findings were not necessarily discussed in that context.

In his influential book, Eberhard (1996) listed a number of mechanisms of CFC and I selected a subset of those traits, which seemed relevant for social insects, together with supporting evidence found in the literature which is summarized in Table 17.2. Although this list is unlikely to be complete and some of these observations might be more convincing than others, it nevertheless provides very encouraging evidence to justify further research.

Table 17.2 A subset of mechanisms of cryptic female choice taken from Eberhard (Eberhard 1996) and empirical observations supporting their presence in social insects
Full size table

17.5.1 Morphologically Based Cryptic Female Choice

Eberhard (1996) pointed out that insect females are generally in control of sperm migration and transport within their bodies and that the relevant morphological structures facilitating these processes are also used for CFC . Based on our current knowledge, this could also be the case in social insects. The sexual organs of queens (and males) are often morphologically complex. They contain structures such as valves, sperm pumps, or narrow ducts for the movement of ejaculates , as well as organs for the temporal storage of sperm prior to transfer to the spermatheca. Although these structures can be expected to have evolved through natural selection to maximize sperm acquisition and storage efficiency, they also offer queens the possibility for CFC. For example, ejaculates are often not directly transferred to the spermatheca, which would be the most efficient mechanism to transfer male gametes to a female storage organ. Instead, they are initially received and temporarily stored in other parts of the queen’s reproductive tract, for example, in the bursa copulatrix or the lateral oviducts (Baer 2003, 2005, 2011). Honeybee queens can actively close both the sting chamber and the bursa copulatrix (Baer 2005; Dade 1962), and they need to actively contract their bursa after having received an individual ejaculate in order to transfer sperm into their lateral oviducts (Koeniger and Koeniger 1991). These specialized reproductive organs could therefore enable honeybee queens to reject entire ejaculates or parts thereof and offer opportunities to study CFC in more detail in the future. A similar mechanism seems to be present in leaf-cutter ants, where males have no physical access to the female’s sexual tract and queens can close the entrance to their sexual organs with a muscle (Baer and den Boer unpublished data).

In species where queens receive more sperm form their mate(s) than required to fill the spermatheca, excess sperm is dumped (Robertson 1995; Baer 2005; Woyke 1983). The process of sperm storage in the Apis mellifera occurs over a period of 40 h. During this process, ejaculates are moved back from the lateral oviducts into the bursa copulatrix through muscular contractions, and some sperm is transferred to the spermatheca. However, more than 95 % of the sperm initially received is expelled, and observed mating frequencies are substantially higher than the number of fathers found in offspring (Baer 2005). The spermathecal duct of honeybee queens is a narrow tube surrounded by muscular tissue (Bresslau 1905; Snodgrass 1984), which provides queens with control over the amount of sperm passing through. Furthermore, a morphological structure present between the spermathecal duct and the spermatheca, known as Bresslau’s sperm pump (Bresslau 1905), is believed to control access of sperm into and out of the spermatheca. However, apart from a detailed description more than 100 years ago, we still lack experimental work to understand its relevance for CFC . Our present knowledge about the mating biology of honeybee queens indicates that they might be able to manipulate ejaculates in multiple ways and during every stage of the mating process: (1) while receiving them, (2) while transporting them to the lateral oviducts, (3) during the storage process, or (4) during fertilization. This suggests that observed paternities are the result of a complex interplay between different mechanisms, which each represents a different level of female choice.

In bumblebees, males transfer an ejaculate consisting of sperm, seminal fluid, and a mating plug into the female’s bursa copulatrix (Duvoisin et al. 1999; Brown and Baer 2005). Sperm is placed at the entrance of the spermathecal duct (Duvoisin et al. 1999) from where it is transferred to the spermatheca. Bumblebee queens also possess a long and narrow spermathecal duct, so the process of storing the sperm into the spermatheca takes up considerably longer than copulation itself (Duvoisin et al. 1999). If bumblebee queens accept additional matings, the first ejaculate is pushed further up into the lateral oviducts (Sauter et al. 2001). Consequently, a queen’s decision to delay remating might influence a first male’s contribution to stored sperm. Interestingly, bumblebee queens seem to possess the necessary morphological structures to store displaced ejaculates. Using morphological structures to delay sperm storage seems also present in the ant Leptothorax gredleri , where the spermathecal duct of the queen is so narrow that sperm have to pass one by one into the spermatheca (Oppelt and Heinze 2007), which takes several hours after mating. Interestingly, only a single male is found to sire worker offspring in this species despite queens mating with up to four males. The same is found in the Argentine ant Linepithema humile , where queens mate with multiple males, but only a single male sires offspring (Keller and Passera 1992). These examples illustrate that paternity distributions in offspring are not reliable predictors of queen mating frequencies (Baer 2011), and CFC could explain mismatches between observed and detected numbers of copulations.

Queens of ants , bees , and wasps can determine the sex of their offspring, because they control whether an egg is fertilized or not. Spermathecal ducts or sperm pumps/valves are expected to control this process, which can already be defined as a form of CFC during egg fertilization. Because males gain direct fitness only by siring queen offspring, reproductive conflicts emerge between the queen and her mate(s) over the sex ratio in sexual offspring (Boomsma 1996). Males prefer a highly queen-based sex ratio, whereas queens prefer an equal investment into reproductive offspring. Sex rations vary greatly between social insect species, indicating that such reproductive conflicts are resolved differently depending on the species and it’s mating system. Furthermore, queens can produce virgin queens asexually in some species and thereby manipulate male fitness, for example, in the ants Cataglyphis cursor (Doums et al. 2013) or Platythyrea punctata (Kellner and Heinze 2011). Similarly, in the little fire ant, queens use sperm to produce workers only, but virgin queens develop without any genetic contributions of males (Fournier et al. 2005). Although these examples might not be seen as classical cases of CFC , the conflict between the queen and her mate(s) results in manipulations of a male’s reproductive success. The power to determine the caste of offspring in the absence of males benefits the queens but reduces male fitness. It would therefore be interesting to investigate whether paternity contributions differ in sexual offspring compared to worker offspring, i.e., whether some fathers are more likely to sire queens than others and whether queens can control paternal representation in their sexual offspring. Very little empirical work has been conducted so far to quantify this. In honeybees, queens are reared from rare “royal” subfamilies (Moritz et al. 2005), indicating that some fathers are more likely to sire virgin queens than others. However, because honeybee workers can influence the fate of a fertilized egg, further research is needed to understand the influence of queens versus workers over caste fate. In general, future work is needed to quantify whether paternity contributions differ between worker and sexual offspring and whether workers, which carry paternal genes, manipulate caste determination and paternity in their fathers’ interest.

In summary, there is good evidence that social insect queens are able to manipulate the process of sperm storage using multiple morphological structures within their sexual tracts. Additionally, storing and using sperm can take up considerable time (Oppelt and Heinze 2007; Reichardt and Wheeler 1996; Woyke 1983; Duvoisin et al. 1999) providing queens with the necessary time window to perform CFC .

17.5.2 Molecular Based Cryptic Female Choice

Apart from morphological structures, queens could also use molecules present in various glandular secretions to bias paternity. Queens have a number of glands associated with their sexual tract (Snodgrass 1984; Janet 1904), but very little is known about these secretions or their influence on ejaculates or paternity. As sperm becomes increasingly dependent on the queen’s support, they can be compared to endosymbionts (Baer et al. 2009). A queen’s power over sperm fate could also be used for CFC, if the amount or compositions of these secretions are modified. The spermathecal gland secretions of honeybee queens are provided to store sperm (Klenk et al. 2004) and are biochemically complex (Baer et al. 2009). They contain proteins that are very efficient at keeping sperm alive (den Boer et al. 2009), but seem to have a variety of additional functions, some of which could be linked to CFC , such as for examples proteins with cytotoxic and signaling functions or chaperons (Baer et al. 2009). The protein composition of spermathecal fluid changes substantially once sperm has become stored, indicating that queens interact differentially with newly arriving compared to stored sperm (Baer et al. 2009). Interestingly, sperm respond to these changes in their host environment as well, as indicated by substantial proteomic differences between stored and ejaculated sperm (Poland et al. 2011).

In highly polyandrous species, ejaculates of multiple males co-occur in the queen’s sexual tract resulting in sperm competition. This is the case in honeybees such as A. mellifera as well as in several leaf-cutter ants such as A. colombica and A. echinatior. In these three species, sperm competition occurs in the form of sperm incapacitation , where seminal fluid proteins kill sperm of rival males (den Boer et al. 2010). However, this is not necessarily in the interest of the queen, especially if insufficient numbers or damaged sperm become stored and compromise her fecundity. As expected, A. colombica queens use secretions from their spermathecal glands to neutralize sperm incapacitation and proteins are known to be the molecules responsible for this effect (unpublished data). As the queen controls the release of secretions from her glands into the spermatheca, she can influence sperm competition and thereby manipulate paternity of males. A. colombica therefore provides another intriguing example that paternity in social insects seems determined by multiple traits, which evolved under postcopulatory sexual selection.

As the number of sequenced social insect genomes is substantially growing, research can now take full advantage of state-of-the-art–omics technologies that become increasingly united as part of systems biology. These techniques allow the detection of a large numbers of molecules as well as their abundance in samples of interest. Furthermore, bioinformatics can assign detected molecules to biochemical networks, offering detailed insights into their biological functions on the phenotypic level. These techniques are therefore highly promising tools for future research to study reproductive traits such as CFC , which were so far challenging to address because they occur within the sexual tract of an individual on a very small scale.

17.5.3 Ultimate Consequences of Cryptic Queen Choice

Postcopulatory female manipulations of paternity will finally determine the frequency of fathers in offspring (Eberhard 1996). The relative contributions of different fathers to offspring (paternity skew) can vary quite substantially in social insects. In polyandrous ants , bees , and wasps where queens mate only with one or two males, paternities are normally highly biased towards one male (Jaffe et al. 2012). Inclusive fitness of helpers is therefore maximized in these species, as predicted from kin selection theory. However, if paternity skew is high in social insects, species currently described as monandrous based on molecular paternity analyses might still be polyandrous. Queen multiple mating might therefore be even more common than acknowledged so far, and CFC could provide the necessary morphological or molecular mechanisms to reduce the number of mates down to a single father. Obviously, polyandrous queens with single paternity such as the previously mentioned ants C. cursor (Doums et al. 2013) and P. punctata (Kellner and Heinze 2011) would be primary target species for future research to unravel the mechanisms by which single fathers are determined.

In species where queens mate with a large number of males, paternity skew becomes increasingly equalized (Jaffe et al. 2012). This seems driven by a number of well-documented fitness benefits gained from increased genetic diversity among helpers (see Table 17.1, Baer and Schmid Hempel 1999; Tarpy 2003; Hughes and Boomsma 2005), which are specifically important in large and long-lived insect societies. Obviously, manipulations of ejaculates that bias or equalize paternity skew is in the interest of queens, as her choice of the number of fathers and their individual contributions define both the level of conflicts in her worker offspring, as well as the potential benefits gained through genetic diversity. Social insects might therefore represent a group of insects where selection on female choice might in fact be stronger than male–male competition , an idea that should certainly be investigated in the future. If true, the most successful societies on earth would be characterized by the presence of an astonishing dominance of female power, both over the reproductive process and during the phase of later social living.

17.6 Conclusions

CFC is admittedly poorly investigated in social insects, because research conducted so far was mostly guided by questions derived from kin selection theory. However, as the study of social insect reproduction received increasing scientific attention over recent years, new findings also provided first evidence for the presence of sexual selection in these species. Theoretical considerations predict that social insect queens are choosy and manipulate paternities in their own interest because they need to store large numbers of high-quality sperm that can only be acquired once in a lifetime. Empirical data support this idea, because social insect queens possess morphological structures as well as secreted molecules that both seem involved in CFC . Cryptic queen choice is not only expected from theoretical considerations, but could also provide an explanation for the observed mismatch between queen mating frequencies and the numbers of fathers and their relative abundance in offspring. Consequently, social insects offer exciting opportunities to study the interplay between sexual and kin selection, especially since methodological and technological progress offers rather spectacular opportunities for future experimental work.