the supercolonies formed by so - called ' unicolonial ' ant species are huge cooperative groups  networks of interconnected nests that exchange individuals and share territory peacefully over extensive areas . in extreme cases ,
a whole population can comprise a single supercolony ; for example , the largest supercolony discovered , the mediterranean argentine ant supercolony , covers 6,000 kilometers of southern european coastline , and individuals accept each other as colonymates all the way from italy to the spanish atlantic coast .
a feature of supercolonies compared with the more usual family - based colonies of social insects is the free movement of individuals between nests , and the fact that each nest contains several queens .
consequently , the relatedness between nestmates approaches zero in the argentine ant and many other species , both invasive and non - invasive .
in other words , individuals that share a nest are no more similar genetically than would be individuals chosen at random from the whole population .
the evolution and persistence of unicoloniality poses a dilemma for the widely accepted theory of kin selection as an explanation for social evolution . in all colonial insects , including the unicolonial ants , the success of the colony depends on so - called ' altruistic ' behavior of non - reproductive individuals  the workers  who build the nest , collect food for the sexually reproducing queen and care for the larvae ( brood rearing ) without any apparent benefit to themselves in terms of maximizing their own reproduction , their ' fitness ' .
kin selection theory proposes that such altruism can evolve and persist if the altruists , such as the ant workers , direct help only to individuals that are genetically related to them . in family - based social insect colonies , which usually have a single queen per colony ,
so , even though the workers do not reproduce , their cooperative altruistic behavior increases the chance that genes identical to their own will be transmitted to the next generation ; in other words , altruism increases the ' inclusive fitness ' of the individual worker  the overall reproductive fitness of itself and its close relatives .
in such situations , production of both queens and workers , in addition to males , is in the interest of the workers , queens , and the developing larvae . by this logic , altruism should not persist in a supercolony , where the receivers of help  the nestmate queens  may be no more closely related to the workers in that nest than they are to the rest of the supercolony .
one might expect a complete breakdown of co - operation in such circumstances , with individuals behaving antagonistically towards non - relatives , and natural selection no longer favoring worker traits .
furthermore , in the absence of genetic relatedness , ' selfish ' queens could maximize their fitness by producing larvae that develop into females and males rather than workers  in which case the supercolony , and the trait of unicoloniality , would eventually die out .
the paradox has been investigated by laurent keller and colleagues in a series of papers on native european wood ants of the genus formica [ 3 - 6 ] . in their most recent paper , published in bmc evolutionary biology ( holzer et al . ) , keller and colleagues have looked more closely at the genetic structure of supercolonies of the wood ant formica paralugubris . they find that supercolonies that appear to behave as single units in fact harbor genetic substructure and consist of several extended families living side by side .
such structuring may help to explain the evolutionary maintenance of unicoloniality , while the peaceful coexistence of potentially competing families can also teach us lessons about behavioral adaptations .
even if supercolonies are problematic for kin selection theory when they have grown to cover whole populations , the behavioral rules that underlie their development can be understood through inclusive fitness principles .
a super - colony starts from a family group that extends its network of nests and retains more and more queens ( figure 1 ) , competing against other such colonies .
this is often a successful strategy for monopolizing resources over a large area , and can be favored by kin selection .
but when this successful family - based strategy continues for a long time and the colony grows larger and larger , it paradoxically dilutes the relatedness between individuals and eventually leads to a situation where individual workers may no longer be helping kin .
worker behavior is no longer favored by selection , but because workers lack the means to assess the decline in relatedness they continue to behave according to the previously successful rules .
formation of supercolonies with extremely low relatedness is especially easy for foreign invasive species , whose colonies grow unhindered by conspecific competitors and native parasites , and can extend over hundreds of square kilometers .
but in evolutionary terms , supercolonies are thought to be evolutionary dead - ends as the link between genetic relatedness and behavior has been lost .
relatedness between individuals is high as a result of the small number of founder queens .
but even if relatedness between individuals who do not behave aggressively to each other drops to zero , relatedness to individuals who benefit from worker altruism remains above zero .
factors that increase relatedness between individuals in altruistic interactions include : limited dispersal of sexuals , context - specific discrimination and variation in kin structures over worker life - time .
in contrast to the argentine ant , supercolonies of indigenous european ants , such as certain species of formica wood ants , are found in stable habitats such as boreal or alpine forests and are non - invasive .
but like their invasive counterparts , these supercolonies cover large areas , and workers from distant parts of the supercolony behave unaggressively , like nestmates , towards each other .
holzer et al .   made a detailed genetic and behavioral study of three f. paralugubris supercolonies in the swiss jura mountains , which revealed that the supercolonies are not as genetically heterogeneous as expected , but consist of cryptic genetically related clusters of nests .
these clusters define groups of individuals that are genetically similar at a level comparable to cousins , with the proportion of genes shared approximating to 0.125 .
holzer et al .   suggest that such family substructures might enable workers to direct help to their relatives , and that this might help to maintain the trait of unicoloniality , in line with kin selection theory .
however , the presence of genetically differentiated subunits raises the question of why there is no aggression within the supercolony , and why the clusters keep exchanging individuals . in theory
in other supercolony - forming species studied , within the supercolony aggression seems to be minimized by workers being unable to recognize whether an ant is close kin or not , but f. paralugubris workers behave unaggressively in artificial bioassay encounters even though they do seem able to recognize non - nestmates .
in contrast to most supercolonial ants , aggression is rare in f. paralugubris , even towards individuals from other super - colonies .
the workers seem to treat most conspecifics amicably , even if recognized as ' outsiders ' .
does this mean that the potential inclusive fitness payoffs from the family structuring are not in fact realized ?
whether lack of discrimination is adaptive depends on the inclusive fitness costs of accepting strangers , compared with the costs of aggression and of erroneously behaving aggressively to group members ( figure 2 ) . in f. paralugubris , it seems that workers are prone to making mistakes in recognition , probably because the large number of queens in each family cluster increases genetic diversity within , and decreases differentiation between , the clusters .
this is likely in f. paralugubris , because the population structure found by holzer et al .  
of particular importance in this regard , keller and colleagues previously found that the dispersal of queens in f. paralugubris is limited .
this would help to guarantee that brood care by the young workers is directed towards relatives , even if the workers later disperse .
workers in f. paralugubris increase their inclusive fitness not by territorial aggression , but by rearing related queens and males that disperse and compete with queens and males from less related nests in other clusters over recruitment as a breeder .
holzer et al .   found that especially males are prone to disperse and spread genes across the clusters .
in other words , relatedness will be above zero in the brood - rearing context , which is most important for inclusive fitness , even where territorial aggression is lacking ( figure 1 ) .
the current study by holzer et al .   confirms previous findings by keller and colleagues that family structures are more obvious in queens and the brood than in old workers in another supercolonial formica ant .
this suggests that young workers take care of related broods , even if they drift away from their relatives later in their lives .
whether similar patterns of genetic structure and behavior also occur in invasive supercolonies remains to be seen .
it is also worth noting that acceptance of non - kin in the context of artificial aggression bioassays does not necessarily mean that they are always accepted in natural contexts more directly connected to inclusive fitness .
favoring individuals that are likely to be relatives is adaptive only in contexts where the inclusive fitness benefits of discrimination are large .
if competition for breeding places occurs mainly by the recruitment of sexuals into the breeding population , workers should be most prone to discriminate in contexts more directly linked to their inclusive fitness , such as behavior towards queens and males , and brood care .
thus , worker ants in formica supercolonies might be able to direct the benefits of brood care mainly to the ' right ' individuals as dictated by kin selection theory .
the other side of the non - aggression coin is whether it is harmful to the rearing of related reproductives to allow strangers into the territory .
as long as this does not harm the production of reproductives in your own nest , the risk of aggression is perhaps not worth taking .
this might be the case when resources are abundant and the benefits of territorial defense are subsequently small .
wood ants tend aphids , a resource that thrives when ants are abundant , so resource limitation may not be too severe in their case
. it will be very interesting to see whether the putative family clusters in wood ants also correspond to foraging areas and the flow of resources , and whether ants are more prone to discrimination at food sources , especially when resources are scarce .
study of nutrient flow within a supercolony of the argentine ant has also shown that a seemingly uniform supercolony might actually consist of separate nest networks across which food resources are not shared .
ideally , relatedness should be measured at the level of such functional units , in both native and invasive supercolonies .
the problem of indiscriminate altruism demonstrates how complicated assessing the adaptive benefits of a behavioral strategy can be .
in particular , we need to consider all the contexts in which a behavioral rule is applied .
the ability to readily recognize and respond to environmental information is an important constraint on adaptation , and organisms are not always able to adjust their behavior to make an optimal response to a given situation .
instead , they might need to rely on rules that work on average . in the case of ant workers in a supercolony
, the payoffs of behavioral rules need to be assessed across all the contexts in which they are used , over seasons , over the lifetime of the workers , but also over the lifetime of the supercolony . there may be cases where workers in a supercolony seem to be in a dilemma .
they are obeying the behavioral rules that made their family big and strong , but this has brought them to a situation where the rules no longer pay off .
but this is only a dilemma for evolutionary theory if we assume that adaptation must always be perfect .
even if the rules seem maladaptive in some cases , the evolutionary benefits may have already been reaped in another .
i thank joan strassmann , dave queller , jes s pedersen and tobias uller for inspiring discussions on unicoloniality , and tobias uller for comments on the manuscript as well .
 vast supercolonies of interconnected nests formed by unicolonial ant species are the largest cooperative groups of animals known . 
 research published recently in bmc evolutionary biology reveals that a supercolony can be more genetically structured than previously thought , comprising several extended families . 
 surprisingly , the families coexist peacefully , even though they seem to recognize each other as non - kin . 
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Ant supercolonies, a dilemma for social evolutionary theory
Are supercolonial wood ants indiscriminate cooperators?
Does lack of discrimination pay off?
Is lack of discrimination more apparent than real?
Is there really a dilemma?
Acknowledgements