BACKGROUND
highly social insect groups have had enormous ecological success  <cit> , yet eusociality has evolved very infrequently  <cit> , raising the question of what barriers there might be to its origin.
furthermore, soldier castes have been lost in some thrips  <cit>  and aphids  <cit> , resulting in the loss of eusocial nesting strategies.
in halictine bees there have been three origins of sociality but as many as twelve losses, suggesting that in an evolutionary sense complex sociality may be difficult to gain, but easy to lose  <cit> .
in contrast, there have been no losses of sociality in the ants, termites, vespid wasps and corbiculate bees, all of which evolved sociality >  <dig> mya  <cit> .
in both thrips and halictid bees, sociality has evolved much more recently than the cretaceous origins of sociality in ants, termites, corbiculate bees and vespid wasps.
mcleish et al.
<cit>  showed that sociality in gall forming thrips evolved less than  <dig> mya, and brady et al.
<cit>  showed that the three origins of sociality in halictines are also relatively recent .
this raises the question of whether losses of sociality are more likely in lineages where sociality is relatively recent, compared to older social lineages that may have reached a 'point of no return'.
the notion of a 'point of no return' was first suggested in the early 1970s by wilson  <cit>  and proposes that, given suitable evolutionary time, the multiple and integrated adaptations associated with highly complex social behavior may preclude reversions to less complex or non-social life cycles.
wilson and hölldobler  refer to this evolutionary point as one where it is either "impossible, or at least difficult and uncommon", for a eusocial species to revert back to a more primitively social or non-social form of organization, and speculate that it coincides with the evolution of an anatomically distinct worker caste.
the conjecture is important because it proposes a degree of irreversibility in social evolution due to integration among adaptations in multiple traits, a situation akin to the idea of phylogenetic inertia arising from bauplan constraints  <cit> .
importantly, the 'point of no return' hypothesis seems to be the only one proposed for the lack of reversions to solitary life-cycles in the major social insect groups and as such is an almost default paradigm.
this is surprising, given the amount of debate on other aspects of social evolution.
yet the point of no return hypothesis lacks a clearly stated underlying mechanism for why reversions are precluded and, as such, is not truly falsifiable.
nevertheless it is open to indirect assessments: in particular, demonstrating that very long term maintenance of sociality does not require social complexity or a distinct worker caste would indicate the possibility of alternative explanations for long term maintenance of sociality.
indirectly assessing whether points of no return depend on social complexity and distinct worker castes can be achieved by examining patterns of origins and losses in taxa where these two traits are absent or variable.
brady et al.
's  <cit>  study of origins and losses in halictine bees provides one such assessment.
halictine bees provide special insights into social evolution because, unlike most other eusocial groups, adult females in all species are totipotent and capable of producing brood  <cit> .
thus, individual females are not constrained to group nesting, so that evolution is able to 'explore' non-social options.
only one other group of bees, tribe allodapini , is speciose, exhibits diverse range in forms of sociality and, in virtually all species, all females are totipotent  <cit> .
until recently it was thought that sociality in allodapines had arisen comparatively recently among the extant lineages  <cit>  from a subsocial ancestor , and this ancestral trait had been retained in a phylogenetically basal african allodapine, halterapis nigrinervis  <cit> .
at the same time, nesting biology of the speciose halterapis group from madagascar was unknown.
recent molecular analyses  <cit>  show that the african halterapis is not basally situated in the allodapine phylogeny, and that the african and malagasy members of this genus are in fact paraphyletic, implying a need for future taxonomic revision.
using both bayesian and penalized likelihood approaches, schwarz et al.
<cit>  indicated an origin of the allodapine tribe as being > <dig> mya but pointed out that this is likely to be a highly conservative estimate.
recent behavioral studies also show that the malagasy halterapis display complex social behavior  <cit> .
these findings indicate that sociality is plesiomorphic for the allodapines, and despite the relatively ancient origin of the tribe, the african halterapis is the only allodapine to have potentially lost complex social behavior.
RESULTS
the initial study of h. nigrinervis  <cit>  was based on a sample size of only eleven nests and multifemale colonies comprised a single inseminated female along with one to several uninseminated females that were presumed to be recently eclosed adults and soon to disperse.
our sample comprised a total of  <dig> h. nigrinervis nests.
seven nests were stored in ethanol and the remaining  <dig> in kahle's solution for dissection.
these samples  indicate that h. nigrinervis is indeed social because:  approximately half  of the nests collected were multifemale;  multifemale nests were far more likely to contain brood  and the number of young brood  increased with the number of adult females, indicating enhanced brood production when multiple adult females are present within a nest ;  sex ratios were strongly female-biased, consistent with positive kin interactions leading to local resource enhancement  <cit> ; and  ovary sizes of nestmates were strongly influenced by intra-colony body-size rank  indicating the existence of reproductive hierarchies.
these results mean that h. nigrinervis is social and that there are consequently now no known losses of sociality in allodapines.
consensus phylogenies from three partitioned bayesian analyses of sequence data all had identical topologies and almost identical branch lengths.
the consensus cladogram with posterior support values is given in figure a <dig> .
ingroup generic-level bifurcations were highly supported and consistent with other sequence-based studies of allodapines  <cit> .
we also analysed sequence data with a maximum parsimony approach and that gave broadly consistent results to our bayesian analyses and previous phylogenetic studies  <cit>  .
the only inconsistencies between our bayesian, our mp results, and those of previous studies, involve nodes close to terminal taxa.
we used penalized likelihood transformation of the bayesian phylograms to produce a chronogram , which also indicates the geographic distribution of the major clades.
when the basal node  is conservatively set at  <dig> mybp, the point estimate for the earliest divergence of allodapine clades is  <dig> mybp, with a lower  95% limit of  <dig> mybp.
because sociality occurs in all the extant allodapine lineages, it must be a plesiomorphic trait and must therefore have originated by at least  <dig> mybp, and probably much earlier given the very conservative nature of all three calibration points.
this age-estimate is similar to that of schwarz et al.
<cit>  and suggests that the age of the allodapine root node is approximately twice the ages for estimated origins of sociality in halictine bees  <cit> .
as an independent assessment of approximate divergence ages, we compared pairwise maximum likelihood distances for allodapine species whose most recent common ancestor  was at the root allodapine node with those for halictine species whose mrca was at one of the three origins of sociality in halictines  <cit> , which were dated at 20– <dig> mya  <cit> , using a sequenced fragment of ef-1α common to both groups.
substitutional parameters for this gene fragment are almost identical for halictines and allodapines  indicating that evolutionary dynamics for this gene fragment are very similar for the two bee groups.
if evolutionary rates are also comparable in the two groups, the resulting distances suggest that the root node of the allodapines is about twice the age of the origins of sociality suggested for the halictines , which concords very closely with our penalised likelihood analysis.
discussion
our results are important because they suggest that, compared to halictine bees, allodapines have a much older origin of sociality but show no losses of sociality.
in this respect, they are more similar to eusocial lineages such as ants, corbiculate bees, vespid wasps and termites, which also have ancient social origins with no losses.
this raises the question of what factors may prevent losses of sociality over very large time scales.
lack of reversions to solitary living in allodapines cannot be explained by arguments that they are restricted to habitats or climatic regimes that favour sociality.
the ecological diversification of allodapines covers habitats such arid gibber deserts, savannas, bushvelds, coastal heathlands, equatorial and subtropical rainforests, and sub-alpine regions  <cit> .
colony phenology ranges from highly seasonal univoltine egg production schedules  <cit> , all the way through to asynchronous development patterns where egg production, brood maturation, and foundress dispersal occur year round  <cit> .
the lack of reversions to purely solitary living is even more notable because of the near-ubiquity of female totipotency in allodapines  <cit> , which means that physiological factors do not preclude independent living.
we argue that the absence of reversions to solitary living is due to ecological consequences of the way in which allodapines rear their brood, and this may also help explain some broader trends found in ants, termites and vespid wasps.
unlike virtually all other bees, which rear their brood in fully provisioned sealed cells, allodapines rear brood in un-partitioned and unsealed tunnels.
because brood lack the physical protection of an enclosed cell, they are highly vulnerable to predation in the absence of an adult guard.
indeed, the major benefit of group living in allodapines is avoidance of total brood failure  <cit> , and this benefit is greatest when comparing single- with two-female colonies, because brood in the former colonies are unprotected when the sole adult is foraging  <cit> .
this vulnerability of brood is heightened by orphaning, since not only will post-feeding brood be unprotected, but feeding-stage larvae will be unable to complete their development due to lack of food.
in the event of orphaning, potential alloparents could reap large indirect fitness gains by simply protecting post-feeding brood or completing the feeding of partially reared larvae  <cit> , as well as direct benefits from inheriting a nest along with a subsequent cohort of potential alloparents.
in fact, alloparental rearing in the absence of possible mothers is recorded from diverse species  <cit>  indicating that it does not require maternal coercion.
at the same time, mothers would gain benefits from permitting some daughters to remain in the nest as insurance against orphaning  <cit> .
in contrast, the vulnerability of brood and the benefits of alloparental care in cell-provisioning insects are quite different.
mass provisioning of brood in cells means that mothers sequester their investments as completed units over time, and these cells provide brood with some physical protection from enemies at the nest.
in addition, if orphaning does occur, immatures sealed in cells will have sufficient food for their complete development, further reducing the scope for benefits from alloparental care.
whilst protecting the nest from both predation and parasitism is important for all bees  <cit> , allodapine brood are particularly vulnerable due to the lack of cells  <cit> .
continuous protection from predation is impossible for a solitary nesting allodapine, as the nest is completely unguarded during the female's foraging trips.
we suggest this vulnerability is a compelling explanation for why there have been no reversions to purely solitary living in allodapines but multiple losses in halictines.
it has been argued that the lack of reversions to solitary nesting in ants and termites may reflect the evolution of social complexity to a 'point of no return'  <cit> , where a species is no longer able to live solitarily.
however, there are numerous ant groups where newly founded colonies involve non-claustral queens rearing their first brood cohort to maturity without help from workers  <cit> , suggesting that competency for solitary brood rearing per se exists in many taxa.
for example, within many ant subfamilies , there are species where colony foundation almost exclusively involves a single brood-rearing foundress and very small ultimate colony sizes  <cit> .
queens must forage in order to feed their first clutch of brood, indicating that solitary-founding females in these species are capable of foraging effectively enough to rear through their first generation of brood alone  <cit> .
many of the aforementioned subfamilies also contain multiple independent losses of the queen caste, which has resulted in numerous species displaying ubiquitous female totipotency  <cit>  but without any transitions to purely solitary living.
in social taxa where females can successfully rear brood to maturity in a non-claustral fashion without help of a worker caste, explanations for a lack of reversions to solitary living must involve something other than incompetence for independent brood rearing.
ants share one key life history trait with allodapines, namely that brood are progressively reared in unsealed communal tunnels.
in social vespid wasps, larvae are also reared to pupation in unsealed cells, and while termite young are not 'provisioned' by adults, they also develop in unsealed chambers and are highly vulnerable in the absence of adults.
in all these taxa, protection of brood depends heavily on adults, and this contrasts with halictines where nearly all species sequester their fully provisioned brood in closed cells.
losses of sociality would remove the brood protection that group living confers in the former groups, but the physical protection of cells in halictines would allow some protection of brood in solitary nesting halictines to be maintained.
lastly, we have argued that the 'point of no return' paradigm has been framed in a way that does not readily allow falsification.
we believe that our conjecture provides two predictions that permit empirical assessment:  that reversions from social to solitary brood rearing are more likely in clades where constraints to solitary living are low; and  that reversions from social to solitary can occur even after specialised worker/soldier castes have evolved, provided that ecological constraints for independent reproduction are relaxed.
halictine bees and gall-forming thrips comprise two groups where these predictions could be tested, but there are likely to be many other taxa as well.
CONCLUSIONS
our findings suggest a very different framework for understanding social evolution from that argued in some recent studies  <cit>  that emphasize the importance of mechanistic approaches involving physiology, regulatory circuits and genetic-networks.
in particular, hunt and amdam  <cit>  suggest "...that social evolution in insects can be fully – and finally – understood" by such mechanistic approaches.
although such approaches may help explain the ontogeny of worker-like behaviour, our results indicate that retention of such behaviour is likely to be due to ecological and life-history factors, and these have the potential to determine very long term patterns of social evolution.
the ecological dimensions of sociality cannot be ignored when trying to understand its origins and long-term maintenance.

 highly social insect groups have had enormous ecological success, yet eusociality has evolved very infrequently, raising the question of what barriers there might be to its origin. furthermore, soldier castes have been lost in some thrips and aphids, resulting in the loss of eusocial nesting strategies. in halictine bees there have been three origins of sociality but as many as twelve losses, suggesting that in an evolutionary sense complex sociality may be difficult to gain, but easy to lose. in contrast, there have been no losses of sociality in the ants, termites, vespid wasps and corbiculate bees, all of which evolved sociality > mya. in both thrips and halictid bees, sociality has evolved much more recently than the cretaceous origins of sociality in ants, termites, corbiculate bees and vespid wasps. mcleish et al. showed that sociality in gall forming thrips evolved less than mya, and brady et al. showed that the three origins of sociality in halictines are also relatively recent. this raises the question of whether losses of sociality are more likely in lineages where sociality is relatively recent, compared to older social lineages that may have reached a'point of no return'. the notion of a'point of no return' was first suggested in the early 1970s by wilson and proposes that, given suitable evolutionary time, the multiple and integrated adaptations associated with highly complex social behavior may preclude reversions to less complex or non-social life cycles. wilson and hölldobler refer to this evolutionary point as one where it is either "impossible, or at least difficult and uncommon", for a eusocial species to revert back to a more primitively social or non-social form of organization, and speculate that it coincides with the evolution of an anatomically distinct worker caste. the conjecture is important because it proposes a degree of irreversibility in social evolution due to integration among adaptations in multiple traits, a situation akin to the idea of phylogenetic inertia arising from bauplan constraints. importantly, the'point of no return' hypothesis seems to be the only one proposed for the lack of reversions to solitary life-cycles in the major social insect groups and as such is an almost default paradigm. this is surprising, given the amount of debate on other aspects of social evolution. yet the point of no return hypothesis lacks a clearly stated underlying mechanism for why reversions are precluded and, as such, is not truly falsifiable. nevertheless it is open to indirect assessments: in particular, demonstrating that very long term maintenance of sociality does not require social complexity or a distinct worker caste would indicate the possibility of alternative explanations for long term maintenance of sociality. indirectly assessing whether points of no return depend on social complexity and distinct worker castes can be achieved by examining patterns of origins and losses in taxa where these two traits are absent or variable. brady et al.'s study of origins and losses in halictine bees provides one such assessment. halictine bees provide special insights into social evolution because, unlike most other eusocial groups, adult females in all species are totipotent and capable of producing brood. thus, individual females are not constrained to group nesting, so that evolution is able to'explore' non-social options. only one other group of bees, tribe allodapini, is speciose, exhibits diverse range in forms of sociality and, in virtually all species, all females are totipotent. until recently it was thought that sociality in allodapines had arisen comparatively recently among the extant lineages from a subsocial ancestor, and this ancestral trait had been retained in a phylogenetically basal african allodapine, halterapis nigrinervis. at the same time, nesting biology of the speciose halterapis group from madagascar was unknown. recent molecular analyses show that the african halterapis is not basally situated in the allodapine phylogeny, and that the african and malagasy members of this genus are in fact paraphyletic, implying a need for future taxonomic revision. using both bayesian and penalized likelihood approaches, schwarz et al. indicated an origin of the allodapine tribe as being > mya but pointed out that this is likely to be a highly conservative estimate. recent behavioral studies also show that the malagasy halterapis display complex social behavior. these findings indicate that sociality is plesiomorphic for the allodapines, and despite the relatively ancient origin of the tribe, the african halterapis is the only allodapine to have potentially lost complex social behavior.  the initial study of h. nigrinervis was based on a sample size of only eleven nests and multifemale colonies comprised a single inseminated female along with one to several uninseminated females that were presumed to be recently eclosed adults and soon to disperse. our sample comprised a total of h. nigrinervis nests. seven nests were stored in ethanol and the remaining in kahle's solution for dissection. these samples indicate that h. nigrinervis is indeed social because: approximately half of the nests collected were multifemale; multifemale nests were far more likely to contain brood and the number of young brood increased with the number of adult females, indicating enhanced brood production when multiple adult females are present within a nest; sex ratios were strongly female-biased, consistent with positive kin interactions leading to local resource enhancement; and ovary sizes of nestmates were strongly influenced by intra-colony body-size rank indicating the existence of reproductive hierarchies. these results mean that h. nigrinervis is social and that there are consequently now no known losses of sociality in allodapines. consensus phylogenies from three partitioned bayesian analyses of sequence data all had identical topologies and almost identical branch lengths. the consensus cladogram with posterior support values is given in figure a. ingroup generic-level bifurcations were highly supported and consistent with other sequence-based studies of allodapines. we also analysed sequence data with a maximum parsimony approach and that gave broadly consistent results to our bayesian analyses and previous phylogenetic studies. the only inconsistencies between our bayesian, our mp results, and those of previous studies, involve nodes close to terminal taxa. we used penalized likelihood transformation of the bayesian phylograms to produce a chronogram, which also indicates the geographic distribution of the major clades. when the basal node is conservatively set at mybp, the point estimate for the earliest divergence of allodapine clades is mybp, with a lower 95% limit of mybp. because sociality occurs in all the extant allodapine lineages, it must be a plesiomorphic trait and must therefore have originated by at least mybp, and probably much earlier given the very conservative nature of all three calibration points. this age-estimate is similar to that of schwarz et al. and suggests that the age of the allodapine root node is approximately twice the ages for estimated origins of sociality in halictine bees. as an independent assessment of approximate divergence ages, we compared pairwise maximum likelihood distances for allodapine species whose most recent common ancestor was at the root allodapine node with those for halictine species whose mrca was at one of the three origins of sociality in halictines, which were dated at 20– mya, using a sequenced fragment of ef-1α common to both groups. substitutional parameters for this gene fragment are almost identical for halictines and allodapines indicating that evolutionary dynamics for this gene fragment are very similar for the two bee groups. if evolutionary rates are also comparable in the two groups, the resulting distances suggest that the root node of the allodapines is about twice the age of the origins of sociality suggested for the halictines, which concords very closely with our penalised likelihood analysis. discussion our results are important because they suggest that, compared to halictine bees, allodapines have a much older origin of sociality but show no losses of sociality. in this respect, they are more similar to eusocial lineages such as ants, corbiculate bees, vespid wasps and termites, which also have ancient social origins with no losses. this raises the question of what factors may prevent losses of sociality over very large time scales. lack of reversions to solitary living in allodapines cannot be explained by arguments that they are restricted to habitats or climatic regimes that favour sociality. the ecological diversification of allodapines covers habitats such arid gibber deserts, savannas, bushvelds, coastal heathlands, equatorial and subtropical rainforests, and sub-alpine regions. colony phenology ranges from highly seasonal univoltine egg production schedules, all the way through to asynchronous development patterns where egg production, brood maturation, and foundress dispersal occur year round. the lack of reversions to purely solitary living is even more notable because of the near-ubiquity of female totipotency in allodapines, which means that physiological factors do not preclude independent living. we argue that the absence of reversions to solitary living is due to ecological consequences of the way in which allodapines rear their brood, and this may also help explain some broader trends found in ants, termites and vespid wasps. unlike virtually all other bees, which rear their brood in fully provisioned sealed cells, allodapines rear brood in un-partitioned and unsealed tunnels. because brood lack the physical protection of an enclosed cell, they are highly vulnerable to predation in the absence of an adult guard. indeed, the major benefit of group living in allodapines is avoidance of total brood failure, and this benefit is greatest when comparing single- with two-female colonies, because brood in the former colonies are unprotected when the sole adult is foraging. this vulnerability of brood is heightened by orphaning, since not only will post-feeding brood be unprotected, but feeding-stage larvae will be unable to complete their development due to lack of food. in the event of orphaning, potential alloparents could reap large indirect fitness gains by simply protecting post-feeding brood or completing the feeding of partially reared larvae, as well as direct benefits from inheriting a nest along with a subsequent cohort of potential alloparents. in fact, alloparental rearing in the absence of possible mothers is recorded from diverse species indicating that it does not require maternal coercion. at the same time, mothers would gain benefits from permitting some daughters to remain in the nest as insurance against orphaning. in contrast, the vulnerability of brood and the benefits of alloparental care in cell-provisioning insects are quite different. mass provisioning of brood in cells means that mothers sequester their investments as completed units over time, and these cells provide brood with some physical protection from enemies at the nest. in addition, if orphaning does occur, immatures sealed in cells will have sufficient food for their complete development, further reducing the scope for benefits from alloparental care. whilst protecting the nest from both predation and parasitism is important for all bees, allodapine brood are particularly vulnerable due to the lack of cells. continuous protection from predation is impossible for a solitary nesting allodapine, as the nest is completely unguarded during the female's foraging trips. we suggest this vulnerability is a compelling explanation for why there have been no reversions to purely solitary living in allodapines but multiple losses in halictines. it has been argued that the lack of reversions to solitary nesting in ants and termites may reflect the evolution of social complexity to a'point of no return', where a species is no longer able to live solitarily. however, there are numerous ant groups where newly founded colonies involve non-claustral queens rearing their first brood cohort to maturity without help from workers, suggesting that competency for solitary brood rearing per se exists in many taxa. for example, within many ant subfamilies, there are species where colony foundation almost exclusively involves a single brood-rearing foundress and very small ultimate colony sizes. queens must forage in order to feed their first clutch of brood, indicating that solitary-founding females in these species are capable of foraging effectively enough to rear through their first generation of brood alone. many of the aforementioned subfamilies also contain multiple independent losses of the queen caste, which has resulted in numerous species displaying ubiquitous female totipotency but without any transitions to purely solitary living. in social taxa where females can successfully rear brood to maturity in a non-claustral fashion without help of a worker caste, explanations for a lack of reversions to solitary living must involve something other than incompetence for independent brood rearing. ants share one key life history trait with allodapines, namely that brood are progressively reared in unsealed communal tunnels. in social vespid wasps, larvae are also reared to pupation in unsealed cells, and while termite young are not'provisioned' by adults, they also develop in unsealed chambers and are highly vulnerable in the absence of adults. in all these taxa, protection of brood depends heavily on adults, and this contrasts with halictines where nearly all species sequester their fully provisioned brood in closed cells. losses of sociality would remove the brood protection that group living confers in the former groups, but the physical protection of cells in halictines would allow some protection of brood in solitary nesting halictines to be maintained. lastly, we have argued that the'point of no return' paradigm has been framed in a way that does not readily allow falsification. we believe that our conjecture provides two predictions that permit empirical assessment: that reversions from social to solitary brood rearing are more likely in clades where constraints to solitary living are low; and that reversions from social to solitary can occur even after specialised worker/soldier castes have evolved, provided that ecological constraints for independent reproduction are relaxed. halictine bees and gall-forming thrips comprise two groups where these predictions could be tested, but there are likely to be many other taxa as well.  our findings suggest a very different framework for understanding social evolution from that argued in some recent studies that emphasize the importance of mechanistic approaches involving physiology, regulatory circuits and genetic-networks. in particular, hunt and amdam suggest "...that social evolution in insects can be fully – and finally – understood" by such mechanistic approaches. although such approaches may help explain the ontogeny of worker-like behaviour, our results indicate that retention of such behaviour is likely to be due to ecological and life-history factors, and these have the potential to determine very long term patterns of social evolution. the ecological dimensions of sociality cannot be ignored when trying to understand its origins and long-term maintenance. 
we argue that allodapine sociality has been maintained by ecological constraints and the benefits of alloparental care, as opposed to behavioural, genetic or morphological constraints to independent living. allodapine brood are highly vulnerable to predation because they are progressively reared in an open nest, which provides potentially large benefits for alloparental care and incentives for reproductives to tolerate potential alloparents. the major lineages of eusocial insects, the ants, termites, stingless bees, honeybees and vespid wasps, all have ancient origins with no reversions to solitary behaviour. h. nigrinervis was thought to be the only allodapine that was not social, and our findings therefore indicate that there have been no losses of sociality among extant allodapine clades. we argue that similar vulnerabilities may also help explain the lack of reversions to solitary living in other taxa with ancient social origins.
 the major lineages of eusocial insects, the ants, termites, stingless bees, honeybees and vespid wasps, all have ancient origins with no reversions to solitary behaviour. this has prompted the notion of a'point of no return' whereby the evolutionary elaboration and integration of behavioural, genetic and morphological traits over a very long period of time leads to a situation where reversion to solitary living is no longer an evolutionary option.  we show that in another group of social insects, the allodapine bees, there was a single origin of sociality > mya. we also provide data on the biology of a key allodapine species, halterapis nigrinervis, showing that it is truly social. h. nigrinervis was thought to be the only allodapine that was not social, and our findings therefore indicate that there have been no losses of sociality among extant allodapine clades. allodapine colony sizes rarely exceed females per nest and all females in virtually all species are capable of nesting and reproducing independently, so these bees clearly do not fit the'point of no return' concept.  we argue that allodapine sociality has been maintained by ecological constraints and the benefits of alloparental care, as opposed to behavioural, genetic or morphological constraints to independent living. allodapine brood are highly vulnerable to predation because they are progressively reared in an open nest, which provides potentially large benefits for alloparental care and incentives for reproductives to tolerate potential alloparents. we argue that similar vulnerabilities may also help explain the lack of reversions to solitary living in other taxa with ancient social origins. 