The wood-dwelling termites are very unusual among social insects in that there are low incentives to help as their young are rather precocial. As hemimetabolous insects the young instars are quite independent. They can move around and do not rely on intensive brood care 10. Especially, as the colony lives inside its food, there is no necessity for costly foraging and food is easily accessible to all individuals 16. Thus, individuals only need to be infested with gut symbionts to exploit the common wood resource and these reciprocal infestations are less costly. As a consequence, the older offspring have few opportunities to reduce the work load of reproductives and hence can hardly gain indirect benefits by raising siblings 1516. Correspondingly, the annual growth rates of wood-dwelling termite colonies are quintessentially like those of solitary insects. Only 20 to 100 offspring per year are produced 21.

All this changes with a transition to a life style where individuals forage outside the nest, i.e. with the transition to foraging termites. Although many of these species still nest in dead wood, they all forage outside for additional food. In foraging termites, young depend on brood care by food provisioning 37. This can be handed over from parents to older offspring (which become true workers) and the latter can further reduce the work load of reproductives by providing them with food as well 3839. So reproductives can concentrate and specialize on reproduction and annual colony growth rates increase reaching extreme values in fungus-growing termites with queens laying up to 40,000 eggs daily 3740. Accompanying specialization on reproduction, true physogastry (i.e., the enlargement of the abdomen through an increase in the number of functional ovarioles and fat bodies) evolved in queens of foraging termites 41. On the other side, true termite workers evolved morphological adaptations to foraging and food provisioning 39. For instance, sclerotization of the cuticle, which is largely absent in false workers, protects true workers against their hostile foraging environment. However, this comes with a cost as it hinders subsequent molts and thus prevents further development 5.

Wood-dwelling termites can also be further distinguished. They can inhabit two distinct nesting environments which might exert somewhat different selective pressures on social evolution: drywood termites (Kalotermitidae) generally nest in sound wood, while soft wood nesters (most dampwood termites, Termopsidae) occupy nests that are at least partially decayed by fungus 5. In the former, the nest is a solid rather parasite- and pathogen-free environment, while the nests of dampwood termites are afflicted by high microbial loads 42. Although data are scare so far and data for more species need to be collected, available evidence indicates that this might have consequences for brood care. In the drywood termites Cryptotermes secundus, Cryptotermes cynocephalus and Cryptotermes domesticus false workers do not care for eggs and young at all 10 (unpubl. data). From the second instar larvae onward individuals were seen to feed themselves. Eggs and first instar larvae are not cared for, they are not carried around, piled up or licked; but also they do not grow obviously until the next molt, which might suggest that they utilize body reserves. Similar data for the dampwood termite Zootermopsis angusticollis indicate that the brood is allogroomed (although other brood care like feeding is also absent) 43. Further studies are clearly needed to illucidate whether this difference in grooming behaviour and pathogen load between damp- and drywood termites proves to be consistent. Based on the more basal phylogenetic position of the dampwood termites 4829 and the fact that the termites' sister taxon, the woodroaches, inhabit damp wood 2527, allogrooming to remove pathogen might have been the first and only component of brood care in the early evolution of termite's sociality that was already present in the termite's ancestors.

In contrast to the termites, social Hymenoptera are holometabolous insects. Their young are altricial, grub-like larvae which strongly depend on brood care, especially as social Hymenoptera always evolved from species with progressive food provisioning of the progeny 131444. This provides ample opportunities for alloparental care. Most modern ants (and many predatory wasps) have adults subsisting mostly on floral nectar or hemiptera exudates while hunting prey (or carrion) for the young 12. As progressive food provisioners the adult female offspring can considerable reduce the work load of their mothers by foraging to feed their younger siblings (and their mother) so that mothers can concentrate on egg-production. Thus, ant workers can gain important indirect fitness benefits by specializing in the most costly investment in altricial young: food provisioning.

Another idiosyncratic property of wood-dwelling termites (and their ancestors) that largely distinguishes them from social Hymenoptera is their bonanza type food resource. Similarly, soil fed by soil feeding termites seems to constitute a bonanza food source but in contrast to wood it is too nutrient poor to live in it and grow to maturity. The nest of wood-dwelling termites, however, constitutes a resource that generally largely outlasts the lifetime of the founding primary reproductives. Hence offspring can stay with their parents as there is no local resource competition which normally selects for offspring dispersal 45. Only when the wood block becomes exploited, resource competition shifts from a global (competition between colonies) to a local scale (competition among colony members) 4647 and in line with this under limited food conditions, false workers develop into winged sexuals that leave the nest during the annual nuptial flight to found new colonies 4849505152. This is possible because the termites can sense food availability via vibrations generated during gnawing that act as reliable indicators of wood size 53. During the developmental period from false worker via several nymphal instars (instars with wing buds) to winged sexual, which lasts several months 19, individuals of the drywood termite Cryptotermes secundus behave increasingly competitive. The degree of reciprocal proctodeal trophallaxis (= anal feeding) among false workers declines and each individual spends more time feeding wood 54. Additionally, when proctodeal trophallaxis occurs it is preferentially directed at closer kin showing conditional nepotism in this species 55.

All this contrasts with most social Hymenoptera/ants which do not inhabit a bonanza type food resource. Social Hymenoptera seem to overcome local resource competition within a colony – and thus selection for dispersal – by increasing food intake through increased numbers of foragers 44. 'Pay to stay' theory predicts that when staying of individuals poses costs to the dominant breeder rent payment can be selected 5657. Subordinate individuals (offspring, workers) might have to pay, for example in the form of foraging, in order to be allowed to stay in the colony 5657. Thus, helping to raise offspring might to some degree in some social Hymenoptera be a rent payment. This might especially apply to wasps or pleometropic foundress associations 58 where adult females face a threat of eviction but gain from staying in a group and where subordinates/helpers are not/less related to raised offspring.

Although there is no local resource competition in wood-dwelling termites when food is abundant, offspring competition can arise over reproductive inheritance, when the colonies' reproductives become unhealthy or die 59606162. Then false workers which can develop into neotenic replacement reproductives compete for inheritance of the breeding position. In drywood termites such competition comes about in two manifestations 4959: (a) several false workers become neotenic replacement reproductives and they fight each other until one pair of reproductives is left; (b) only one replacement reproductive develops that immediately seems to inhibit the development of other false workers into replacement reproductives. Which individual will become the heir is apparently determined in a 'fair' process that is stable against cheating: During the intermolt period (note, false workers are larvae) there seems to be a very short phase of competence during which false workers are sensitive to the absence of reproductives 6364. As all false workers pass through this period, but only one or a few individuals (depending on colony size) are competent at any given time, each individual has a fair chance to inherit the breeding position.

This situation of local competition over inheritance might also counterselect altruistic helping and thus might further explain why false workers do not help in raising siblings. Taylor 46 and West and co-workers 65 have shown that under conditions of intense local competition (local competition = 1, global competition = 0) between relatives, as it occurs during the inheritance process, the benefits of altruism and the costs of competition cancel out each other so that altruistic helping will not be selected. Thus even though sharing resources with relatives is not costly because food is abundant, individuals should not invest in raising their strongest future competitors over the breeding position. The competition model further predicts that if intense local competition occurs among non-relatives spiteful behaviour might even be selected. Interestingly, in Cryptotermes interactions among non-relatives occur when two unrelated colonies that were independently founded in the same tree fuse during colony expansion 66. It will be interesting to test whether non-relatives behave spitefully under such conditions.

Due to their different ancestry, termites and social Hymenoptera have different modes of sex determination. Social Hymenoptera are haplodiploid. Males develop from unfertilized eggs and are haploid, while females develop from fertilized diploid eggs. In contrast, termites are diploid with both sexes developing from fertilized diploid eggs. This results in symmetrical relatedness associations within monogamous termite colonies where the relatedness among fullsiblings and between parents and their offspring are identical (always r = 0.5). By contrast, in Hymenoptera fullsisters are more closely related to each other (r = 0.75) than parents are to their offspring (r = 0.5) or sisters are to their brothers (r = 0.25). This has important consequences: First, the unusually high relatedness between fullsisters in social Hymenoptera was initially thought to explain the multiple origins of eusociality in this order and the female preponderance in these colonies (167; for more recent discussions: 223468). However this haplodiploidy hypothesis faded as it became clear that haplodiploidy will only promote altruism relative to diploidy under rather restricted conditions 68. Second, haplodiploidy creates relatedness asymmetries which result in conflicts between queens and workers over the sex ratio of sexuals and male production 69. Similar conflicts are lacking in termites 61. Third, Teyssèdre and co-workers 70 recently showed that the spread of altruism in diploids requires a pleiotropic link between altruism and enhanced productivity. By contrast, in haplodiploid organisms altruism within families that even lowers the productivity may spread, provided daughters sacrifice their own reproduction to raise full-sisters. A link between altruism and enhanced colony efficiency is unlikely when a mutant causing altruism occurs in a solitary organism. However, if such an altruistic mutant evolves in an already established group of coexisting relatives than it could automatically increase group productivity. This might have actually happened during the evolution of termites' eusociality 16. As recent results indicate for wood-dwelling termites, in the termites' ancestors offspring probably stayed as false workers in the nest that did not invest more in raising siblings than woodroaches. So, family groups formed in which only in a second step an altruist mutant, the soldier caste, evolved. By defending the colony they considerably increased the group's reproductive success. Thus, altruism would be linked to enhanced productivity because large, less altruistic family groups already existed.