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The size-complexity rule posits that the evolution of larger cooperative groups should favour more division of labour. Examples include more cell types in larger multicellular organisms, and more polymorphic castes in larger eusocial colonies. However, a correlation between division of labour and group size may reflect a shared response of both traits to resource availability and/or profitability. Here, this possibility was addressed by investigating the evolution of sterile caste number (worker and soldier morphotypes) in termites, a major clade of eusocial insects in which the drivers of caste polymorphism are poorly understood. A novel dataset on 90 termite species was compiled from the published literature. The analysis showed that sterile caste number did increase markedly with colony size. However, after controlling for resource adaptations and phylogeny, there was no evidence for this relationship. Rather, sterile caste number increased with increasing nest-food separation and decreased with soil-feeding, through changes in worker (but not soldier) morphotype number. Further, colony size increased with nest-food separation, thus driving the false correlation between sterile caste number and colony size. These findings support adaptation to higher energy acquisition as key to the rise of complex insect societies, with larger size being a by-product.

Larger species tend to feed on abundant resources, which nonetheless have lower quality or degradability, the so-called Jarman-Bell principle. The "eat more" hypothesis posits that larger animals compensate for lower quality diets through higher consumption rates. If so, evolutionary shifts in metabolic scaling should affect the scope for this compensation, but whether this has happened is unknown. Here, we investigated this issue using termites, major tropical detritivores that feed along a humification gradient ranging from dead plant tissue to mineral soil. Metabolic scaling is shallower in termites with pounding mandibles adapted to soil-like substrates than in termites with grinding mandibles adapted to fibrous plant tissue. Accordingly, we predicted that only larger species of the former group should have more humified, lower quality diets, given their higher scope to compensate for such a diet. Using literature data on 65 termite species, we show that diet humification does increase with body size in termites with pounding mandibles, but is weakly related to size in termites with grinding mandibles. Our findings suggest that evolution of metabolic scaling may shape the strength of the Jarman-Bell principle., (© 2020 The Authors. Evolution © 2020 The Society for the Study of Evolution.)

The size–complexity rule posits that the evolution of larger cooperative groups should favour more division of labour. Examples include more cell types in larger multicellular organisms, and more polymorphic castes in larger eusocial colonies. However, a correlation between division of labour and group size may reflect a shared response of both traits to resource availability and/or profitability. Here, this possibility was addressed by investigating the evolution of sterile caste number (worker and soldier morphotypes) in termites, a major clade of eusocial insects in which the drivers of caste polymorphism are poorly understood. A novel dataset on 90 termite species was compiled from the published literature. The analysis showed that sterile caste number did increase markedly with colony size. However, after controlling for resource adaptations and phylogeny, there was no evidence for this relationship. Rather, sterile caste number increased with increasing nest–food separation and decreased with soil-feeding, through changes in worker (but not soldier) morphotype number. Further, colony size increased with nest–food separation, thus driving the false correlation between sterile caste number and colony size. These findings support adaptation to higher energy acquisition as key to the rise of complex insect societies, with larger size being a by-product. [ABSTRACT FROM AUTHOR]

Animals have evolved a number of antipredator strategies, which can reflect both intrinsic and extrinsic factors related to predation risk. In many termite species, a chemically armed soldier caste known as 'nasute' engages in patrols upon breaching of foraging galleries, suggesting an adaptive response to imminent risk of mortality. However, the drivers of this collective behaviour are poorly understood. Here, I describe and test a qualitative model of patrolling behaviour upon gallery breaching by the arboreal termite Nasutitermes corniger (Termitidae: Nasutitermitinae). Patrolling behaviour is postulated to change throughout colony ontogeny, thus scaling with colony size. Further, this pattern is expected to be modulated by extrinsic cues for predation risk. Accordingly, patroller number increased with tree circumference at breast height, a surrogate for colony size. However, for any tree size, more soldiers engaged in patrols after repeated breaching, suggesting sensitization to structural damage. In parallel, patrolling rate (i.e., patroller number per time unit) was higher in smaller trees due to a disproportionate increase in patrol duration with patroller number. However, this response only occurred after repeated breaching; otherwise, patrolling rate was constant across tree sizes. Overall, patrolling behaviour by N. corniger seems to reflect intrinsic risks related to colony growth, adjusted by the extrinsic risk imposed by structural damage. [ABSTRACT FROM AUTHOR]

1. Metabolic rate ( B) is a fundamental property of organisms, and scales with body mass ( M) as B = α Mβ. There has been much debate on whether scaling parameters should be viewed as constants or variables. However, there is increasing evidence that ecological differentiation can affect both α and β. 2. In colonial organisms such as social insects, individual metabolism is integrated at the colony level. Theory and data suggest that whole-colony metabolism partly reflects individual-level metabolic and life-history scalings, but whether these have been affected by ecological diversification is little known. 3. Here, this issue was addressed using termites. Data from the literature were assembled to assess the interspecific scalings of individual metabolic rate with individual mass, and of individual mass with colony mass. Concurrently, it was tested whether such scalings were affected by two key ecological traits: lifestyle and diet. 4. Individual-level metabolic scaling was affected by diet, with β = 1.02 in wood feeders and 0.60 in soil feeders. However, there was no difference in α. Further, individual mass scaled to the 0.25 power with colony mass, but forager species had larger colonies and smaller individuals relative to wood-dwelling, sedentary ones, thus producing a grade shift. 5. Our results show that ecological diversification has affected fundamental metabolic and life-history scalings in termites. Thus, theory on the energetics and evolution of colonial life should account for this variability. [ABSTRACT FROM AUTHOR]