Table 1.

Comparisons of host–parasite traits identify patterns of parasite trait evolution: putatively ancestral traits may be reduced or completely lost (deletion) or exaggerated (augmentation); new traits that are not found in the host may evolve (gain), or traits may be unmodified (retention). Through comparative analyses of host–parasite pairs, these different types of trait evolution can provide insights into host sociality. The kinds of process (trait retention, modification, loss and gain) are reported together with key examples from the literature and the possible insights they provide into the ultimate and proximate mechanisms of host sociality. Here we focus on what can be learned in terms of traits lost, gained and/or modified during the transition to the parasitic lifestyle; however, it is important to acknowledge that both host and parasite adaptations through loss/gain/modification can occur also for particular life history aspects that are not related to host–parasite interactions nor to sociality. For example, social parasites may retain traits that are not related to sociality, such as thermoregulation and grooming. Similarly, social parasites will modify/acquire new traits also for other selective pressures, such as different environmental requirements (e.g. different overwintering habits [25]). The power of these analyses, therefore, lies in analysing replicated events of change in host–parasite pairs across the phylogeny of Hymenoptera, where social parasites have evolved.

change in trait	key examples in the literature	insights into host society
deletion model
complete deletion (figure 2a) the trait is no longer present	— loss of nest founding behaviour and worker caste [1,12] — loss of wax production and pollen collecting apparatus on the hind leg in bumblebees [36] — loss of multiple mating in ants and wasps [30,37]	complete and partial losses and/or reduction expose traits required for free-living social organisms that are not beneficial for a parasitic lifestyle, or that would actively reduce the fitness of the parasite. As reduction/loss is likely matched by reduction/loss of the regulatory mechanisms, host–parasite comparisons for these traits have the potential to reveal the mechanistic pathways (e.g. hormonal, molecular) underlying these social traits
partial deletion and reduction (figure 2b) the expression of the trait is reduced (e.g. performed behaviour at a lower rate) or its value is reduced (e.g. a less pronounced morphological feature)	— reduced expression of colony maintenance activities, such as cell building, thermoregulation, colony defence [38] — reduced antibiotic production in Acromyrmex insinuator social parasites [39]
augmentation model
gain (figure 2c): a new trait has evolved de novo	— usurping restlessness in a paper-wasp social parasite [40] — appeasement pheromone in the brood of paper-wasp social parasite [41]	this trait is important for parasitic lifestyle. It might reveal unexpected host traits (e.g. sensory biases that can be exploited by the parasite) that would otherwise have gone unnoted if the host was studied alone
increase (figure 2d) trait expression is increased (e.g. performed behaviour at a higher rate) or trait value is increased (e.g. a more pronounced morphological feature)	— increased development of physical weaponry, such as sting, mandibles and size [12,38] — enlarged glands for secretion of semiochemicals (Dufour gland, venom glands and Van der Vecht organ in bumblebee and/or wasp parasites) [36,42] — increased rate of performance of stroking behaviour in paper-wasp parasites, to obtain chemical integration [12]	augmented mandibles and stings in parasites help confirm the importance of these weapons in dominance interactions and conflict resolutions by the host; enlarged glands support their role in chemical integration. Parasites are likely to be exploiting these traits, and there is selection for them to invest more heavily in them than the host because of their critical importance in infiltrating (and controlling) host societies
functional shift the trait remains the same but it acquires a different function in the social parasite	— Acromyrmex insinuator parasite workers acquire the role of suppressing host reproduction [43]	this discloses how host social traits can be co-opted for new functions by social parasites, thus suggesting alternative use of old traits
retention model
retention (figure 2e) the trait is maintained	— dominance behaviour: bumblebees, paper-wasps [12,44]	such traits might be essential for the integration into host social life. Dominance, for example, is often crucial in the host society to maintain reproductive control. It helps corroborate any putative mechanism identified from the host system