within animal families can be given: túngara frogs (engystomops pustulosus) create 'foam nests' during breeding in which embryogenesis occurs; these foam nests are imbued with ranaspumin proteins which provide defence against microbial attack and act as a detergent. the three-spined stickleback (gasterosteus aculeatus), grass goby (zosterisessor ophiocephalus), fringed darter (etheostoma crossopterum) and two species of blenny also use chemical strategies to defend their eggs from microbes. intriguingly, microbes themselves have been found to have social immune systems: when a population of staphylococcus aureus is infected with gentamicin, some individuals (called small colony variants) begin to respire anaerobically, lowering the ph of the environment and thus conferring resistance to the antibiotic to all other individuals-including those s. aureus individuals who did not switch phenotype. an analogy can be drawn here with the social fever in bees described above: a subset of individuals in a population change their behaviour and in doing so provide population-wide resistance. using richard dawkins's concept of the extended phenotype, the healthcare systems developed by humans could be seen as a form of social immunity. study species the majority of studies on social immunity have been on eusocial insects. for example, sylvia cremer's work uses ants as a model system whilst rebeca rosengaus works with termites. outside of eusocial insects, one emerging model system is the burying beetle nicrophorus vespilloides. nicrophorus vespilloides two n. vespilloides preparing a carcass already a model system in evolutionary ecology due to their extensive parental care, burying beetles like n. vespilloides hunt for small vertebrate carcasses which they then bury before intricately preparing it as a resource for its larvae to breed on-these carcasses are scarce and ephemeral yet are necessary for burying beetles' reproductive success. carcasses are highly contested resources with challenges being launched by other burying beetles and other scavenging species, as well as microbial decomposers. older carcasses have a higher microbial load and thus have a lower quality as a breeding resource: larvae raised on these carcasses are smaller and in a worse nutritional state–at adulthood these beetles were also smaller, which in n. vespilloides reduces fitness. daniel rozen et al. demonstrated in 2008 that n. vespilloides preferentially chooses newer carcasses (which tend to have a lower microbial load) over old carcasses, and if it is not possible to acquire one of these higher quality carcasses that they use pre and post-hatching parental care to reduce the challenge posed by microbes. sheena cotter and rebecca kilner demonstrated that part of this anti-microbial parental care involved both parents smearing the carcass with antibacterial anal exudates: their 2009 work demonstrated that when beetles encounter a carcass they upregulate the antibacterial activity of their anal exudate by actively altering its composition (lysozyme-like activity increases, phenoloxidase activity decreases), and that the specifics of this social immune system differed between the sexes: female exudate has greater antibacterial activity than males; widowed males increased the antibacterial activity of their exudate whilst a reduction was seen in widowed females. cotter et al. went on to show the costliness of this social immune response-by providing females with microbe-infested carcasses, they found that the upregulation of antibacterial activity that followed led to a 16% decrease in lifetime reproductive output. this significant reduction in fitness, due to both increased mortality and age-related dropoff in fecundity, explains why the antibacterial activity of the exudate is only induced and not present constitutively. further work revealed how a trade-off existed between investment in personal immunity vs investment in social immunity, i.e., upon injury, n. vespilloides upregulates its personal immune response whilst concomitantly reducing its social immune response. recently, the kilner group identified a gene associated with social immunity in n. vespilloides: the expression rate of lys6, a lysozyme, increases 1,409 times when breeding, and goes from the 5,967th most abundant transcript in the transcriptome of gut tissue to the 14th; it was also demonstrated that expression rates of lys6 covary with the antibacterial activity of the anal exudate. social immunity efforts peaks during middle-age, in contrast to efforts in personal immunity increasing or being maintained with age in breeding burying beetles. the exudate of the larvae themselves also contains antibacterial substances, with activity peaking at hatching and declining as the larvae age. rfemoving parents results in a downregulation of antibacterial effort, possibly due to the need to invest energy in other more important tasks that arise due to parental absence. evolution comparison with personal immunity many researchers have noticed marked parallels between the more familiar personal immune systems of individual organisms (e.g. t and b lymphocytes) and the social immune systems described above, and it is generally appreciated among ecological immunologists that rigorous comparative work between these two systems will increase of understanding of the evolution of social immunity. whilst the specific physiological mechanisms by which immunity is produced differ sharply between the individual and society, it is thought that at a "phenomenological" level the principles of parasite threat and response are similar: parasites must be detected rapidly, responses should differ depending on the parasite in question, spread of the infection must be limited and different components of the individual/society should be afforded different levels of protection depending on their relative fitness contribution. cremer was the first to do this systematically, and partitioned immunological phenomena into three categories: border defence (intake avoidance), soma defence (avoid establishment within non-reproductive components of an individual/society) and germ-line defence (avoid infection of the reproductive components of an individual/society). example analogies from cremer's paper are: border defence: clotting in an individual's wounded blood vessels can be compared to the entrance-plugging behaviour of special ant workers in response to a parasite attack the circulatory system of an individual can be compared to nutrient distribution in social insect colonies, where a few forage and then distribute it to the rest of the nest cats and dog self-grooming can be compared to allogrooming soma defence: granulomas form to contain diseases that individual immune systems are struggling to eliminate; this can be compared to the social encapsulation seen against small hive beetles in honeybees apoptosis in response to disease can be compared to the killing of infected workers or the enforced suicide seen in temnothorax germ-line defence: the special protection afforded to reproductive organs (e.g. blood-ovary/blood-testes barrier, increased number of immune cells relative to non-reproductive organs) can be compared to the royal chamber found in social insects, where due to centrifugal polyethism the queen(s), and sometimes king(s), are cared for by young workers who have remained inside the nest their whole life and thus have a lower probability of parasite infection other similarities include the immunological memory of the adaptive immune system in vertebrates and the observation that a similar collective memory (operating with a yet-to-be-explained mechanism) occurs in some insect societies e.g. individual z. angusticollis survive m. anisopliae infections significantly more when they have been in contact with a previously infected conspecific, a 'social transfer of immunity' or 'social vaccination'. transplant rejection caused by non-self major histocompatibility complexes is frequently thought to be a byproduct with no evolutionary function, however cremer cites cases (such as the colonial star ascidian (botryllus schlosseri)) where recognizing foreign cells may have evolved as an adaptation - if so, then this could be analogous to the self-recognition systems in social insects which prevent brood parasitism and the worker policing behaviours which suppress 'social tumours'. specific immune cells in animals 'patrol' tissues looking for parasites, as do worker-caste individuals in colonies. cotter and kilner argue that not only is social immunity a useful concept to use when studying the major transitions in evolution (see below), that the origin of social immune systems might be considered a major transition itself. role in the evolution of group living the transition from solitary living to group living (identified by john maynard smith as one of the seven major transitions in evolutionary history) brought with it many fitness benefits (increased anti-predator vigilance, foraging benefits etc.) and the opportunity to exploit a vast array of new ecological niches, but group living also has its pitfalls. numerous studies have demonstrated an increase in contact-transmitted parasite load with group size increase, and thus research has been done on the role of social immunity in the evolution of early group living. empirical evidence already exists, from both interspecific and intraspecific comparative studies, that an increase in population density drives an increase in personal immune effort (density-dependent prophylaxis). however, there is also good evidence that the evolution of social immunity leads to a trade-off between effort into personal immune responses vs. effort in social immune responses - physiological and genomic studies have shown that social conditions can lead to a reduction in personal immune effort. personal immunity in the australian plague locust (chortoicetes terminifera) decreases upon an increase in population density and increases when artificially isolated. genomic studies reveal that infected solitary s. gregaria express more genes involved in immunity than infected individuals in the gregarious phase, bombus terrestris workers also upregulate immune-related genes when experimentally isolated and there are three times more immune-related gene families in solitary insects than in the eusocial honeybees. joël meunier argued that the two seemingly contradictory relationships between personal immune effort and population density were a function of two assumptions implicit in the prediction that there should be a negative correlation between personal immune effort and group living: "group living is always associated with the expression of social immunity" – this is false; worker termites (z. angusticollis) do not discriminate between infected and uninfected conspecifics, pharaoh ant colonies (monomorium pharaonis) choose to move into infected nests over uninfected ones and queen wood ant (formica paralugubris) are not repelled but actually attracted to habitats contaminated with entamopathogenic fungi. "social immune responses are costly for producers" – only in one species, nicrophorus vespilloides, has this assumption been tested - a bacterial challenge to the larval resource led to a social immune response by the mother, and this response did lead to a reduction in lifetime reproductive success (i.e. there was a cost). whilst advising that further studies in lots of different eusocial and non-eusocial taxa are required to better assess the validity of these assumptions, meunier notes that the existence of a trade-off between personal and social immunity could be masked or erroneously 'discovered' in a population/species due to individual variation (e.g. low-quality individuals may not be able to afford relatively high investment into both immune systems), and thus recommends that the intrinsic quality of individuals should be controlled for if valid conclusions are to be drawn. to assess what current knowledge of social immune systems suggested about whether social immunity was a byproduct or driver of complex group living, meunier delineated 30 different mechanisms of social immunity found in eusocial insects and looked for counterparts to these in presocial and solitary insects. supporting the hypothesis that social immunity was a driver and not a by-product of complex group living, ten mechanisms had counterparts in presocial insects and four in solitary species (though this does not imply that some mechanisms may evolve as a byproduct). evidence that social immunity mechanisms are selected for at least somewhat due to collective benefits is lacking though – possibly due to the difficulty in isolating the immune benefits from the other benefits that social immunity mechanisms often bestow (e.g. allogrooming inhibits the establishment of ectoparasites, but also improves the accuracy of nest mate recognition due to the sharing and thus homogenization of chemical signatures between group members), and the difficulty in experimentally separating direct fitness from indirect fitness, potentiated in eusocial taxa where sterile/non-reproductive individuals predominatee. more studies on presocial taxa would allow for phyletic analyses to recover the actual path of evolution that different mechanisms of social immunity took. role in the evolution of polyandry the origin of polyandry in nature and its adaptive value is a subject of ongoing controversy in evolutionary biology, partly due to the seemingly numerous costs it places on females - additional energetic and temporal allocation to reproduction, increased risk of predation, increased risk of sexually transmitted diseases and increased risk of physical harm caused by copulation/sexual coercion – for eusocial insects, the effects polyandry has on the colony member's coefficient of relatedness is also important, as reducing the relatedness of workers limits the power of kin selection to maintain the ultracooperative behaviours which are vital to a