Trophallaxis

Trophallaxis is the transfer of food or other fluids among members of a community through mouth-to-mouth (stomodeal) or anus-to-mouth (proctodeal) feeding. Along with nutrients, trophallaxis can involve the transfer of molecules such as pheromones, organisms such as symbionts, and information to serve as a form of communication. Trophallaxis is used by some birds, gray wolves, vampire bats, and is most highly developed in eusocial insects such as ants, wasps, bees, and termites.

Etymology
Tropho- (prefix or suffix) is derived from the Greek trophé, meaning 'nourishment'. The Greek 'allaxis' means 'exchange'. The word was introduced by the entomologist William Morton Wheeler in 1918.

Evolutionary significance
Trophallaxis was used in the past to support theories on the origin of sociality in insects. The Swiss psychologist and entomologist Auguste Forel also believed that food sharing was key to ant society and he used an illustration of it as the frontispiece for his book The Social World of the Ants Compared with that of Man. Proctodeal trophallaxis allowed termites to transfer cellulolytic flagellates that made the digestion of wood possible and efficient. Besides sociality, trophallaxis has evolved within many species as a method of nourishment for adults and/or juveniles, kin survival, transfer of symbionts, transfer of immunity, colony recognition and foraging communication. Trophallaxis has even evolved as a parasitic strategy in some species to obtain food from their host. Trophallaxis can also result in the spreading of chemicals, such as pheromones, throughout a colony, which is significant in social colony functioning.

Species have evolved anatomy to allow them to participate in trophallaxis, such as the proventriculus in the crops of Formica fusca ants. This structure acts as a valve to enhance food storage capacity. Likewise, the honey bee Apis mellifera is able to protrude their proboscis and sip nectar from the open mandibles of the donor bee. Certain mechanisms have also evolved to initiate food sharing, such as the sensory exploitation strategy that has evolved in the common cuckoo brood parasites. These birds have evolved brightly coloured gapes that stimulate the host to transfer food.

Invertebrates
Trophallaxis is a form of social feeding in many insects that contributes to the formation of social bonds. Trophallaxis serves as a means of communication, at least in bees, like M. genalis, and ants. Trophallaxis in M. genalis is part of a social exchange system, where dominant bees are usually the recipients of food. It increases longevity of bees that have less access to food and decreases aggression between nest mates. In the red fire ant, colony members store food in their crops and regularly exchange this food with other colony members and larvae to form a sort of "communal stomach" for the colony. This is also true for certain species of Lasioglossum, such as the sweat bee Lasioglossum hemichalceum. L. hemichalceum will often exchange food with other members regardless of whether they are nestmates or not. This is because cooperation among non-relatives offers more benefit than cost to the group.

Many wasps, like Protopolybia exigua and Belonogaster petiolata, exhibit foraging behavior where adults perform trophallaxis with adults and between adults and larvae. P. exigua carry nectar, wood pulp and macerated prey in its crop from the field to the nest for transfer; for larvae survival they carry amounts of prey proportional to the amount of larvae in the nest. Voluntary trophallaxis in Xylocopa pubescens bees has led to the nest guarding behavior that the species is known for. This bee species allows one adult to forage and bring nectar back for the rest of the nest population as a way to continually defend the nest while obtaining nutrients for all members of the colony.

In termites, proctodeal trophallaxis is crucial for replacing the gut endosymbionts that are lost after every molt. Gut symbionts are also transferred by anal trophallaxis in wood-eating termites and cockroaches. Transfer of gut symbionts in these species is essential to digest wood as their food source. Carpenter ants transfer immunity through trophallaxis by the direct transfer of antimicrobial substances, increasing disease resistance and social immunity of the colony.

In some species of ants, it may play a role in spreading the colony odour that identifies members.

Honey bee foragers use trophallaxis in associative learning to form long-term olfactory memories, in order to teach nest mates foraging behavior and where to search for food.

In addition, Vespula austriaca wasps also engage in trophallaxis as a form of parasitism with its host to obtain nutrients. V. austriaca is an obligate parasite species that invades the nests of host species and obtains food by constraining the host with their legs and forcing trophallaxis.

Vertebrates
Vertebrates such as some bird species, gray wolves, and vampire bats also feed their young through regurgitation of food as a form of trophallaxis. Food sharing in vertebrates is a form of reciprocity demonstrated by many social vertebrates.

Wild wolves transport food in their stomach to pups and/or breeding females and share it by regurgitation, as a form of trophallaxis. The recipient wolves often lick or sniff the donor wolf's muzzle to activate regurgitation and receive nutrients. Vampire bats share blood with kin by regurgitation as a means of increasing their fitness through kin selection.

Birds regurgitate food and directly transfer it into the mouths of their offspring as a part of parental care, such as the "crop milk" that is transferred by mother ring doves into the mouths of their young. The cuckoo brood parasite is another bird species that engages in trophallaxis. The cuckoo bird uses mimicry, such as mimicking the eggshell colors and patterns of the host's eggs, to place their young in the nest of host species where they will be fed and reared at no expense to the cuckoo mother. The cuckoo young can often mimic the begging call of an entire nest of the host species' young and have evolved intensely colored gapes; both of which act as supernormal stimuli, inducing the host bird to deliver food to them over their own young via trophallaxis.