Harmony and Conflict in the Living World - Alexander F. Skutch [37]
By placing food in each other's mouths, animals establish a still closer bond. Birds of many kinds feed their mates, most frequently as the breeding season approaches and while incubation is in progress. Usually the male gives food to the female; but occasionally she passes a morsel to him, as I have seen in the White-flanked Antwren and the Tawny-bellied Euphonia and L. de K. Lawrence (1968) saw in the Evening Grosbeak. By repeatedly feeding the male Andean Hillstar that has entered her territory in the high Peruvian Andes to court her, the female overcomes his timidity in a strange situation, without known parallel in the hummingbird family (Dorst 1962). More rarely, birds feed companions other than their mates: a group of Cedar Waxwings may pass a berry back and forth until finally one swallows or drops it. Scattered through the literature are instances of impaired adult birds remaining alive and in fair condition, apparently supplied by their companions with all the food they needed: a blind American White Pelican, a blind Indian Crow, a Brown Booby with only one wing, a Magnificent Frigatebird in similar plight, a Fiery-billed Aracari with a badly deformed beak, an ailing Gray Wood-Swallow, and a wounded Fijian White-breasted Wood-Swallow. It is not evident that all these crippled or sick birds were actually seen to receive food from others; but in British Columbia N. A. M. Verbeek and R. W. Butler (1981) repeatedly saw a male Northwestern Crow feed a femalenot his matewho had a blind eye and a deformed bill.
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The importance of food as a social bond reaches its climax among the most highly social of all animals, the termites and the social Hymenoptera. W. M. Wheeler (1928) applied the name "trophallaxis" to the continual exchange of food, or at least of gustatory satisfactions, which he regarded as the most compelling attraction between all the members of an insect community. Termites are constantly feeding each other, with nourishment extruded either from the mouth or from the opposite end of the body, a practice that led Maeterlinck (1927) to characterize a termitary as a "collective coprophagy." By these exchanges, they infect each other with the intestinal protozoa without which many species of termites cannot assimilate their ligneous meals. In certain termites, the queen of a colony, whose huge swollen abdomen has become a factory for turning out endless eggs, exudes from her skin a substance so highly relished by the workers who feed her that sometimes, in their eagerness for more, they tear little strips from her cuticle to reach the underlying source. Small brown scars mark the spots where she has been wounded by her progeny.
The larvae of certain ants and wasps secrete from their salivary glands, or from relatively enormous glandular growths surrounding the mouth, substances that their nurses greedily lick up after feeding them. Naturally, the quantity of nourishment given by the attendants to the larvae far exceeds what they receive from them; otherwise, the young insects could not grow. But the larval secretions are so highly attractive that the workers will relinquish much food to obtain them; as a farmer will sometimes sell pounds of his produce in order to buy a few ounces of some delicacy. Adult ants of the same colony habitually feed each other with nutriment regurgitated from their "social stomachs." Wheeler held that neither affection nor cleanliness is the motive for the mutual licking in which ants indulge;