Page:EB1911 - Volume 18.djvu/520

 unpleasant experience, helps in fact to establish a psychical association between a particular style of coloration and a nasty taste or a painful wound. This being so, it is evident that if all the distasteful species in a given area are differently coloured, some individuals of all the species will be annually sacrificed to the experimental tasting of inexperienced foes before the numerous lessons have been learnt. But if all the species in question resemble each other the resemblance will be mutually beneficial to them because the association between the two attributes they have in common, namely distastefulness and a particular scheme of colour, will be rapidly established. One lesson only, instead of many, has to be learnt; and once learnt at the expense of a few individuals of one or two species it will thereafter be applied indiscriminately to all. This type of mimicry has been well defined by Professor E. B. Poulton as the unification of warning colours.

The term mimicry has also been applied to resemblances of a different kind from the two enumerated above—resemblances, that is to say, by which predaceous species are supposed to be enabled to approach or mix without detection with animals they prey upon or victimize in other ways. To this end the resemblance may be actually to the species victimized or preyed upon or else to a species which the species preyed upon does not fear. This phenomenon is termed “aggressive mimicry” as opposed to

the Batesian and Müllerian phenomena, which are termed “protective mimicry.” A few possible cases of aggressive mimicry are enumerated in the following summary of some of the recorded cases of mimicry in different classes of the animal kingdom; but the phenomenon is of comparatively rare occurrence, and the supposed instances may be susceptible of other interpretations, excluding them altogether from mimicry, or bringing them under the Batesian or Müllerian interpretation of the phenomenon.

Among mammalia there are no certain cases of mimicry known. It has been claimed that the resemblance between some of the Oriental tree-shrews of the genus Tupaia and squirrels comes under the category of aggressive mimicry, the tupaias being enabled by their likeness to approach and pounce upon small birds or other animals which, mistaking them for the vegetable-feeding squirrels, make no effort to get out of the way. But this hypothesis cannot be accepted as furnishing a satisfactory explanation of the likeness. For in the first place there seems to be no good reason for thinking that the Tupaias feed to any considerable extent upon prey of that kind, and in the second place the resemblance is due to characters which may be merely adaptations to a similar mode of life. A long and bushy tail, for instance, is a useful balancer and is a not uncommon feature in mammals which lead an active arboreal life. Similarly the dull coloration of the two sets of animals is very possibly procryptic and serves to hide both shrews and squirrels from enemies. Hence there seem to be good reasons for regarding the likeness in question as due to similarity in habitat and not as mimetic.

In East and South Africa there is a genus of Mustelidae known as Ictonyx (Zorilla) which possesses a foetid odour and is warningly coloured with black and white bands after the manner of skunks. There also occurs in South Africa another member of this family (Poecilogale albinucha), which is very similarly coloured. It is possible that this resemblance is mimetic in the Batesian sense of the word, and that the Poecilogale, if inoffensive, profits by its likeness to the highly offensive and warningly coloured Ictonyx. But, on the other hand, Poecilogale may itself be a protected form since subcaudal stink-glands are commonly found in species of the, weasel tribe. If this be the case the two species probably furnish an instance of true Müllerian mimicry. In South America there is considerable superficial resemblance between the little bush dog (Speothos venaticus) of Guiana and Brazil and the large weasel-like animal of the same countries—the tayra (Galera barbara). The tayra is, when adult, black beneath and on the legs, and not uncommonly has a considerable quantity of greyish hair on the head. In these particulars, as well as in size and shortness of leg, the dog resembles the weasel; and since there are good reasons for believing that the latter is protected alike by ferocity and stink-glands, it is quite possible that the dog, of unusual coloration and form for the Canidae, is protected from the attacks of pumas, jaguars and ocelots by his likeness to the tayra.

A few cases of mimicry have been recorded in birds. The common cuckoo and some other species inhabiting Africa and Asia closely resemble sparrow-hawks. Some cuckoos are singular for their habit of using the nests of smaller birds to lay their eggs in, so that the young may be reared by foster-parents; and it has been suggested that the object of the likeness exhibited to the hawk is to enable the cock cuckoo either to frighten the small birds away from their nests or to lure them in pursuit of him, while the hen bird quietly and without molestation disposes of her egg. The fact that both sexes of the cuckoo resemble the hawk does not necessarily prove this suggested explanation to be false; but if it be true that the smaller passerine birds are duped by the similarity to the bird of prey, it may be that the cuckoos themselves escape molestation from larger hawks on account of their resemblance to the sparrow-hawk. Another species of this group, the black cuckoo of) India, apparently mimics the black drongo-shrike (Dicrurus ater), the resemblance between the two species being very close. The drongo is a fierce and powerful bird which will not tolerate a strange bird of the size of a cuckoo near its nest, yet on account of its resemblance to the drongo, the hen cuckoo is enabled, it has been claimed, to lay her egg in the nest of the drongo, which mistakes the cuckoo for one of its own kind. In this case also both sexes of the cuckoo mimic the drongo, whereas according to the theory it would be necessary for the hen bird alone to do so. This suggests that the resemblance to the pugnacious drongo may be beneficial in protecting the defenceless cuckoo from enemies.

Some observations, however, of Guy Marshall on the inedibility of certain birds suggest that the resemblance between cuckoos and hawks on the one hand and cuckoos and drongos on the other may be susceptible of another explanation in full agreement with the theory of mimicry as propounded by Bates. He found that a South African drongo (Dicrurus (Buchanga) assimilis) was rejected after one or two attempts to eat it by a hungry mongoose (Herpestes galera) which had been starved for purposes of the experiment. The drongo is blue and black and is, he believes, warningly coloured. The same mongoose also refused to eat a kestrel (Cerchneis rupicoloides) and a hobby (Falco subbuteo), although it devoured certain