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Dimorphism is a term originally used by botanists
to express the fact that certain plants a difference, whether in form or
colour, more or less considerable exists between individuals belonging
to the same species, this difference not being attributed to local influences
or of the kind called accidental, but yet one that is constantly exhibited.
As analogous cases are observable in animals, the term has been adopted
by zoologists, and, disregarding other classes, it will at once be perceived
that among Birds there are two kinds of dimorphism - one depending upon
gender, in which the characters of the male and female may differ in very
many ways, and the other which is apparently quite independent of gender
distinction. Of this last kind, which seems to approach most closely to
the Dimorphism of botanists, there are not many undisputed instances.
Colour Dimorphism
The best known is that of some species of SKUA, in
which a parti-coloured bird may be frequently found mated with one that
is (so to speak) whole-coloured-in some cases the former being the male,
the latter the female, and in others just the contrary, it rarely happening
that both partners are alike in plumage. Another case is that
afforded by the GUILLEMOT, for at nearly everyone of its breeding - resorts
a portion of the tenants (perhaps one in a score) will be found to have
a white circle round the eye and a white line stretching backward from
it - these Ringed or Bridled Guillemots being of either gender and apparently
paired with birds of normal plumage (At one time these Ringed or Bridled
Guillemots were looked upon as a distinct species, called Uria Lacrymans,
but that view has been abandoned. Similarly the dark, whole-colored
examples of the common species of SKUA were originally described as forming
a separate species, Lestris richardsoni, but though the name was
retained by many writers for some while, by the end of the 19th Century
it appears to have been generally understood that this bird was not a distinct
species).
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Among birds examples of gender Dimorphism are so
numerous as to make it almost the rule. Yet, as already stated and as is
widely known, this kind of Dimorphism manifests itself in very many ways-the
commonest being that of general coloration, instances of which will occur
to everyone; but apart from that the coloration of particular parts is
scarcely less often divergent in the two genders, while differences of the
form or development of certain portions of the plumage are also very abundant,
as witness the occipital plumes in the male of many birds, while the extraordinary
elongation of the feathers of the lower back in the PEACOCK, of those on
the side of the breast in the BIRD OF PARADISE, or of the tail in the BLACKCOCK
are notorious.
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Passing to characteristics which may be of greater significance,
we have spurs on the metatarsus or near the wrist, the former only among
the Gallinae, but the latter found in birds of several groups that
are not closely allied. These are generally and justly admitted to be weapons,
and hardly less effective are the knobs which occupy the like position
in other forms, those of the male n the now extinct Rodriguez solitaire
(Pezophaps solitaria) being perhaps the most remarkable.
Gender Dimorphism of the bill
has been already noticed, and it extends in various ways to the head, wattles,
frontal plates, protuberances that are permanent or only temporarily erectile,
which are far too numerous to mention; but other much more special peculiarities
are the sublingual (i.e. under the tongue) bag of the Musk Duck (Biziura
lobata), the seasonal pouch of the Bustard,
and the inflatable sacs of the Prairie-fowls (GROUSE), while the convolutions
and enlargements of the trachea in many birds (e.g. MANUCODE) though
not externally visible produce an audible gender Dimorphism.
Size Dimorphism
Gender Dimorphism in size is also manifested among
birds-and this in both directions. To humans, it may seem natural that
the male would be the stronger and therefore the bigger gender, and among
Mammals he generally is. But in Birds this is by no means so much
the rule, the male being very considerably larger than the hen only in
certain Gallinaceous and Ratite groups, most of which are polygamous, and
hence a possible explanation may be afforded.On the other hand, although
a case in which the female is larger than the male is rarely found among
Mammals, instances occur among Reptiles (notably in Tortoises and Snakes)
and very frequently among Amphibians and Fishes. Among Birds it is almost
universal with the Accipitres. It also occurs in the Dotterel and several of the Limicolae, such as the GODWIT, PHALAROPE,
and Rhynchaea or painted SNIPE, as well as in some of the Turnicidae
(HEMIPODE).
No single explanation that will fit all these cases seems possible; but
in those of the Limicolae just mentioned, it is to be remarked that
the females are not only larger but are more conspicuously coloured than
the males, which latter are believed to perform exclusively the duty of
incubation. In the lower classes of Vertebrates the production of the often
numerous eggs may be the original cause of the greater size of the females.
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