In the species dissected, the only variable muscle that I consider
significant in revealing relationships is _M. pseudotemporalis
profundus_. It is markedly enlarged in the White-winged Dove in
relation to the homologous muscle in the Mourning Dove. The muscle is
enlarged in such a manner that a lateral expansion of its mass is
apparent in superficial or dorsal view (compare figures 15 and 16).
This, of course, indicates a muscle with powerful contraction, which
has been unable to enlarge its circumference symmetrically because the
eye is immediately dorsal to the muscle. Therefore it has expanded
laterally. Ventral expansion is blocked by the presence of other
muscles, and medially there is no surface for the insertion of
additional fibers on the orbital process of the quadrate.
The jaw musculature has been known for some time to be highly adaptive
(Beecher, 1951a and b, 1953; Bowman, 1961; Burt, 1930; Engels, 1940 and
Goodman and Fisher, 1962) and it would not be unreasonable, I think, to
expect the jaw muscles of closely related species with similar habits
to be similar. The beak of the White-winged Dove is longer in
proportion to the length and height of the skull (exclusive of the
beak) than is the beak of the Mourning Dove. The lengthened beak is
probably an adaptation for nectar-feeding, which has been documented by
McGregor, Alcorn and Olin (1962:263-264) while investigating
pollinating agents of the Saguaro Cactus (_Cereus giganteus_), and by
Gilman (1911:53) who observed the birds thrusting their bills into the
flowers of the plant. Gilman indicated, however, that he could not be
sure if the birds were seeking insects, pollen, or nectar. In any event
the lengthened bill probably facilitates getting food by birds that
probe parts of flowers. Hensley (1954:202) noted that both Mourning and
White-winged doves were "exceptionally fond of this source of
nourishment." But he also points out an "interesting correlation"
between the presence of the white-wings in the desert and the flowering
of the saguaro. During his studies the appearance of the first
white-wing preceded the opening of the first saguaro flower by two
days. The flowering and fruiting season lasted until August, the month
of termination of the white-wing breeding season.
Since Hensley makes the correlation solely with the white-wings, I
assume that there is no other obvious correlation between plants and
birds among the remainder of the avifauna of the desert. Probably the
Mourning Dove has failed to adapt to nectar-feeding as yet, and the
White-winged Dove is the primary exploiter of this food niche. It
should be noted, also, that the head of the Mourning Dove is smaller
than the white-wing's, and perhaps there is no need for an elongated
beak for reaching deeply into the flowers.
The lengthened bill should produce no difficulties in protraction of
the upper mandible and depression of the lower for the reason that in
the dove there is no known resistance offered to these movements. The
genus _Icterus_ furnishes an example wherein resistance is met in the
process of opening the mandibles; individuals of this genus thrust
their closed bill into certain fruits and forcibly open their mandibles
against the resistance of the pulp by strong protraction and
depression, thus permitting the juices of the fruit to lake and
ultimately to be consumed (Beecher, 1950:53). Beecher refers to the
technique used in fruit-eating as "gaping." The result of gaping in
_Icterus_ should be the presence of a more massive set of muscles
concerned with protraction and depression than is found in non-gaping
groups. Beecher found the situation to be exactly as expected in that
genus and in other genera which also gape. Meadowlarks (_Sturnella_)
and caciques (_Archiplanus_) gape and pry in soil and wood respectively
(Beecher, 1951a:422 and 426).
The lengthened beak would be a problem when the White-winged Dove
attempted to pick up objects such as seeds, which do in fact constitute
the largest percentage of its diet in spite of its nectar-feeding
habit. A similar situation exists in the genus _Icterus_, which is
primarily adapted for gaping even though it shows a preference for
insects when they are abundant (Beecher, 1950:53). The lengthened beak
could be compensated for by (A) migration of the anterior end of the
jugal bar toward the rostral tip of the bill and away from the
fronto-nasal hinge with a simultaneous enlargement of the adductor
muscles of the lower mandible, or (B) enlargement of the one muscle
that functions simultaneously as an efficient retractor of the upper
mandible and adductor of the lower mandible, namely _M.
pseudotemporalis profundus_. _Mm. pterygoideus dorsalis et lateralis_
perform the same function, but because of their position on the lower
mandible they, apparently, are stronger retractors of the upper
mandible than they are adductors of the lower.
It will be recalled that the jugal bar bears the same, or nearly the
same, relationship to the cranium in the white-wing as it does in the
Mourning Dove and that the heads, excluding the beaks of both species,
are of nearly the same proportions. Also, _Mm. adductor mandibulae
externus_ and _pseudotemporalis superficialis_, the chief adductor
muscles of the lower mandible, were not noticeably enlarged in the
white-wing. It is also important to note that other combinations of
migration of bone and/or enlargement of muscles could successfully
solve the problem of providing sufficient leverage for the proper
functioning of the lengthened mandibles, but it is my thesis that the
second alternative sufficed for seed-eating habits and that that is the
adaptation that was established; it is, in fact, the only one present
in the White-winged Dove.
It is unlikely that this enlarged muscle and beak are the remains of
another series of jaw muscles that have converged toward the condition
in Mourning Doves. Columbids are almost unquestionably monophyletic,
and two lines would have had to diverge and then converge. There is no
evidence for such an evolutionary occurrence.