University of Kansas Publications

Museum of Natural History



Volume 12, No. 12, pp. 521-551, 22 figs.

October 25, 1963

Jaw Musculature

Of the Mourning and White-winged Doves

BY


ROBERT L. MERZ

University of Kansas

Lawrence

1963

University of Kansas Publications, Museum of Natural History



Editors: E. Raymond Hall, Chairman, Henry S. Fitch,

Theodore H. Eaton, Jr.



Volume 12, No. 12, pp. 521-551, 22 figs.

Published October 25, 1963





University of Kansas

Lawrence, Kansas





PRINTED BY

JEAN M. NEIBARGER, STATE PRINTER

TOPEKA, KANSAS

1963



29-7865

[Pg 523]

Jaw Musculature

Of the Mourning and White-winged Doves

BY



ROBERT L. MERZ

For some time many investigators have thought that the genus
Zenaida, which includes the White-winged and Zenaida doves, and
the genus Zenaidura, which includes the Mourning, Eared, and
Socorro doves (Peters, 1937:83-88), are closely related, perhaps
more closely than is indicated by separating the several species
into two genera. It is the purpose of this paper to report investigations
on the musculature of the jaw of doves with the hope that,
together with the results of other studies, the relationships of the
genera Zenaida and Zenaidura can be elucidated.

METHODS AND MATERIALS

In order to determine in each species the normal pattern of musculature
of the jaws, heads of 13 specimens of doves were dissected (all material is in
the Museum of Natural History of The University of Kansas): White-winged
Doves (Zenaida asiatica), 40323, 40324, 40328, 40392, 40393; Zenaida Doves
(Z. aurita), 40399, 40400; Mourning Doves (Zenaidura macroura), 40326,
40394, 40395, 40396, 40397, 40398.

Thirty-seven skulls from the collection of the Museum of Natural History
of The University of Kansas and two skulls from the United States National
Museum were measured. The measurements are on file in the Library of The
University of Kansas in a dissertation deposited there by me in 1963 in partial
fulfillment of requirements for the degree of Master of Arts in Zoology. Specimens
used were: White-winged Doves, KU 19141, 19142, 19143, 19144,
19145, 19146, 19147, 23138, 23139, 24337, 24339, 24341, 23592, 23593,
24340, 31025, 31276; Mourning Doves, KU 14018, 14781, 15347, 15533,
15547, 15550, 15662, 15778, 15872, 16466, 17782, 17786, 17788, 17795,
19153, 19242, 20321, 21669, 22394, 22715; Eared Doves (Zenaidura auriculata),
USNM 227496, 318381. Additionally, the skulls of the Zenaida Doves
mentioned above were measured. All measurements were made with a dial
caliper and read to tenths of a millimeter.

ACKNOWLEDGMENTS

My appreciation is extended to Professor Richard F. Johnston, who advised
me during the course of this study, and to Professors A. Byron Leonard and
Theodore H. Eaton for critically reading the manuscript.

I would like also to acknowledge the assistance of Dr. Robert M. Mengel
and Mr. Jon C. Barlow for suggestions on procedure, and Mr. William C.
Stanley, who contributed specimens of Mourning Doves for study. Mr. Thomas[Pg 524]
H. Swearingen offered considerable advice on production of drawings and
Professor E. Raymond Hall suggested the proper layout of the same and gave
editorial assistance otherwise, as also did Professor Johnston.

MYOLOGY

The jaw musculature of doves is not an imposing system. The
eating habits impose no considerable stress on the muscles; the
mandibles are not used for crushing seeds, spearing, drilling, gaping,
or probing as are the mandibles of many other kinds of birds.
Doves use their mandibles to procure loose seeds and grains, which
constitute the major part of their diet (Leopold, 1943; Kiel and
Harris, 1956: 377; Knappen, 1938; Jackson, 1941), and to gather
twigs for construction of nests. Both activities require but limited
gripping action of mandibles. The crushing habit of a bird such as
the Hawfinch (Coccothraustes coccothraustes), on the other hand,
involves extremely powerful gripping (see, for example, Sims,
1955); the contrast is apparent in the development of the jaw musculature
in the two types. Consequently, it is not surprising to find a
relatively weak muscle mass in the jaw of doves, and because the
musculature is weak there are few pronounced osseous fossae,
cristae and tubercles. As a result, the bones, in addition to being
small in absolute size, are relatively weaker when compared to
skulls of birds having more distinctive feeding habits which require
more powerful musculature.

The jaw muscles of the species dissected for this study are, in
gross form, nearly identical from one species to another. Thus, a
description of the pertinent myology of each species is unnecessary;
one basic description is hereby furnished, with remarks on the
variability observed between the species.

The terminology adopted by me for the jaw musculature is in
boldfaced italic type. Synonyms are in italic type and are the
names most often used by several other writers.

The terminology adopted by me is that of Lakjar (1926) except that the
divisions of M. depressor mandibulae are designated by the Latinized equivalents
of the names used by Rooth (1953:261-262).

M. pterygoideus ventralis lateralis.—The origin is fleshy and by aponeurosis
on the ventral side of the palatine anterior to the palatine fossa. The insertion
is fleshy on the ventromedial surface of the lower mandible and continues
along the anteromedial surface of the internal angular process to its distal tip.
A few fibers leave pars lateralis and insert on an aponeurosis which receives
also all the fibers of M. pterygoideus dorsalis lateralis. The latter fact may
have prompted Rooth (1953:257) to make the statement that the fibers
originating on the dorsal part of the palatine inserted more laterally than those
originating on the ventral side. Rooth worked with Columba palumbus, the
Woodpigeon, and his description concerned M. adductor mandibulae internus
pterygoideus, which is composed of Mm. pterygoideus ventralis et dorsalis
of Lakjar (1926). His assertion that ventral fibers, that is to say, fibers arising
on the ventral surface of the palatine, insert medially does not appear to be
completely true for doves.

Aponeuroses cover most of the lower surface of the muscle and one or two
nerves extend into the substance of the muscle. The nerves run from the[Pg 526]
anterior edge of M. pterygoideus dorsalis medialis and farther posteriorly from
a separation in the muscle.

M. pterygoideus ventralis medialis.—The origin is by aponeurosis from
the ventral surface of the palatine and fleshy from the palatine fossa. The
aponeurosis is the same that gives origin to the fibers of pars lateralis. Part
of the aponeurosis becomes tendonlike in the middle of M. pterygoideus ventralis
and separates its two divisions. The insertion is fleshy on the lower
one-third of the anterior surface of the internal angular process of the lower
mandible, and by two tendons on the distal tip of that process. Many of the
fibers of pars medialis insert on the tendons. The fibers at their insertion are
not distinctly separate from those of pars lateralis and there is considerable
mingling of the fibers. Consequently, the medial part of M. pterygoideus
ventralis cannot be removed as a part distinct from the lateral part (figs. 1,
4, 10, 21 and 22).

Ordinarily M. pterygoideus ventralis does not cross the ventral edge of the
lower mandible, but in one white-wing the muscle was slightly expanded on
the right side and it could be seen in lateral view. The homologous muscle
in Columba palumbus apparently is consistently visible in lateral view. (See
Rooth, 1953, fig. 6.)

M. pterygoideus dorsalis medialis.—The origin is fleshy on the dorsolateral
surface of the palatine immediately anterior to the pterygoid and also on the
anterior, dorsolateral, posterior and ventromedial surfaces of the pterygoid.
The insertion is fleshy on the ventromedial surface of the lower mandible and
the anterior surface of the internal angular process immediately dorsal to the
insertion of M. pterygoideus ventralis lateralis.

M. pterygoideus dorsalis lateralis.—The origin is fleshy from the dorsolateral
surface of the palatine, anterior to the origin of pars medialis and the
insertion is by means of an aponeurosis on the medial surface of the lower
mandible, lateral to the insertion of M. pterygoideus ventralis lateralis. The
aponeurosis crosses the medial side of the insertion of M. pterygoideus dorsalis
medialis. The fibers run in a posteroventrolateral direction and insert on the
ventromedial side of the aponeurosis (figs. 1, 6, 8, 9, 13-22).

In one individual, a Mourning Dove, the origin of pars lateralis of M. pterygoideus
dorsalis extended to the pterygoid. With this one exception the
muscle was uniform throughout the several species.

M. adductor mandibulae externus.—This is the most complex muscle in
the jaw owing to its system of tendons and aponeuroses. Three divisions of
this muscle were described by Lakjar (1926:45-46) and the divisions appear
to be distinguishable in the doves, but there is no clear line of demarcation
for any of the parts and the following description is based upon my own attempts
to delineate the muscle.

M. adductor mandibulae externus superficialis.—The origin is fleshy from
the most lateral area of the temporal fossa. Dorsally the origin is bounded by
the base of the postorbital process and ventrally by the temporal process. The
fibers converge upon a tendon that passes beneath the postorbital ligament
and runs anteriorly among the fibers of pars profundus. The insertion is
tendinous on the dorsal surface of the lower mandible in common with the
dorsal aponeurosis of pars profundus. The insertion is immediately anterior[Pg 527]
to the ventral aponeurosis of pars profundus near the medial edge of the
dorsal surface on a tubercle at the posterior end of the dorsal ridge of the
lower mandible.

M. adductor mandibulae externus medialis.—The origin is by a flat, heavy
tendon from the temporal process. The tendon is attached almost vertically
on the temporal process. It twists approximately 130° as it runs anteriorly,
and becomes a thin aponeurosis, which gives rise on its dorsal and ventral
surfaces to many fibers that insert in a fan-shaped area on the mandibular
fossa. Fibers from the dorsal and dorsomedial sides of the heavy tendon run
rostrad and insert on the ventral surface of the dorsal aponeurosis of pars
profundus. From the ventral surface the most posterior fibers converge on
an aponeurosis that inserts on a transverse crista on the dorsal surface of the
mandible immediately lateral to the ventral aponeurosis of pars profundus and
dorsal to the insertion of M. adductor mandibulae posterior. The more anterior
fibers insert fleshily on the mandibular fossa. The tendon of origin is
actually one with the ventral aponeurosis of pars profundus, which is situated
in a horizontal plane. The insertion is primarily a fleshy attachment on the
mandibular fossa. Some of the fibers that arise on the dorsomedial and
lateral surfaces of the tendon of origin attach to another tendon, which inserts
in the midline of the mandibular fossa on a small tubercle near the anterior
end. Also, there is insertion by an aponeurosis anterior to M. adductor mandibular
posterior as stated above. Fibers attach to the dorsal and ventral side
of the aponeurosis.

M. adductor mandibulae externus profundus.—The origin is fleshy from
the medial surface of the temporal fossa, the posterior wall of the orbit and
the otic process of the quadrate. The origin is bounded laterally by the
origin of pars superficialis and medially by the origin of M. pseudotemporalis
superficialis. Ventrally the muscle lies against its own ventral aponeurosis,
which originates on the posterior wall of the orbit immediately above the articulation
of the otic process of the quadrate, and which also receives many fibers
from the surface of the quadrate. The insertion is primarily by means of
two aponeuroses. The most dorsal aponeurosis inserts on a tubercle at the
posterior tip of the dorsal edge of the mandible. The lateral tendon of
M. pseudotemporalis superficialis converges with the aponeurosis. It is superficial
and there are no fibers on its dorsal surface. The ventral aponeurosis
inserts on a crista immediately below the insertion of the dorsal aponeurosis.
It receives fibers on its ventral surface from the otic process of the quadrate,
and on its dorsal surface gives rise to fibers that insert on the dorsal aponeurosis
(figs. 2, 3, 5, 9, 10, 11, 13-18).

The tendon of insertion of pars medialis of M. adductor mandibulae externus
does not become a superficial aponeurosis posteriorly in the Zenaida Dove as
it does in the Mourning and White-winged doves.

M. pseudotemporalis profundus.—The origin is fleshy from the medial and
partially from the dorsal surface of the lower mandible. The origin is almost
completely anterior to and partly dorsal and ventral to the medial (most
anterior) insertion of M. pseudotemporalis superficialis. The anterior margin
of the origin is at the point where the mandibular ramus of the trigeminal
nerve enters the mandible. Posteriorly the origin is bounded by the insertion[Pg 528]
of M. adductor mandibulae posterior, and ventrally by a ridge that is situated
about halfway down the medial side of the mandible. The insertion is by
aponeurosis on the tip of the orbital process of the quadrate and fleshily on
the anterior surface of the same process. The aponeurosis extends about
three-fifths of the distance along the muscle and it is dorsal or superficial
to all of the fibers. Many fibers insert on the ventral side of the aponeurosis
(figs. 1, 5, 13, 14, 15, 16, 21 and 22).

This muscle is the most variable of all the jaw muscles. In the Mourning
Dove the muscle appears rather slender in dorsal view and in the White-winged
Dove has an enlarged lateral belly that gives the appearance of a
thicker muscle. In the Zenaida Dove M. pseudotemporalis profundus is intermediate
in shape between those of the other two species. This muscle will be
discussed in detail later.

M. protractor pterygoidei.—The origin is fleshy from the junction of the
sphenoidal rostrum and the interorbital septum. Fibers converge on the
pterygoid in anteroventrolateral and posteroventrolateral directions. The posterior
edge of the muscle is in contact with M. protractor quadrati with which
its fibers mingle. The insertion is fleshy on the posterior surface of the lateral
half of the pterygoid to its articulation with the body of the quadrate (figs.
6, 8, 9, 11, 13-20).

M. depressor mandibulae superficialis medialis.—The origin is fleshy from
the lateral edge of the basioccipital where the muscle is attached to Ligamentum
depressor mandibulae and extends in a lateral direction to a point
where the structures involved turn dorsad. The insertion is by fibers and a
light aponeurosis on the crista that is situated on the posteroventromedial edge
of the lower mandible.

M. depressor mandibulae superficialis lateralis.—The origin is fleshy from
the squamosal region, slightly posteroventral to the origin of M. adductor mandibulae
externus superficialis. A thin aponeurosis lies medial to the muscle
fibers. The insertion is by means of an aponeurosis that becomes tendonlike
along the posteroventrolateral crista and the posteriormost part of the ventral
edge of the lower mandible.

M. depressor mandibulae medialis.—The origin is fleshy from the lateral
and ventral surfaces of Ligamentum depressor mandibulae. The insertion is
fleshy on the posterior surface of the lower mandible, posterodorsal to the
insertions of partes superficialis medialis et lateralis (figs. 4, 9, 10, 13 and 14).

The parts of M. depressor mandibulae are difficult to distinguish from one
another because of considerable intermingling of fibers.

M. pseudotemporalis superficialis.—The origin is fleshy from the posterior
wall of the orbit, dorsal to the foramen of the trigeminal nerve, lateral to the
origin of M. protractor quadrati and medial to M. adductor mandibulae externus
profundus. The insertion is by means of an aponeurosis that bifurcates
at the point of contact with the mandibular ramus of the trigeminal nerve,
which is at the level of the orbital process of the quadrate (except in the
Mourning Dove where the division is more anterior), and which inserts as two
tendons on the dorsomedial edge of the lower mandible posterior to the
insertion of M. pseudotemporalis profundus. The lateral tendon is superficial
to the dorsomedial edge of M. adductor mandibulae externus, and converges
with the aponeurosis of pars profundus of that muscle and inserts with it on[Pg 529]
a tubercle near the dorsomedial edge of the mandible anterior to the insertion
of M. adductor mandibulae posterior as mentioned before. The anterior half
of the medial tendon lies ventral to the lateral edge of M. pseudotemporalis
profundus and the mandibular ramus of the trigeminal nerve. All of the
fibers of the muscle insert on the posteroventral surface of the aponeurosis
before it divides. Part of M. pseudotemporalis profundus also lies ventral to
the medial tendon of M. pseudotemporalis superficialis and, in effect, the
tendon is imbedded in the substance of M. pseudotemporalis profundus as it
proceeds anteriorly. The trigeminal nerve leaves a slight impression on the
ventral surface of the muscle near its origin (figs. 1, 3, 11, 13, 14, 15 and 16).

M. adductor mandibulae posterior.—The origin is fleshy from the anterodorsal
and anterior surfaces of the quadrate body, from the anterodorsolateral,
medial and anterior surfaces of the orbital process of the quadrate. The
muscle also has an origin from the otic process of the quadrate, partly fleshy
and partly by a slight aponeurosis. The insertion is fleshy on the dorsal and
lateral surfaces of the mandible immediately anterior to the articulating
surface. This muscle also has extensive insertion on the medial side of the
lower mandible dorsal to the insertion of M. pterygoideus dorsalis medialis and
posterior to the origin of M. pseudotemporalis profundus (figs. 1, 3, 5, 17,
18, 19 and 20).

The fibers of M. pseudotemporalis profundus can be distinguished from
the fibers of M. adductor mandibulae posterior because the pterygoideus nerve
passes between the two (Lakjar, 1926:55). Rooth (1953:255-256) considers
as part of this muscle the ventral aponeurosis of pars profundus of M. adductor
mandibulae externus and all the fibers ventral to it. But I could not justify
the inclusion of that aponeurosis as part of M. adductor mandibulae posterior
in the doves because none of the fibers of M. adductor mandibulae posterior
as I have described it were attached to that particular aponeurosis.

M. protractor quadrati.—The origin is fleshy from the posterior wall of
the orbit medial to the foramen of the trigeminal nerve and also medial to the
origin of M. pseudotemporalis superficialis. The origin describes an arc in
the horizontal plane until it reaches the interorbital septum and the optic
nerve. The insertion is fleshy on the posteromedial edge of the body of the
quadrate and the orbital process of the quadrate and on the otic process of
the quadrate. The muscle also inserts on the ventromedial surface of the
orbital process of the quadrate and the adjacent area of the body of the
quadrate (figs. 5, 7, 9, 11, 13-18).

M. protractor quadrati possesses many fibers that arise from M. protractor
pterygoidei. Consequently, it is difficult to determine the exact extent of the
origin or the insertion of either muscle.

ACTION OF JAW MUSCLES

M. pterygoideus ventralis.—Contraction of this muscle retracts the upper
mandible by moving the palatine posteriorly, and simultaneously adducts the
lower mandible.

M. pterygoideus dorsalis.—This muscle functions in essentially the same
manner as M. pterygoideus ventralis. The result of having a part of its origin
on the pterygoid as well as on the palatine is to facilitate retraction of the
upper mandible.[Pg 530]

M. adductor mandibulae.—This is the chief adductor of the lower mandible
and the muscle functions solely in that capacity. In birds having great
crushing ability, this muscle is much larger and more powerful and the skull
is reinforced behind the quadrate in order to withstand the pressure of the
lower mandible against the quadrate during adduction (Sims, 1955:374 and
Bowman, 1961:219-222).

M. pseudotemporalis profundus.—With origin and insertion on highly
movable bones, this muscle, when it contracts, retracts the upper mandible
and adducts the lower mandible. Like the pterygoid muscles, this muscle,
by itself, would allow the bird to grasp objects by means of its mandibles.
However, M. pseudotemporalis profundus could produce a more powerful grip
because it takes origin farther anteriorly on the lower mandible.

M. protractor pterygoidei.—Contraction of M. protractor pterygoidei pulls
the pterygoid anteromedially and causes it to slide forward along the sphenoidal
rostrum. This action aids in protraction of the upper mandible.

M. depressor mandibulae.—The depressor of the lower mandible is the
sole muscle other than M. geniohyoideus involved in the function of abducting
the lower jaw of doves. Its size can be correlated especially well with feeding
habits of the bird. Other birds that force their closed mandibles into fruit,
wood or the earth and then forcibly open them, belong to groups possessing
enlarged depressors. Contraction of the muscle pulls the postarticular (retroarticular)
process upward with the resultant downward movement of that
part of the mandible which is anterior to the articulation. Since there is no
"gaping" in doves the muscle is only large enough to overcome the inherent
tone of the relaxed adductor muscles.

In some non-passerine species as well as in certain passerines the muscle
also serves to raise the upper jaw by acting on the quadrate, which is capable
of rotating vertically on its otic process. Especially in the gapers, where
resistance is offered near the tip of the lower mandible, contraction of the
muscle pulls the entire mandible dorsad thus forcing the jugal and palatal struts
forward (Zusi, 1959:537-539). The action supplements that of Mm. protractor
pterygoidei et quadrati and is enhanced by anterior migration of the
origin of M. depressor mandibulae.

There is no lifting action involved in contraction of the depressor muscle
in doves for two reasons—(A) the origin of the muscle is situated much too
far posteriorly on the skull, and, more important, (B) the quadrate is not
hinged for vertical movement. As will be discussed later, it moves only in
a horizontal plane.

M. pseudotemporalis superficialis.—Like M. adductor mandibulae, this
muscle performs only the one function of adducting the lower mandible, and
like M. pseudotemporalis profundus it is a synergist of that muscle.

M. adductor mandibulae posterior.—Although this muscle undoubtedly
acts as an adductor of the lower mandible, I believe that, because of its disadvantageous
insertion so near the articulation, its main function must be
concerned with firming the mandible against the quadrate. This is to say
that its function is partially that of a ligament.

M. protractor quadrati.—When M. protractor quadrati contracts, the quadrate
bone is swung medially. This action, as mentioned previously, results in[Pg 531]
protraction of the upper jaw, and, as a consequence, its action supplements the
action of M. protractor pterygoidei.

CRANIAL OSTEOLOGY

The ability of most birds to protract the upper mandible, and
the structure of the skull which enables them to do so are responsible
for common reference to the skull as "kinetic" (Beecher, 1951a:412;
Fisher, 1955:175). The movement is effected by muscular action
on a series of movable bones that exert their forward force on the
upper mandible, which in turn swings on a horizontal hinge, the
"naso-frontal hinge," at the base of the beak. The bone initiating
the movement is the quadrate, which is hinged posteriorly by its
otic process and which ordinarily swings up or down depending
on the muscle or muscles being contracted at any given moment.
The upward swing of the quadrate pushes the jugal bar, which is
attached to its lateral tip, along its longitudinal axis, in an anterodorsal
direction, and the force is transferred to the upper mandible,
which is thereby elevated. A synergetic mechanism is simultaneously
initiated by the same bone—the quadrate. Since the quadrate
body articulates with the pterygoid, the upward movement forces
the pterygoid to slide along a ridge in the ventral midline of the
cranium, the sphenoidal rostrum, thus pushing the palatine forward
and exerting an upward push on the upper mandible.

In the columbids the quadrate has a bifurcated otic process that
functions as the hinge. The posterior tips of the forks are situated
almost vertically (one above the other) and the movement of the
quadrate is not so much up and down, or vertical, as it is horizontal
(fig. 12). When the quadrate moves medially the upper mandible
is protracted; a lateral movement results in retraction. There is a
slight, almost negligible, upward movement of the quadrate. The
movements of the various bony elements were observed on a skull
that had been made flexible by boiling in water for a minute as
suggested by Beecher (1951a:412).

Also in the columbids the naso-frontal hinge is not constructed
in the same manner as it is in many other birds as there is not a
simple hinge across the entire base of the beak. In fact, there is
no true hinge at all in the area of the nasals, but those bones are
extremely thin and they bend or flex under pressure. Actually,
the hinge is double or divided. One part is on either side of the
nasals. The hinges are situated at the posterodorsal tips of two
thin processes of the maxillary bones and the appearance is not
unlike that of half a span of a suspension bridge having the hinges[Pg 532]
at the tops of the towers. Several other species of birds share
this type of hinge construction with columbids.

The movement of the lower jaw is, of course, the primary operation
involved in opening the mouth. The lower jaw possesses a
deep fossa at its posterior end, or on its posterodorsal surface,
which articulates with the body of the quadrate bone. The length
of that part of the mandible extending behind the articulation is
directly correlated with the resistance offered the mandible in
opening, since it is on the posterior extension that the depressor
of the lower mandible inserts. The larger the muscle the more
surface is needed for attachment. Also the added length of the
mandible posterior to the articulation serves as a lever in opening
the mandible, and the fulcrum is moved relatively farther forward.

In birds lacking resistance to abduction of the lower mandible, as
in doves, it is nevertheless necessary for a slight postarticular
process to remain for the insertion of a small depressor muscle
which, as mentioned previously, is necessary to counteract the relaxed
adductor muscles of the lower jaw.

There are many exceptions to the rule that birds have kinetic
skulls, and usually a secondary fusion and reinforcement of bones
around the hinge has limited or eliminated all movement. Sims
(1955) describes the Hawfinch's immobile upper jaw, which is
used as a powerful press in cracking the stones of fresh fruit.
Skulls of woodpeckers have been modified somewhat in the same
manner as a result of their foraging and nesting habits (Burt, 1930).

The two most distantly related members of the genera under
investigation are the White-winged Dove, Zenaida asiatica, and the
Mourning Dove, Zenaidura macroura. They were chosen to demonstrate
differences and likenesses in proportions of members of the
genera.

Ten measurements were taken on each skull, but simple observation
reveals that, in relation to total length of the skull, the beak of
the White-winged Dove is longer than that of the Mourning Dove.
Tip of upper mandible to base of beak averaged 48.6 and 42.9
per cent of the total length of the skull in the White-winged Dove
and Mourning Dove, respectively. The position of the jugal bar
has remained about the same with respect to the cranial part of
the skull, and the entire cranial part of the skull is almost the same
shape in the species studied.

Likewise, in the White-winged Dove the distance from the
anterior tip of the lower mandible to the anterior part of M. adductor[Pg 533]
mandibulae externus is relatively longer in relation to the length
of the lower mandible than in the Mourning Dove. Finally, the
position of the jugal with respect to the naso-frontal hinge is about
the same in the two species.

Measurements and calculations indicate that the longer beak of
the White-winged Dove as compared with the Mourning Dove is
a function of the beak itself, not of differences in other parts of the
skull. Measurements of skulls of Eared and Zenaida doves support
this view.

OTHER MORPHOLOGICAL FEATURES

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[Pg 534]
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[Pg 535]
(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.

GENERIC RELATIONSHIP

An attempt will be made here to summarize all the available
evidence, direct or indirect, which bears on the problem of relationship
of these genera. The original dissections which are discussed
in this report are only valuable as one more bit of evidence concerning
one characteristic that aids in clarification of generic relationship,
and it is only in conjunction with other evidence that any
satisfactory conclusion may be forthcoming.

Morphology

My dissections demonstrated that, in relation to the size of the
doves, the jaw musculature of all the specimens investigated was
so nearly alike that only one major difference was detected. M.
pseudotemporalis profundus appeared to be enlarged in the White-winged
Dove. This might have been predicted, since the white-wing[Pg 536]
was also shown to possess an elongated beak, presumably an
adaptation for nectar-feeding, which would necessitate additional
muscle development in order to compensate for the added length.
Measurements recorded from several skulls indicated that the
heads of the birds (excluding the beak) are nearly proportional.

Perhaps plumage patterns are the most widely used characters
for determining generic relationships of birds. Ridgway (1916:339-385)
followed the columbid classification of Salvadori (1893)
using plumage patterns and body proportions to distinguish between
the genera. In the genus Zenaidura he included the unique
specimen Zenaidura yucatanensis, and he placed auriculata in
Zenaida. The White-winged Dove was referred to a separate genus,
Melopelia. He described the genus Zenaidura in the following
manner:

He noted that the plumage of Zenaida was almost precisely as
described for Zenaidura. Also, although all members of Zenaida
reputedly possessed twelve rectrices, a characteristic of the genus,
it was later found that auriculata possessed fourteen rectrices. The
species was promptly placed in the genus Zenaidura by Peters
(1934:213-215). In plumage and coloration, Melopia was described
as similar to Zenaida and Zenaidura but without black spots on the
wings.

The White-winged Dove also has twelve rectrices, but Bond
(1940:53) and Goodwin (1958:330-334) considered the number
and shape of rectrices to be of minor importance when compared
to the homologous markings of the plumage. Goodwin stated that
his conclusion was emphasized by the fact that the tail of auriculata
is intermediate in length and shape between those of macroura and
aurita. In summary Goodwin "lumped" the genera Zenaida and
Zenaidura under the genus Zenaida.

Nidification

It has been adequately documented that members of these
genera closely resemble one another in their nesting and egg-laying
habits. Bent (1932:407, 417), Davie (1889:157), Goss (1891:242)[Pg 537]
and Nice (1922:466) have described the two, white eggs of the
clutch of the Mourning Dove. They have also noted that their
nests are composed mainly of twigs and may be constructed in
trees, shrubs or on the ground. The Eared Dove has nearly identical
habits (Bond, 1961:104), and a similar situation exists with the
Zenaida Dove (Audubon, 1834:356; Bent, 1932:418-419).

Like the other species, White-winged Doves lay two white or
buffy eggs per clutch and build frail nests of sticks (Bent, 1932:431;
Wetmore, 1920:141; Baird, Brewer and Ridgway, 1905:377).

The point to be made here is simply this: If the species in
question are to be considered congeneric then it might reasonably
be expected that they would display some similarity in nidification
and egg-laying. If their habits varied considerably it would not
necessarily mean that their relationship was more distant, but
similarities can usually be considered indicative of affinities because
they are the phenotypic expression of the partially unaltered genotype
of the common ancestor.

Interbreeding

Intergeneric crosses of columbids in captivity are common, but
in nature there is little evidence that even interspecific crosses
occur. Only one apparent hybrid between members of the genus
Zenaida and genus Zenaidura has ever been discovered. The individual
was taken on the Yucatan peninsula of Mexico, and was described
and named as a new species (Zenaidura yucatanensis).

Salvadori (1893:373), Ridgway (1916:353) and Peters (1934:213-215)
agree that Zenaidura yucatanensis Lawrence is a hybrid
between Zenaidura macroura marginella and Zenaida aurita yucatanensis.
Ridgway (1916:355), however, notes that "... If Zenaidura
yucatanensis Lawrence should prove to be really a distinct
species, and not a hybrid ... unquestionably Zenaida and
Zenaidura can not be separated generically, since the former is in
every way exactly intermediate between the two groups." In the
event that the unique type is a hybrid, the very fact of its existence
supports the hypothesis that the genera are more closely related
than is currently recognized.

Serology

There have been no investigations having as their sole purpose
the clarification of the relationship of the genera Zenaida and Zenaidura.
But some work has involved the comparison of the antigenic
content of individual columbids with the antigenic content
of a member of another species of the same family.[Pg 538]

Irwin and Miller (1961) tested, along with other columbids,
members of Zenaida and Zenaidura for presence of, 1) species-specific
antigens of Columba guinea (in relation to Columba livia)
which are designated A, B, C and E, and, 2) species-specific antigens
of C. livia (in relation to C. guinea) which are designated
A´, B´, C´ and E´.

In the first test all five species of Zenaida and Zenaidura possessed
antigens A and C, and all but auriculata possessed E. None of the
species gave evidence of the presence of the B antigen of C. guinea
in their blood. In the latter test only macroura had A´, only asiatica
had B´ (aurita was not tested for B´), and none had C´ or E´.

These results would indicate that the five species are similar
regarding antigenic content of the blood, and the variation is not
consistent within one or the other genus as presently known.

SUMMARY AND CONCLUSION

The avian genus Zenaida is currently considered to be distinct
from the genus Zenaidura by most columbid taxonomists. The jaw
muscles of six Mourning Doves (Zenaidura) and five White-winged
Doves (Zenaida) were investigated as to differences and similarities
that might clarify the relationships of the genera. The sizes and
proportions of skulls were also considered in 37 Mourning and
White-winged doves and two Eared Doves. Larger size of M.
pseudotemporalis profundus, the muscle that functions simultaneously
as an adductor of the lower jaw and retractor of the upper
jaw, in the White-winged Dove was the character found in the jaw
musculature that could be used to support the contention that
Zenaidura and Zenaida represent distinct genera. Larger size of
this muscle in the white-wing seems to be related to its elongated
beak. The long beak apparently is used for nectar-feeding in
flowers of the Saguaro Cactus.

Excluding the beak, skulls of the white-wing and Mourning
doves are of nearly the same shape. Previous investigators have
shown that in Zenaida and Zenaidura plumage patterns are similar,
nesting habits and eggs are nearly identical, blood proteins are
similar, and one "intergeneric" hybridization in nature is known.

Consequently, it is concluded that species of the two alleged
genera are congeneric, and I agree with Goodwin (1958) that the
genus Zenaida (Bonaparte, 1838:41) should include the Mourning
Dove, Eared Dove, Socorro Dove, Zenaida Dove, and White-winged[Pg 539]
Dove. Their Latin binomina are Zenaida macroura, Zenaida auriculata,
Zenaida graysoni, Zenaida aurita, and Zenaida asiatica, respectively.

Fig. 1. Medial view of left ramus of lower mandible of Mourning Dove. × 2-1/2.



Fig. 2. Lateral view of right ramus of lower mandible of Mourning Dove. × 2-1/2.

[Pg 540]

Fig. 3. Dorsal view of lower mandible of Mourning Dove. × 2-1/2.



Fig. 4. Ventral view of lower mandible of Mourning Dove. × 2-1/2.

[Pg 541]

Fig. 5. Dorsal view of right quadrate of Mourning Dove. × 5.



Fig. 6. Dorsal view of right pterygoid of Mourning Dove. × 5.



Fig. 7. Ventral view of right quadrate of Mourning Dove. × 5.



Fig. 8. Ventral view of right pterygoid of Mourning Dove. × 5.

[Pg 542]

Fig. 9. Right lateral view of skull of Mourning Dove. × 2-1/2.



Fig. 10. Ventral view of skull of Mourning Dove. × 2-1/2.

[Pg 543]

Fig. 11. Cross section of skull of Mourning Dove; anterior
view. × 2-1/2.



Fig. 12. Dorsal view of right quadrate of Mourning Dove
showing movement which protracts the upper mandible
(broken line). × 5.

[Pg 544]

Fig. 13. Right lateral view of the jaw musculature of the White-winged Dove;
superficial layer, × 5.



Fig. 14. Right lateral view of the jaw musculature of the Mourning Dove;
superficial layer. × 5.

[Pg 545]

Fig. 15. Dorsal view of the jaw musculature of the White-winged Dove
(right side); superficial layer. × 5.



Fig. 16. Dorsal view of the jaw musculature of the Mourning Dove (right
side); superficial layer. × 5.

[Pg 546]

Fig. 17. Dorsal view of the jaw musculature of the White-winged Dove (right
side); middle layer. × 5.



Fig. 18. Dorsal view of the jaw musculature of the Mourning Dove (right
side); middle layer. × 5.

[Pg 547]

Fig. 19. Dorsal view of the jaw musculature of the White-winged Dove
(right side); deep layer. × 5.



Fig. 20. Dorsal view of the jaw musculature of the Morning Dove
(right side); deep layer. × 5.

[Pg 548]

Fig. 21. Ventral view of the jaw musculature of the White-winged Dove
(M. depressor mandibulae not shown). × 5.



Fig. 22. Ventral view of the jaw musculature of the Mourning Dove (M.
depressor mandibulae not shown). × 5.

[Pg 549]

LITERATURE CITED

Adams, L. A.

1919. A memoir on the phylogeny of the jaw muscles in recent and fossil
vertebrates. Annals New York Acad. Sci., 28:51-166.



Audubon, J. J.

1834. Ornithological biography. Vol. II. Adam & Charles Black, Edinburgh,
xxxii + 588 pp.



Baird, S. F., Brewer, T. M., and Ridgway, R.

1905. The land birds of North America. Little, Brown, and Company,
Boston, 560 + xxvii pp.



Beecher, W. J.

1950. Convergent evolution in the American orioles. Wilson Bull. 62:51-86.

1951a. Adaptations for food-getting in the American blackbirds. Auk,
68:411-440.

1951b. Convergence in the Coerebidae. Wilson Bull., 63:274-287.

1953. A phylogeny of the oscines. Auk, 70:270-333.



Bent, A. C.

1932. Life histories of North American gallinaceous birds. Bull. U. S.
Nat. Mus., 162:xi + 490 pp., 93 pls.



Bonaparte, C. L.

1838. Geographical and comparative list of the birds of Europe and
North America. John Van Voorst, London, vii + 68 pp.



Bond, J.

1961. Birds of the West Indies. Houghton Mifflin Company, Boston. 256
pp., 8 pls., 186 figs.



Bowman, R. I.

1961. Morphological differentiation and adaptation in the Galapagos
finches. Univ. California Publ. Zool., 58:vii + 302 pp., 22 pls.,
74 figs., 63 tables.



Burt, W. H.

1930. Adaptive modifications in the woodpeckers. Univ. California Publ.
Zool., 32:455-524.



Cain, A. J.

1956. The genus in evolutionary taxonomy. Syst. Zool., 5:97-109.



1959. Taxonomic concepts. Ibis, 101:302-318.



Davie, O.

1889. Nests and eggs of North American birds. Hann & Adair, Columbus,
455 + xii pp., 13 pls.



Edgeworth, F. H.

1935. The cranial muscles of vertebrates. Cambridge Univ. Press, viii +
493 pp., 841 figs.



Engels, W. L.

1940. Structural adaptations in thrashers (Mimidae: genus Toxostoma)
with comments on interspecific relationships. Univ. California
Publ. Zool., 42:341-400, 24 figs., 11 tables.



Fisher, H. I.

1955. Some aspects of the kinetics in the jaws of birds. Wilson Bull.,
67:175-188, 4 figs., 6 tables.



Gadow, H.

1891. Vogel: I. Anatomischer Theil. Bronn's Klassen und Ordnungen des
Thier-Reichs. C. F. Winter, Leipzig, 6:1-1,008, many figs., 59 pls.



Gilman, M. F.

1911. Doves on the Pima Reservation. Condor, 13:51-56.



Goodman, D. C., and Fisher, H. I.

1962. Functional anatomy of the feeding apparatus in waterfowl. Southern
Illinois Univ. Press, Carbondale, xii + 193 pp.
[Pg 550]


Goodwin, D.

1958. Remarks on the taxonomy of some American doves. Auk, 75:330-334.



Goss, N. S.

1891. History of the birds of Kansas. Geo. W. Crane & Co., Topeka,
692 pp., 35 pls.



Hensley, M. M.

1954. Ecological relations of the breeding bird populations of the desert
biome of Arizona. Ecol. Monographs, 24:185-207.



Hofer, H.

1950. Zur Morphologie der Kiefermuskulatur der Vogel. Zool. Jahrb.
Jena (Anat.), 70:427-556, 44 figs.



Irwin, M. R., and Miller, W. J.

1961. Interrelationships and evolutionary patterns of cellular antigens in
columbidae. Evolution, 15:30-43.



Jackson, A. S.

1941. The mourning dove in Throckmorton County, Texas, Unpubl.
manuscript (Abstract).



Kiel, W. H., Jr., and Harris, J. T.

1956. Status of the white-winged dove in Texas. Trans. 21st N. Amer.
Wildl. Conf., pp. 376-388.



Knappen. P.

1938. Preliminary report on some of the important foods of the mourning
dove in the southeastern U. S. Trans. 3rd N. Amer. Wildl. Conf.,
pp. 776-781.



Lakjer, T.

1926. Studien Uber die Trigeminus-versorgte Kaumuskulatur der Sauropsiden.
C. A. Reitzel Buchhandlung, Kopenhagen, 154 pp., 26 pls.



Leopold, A. S.

1943. Autumn feeding and flocking habits of the mourning dove in
southern Missouri. Wilson Bull., 55:151-154.



McGregor, S. E., Alcorn, S. M., and Olin, G.

1962. Pollination and pollinating agents of the saguaro. Ecology, 43:259-267.



Nice, M. M.

1922. A study of the nesting of mourning doves. Auk, 39:457-474;
40:37-58.



Peters, J. L.

1934. The classification of some American pigeons. Condor, 36:213-215.



1937. Check-list of birds of the world. Vol. III. Harvard Univ. Press,
Cambridge, xiii + 311 pp.



Rooth, J.

1953. On the correlation between the jaw muscles and the structure of
the skull in Columba palumbus. Kon. Ned. Akad. Wet.; Proc.
Sect. Sci., Vol. LVI, serie C, pp. 251-264.



Salvadori, T.

1893. Catalogue of birds in the British Museum, 21:xvii + 676 pp., 15
pls. + 17 pp.



Shufeldt, R. W.

1890. The myology of the raven. MacMillan & Co., London, xix + 343
pp., 76 figs.
[Pg 551]


Sims, R. W.

1955. The morphology of the head of the hawfinch. Bull. Brit. Mus.
(Nat. Hist.) Zool., 2:369-393.



Wetmore, A.

1920. Observations of the habits of the white-winged dove. Condor,
22:140-146.



Zusi, R. L.

1959. The function of the depressor mandibulae muscle in certain passerine
birds. Auk, 76:537-539.

Transmitted June 3, 1963.