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Title: Jaw Musculature of the Mourning and White-winged Doves

Author: Robert L. Merz

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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




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 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.

     ~_M. pterygoideus ventralis:_~ part of Mm. pterygoidei, Gadow,
     1891:323-325, table 26, figs. 1, 2, 3 and 4, and table 27,
     fig. 3--part of M. pterygoideus internus, Shufeldt, 1890:20,
     figs. 3, 5, 6, 7 and 11--part of M. adductor mandibulae
     internus, Edgeworth, 1935:58, figs. 605c and 607--part of M.
     pterygoideus anterior, Adams, 1919:101, pl. 8, figs. 2 and 3.

     ~_M. pterygoideus dorsalis:_~ part of Mm. pterygoidei,
     Gadow, 1891:323-325, table 26, fig. 7 and table 27, figs. 1
     and 3--part of M. pterygoideus internus, Shufeldt,
     1890:20--part of M. adductor mandibulae internus, Edgeworth,
     1935:58, fig. 605c--? part of M. pterygoideus anterior,
     Adams, 1919:101, pl. 8, figs. 2 and 3.

     ~_M. adductor mandibulae externus:_~ _a_) ~_pars
     superficialis:_~ parts 1 and 2 of M. temporalis, Gadow,
     1891:320-321--part of M. temporal, Shufeldt, 1890:16,
     figs. 5 and 7--part of M. adductor mandibulae externus,
     Edgeworth, 1935:58-60--M. capiti-mandibularis medius and
     profundus, Adams, 1919:101, pl. 8, fig. 1.

     _b_) ~_pars medialis:_~ ? parts 1, 2 and 3 of M. temporalis,
     Gadow, 1891:320-322--part of M. masseter and ? part of M.
     temporal, Shufeldt, 1890:16-18, figs. 5, 6, 7 and 11--part
     of M. adductor mandibulae externus, Edgeworth,
     1935:58-60--M. capiti-mandibularis superficialis, first
     part, Adams, 1919:100-101, pl. 8, fig. 1.

     _c_) ~_pars profundus:_~ part 2 of M. temporalis, Gadow,
     1891:321, table 27, fig. 2--part of M. temporal and ? part
     of M. masseter, Shufeldt, 1890:16-18--part of M. adductor
     mandibulae externus, Edgeworth, 1935:58-60--? part of M.
     capiti-mandibularis medius and all of pars superficialis,
     second part, Adams, 1919:100-101.

     ~_M. pseudotemporalis profundus:_~ M. quadrato-maxillaris,
     Gadow, 1891:322-323--M. pterygoideus externus, Shufeldt,
     1890:20-21, figs. 3, 5 and 11--part of M. adductor mandibulae
     medius, Edgeworth, 1935:58-59--? part of M. pterygoideus
     posterior, Adams, 1919:101, pl. 8, figs. 2 and 3.

     ~_M. protractor pterygoidei:_~ part 4b of M. temporalis,
     Gadow, 1891: 322-323, table 27, fig. 4--part of M.
     entotympanious, Shufeldt, 1890:19-20, figs. 3 and 11--part
     of M. spheno-pterygo-quadratus, Edgeworth, 1935:57.

     ~_M. depressor mandibulae:_~ M. digastricus s. depressor
     mandibulae, Gadow, 1891:318-319--M. biventer maxillae,
     Shufeldt, 1890:18-19, figs. 3, 4, 5, 6, 7 and 11.

     ~_M. pseudotemporalis superficialis:_~ M. spheno-maxillaris,
     Gadow, 1891:323--part of M. temporal, Shufeldt, 1890:16--part
     of M. pseudotemporalis, Hofer, 1950:468-477--part of M.
     adductor mandibulae medius, Edgeworth, 1935:277.

     ~_M. adductor mandibulae posterior:_~ ? part of M. temporal,
     Shufeldt, 1890:16--part of M. adductor mandibulae medius,
     Edgeworth, 1935:58-59--? part of M. pterygoideus posterior,
     Adams, 1919:101, pl. 8, figs. 2 and 3.

     ~_M. protractor quadrati:_~ part 4a of M. temporalis, Gadow,
     1891:322-323, table 27, fig. 4--part of M. entotympanicus,
     Shufeldt, 1890:19-20, figs. 3 and 11--part of M.
     spheno-pterygo-quadratus, Edgeworth, 1935:57.

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 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 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 deg. 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 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 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.

~_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 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 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 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 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.


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 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:

     "Plumage of head, neck and under parts soft and blended;
     bare orbital space moderate, broadest beneath eyes.
     Coloration plain, the proximal secondaries (sometimes
     adjacent wing-coverts and scapulars also) spotted with
     black; rectrices (except middle pair) with a black band
     across postmedian portion, the apical portion paler gray
     than basal portion, sometimes white; a small black
     subauricular spot; adult males with head, neck and anterior
     under parts more or less vinaceous and sides of neck glossed
     with metallic purple."

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) 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.

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 Dove. Their Latin
binomina are _Zenaida macroura_, _Zenaida auriculata_, _Zenaida
graysoni_, _Zenaida aurita_, and _Zenaida asiatica_, respectively.

[Illustration: FIG. 1. Medial view of left ramus of lower
mandible of Mourning Dove. x 2-1/2.

FIG. 2. Lateral view of right ramus of lower mandible of
Mourning Dove. x 2-1/2.]

[Illustration: FIG. 3. Dorsal view of lower mandible of
Mourning Dove. x 2-1/2.

FIG. 4. Ventral view of lower mandible of Mourning Dove.
x 2-1/2.]

[Illustration: FIG. 5. Dorsal view of right quadrate of
Mourning Dove. x 5.

FIG. 6. Dorsal view of right pterygoid of Mourning Dove. x 5.

FIG. 7. Ventral view of right quadrate of Mourning Dove. x 5.

FIG. 8. Ventral view of right pterygoid of Mourning Dove. x 5.]

[Illustration: FIG. 9. Right lateral view of skull of Mourning
Dove. x 2-1/2.

FIG. 10. Ventral view of skull of Mourning Dove. x 2-1/2.]

[Illustration: FIG. 11. Cross section of skull of Mourning Dove;
anterior view. x 2-1/2.

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

[Illustration: FIG. 13. Right lateral view of the jaw musculature of
the White-winged Dove; superficial layer, x 5.

FIG. 14. Right lateral view of the jaw musculature of the Mourning
Dove; superficial layer. x 5.]

[Illustration: FIG. 15. Dorsal view of the jaw musculature of the
White-winged Dove (right side); superficial layer. x 5.

FIG. 16. Dorsal view of the jaw musculature of the Mourning Dove
(right side); superficial layer. x 5.]

[Illustration: FIG. 17. Dorsal view of the jaw musculature of the
White-winged Dove (right side); middle layer. x 5.

FIG. 18. Dorsal view of the jaw musculature of the Mourning Dove
(right side); middle layer. x 5.]

[Illustration: FIG. 19. Dorsal view of the jaw musculature of the
White-winged Dove (right side); deep layer. x 5.

FIG. 20. Dorsal view of the jaw musculature of the Morning Dove (right
side); deep layer. x 5.]

[Illustration: FIG. 21. Ventral view of the jaw musculature of the
White-winged Dove (_M. depressor mandibulae_ not shown). x 5.

FIG. 22. Ventral view of the jaw musculature of the Mourning Dove
(_M. depressor mandibulae_ not shown). x 5.]




LITERATURE CITED


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_Transmitted June 3, 1963._




       *       *       *       *       *

Transcriber's Notes

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