The erection mechanism of the ratite penis

Abstract

The erection mechanism of the penis in most vertebrates is blood vascular. A major evolutionary transition occurred in birds, where the erection mechanism changed from blood vascular to lymphatic. Within birds, however, the erection mechanism of the ratite penis has remained unknown. Early work suggested that the erection mechanism in ostrich Struthio camelus was blood vascular while no description existed for the emu Dromaius novaehollandiae or the rhea Rhea ameri-cana. Because the penis in all other described birds has a lymphatic erection mechanism, clarifying that the erection mechanism of ratites is of great importance to understanding one of the major evolutionary transitions of penis morphology within amniotes. Here, we show that the erection mechanism of ratites is lymphatic, confirming that the evolutionary transition to lymphatic erection occurred in the last common ancestor of Aves.

Full text

The erection mechanism of the ratite penis
Patricia L. R. Brennan & Richard O. Prum
Department of Ecology and Evolutionary Biology and Peabody Museum of Natural History, Yale University, New Haven, CT, USA
Keywords
genital evolution; penis morphology;
lymphatic erection.
Correspondence
Patricia L. R. Brennan. Current address:
Department of Biology, University of
Massachusetts, Amherst, MA 01003, USA.
Tel: +413-545-1696; Fax: +413-545-3243
Email: pbrennan@bio.umass.edu
Editor: Virginia Hayssen
Received 24 May 2011; revised 25 July
2011; accepted 28 July 2011
doi:10.1111/j.1469-7998.2011.00858.x
Abstract
The erection mechanism of the penis in most vertebrates is blood vascular. A
major evolutionary transition occurred in birds, where the erection mechanism
changed from blood vascular to lymphatic. Within birds, however, the erection
mechanism of the ratite penis has remained unknown. Early work suggested that
the erection mechanism in ostrich Struthio camelus was blood vascular while no
description existed for the emu Dromaius novaehollandiae or the rhea Rhea ameri-
cana. Because the penis in all other described birds has a lymphatic erection
mechanism, clarifying that the erection mechanism of ratites is of great impor-
tance to understanding one of the major evolutionary transitions of penis mor-
phology within amniotes. Here, we show that the erection mechanism of ratites is
lymphatic, confirming that the evolutionary transition to lymphatic erection
occurred in the last common ancestor of Aves.
Introduction
Most birds lack a penis, but both the Paleognathes and the
Galloanseridae are among the few avian groups that retain it
(Montgomerie & Briskie, 2007).
The avian penis is likely homologous with the reptile penis
(Jones, 1915; King, 1981); however, several characters of the
penis have changed between the two groups. One important
difference is the erection mechanism, which in reptiles is gen-
erally described as blood vascular (Jones, 1915; Gerhardt,
1933), whereas in birds is generally considered lymphatic
(King, 1981). The lymph needed for lymphatic erection to take
place in birds is produced in the paralymphatic bodies [also
referred to as the lymphobulbus phalli (King, 1993) and the
vascular body of the phallus (Oliveira & Mahecha, 2000)],
which are ellipsoid spongy organs located alongside the
urodeum (or middle compartment of the cloaca).
Within birds, the erection mechanism of the Galloanseridae
has shown to be lymphatic, whereas the erection mechanism
of the ratites has remained contentious (King, 1981; Mont-
gomerie & Briskie, 2007). Early reports suggested that the
erection mechanism of the ostrich could be blood vascular
(Müller, 1836; Boas, 1891), whereas others have suggested
that the erection mechanism is lymphatic (Grimpe, 1923; cited
by Gerhardt, 1933). The presence of a ‘spongy cushion’ lying
ventrally between the urodeum and the base of the phallus in
the ostrich was reported by Grimpe (1923; cited by Gerhardt,
1933), which Gerhardt considered to be the paralymphatic
bodies. However, the original reference has been lost, and no
further work on establishing the presence or absence of para-
lymphatic bodies has been carried out. King (1981, p. 137)
declared that ‘the lack of reliable information about the pres-
ence or absence of a vascular body in the ratites remains one
of the most striking defects in our knowledge of morphology
in birds’.
The erection mechanism of all large-bodied ratites has
remained unknown despite recent work on cloacal morphol-
ogy in the ostrich. A recent examination of the ostrich cloaca
did not comment on the function or components of the
phallus (Warui, Erlwanger & Skadhauge, 2009). Detailed
work on the ostrich’s blood supply reported that thin
branches of the pudendal artery are found near the base of the
phallus where they form an arterial plexus or network (Elias,
Aire & Soley, 2007, 2008).
Recently, two studies showed a clear presence of paralym-
phatic bodies in the tinamous (family tinamidae), the closest
relatives of the ratites: in the intromittent penis of the
spotted tinamou Nothura maculosa (Oliveira & Mahecha,
2000) and in the non-intromittent penis of the genus Cryp-
turellus (family tinamidae) (Brennan et al., 2008). Although
tinamous were recently placed within the order ratites
(Harshman et al., 2008), this hypothesis is not universally
supported.
The phallus of large-bodied ratites has a fixed portion that
attaches to the floor of the proctodeum (the cloacal compart-
ment nearest the vent), and a free portion that constitutes the
body of the penis. The body of the penis in ratites has three
major components: a pair of fibrous bodies that begin in the
fixed portion of the penis (the left one being larger than the
right), a core of elastic tissue known as the elastic vascular
body, and an external channel through which sperm travels
(sulcus spermaticus) (King, 1981).
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Journal of Zoology •• (2011) ••–•• © The Authors. Journal of Zoology © 2011 The Zoological Society of London 1
been

The penis of the ostrich is a single shaft that bends to the left
because of the asymmetry in size of the fibrous bodies. The
fibrous bodies have been reported to lack any erectile spaces
(Müller, 1836), although King (1981) doubts this conclusion.
During copulation and defecation, the penis protrudes from
the floor of proctodeum where it usually rests (Gerhardt,
1933; Fig. 1a and b). Movement of the penis is achieved
through the action of the muscles that lift and retract the
phallus (musculus levator phalli and musculus retractor phalli).
The ostrich penis lacks an invaginated portion of the penis,
and therefore, it is classified as a type A phallus (lacking a
blind tubular cavity; King, 1981).
The penis of the emu (Fig. 1d) was described in some detail
by Müller (1836), Boas (1891) and Gerhardt (1933), again
without noting the erection mechanism. The emu penis has
asymmetric fibrous bodies and an eversible region at the tip
(Fig. 1e). The tip is invaginated at rest, and it is therefore
classified as a type B phallus (possessing a blind tubular cavity;
King, 1981).
The rhea penis was described in detailed by Müller (1836),
without mention of the erection mechanism, while Gerhardt
(1933) suggested that it could be lymphatic, but without any
evidence to support his assertion. The penis in rhea has asym-
metric fibrous bodies and a blind tubular cavity (type B
phallus; King, 1981). Some males have a fully inflatable and
eversible corkscrew-shaped phallus that can vary greatly in
length, while other males lack the corkscrew shape altogether
(Góes, 2004; Góes et al., 2010).
We examined the penis of one adult fully reproductive male
ostrich and three adult fully reproductive male emu, and we
report the unequivocal presence of paralymphatic bodies. We
also analyzed published pictures of everting rhea penises from
Góes (2004) that show lymph flow into the penis, and we find
support for the hypothesis that lymphatic penis erection is a
synapomorphy of Aves. We also use histology and direct
manipulation of fresh penises to suggest how the lymphatic
erection mechanism works in these large ratites.
Methods
Dissection of the penis structures was done on a fresh emu
cloaca (from a private farm in Connecticut, US) and on a
frozen ostrich cloaca (from a private farm in New Mexico,
US). The males were adults in reproductive condition. The
fresh emu specimen (3-year-old male Yale Peabody Museum
139717) had just begun to sit on eggs when the organs were
collected. Two other emu specimens (2-year-old males) were
collected from a private farm in Tennessee, US, and their
cloacas were preserved in formalin 10% prior to shipment. The
ostrich male was an adult (6 years old), in full reproductive
condition as determined by its red legs and red-bill coloration.
The cloacal tissue was frozen and shipped overnight for dis-
section. All the specimens were sacrificed by the farmers, and
their organs were donated to our research. After removing all
the connective tissue, we removed muscle bundles to search
for the paralymphatic bodies.
We examined cross-sectional histological slides of the tissue
at the base, middle and tip of the body of the penis of one emu
male and the ostrich specimen, that were preserved in buffered
formalin 10% and stained both hematoxylin and eosin and
Mason’s trichrome (following Gray, 1954), primarily to dis-
tinguish connective tissue, blood vessels and smooth muscle.
Results
In both emu and ostrich, we found paralymphatic bodies
located on either side of the urodeum, underneath the cloacal
muscles (Fig. 1c and f). In ostrich, they are large ellipsoids of
a spongy consistency measuring 8 cm ¥4 cm (Fig. 1c), while
in emu, they measure 4.0 cm ¥2.5 cm (Fig. 1f).
In cross-section, the ostrich penis has a clear concentric
layer of collagen fibers running parallel to one another directly
under the epidermis (Fig. 2a). The area just below this organ-
ized collagen presents heavy vascularization (Fig. 2b). The
ostrich penis is always stiff even in the absence of inflation,
except at the tip where the tissue is more flexible. This stiffness
results from the fibrous bodies, which are composed of a dense
collagen matrix that is largely disorganized, except in the areas
surrounding the many narrow lymphatic spaces, where the
fibers are arranged running in parallel to one another. The
more flexible tissue toward the tip of the penis is composed
Figure 1 The penis of ostrich (a–c) and emu (d–f). The ostrich penis
protrudes from the cloaca during micturition and defecation (a). Its
conical structure bends towards the left; the left fibrolymphatic body
(l.f.b.) is larger than the right (r.f.b.) (b). The paralymphatic bodies (p.b.)
are paired and located under the urodeum, here shown from the ventral
aspect (c). The emu penis has a typical spiral shape (d), and the tip of
the penis is eversible, with a single spiral (e). The paralymphatic bodies
are located under the urodeum (f). * Dorsal aspect of the cloaca. S.s,
Sulcus spermaticus. Scale bar: 2 cm. I, intestine; c.m., cloacal muscles.
The erection mechanism of the ratite penis P. L. R. Brennan & R. O. Prum
2Journal of Zoology •• (2011) ••–•• © The Authors. Journal of Zoology © 2011 The Zoological Society of London

primarily of the elastic vascular body. The boundary between
the tissue of the fibrous bodies and the elastic vascular body is
not discrete, but rather a gradual change in the collagen
density and fiber packing. In the elastic body, the fibers are less
dense, and the lymphatic spaces are smaller than those found
within the fibrous bodies (Fig. 2c). Few blood vessels are seen
within the fibrous bodies or the elastic vascular body. The
collagenous connective tissue in the elastic body is irregular,
and fibers run in all directions (Fig. 2c).
In all three emu specimens observed, the eversible portion
of the penis was small, with a single full turn of the sulcus
spermaticus at the tip (Fig. 1e). In cross-section, the collagen
matrix in the emu penis is dense and disorganized just as in
ostrich, but the separation between the fibrous bodies and the
elastic vascular body is well demarcated. In addition, the emu
has a tubular cavity lined with secretory cells (the blind
tubular cavity of a type B penis), surrounded by dense elastic
tissue. In both ostrich and emu, there are abundant fibroblasts
embedded in the connective tissue.
Photographs of everting rhea penises from Goes (2004)
show that in some males, the eversible portion of the penis can
be very long (at least 30–40 cm), and it has a corkscrew shape.
Clear lymphatic fluid can be seen flowing into the penis.
Discussion
The erection mechanism of the ratite penis is lymphatic, not
blood vascular as has been suggested. It is likely that Grimpe
(1923) indeed found the paralymphatic bodies when he
described the ‘spongy cushions’ in the cloaca. The location
and description of the arterial plexus or network found at the
root of the phallus (Elias et al., 2007, 2008) make it likely that
these are surrounding the paralymphatic bodies to provide the
lymph needed for erection. All vertebrates reportedly have a
blood vascular penis-erection mechanism, and the novel lym-
phatic erection in birds is notable (King, 1981). Whether all
reptiles do in fact lack a lymphatic erection mechanism is
unknown, and modern anatomical studies in this group would
shed light on whether the transition from blood to lymph
occurred only in the last common ancestor of Aves.
The lymphatic system is a low-pressure circulatory system
and therefore not ideal where maintenance of an erection is
required. A lymphatic penile-erection mechanism has to be
fundamentally different from blood vascular erection. In
waterfowl, lymph can be seen accumulating at the base of the
cloaca as the male threads on top of the female prior to
copulation. When the male lowers his cloaca to meet the
female’s cloaca, release of the sphincter muscles allows the
lymph to flow freely into the large lumen of the penis causing
explosive eversion (Brennan, Clark & Prum, 2010). Because
ejaculation happens at the moment of maximum eversion, the
lymph is also pushing the seminal fluid from the base of the
ejaculatory groove, along the length of the sulcus spermaticus
to the tip of the penis (Brennan et al., 2010). Immediately after
ejaculation, the penis is flaccid, and it is slowly returned to
the cloaca (Brennan et al., 2010). Natural eversion in the
corkscrew-shape penis of rhea may work in a similar manner
given the large continuous lumen space that fills out with
lymph as evidenced in the photographs we examined.
However, the high density of collagen fibers found throughout
the penis of ostrich and emu, and the absence of a large
continuous lumen space, suggests that lymph does not flood
inside the penis of these species to evert it or elongate to the
extent that this is seen in rhea and waterfowl. We hypothesize
that in ostrich and emu, lymph is acting primarily to engorge
the penis and to transport semen that has been ejaculated from
the seminal papillae along the sulcus spermaticus, to aid in
ejaculation. This would be the same function as that of lymph
flowing into the non-intromittent penis of tinamous and gal-
liformes, where the presence of paralymphatic bodies has been
well established (Nishiyama, 1955; Brennan et al., 2008).
However, a functional study of the role of lymph flow in the
ratite penis is needed to fully understand how the erection
mechanism works in this group of birds.
The body of the ostrich penis occupies most of the procto-
deum, and it must be extruded from the vent to allow defeca-
tion and urination (Fig. 1a; Fowler, 1991). This penis
extrusion is carried out by muscular action of a pair of
muscles, the m. levator phalli. It is likely that in both ostrich
and emu, the penis is similarly protracted by muscular action
when copulation takes place, and that lymph then flows in to
aid in engorgement and ejaculation. Reports that the ostrich
penis changes from 20 cm when flaccid to 40 cm when erect
(Gerhardt, 1933) are likely the result of the full protrusion of
the phallus showing its entire shaft, rather than a significant
change in length. Our attempts to fill the ostrich penis shaft
with fluid resulted only in changes in the diameter of the shaft,
but not elongation of the penis itself. It is clear, however, that
contrary to the report by Müller (1836), the fibrous bodies
have erectile spaces within them, and these likely fill up with
lymph to add to the stiffness of the penis during copulation.
The dense regular arrangement of collagen fibers in the layer
immediately under the skin and within the fibrous bodies sug-
gests that these areas have higher tensile strength and stretch
resistance.
The age of the males and the time of the year when specimens
are collected have important consequences for the description
of genital morphology. For example, waterfowl penis under-
goes dramatic seasonal changes (Hohn, 1960 and P. L. R.
Brennan et al., unpublished data), and it continues to grow
Figure 2 Histological cross-section of ostrich penis. Trichrome stain.
Collagen shown in blue (a). Transition between the outer layer of cir-
cumferential collagen and the fibrous body (f.b.). Keratin (k) in the
epidermis is shown pink (4¥), scale bar 1 mm. (b) Epidermis and some
disorganized collagen immediately below (10¥) with some blood
vessels (b. v.) (c) A lymphatic space (l.s.) within the fibrous body (10¥).
Scale bar (b, c) 200 um.
P. L. R. Brennan & R. O. Prum The erection mechanism of the ratite penis
Journal of Zoology •• (2011) ••–•• © The Authors. Journal of Zoology © 2011 The Zoological Society of London 3

during the lifetime of an individual (P. L. R. Brennan et al.,
unpublished data). Although we know that the phallus of
juvenile ostrich is shorter than in adults (Fowler, 1991), no
studies have been conducted in ratites to determine whether
their penis changes seasonally, or with age as it is the case in
waterfowl. Failure to account for these variables may be the
reason for some of the contradictions in the early literature
describing the penises of ratites. Rhea males differ greatly in
penis morphology; some males are classified as having a small
phallus (<3 cm), whereas others have large penises (>3 cm)
(Góes et al., 2010). Some males with large penises have a
corkscrew spiral while others do not (Góes, 2004; Góes et al.,
2010). Góes et al. (2010) report that all the males they examined
were 3–4 years old, but it is unclear which males were examined
during the breeding season. The penis of all males reportedly
remained small during the nonbreeding season (Góes et al.,
2010). It is likely that the older males during the reproductive
season are the ones that develop a full corkscrew-shape penis
with an elongated eversible portion. Studies of seasonality and
ontogeny of penis development are needed in all ratites.
In conclusion, we report the presence of paralymphatic
bodies in the ratite penis, which combined with the absence
of blood sinuses suggest that the erection mechanism is
lymphatic. In rhea, the lymph likely acts in everting and inflat-
ing the penis as well as in transporting semen. However, lymph
is unlikely to flow in large quantities in the penis of ostrich or
emu to change their length dramatically as both of these species
have dense penises lacking large lumen spaces. We propose that
in these two species, lymph aids primarily in engorging the
phallus and pushing the semen during ejaculation.
Acknowledgements
Emu song farm and Tom Gereg kindly provided the emu
specimens for this research. The ostrich specimen was
acquired from Floeck’s ostrich ranch. Males were euthanized
by the farmers because of injuries (ostrich) or because of
culling of breeding population (emu), and the farmers allowed
us access to reproductive tissues. This research was covered
by the Institutional Animal Care and Use Committee protocol
number 2011–10906 to the Peabody Museum at Yale Univer-
sity. Steven Holt photography granted permission to use the
photograph in Fig. 1a. The Yale University Histopathology
Lab produced the histology slides used in this analysis.
Brennan was funded in part from the National Science Foun-
dation grant # 920344 and a Coe post-doctoral grant from
Yale University. Diane Kelly helped to photograph the his-
tology slides of the ostrich and emu specimens.
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