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Integrated Biology of the Crocodilia

Ben Creisler

A special issue of the journal Integrative and Comparative Biology. I
think at least one of these papers (Farmer) was posted on the DML in
pre-publication form.

Integrated Biology of the Crocodilia

Michael B. Pritz (2015)
Crocodilian Forebrain: Evolution and Development.
Integrative and Comparative Biology 55 (6): 949-961

Organization and development of the forebrain in crocodilians are
reviewed. In juvenile Caiman crocodilus, the following features were
examined: identification and classification of dorsal thalamic nuclei
and their respective connections with the telencephalon, presence of
local circuit neurons in the dorsal thalamic nuclei, telencephalic
projections to the dorsal thalamus, and organization of the thalamic
reticular nucleus. These results document many similarities between
crocodilians and other reptiles and birds. While crocodilians, as well
as other sauropsids, demonstrate several features of neural circuitry
in common with mammals, certain striking differences in organization
of the forebrain are present. These differences are the result of
evolution. To explore a basis for these differences, embryos of
Alligator misissippiensis were examined to address the following.
First, very early development of the brain in Alligator is similar to
that of other amniotes. Second, the developmental program for
individual vesicles of the brain differs between the secondary
prosencephalon, diencephalon, midbrain, and hindbrain in Alligator.
This is likely to be the case for other amniotes. Third, initial
development of the diencephalon in Alligator is similar to that in
other amniotes. In Alligator, alar and basal parts likely follow a
different developmental scheme.


C. G. Farmer (2015)
Similarity of Crocodilian and Avian Lungs Indicates Unidirectional
Flow Is Ancestral for Archosaurs.
Integrative and Comparative Biology 55 (6): 962-971

Patterns of airflow and pulmonary anatomy were studied in the American
alligator (Alligator mississippiensis), the black caiman (Melanosuchus
niger), the spectacled caiman (Caiman crocodilus), the dwarf crocodile
(Osteolaemus tetraspis), the saltwater crocodile (Crocodylus porosus),
the Nile crocodile (Crocodylus niloticus), and Morelet’s crocodile
(Crocodylus moreletii). In addition, anatomy was studied in the
Orinoco crocodile (Crocodylus intermedius). Airflow was measured using
heated thermistor flow meters and visualized by endoscopy during
insufflation of aerosolized propolene glycol and glycerol. Computed
tomography and gross dissection were used to visualize the anatomy. In
all species studied a bird-like pattern of unidirectional flow was
present, in which air flowed caudad in the cervical ventral bronchus
and its branches during both lung inflation and deflation and craniad
in dorsobronchi and their branches. Tubular pathways connected the
secondary bronchi to each other and allowed air to flow from the
dorsobronchi into the ventrobronchi. No evidence for anatomical valves
was found, suggesting that aerodynamic valves cause the unidirectional
flow. In vivo data from the American alligator showed that
unidirectional flow is present during periods of breath-holding
(apnea) and is powered by the beating heart, suggesting that this
pattern of flow harnesses the heart as a pump for air. Unidirectional
flow may also facilitate washout of stale gases from the lung,
reducing the cost of breathing, respiratory evaporative water loss,
heat loss through the heat of vaporization, and facilitating crypsis.
The similarity in structure and function of the bird lung with
pulmonary anatomy of this broad range of crocodilian species indicates
that a similar morphology and pattern of unidirectional flow were
present in the lungs of the common ancestor of crocodilians and birds.
These data suggest a paradigm shift is needed in our understanding of
the evolution of this character. Although conventional wisdom is that
unidirectional flow is important for the high activity and basal
metabolic rates for which birds are renowned, the widespread
occurrence of this pattern of flow in crocodilians indicates
otherwise. Furthermore, these results show that air sacs are not
requisite for unidirectional flow, and therefore raise questions about
the function of avian air sacs.


Sarah W. Keenan and Ruth M. Elsey (2015)
The Good, the Bad, and the Unknown: Microbial Symbioses of the
American Alligator
Integrative and Comparative Biology 55 (6): 972-985

Vertebrates coexist with microorganisms in diverse symbiotic
associations that range from beneficial to detrimental to the host.
Most research has aimed at deciphering the nature of the composite
microbial assemblage’s genome, or microbiome, from the
gastrointestinal (GI) tract and skin of mammals (i.e., humans). In
mammals, the GI tract’s microbiome aids digestion, enhances uptake of
nutrients, and prevents the establishment of pathogenic
microorganisms. However, because the GI tract microbiome of the
American alligator (Alligator mississippiensis) is distinct from that
of all other vertebrates studied to date, being comprised of
Fusobacteria in the lower GI tract with lesser abundances of
Firmicutes, Proteobacteria, and Bacteroidetes, the function of these
assemblages is largely unknown. This review provides a synthesis of
our current understanding of the composition of alligators’
microbiomes, highlights the potential role of microbiome members in
alligators’ health (the good), and presents a brief summary of
microorganisms detrimental to alligators’ health (the bad) including
Salmonella spp. and others. Microbial assemblages of the GI tract have
co-evolved with their vertebrate host over geologic time, which means
that evolutionary hypotheses can be tested using information about the
microbiome. For reptiles and amphibians, the number of taxa studied at
present is limited, thereby restricting evolutionary insights.
Nevertheless, we present a compilation of our current understanding of
reptiles’ and amphibians’ microbiomes, and highlight future avenues of
research (the unknown). As in humans, composition of microbiome
assemblages provides a promising tool for assessing hosts’ health or
disease. By further exploring present-day associations between
symbiotic microorganisms in the microbiomes of reptiles and
amphibians, we can better identify good (beneficial) and bad
(detrimental) microorganisms, and unravel the evolutionary history of
the acquisition of microbiomes by these poorly-studied vertebrates.

Christopher R. Tracy, Todd J. McWhorter, C. M. Gienger, J. Matthias
Starck, Peter Medley, S. Charlie Manolis, Grahame J. W. Webb, and
Keith A. Christian (2015)
Alligators and Crocodiles Have High Paracellular Absorption of
Nutrients, But Differ in Digestive Morphology and Physiology.
Integrative and Comparative Biology 55 (6): 986-1004

Much of what is known about crocodilian nutrition and growth has come
from animals propagated in captivity, but captive animals from the
families Crocodilidae and Alligatoridae respond differently to similar
diets. Since there are few comparative studies of crocodilian
digestive physiology to help explain these differences, we
investigated young Alligator mississippiensis and Crocodylus porosus
in terms of (1) gross and microscopic morphology of the intestine, (2)
activity of the membrane-bound digestive enzymes aminopeptidase-N,
maltase, and sucrase, and (3) nutrient absorption by carrier-mediated
and paracellular pathways. We also measured gut morphology of animals
over a larger range of body sizes. The two species showed different
allometry of length and mass of the gut, with A. mississippiensis
having a steeper increase in intestinal mass with body size, and C.
porosus having a steeper increase in intestinal length with body size.
Both species showed similar patterns of magnification of the
intestinal surface area, with decreasing magnification from the
proximal to distal ends of the intestine. Although A. mississippiensis
had significantly greater surface-area magnification overall, a
compensating significant difference in gut length between species
meant that total surface area of the intestine was not significantly
different from that of C. porosus. The species differed in enzyme
activities, with A. mississippiensis having significantly greater
ability to digest carbohydrates relative to protein than did C.
porosus. These differences in enzyme activity may help explain the
differences in performance between the crocodilian families when on
artificial diets. Both A. mississippiensis and C. porosus showed high
absorption of 3-O methyl D-glucose (absorbed via both carrier-mediated
and paracellular transport), as expected. Both species also showed
surprisingly high levels of L-glucose-uptake (absorbed
paracellularly), with fractional absorptions as high as those
previously seen only in small birds and bats. Analyses of absorption
rates suggested a relatively high proportional contribution of
paracellular (i.e., non-mediated) uptake to total uptake of nutrients
in both species. Because we measured juveniles, and most paracellular
studies to date have been on adults, it is unclear whether high
paracellular absorption is generally high within crocodilians or
whether these high values are specific to juveniles.

Also, a recent paper from another journal:

Sebastian Klenner, Ulrich Witzel, Frank Paris and Claudia Distler (2015)
Structure and function of the septum nasi and the underlying tension
chord in crocodylians.
Journal of Anatomy (advance online publication)
DOI: 10.1111/joa.12404

A long rostrum has distinct advantages for prey capture in an aquatic
or semi-aquatic environment but at the same time poses severe problems
concerning stability during biting. We here investigate the role of
the septum nasi of brevirostrine crocodilians for load-absorption
during mastication. Histologically, both the septum nasi and the
septum interorbitale consist of hyaline cartilage and therefore mainly
resist compression. However, we identified a strand of tissue
extending longitudinally below the septum nasi that is characterized
by a high content of collagenous and elastic fibers and could
therefore resist tensile stresses. This strand of tissue is connected
with the m. pterygoideus anterior. Two-dimensional finite element
modeling shows that minimization of bending in the crocodilian skull
can only be achieved if tensile stresses are counteracted by a strand
of tissue. We propose that the newly identified strand of tissue acts
as an active tension chord necessary for stabilizing the long rostrum
of crocodilians during biting by transforming the high bending stress
of the rostrum into moderate compressive stress.