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Alligators v Crocodiles: What's the difference?

5/15/2016

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Alright, let's sort out once and for all the differences between alligators and crocodiles.  Many assume that the chief difference between the large reptiles is the shape of their heads; alligators having broader, more baguette shaped heads and crocodiles more narrow heads.  This is generally true, as per the illustration below.  However, head/snout shape is only part of the picture.
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Morphological differences in head/snout shape between alligators and crocodiles

​From the standpoint of biological classification, alligators and crocodiles are members of two distinct families.  They are both members of the order crocodilia.  Thus, all alligators and all crocodiles are crocodilians.  

Table 1 Biological classification of the crocodilian family

Classification Alligator | Crocodile
Kingdom Animalia
Phylum Chordata
Class Reptilia
Order Crocodilia
Family Alligatoridae Crocodylidae Gavialidae
Subfamily Alligatorinae Crocodylinae Gavialinae
Genus Alligator Crocodylus Gavialis


​Note an additional biological family in Table 1, the gavialidae.  The gavial, or gharial, is a fish eating (piscivorous) saltwater crocodile native to northern India.  The gharial is a highly endangered species, with a known number of extant individuals only a little over 200.  As can be seen in the photo below, the gharial has a very long, narrow snout specialized for catching and consuming fish.  Very different morphology than other crocodilians indeed.
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The piscivorous gavial, or gharial of northern India
Alligators and crocodiles.  Can you tell which is which?

​Other differences between alligators and crocodiles are the placement of teeth and jaw plan.  Alligators have a larger upper jaw than their lower jaw.  Thus, with jaws closed, an alligator's lower teeth are not visible (part A below).  Crocodile's upper and lower jaws are approximately the same width (part B), and when closed, the upper and lower teeth interlock.
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Tooth placement in alligators (A) and crocodiles (B)

Alligators and crocodiles both have integumentary (skin) sensory organs know as dermal pressure receptors. These highly sensitive receptors can detect the faintest of water perturbations, assisting them in locating and tracking prey.  These receptors are distributed differently in alligators and crocs.  Both have DPRs located in the skin of their upper and lower jaws.  However, crocs also have DPRs distributed about most of their body's surface area.
DPRs (Dermal Pressure Receptors), on the upper/lower jaws, and a close up

There are behavioral differences as well between the two families.  Crocodiles, in general, are more aggressive than alligators.  Alligators will often retreat from humans, where crocodiles may have a greater propensity to attack.  However, one has to be careful in overgeneralizing, as the speciation of crocodilians is very complex and vast.  Thus, depending upon the particular species, there will be many differences  in temperament, aggressiveness, as well as size.
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Masters of Camouflage

4/18/2016

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I know this is my second consecutive post featuring a zoological biology topic, and I really do prefer to mix up my topics more. However, I simply couldn't resist, as the cuttlefish is simply too amazing a species and thus cannot be denied.  The cuttlefish is a cephalopod that bears a resemblence to, and is sometimes mistaken for it's relative the squid.  However, cuttlefish and squid are quite different, both taxonomically and morphologically.
Images of the cuttlefish.  Note the striking color variation.  Cuttlefish are able to change color rapidly due to an extremely intricate network of specialized cells called chromatophores (and others) that contain sacs of pigment activated by muscle contractions.
As I alluded to above, cuttlefish are cephalopods, a contraction from greek meaning "head-foot." To me this nomenclature is counter intuitive, as there are 8 arms and 2 tentacles projecting from their heads.  Nowhere is the term "foot" used in the anatomical description.  

The cephalopods are part of the class of invertebrates that include octopuses, squids, and nautiluses, all grouped under the phylum of mollusk.  The speciation of cuttlefish and other cephalopods is quite complex, as can be seen in this Encyclopedia of Life link.  The common cuttlefish is known as sepia officinalis.


                                             Squid eye                                                                        Cuttlefish eye (note the "W" shape) 
There are any number of excellent articles on the differences between the cuttlefish and other cephalopods/molluscs.  I would like to jump straight into the topic of the post title, that is, camouflage.  Cuttlefish exhibit a capacity to blend into their background by changing color within a half second.  In addition to changing color, they can also modify the texture of their skin. for example, spiky-like protuberances in the photo below:

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A cuttlefish displaying skin protuberances that help it blend into it's surrounding rocky background

The biological mechanism behind this intricate and fast acting camouflage capability includes a network of more than 20 million specialized cells called chromatophores that contain sacs of  pigment
.  As in Figure 1 below, the chromatophore cells are attached to muscle fibers that connect to the cell body radially and are innervated with nerve axons.

 The colors of the chromatophore pigmentation are yellow, red, and brown. There are other cells involved in cuttlefish coloration, but the chromatophores are the most kinetic of the set owing to their nexus with the neuromuscular system.


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Figure 1: Chromatophore

​In the illustration in Figure 2 below, we see on the left a chromatophore cell at rest.  On the right we see the cell activated via expansion of the radial muscle fibers.  Note how the yellow pigment sac in this instance is stretched out by the strands of muscle, increasing it's surface area sufficiently to render the color visible.  
                                A chromatophore cell at rest               Figure 2               An activated chromatophore                                                                          
As might be expected in nature, this mechanism is more complicated in practice and relies upon other supporting pigmentation cells.  Firstly, there are several layers of chromatophore sheets, and below these are oval shaped iridescent cells called iridophores that reflect the colors blue, green, red, and pink. All this structure, finally, undergirded by a network of yet more cells rendering white coloration called leucophores!

​This color 
palette ensures that the cuttlefish can produce virtually any color in the color spectrum.  When you're an extant species belonging to a biological class that originated in the Ordovician era, between 485 and 444 million years ago, there's been much time for evolution to solve certain adaptation problems.
​
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Relative positions of chromatophores, iridophores, and leucophores

​Although the issue is not definitively settled, the current consensus among marine biologists appears to be that cuttlefish process their background environment visually, via their "W" shaped eyes.

That visual sensory input subsequently undergoes highly sophisticated neural processing, sending commands to the radial muscle fibers attached to the chromatophore cell bodies.  Specifically, on how to expand in the correct sequence and combinations in order to produce the desired skin color and body shape. The fact that this process is coordinated at so many levels of organization so quickly staggers the imagination.

​Obviously the ability to innervate over 20 million cells in order to produce complex colors and shapes, informed by background color/textures cues no less, requires gargantuan neural processing power.  It turns out that cuttlefish, along with other cephalopods, have large brains and a high encephalization quotient. 
This metric is thought to be a predictor of sorts of the intelligence of an animal.

The PBS NOVA video embedded below from 2007 offers a fascinating glimpse into the behavior and biology of this magnificent animal, and is well worth the time.
If you don't have sufficient time to view the entire video, make sure to see the section on how chromatophores produce color, around 9:30 into this video:
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Mammalian Night Vision (Tapetum Lucidum)

4/16/2016

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This leopard (Panthera pardus) presents clearly the signature tapetum lucidum "eyeshine"

There's no more impressive sight to witness in nature than the seemingly glowing, backlit eyes of a nocturnal or crepuscular mammal.  Especially in the case of large species such as the leopard in the photo above, it can be an awe inspiring sight to behold.  Not only mammals, however, exhibit the characteristic glow, or eyeshine, but a great many vertebrates.  Actually this phenomenon is not a true glow, but rather a reflection of light.
​
Other vertebrates presenting tapetum lucidum eyeshine:  clockwise from the top:  alligator, a quartet of raccoons, male lion, deer, rabbit, and sportive lemur.

The biological mechanism behind the characteristic nighttime eyeshine comes courtesy of a membrane situated directly posterior to the retina called the tapetum lucidum.  

The highest level explanation of the mechanism behind the tapetum lucidum ​is thus: ambient light in otherwise dark conditions passing through the retina is reflected back to the retina by the tapetum lucidum membrane.  This re-emission via reflection gives the photoreceptors, mainly rods, another chance to gather more photons for image processing.

A diagram is helpful in better understanding this process.  In the right-most section of Figure 1 below, light enters the eye and first contacts the retina (red color coded).  Light passing through the retina impacts the tapetum lucidum (green color) membrane and is reflected back to the retina.  Thus the light sensitive rods get a second chance at absorbing and processing more light.  Generally most vertebrates that have keen night vision have a greater proportion of more light sensitive rods than cones.  Cones are responsible for sensitivity to color.  
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Figure 1

As always, any SMEs (subject matter experts) in biology/zoology feel free to chime in and correct any inaccuracies in my description.  There is much detail involved in the biology of the tapetum lucidum; 
to get an idea of how granular the subject can become, take a glance at this article from the journal Marine Ecology.  ​
​
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