Top: Uterine lining at 5 1/2 months, displaying thin maternal separation from fetus, and high level of placental implantation
Center: Relation of placenta to uterus at 5 weeks and 8.5 months
Bottom: Major arteries and veins of the placenta

Did you know that the placenta is a temporary organ that’s actually created by the fetus, and not the woman?

The human female is a curious creature; like our close great ape cousins, but unlike almost all other mammals, they build up a thick barrier in the uterine wall, to protect against any potential embryo that might implant itself. When there’s no embryo implantation, the thickened wall is shed, in the process known as menstruation.

The thing is, most mammals don’t menstruate. They go into heat, and occasionally shed uterine lining (if the uterus is scratched, or an egg tries to implant but fails, for example), but there’s no regular cycle of bloody discharge relating to breeding. This is because other mammals go through triggered decidualization (developing a uterine lining only when a fertilized egg begins to implant itself), while the great apes (and a couple other convergently evolved families, including bats) experience spontaneous decidualization, where they develop a thick uterine lining during every ovulation, before an egg can even attempt to implant itself.

Why the different linings? Well, it turns out that there are three types of mammal placentas (remember, placentas are developed by the embryo/fetus, not the mother):

1. Epitheliochordal, which is completely superficial, and does not connect in any significant way to the mother’s body. The endometrial epithelium, connective tissue, and uterine epithelium are all preserved and undisturbed in the mother. The fetus is separated from the mother by three layers of tissue. Nutrients and waste are delivered and eliminated through diffusion, rather than direct connection. This group includes equids, swine, and ruminants.
2. Endotheliochordal, which is slightly more invasive to the mother, only preserves the uterine epithelium. Nutrients and waste are not exchanged through direct connection to the mother, but the placenta only leaves one layer of tissue between it and the mother. This group includes cats and dogs.
3. Hemochorial is the most invasive form of placenta in the animal kingdom. The embryo directly hooks itself up to the host (mother’s) blood flow, and leaves no tissue layers between the female and the placenta. This allows much more efficient nutrient transfer to the embryo or fetus, but is also potentially the most harmful to the female since the embryo attaches itself so securely to the uterine wall. The female must develop preemptive measures (a thickened uterine lining) to protect herself from a life-form that is literally driven to take all of the nutrients it needs to develop, and which has adapted to connect itself directly to the host. This group includes elephant shrews, most bats, and most primates.

Interested in more about the science behind reproduction and how amazingly efficient the human embryo is at sucking its host clean, just to obtain its needed resources for development?

PZ Meyers at Pharyngula has an understandable explanation of the article I referenced for this post.

There is also a great site by R. Bowen about the pathophysiology of the reproductive system.

An American Text-Book of Obstetrics for Practitioners and Students. Edited by Richard C. Norris, 1895.

Top: Bichir and trunkfish [top], Electric Catfish [bottom]
Center: Electric “eel” - Electrophorus electricus
Bottom: Indo-Pacific Moray Eel - Muraena nudivomer (now Gymnothorax nudivomer) 

A while ago I saw this Bird and Moon illustration of animals with misleading names, but I kept seeing people asking, “Ok, if they’re not THAT, then what ARE they?” For some reason, I completely forgot that I wanted to cover those questions, but hey, better late than never!

The electric eel isn’t an eel - it’s a knifefish. Knifefish (Gymnotiformes) are actually more closely related to electric catfish (Siluriformes) than they are to true eels (order Anguilliformes), but developed their electroconductive organs through convergent evolution - the first signs of the organ evolution in both the electric eel and the electric catfish appeared after they shared a common ancestor.

In addition to electric eels and electric catfish, electric rays (order Torpediniformes) are the only other “strongly electric” fishes - that is, fish that produce electric shocks over one volt, and use their electrogenerative organs to either stun or kill prey and/or attackers. There are many fish that can produce a small current (“weakly electric”), but it is used for electrolocation and electrocommunication, instead.

Images:
Fishes of Zanzibar: Acanthopterygii. J. Van Voorst, 1866.
The Standard Natural History. John Sterling Kingsley, 1884.
Wild life of the world. Richard Lydekker, 1915.

One of these things is not like the other…

First row: Walrus (Odobenus rosmarus) skeleton
Second row: Hooded seal (Cystopkora cristata) skeleton
Third row: Dugong (Dugong dugon) skeleton, Brazilian sea lion (Otaria flavescens) skeleton.

*Skulls depicted are of species in the same genus as the skeleton.

Sirenia (manatees, dugongs, and sea cows) and Pinnipedia (the seals, walruses, and sea lions) are often seen as very similar, but they came from very different lineages.

While both came from land mammals (just like all sea mammals), the pinnipeds evolved from a bear-like ancestor, who returned to the sea around 28 MYA. They’re Caniformidae, or dog-like Carnivora.

The sirens evolved from the same ancestor as the hyraxes and elephants, and returned to the sea around 50 MYA. They’re only distantly related to Cetaceans and Pinnipeds.

Vergleicheende Osteologie. Edward D’alton, 1821.

The Juvenile Hoatzin (Opisthocomus hoazin)

It should first be noted that all birds are dinosaurs (order Saurischia, clade Theropoda), not just descendents of dinosaurs - modern genetic analysis strongly supports this cladistic organization. But given what we’re too often taught in schools, birds and dinosaurs are hard to reconcile in many peoples’ minds.

The juvenile hoatzin, however, makes it easy to see the reptilian traits that once dominated the early birds, and displays the unused genetic codes that lurk in the genome of modern avians. When they hatch, they’re equipped with lizard-like claws in front of their wings. Their use is described here, but in short, they use them to return to their nest and avoid predators. Their claws disappear by the time they leave the nest, having grown together into the metacarpals that support the wing structure.

Another fascinating trait of the hoatzins is their vegetarianism and their digestive tract. They have gut flora and fermentation similar to ruminants, which no other bird has. This is actually what leads to their being called “stink birds” - they exude a lot of stench with the fermentation process. The gut fermentation is so important to the hoatzin that the flight muscles attached to their keel are significantly reduced, to allow for more space for the stomach. They are weak flyers because of this. After a large meal, an adult hoatzin can spend up to two days doing almost nothing, allowing the leaves and greenery to have their nutrients released by their symbiotic gut flora.

Images:

Top: Attitudes of the juvenile hoatzin while climbing
Second row, left: Hoatzin nest with two eggs - Note proximity to water
Second row, right: Two hoatzin chicks preparing to dive, after appearance of threat from above
Third row, left: Hoatzin chick demonstrating strong swimming abilities
Third row, right: Hoatzin chick demonstrating poor locomotion on land
Bottom: Detail of hoatzin chick climbing, using neck, feet, and claws.

Tropical Wild Life in British Guinea, Vol 1. Curated by William Beebe, 1898.

sirjosephbanksfrs:

biomedicalephemera:

Apteryx owenii - The  Little Spotted Kiwi - In life and superficial lateral dissection

Though at first glance the kiwi appears to not have any wings, the lateral anatomical view with no feathers shows that external wings still exist. Of course, they are rudimentary at best, and useless for flying, but they still serve to balance the bird and are not considered vestigial. The only birds that had no wings were the giant Moas, also of New Zealand.

Transactions of the Zoological Society of London, Vol III. 1849.

Kiwis have minute wings, not considered to be vestigial because they serve the purpose of helping the animal to balance.

This is actually pretty true (that the tiny things DO have a function) in the Little Spotted Kiwi and the Brown Kiwi - you can see them being used like that in their natural habitat, which is generally rough terrain.

But I think a case could be made for them being truly vestigial in the Great Spotted kiwi. The size of the wings in the Great Spotted is smaller than even the wings of the Little Spotted, the weight is differently distributed in the two species (with a heavier hind-end in the Great), and the Great Spotted has proportionally larger leg muscles. Not to mention they also live in a flatter and less rough habitat, but I’m not sure how much that played into the differences in body structure.

That said, you’ll always have people arguing that even the most useless remnant of a limb or organ isn’t vestigial, because the body works around what it has - even if you watched the process of, say, horse leg evolution, and realized that eventually the remnants of the fibula probably won’t exist anymore, you can say that they’re not vestigial, because the muscles of the horse still partially attach to the fused parts of that bone.

Top: Canada Lynx (Lynx candensis)
Bottom: Bobcat (Lynx rufus)

Despite both being members of the Lynx genus, the bobcat and Canada lynx did are not as closely related as one might think.

The Eurasian lynx (Lynx lynx) first arrived in North America approximately 2.6 million years ago, crossing over the Bering Land Bridge, and moved far to the south, eventually settling in the Southern half of North America and evolving into what we know as the modern bobcat by the time of the earliest human settlers to the land.

During the last Ice Age (22,000 years ago), the Eurasian lynx once again crossed into North America, along with the Homo sapiens who would eventually populate the continent. This second population evolved into the Canada lynx, which is much more closely related to their progenitors than the bobcats are.

The fur patterns of the bobcat vary drastically from region to region. Some southern bobcats are spotted almost identically to the ocelot, while others in the north are much closer to the faded-spot and grey-white coat of the Canada lynx. All bobcats are generally smaller than the lynx, and have tails about twice the length of other lynx species. They’re also generally more adaptable, as they will act as opportunistic predators when the need arises.

The Canada lynx can be discerned from the bobcat by, in addition to its size and tail, its distinctive striped ruffed collar, and tufts of fur above the ears.

Wild Animals of the World. Edward W. Nelson for the National Geographic Society, 1918.

Ways to Die: Snake Venom

The vast majority of snakes that one encounters in the wild (unless you live in Australia or India) are either non-venomous to humans or want nothing to do with you.

However, should you stumble upon a rattlesnake nest or coral snake hole while texting in the middle of nowhere, there will probably be a combination of different enzymes and polypeptides pumped into your body, via the modified parotid salivary glands (right below the ear in humans) that snakes have evolved over the ages, to disable their prey. Of course, you’re not prey, but you stepped on a snake while texting. It has every reason to envenomate you.

While all snakes have multiple active enzymes in their venom, all snakes dangerous to humans have either neurotoxins or cytotoxins as a significant component in their venom. For the most part, elapids (such as the cobras and mambas) create neurotoxins, while the viperids (such as vipers, adders, and rattlesnakes) create cytotoxins.

Neurotoxins

• Dendrotoxins: Inhibit neurotransmission by blocking the exchange of positive and negative ions across the pre-synaptic neuronal membrane, causing paralysis. Found in some rattlesnakes (such as the Mojave) and mambas.
• Fasciculins: Destroys acetylcholinesterase (AChE) in synaptic clefts of nerves. Without AChE, acetylcholine (ACh) is not broken down, and remains bound to the postsynaptic vesicles of the nerve, leading to constant contraction of the related muscles. This is called tetany or tetanic paralysis. Found only in mambas.
• α-neurotoxins: Very large group of toxins that mimic ACh and bind to post-synaptic vesicles, leading to numbness and paralysis. Found in cobras, kraits, and sea snakes. 

Cytotoxins

• Cardiotoxins: Target muscle cells and cause depolarization. If enough of these components reach the heart, the depolarization can cause irregular heartbeat or spontaneous stopping of the heart. Can cause fasciculations in skeletal muscles. Found in the Naja genus, and in King Cobras. Minor but important component of mamba venom.
• Phospholipases: Proteins that target the phospholipid bilayer of cells, causing cellular rupture. Can cause extreme blistering at site of bite. Relatively uncommon, found in the Japanese Habu.
• Hemotoxins: Burst red blood cells (hemolysis), causing thin blood, internal bleeding, and blood clots due to the massive clotting response. Found to some degree in almost all vipers, as well as some cobras.

Images:
Top: Bungaris fasciatus - Banded Krait. An elapid, and the largest of the kraits. Has neurotoxic venom. [source]
Center Right: Hydrophis robusta [now Hydrophis spiralis] - Yellow Sea-Snake. The longest sea snake, at 3 m (9.8 ft). A member of the Hydrophiinae, separate from other elapids. Though they have some of the most toxic venom in the world, bites are extremely uncommon and often unnoticed. [source]
Center Left: Vipera russellii - Russell’s Viper. A particularly aggressive viperid. Necrosis and amputation following envenomation not uncommon, due to hemolysis and local cell damage. [source]
Bottom: Vipera caudisona [now Crotalus horridus] - Timber Rattlesnake. A venomous viperid endemic to the United States. Primarily hemotoxic venom, very low fatality rate, but very painful bites. [source]