Why are mammals rarely poisonous?

Even people with a lack of biological knowledge can name many poisonous vertebrates, but few people have heard…

Even people with a lack of biological knowledge can name many poisonous vertebrates, but few people have heard of poisonous mammals-indeed, of the 5,000 species of mammals known, no more than 10 are truly poisonous. Species, and most of them are located on the opposite edge of the branch, this is a phenomenon worth considering.

Among all mammals that attack with toxins, the most important lineage is the “True No Caecum”. The status of this group is undetermined and includes many members of the pre-Insectivora order. Many of these species have similar fangs, such as the northern short-tailed shrew (Blarina brevicauda) and the southern European water shrew (Neomys anomalus). The European mole (Talpa europaea), and 4 species of Solenodontidae (Solenodontidae).

A Haitian groin shrew (Solenodon paradoxus), like many small mammals in the New World, is endangered by species invasion

For the representative groove-tooth shrews, they live in Cuba and Haiti. They are only about 30 cm long, but they are already huge in the local area. “Gully teeth” refers to the two sharp lower incisors with a groove on the back leading to the submandibular gland. The saliva contains neurotoxins, which can paralyze larger prey, which allows them to hunt in addition to insects. Big frog and lizard.

Why are mammals rarely poisonous?
Why are mammals rarely poisonous?

On the inside of the lower jaw of the grooved shrew, the grooves on the lower incisors are visible

Several other venomous species of the cecum species have similar characteristics: the incisors are grooved or concave, and the saliva is poisonous, which can paralyze larger prey. The northern short-tailed shrew can even catch mice larger than itself-this is in progress. It has a distinct specimen value.

According to the gene comparison, the evolutionary relationship between the true non-cecum (marked as true blindness and big eyes) and the closely related group in mammals

The true non-caecum is a fairly old group of mammals, still such as groove-tooth shrews. They have been distinguished from other mammal groups in the Cretaceous about 78 million years ago, living in isolation on isolated islands in the Caribbean Sea. In the above, many ancient traits are preserved-there are indeed many similar groove teeth in the fossils of early mammals, which implies that mammals were once as good at using poison as other vertebrates, but the existing species generally gave up this ability. Only preserved in certain marginal and ancient taxa.

For this change, the more common explanation is that as a warm-blooded animal, mammals have a fairly high metabolic rate. Early species were difficult to hunt larger prey because of their small size. They could get more food at one time with poison, which enhanced their adaptation. Sex.

But as mammals become the masters of the earth in the Cenozoic and become larger and larger, venom becomes more and more difficult to use: it takes time to accumulate venom, and it takes time to venom attack, and the larger the prey, the more venom is needed. The waiting time is also longer. As the largest venomous creature, the Komodo dragon may require several weeks to track its prey once, but few mammals can afford the same waiting.

So in general, when the target is smaller and moving slowly, attacking with venom may be a good strategy, but for larger, faster prey, the venom seems to be a loss. The latter is the normal life of mammals.

Fortunately, a faster metabolism and a developed nervous system also bring powerful strength and agile responses. Fighting is a more effective predatory skill. Venom has completely become a cumbersome trait and has been abandoned by evolution – as evidence, Venomous snakes tend to capture small prey, while pythons that capture large prey use violent squeezing without fangs.

Higher metabolic rate and constant body temperature make mammals react extremely quickly, exercise longer, and are better at fighting

In addition to fangs, some mammals have a special “venom distance”. The only modern example is the platypus. Regardless of whether they are male or female, the root bone (that is, the heel bone) has a special distance at birth, like sharp claws, but only males can maintain this structure after they reach adulthood, and they are quite powerful and can bear their own body weight.

A partial photo of the ventral surface of a male platypus. The big thorn is the distance. The hole on the left is the cloaca hole shared by digestion, excretion and reproduction-so the platypus belongs to the order of the monotreme

June to October is the breeding season, a kidney-like venom gland connected to this big thorn will become active, allowing the male platypus to temporarily have the ability to inject venom. The few milliliters of their venom each time contain at least 19 peptides, which can be divided into three categories: defensin peptides (OvDLPs), derived from antimicrobial peptides that are ubiquitous in the vertebrate immune system, and can be embedded in the cell membrane system C-type natriuretic peptides (OvCNPs), which are related to body fluid balance, can trigger vasodilation and lower blood pressure; nerve growth factor (OvNGF), in addition to the growth and survival of neurons, also controls islets Activities with the immune system.

A close-up of the platypus venom, showing a hole to drain the venom

This kind of venom has a great relationship with the immune system. It has a similar origin to that of reptiles and is an example of convergent evolution; but it is mainly used for fighting between males and can cause severe pain and even temporary paralysis to invaders. , Help yourself establish territory. Considering that other monotreme mammals such as echidna also have this fighting distance, but lack of venom glands, deterministic selection has played an important role in the evolution of platypus venom distance.

The venomous distance also has an ancient origin. The lactating megalodon (Megaconus mammaliaformis) lived in the Middle Jurassic period 165 million years ago. It is a collateral line of all the existing mammalian ancestors. They have a venomous distance on their hind feet; Zhang Zhangheotherium quinquecuspidens (Zhangheotherium quinquecuspidens) lived in the Early Cretaceous period 125 million years ago. It is a sideline of the last common ancestor of marsupials and placenta, and their hindfoot fossils also have a clear poisonous distance, just like the platypus. .

Reconstruction and skeleton diagrams of Megaconus mammaliaformis (Megaconus mammaliaformis), pay attention to the distance on the hind feet

This to a large extent implies that toxic distance was once a common trait in early mammals, but marsupials and placenta did not continue this trait. The reason for this is not clear. This may be related to the structure of the limbs-most of the Mesozoic. The movement of mammals is close to crawling, and the hind limbs can be embraced backwards. Platypus still retains such hip joints, but the limbs of marsupials and placenta are perpendicular to the ground, which makes the heels of the hind limbs no longer easy to stab the enemy; or just Coincidentally-kangaroos and our common ancestor happened to be the group that lacked poison distance, so the offspring also lacked this trait.

In addition to injections, there is another way to use poison, which is to accumulate a large amount of toxins in the tissues or skin, allowing predators to eat and gain wisdom, just like puffer fish or poison dart frogs. This is a purely passive defense that does not really exist in living mammals, but there are several controversial examples.

The slow loris of the protomonkey suborder Loris are quite basal in the primates. They have specialized sweat glands on their wrists, which can secrete secretory globulins that cause allergic reactions, as well as some volatile substances. Loris usually lick this gland. The body is attached to the comb-like lower incisors and used to comb their own and children’s hair, so biting them or being bitten by them may cause allergies and severe pain-but they unfortunately live in Southeast Asia, suffering habitat destruction, The triple deadly threat of traditional medicine medicinal hunting and pet trade is now threatened with extinction.

Promonas generally have comb-shaped lower incisors for combing hair

Pet dealers cut slow loris teeth-slow loris are very popular among cute pet lovers in the Internet era, which has significantly accelerated their wild demise

Two other more controversial examples are the hedgehog and the African crested rat. Hedgehogs are also members of the true non-cecal class. They lack toxins, but they often kill toads and smear their needles with toads’ toxins. The rodent African crested rat chews the bark of the oleander and applies the poisonous sap to its hair. This area is also marked by white hair to warn predators.

Portrait of the African Crested Rat (Lophiomys imhausi), the white bordered area is coated with oleander’s poisonous hair

Toxins cannot be preserved by fossils, but it can still be speculated that the reason why modern mammals do not accumulate toxins in their bodies is similar to their lack of fangs. Flexible muscles and developed nervous systems have given mammals a wealth of defense strategies.

Moreover, the toxins accumulated in the tissues of vertebrates are usually unable to synthesize by themselves. Whether puffer fish, poison dart frogs or salamanders, toxins come from food enrichment, which will not only increase the metabolic cost, but also limit their feeding range and have a strong effect on metabolism. For the mammals, the gain is more than the loss.

Finally, although mammals lack venom and stingers, they have unprecedentedly “invented” poisonous gas-most mammals have anal glands to lubricate the rectum, while the anal glands of the Mustelidae and Skunk families of the carnivorous order are particularly developed. Its oily secretions contain many low-molecular-weight mercaptan substances, which are released like spray when encountering danger, creating a dark and foul-smelling atmosphere, and enough to make predators temporarily blind.

This strategy also works in several other slow-moving mammals, such as pangolins and long-nosed armadillos. Compared with awkwardly accumulating large amounts of toxins, such a small number of efficient tricks can better reflect the wisdom and cunning of mammals.

Pangolins have thick scales, amazing burrowing speed, and skunk-like anal glands-if it weren’t for Chinese medicine and Chinese food markets, they would be very happy


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