It can be argued that the greatest concern for every living thing on this planet is reproduction. The ultimate goal for every organism is the passing on of ones genes and the prosperity of their species. Organisms take many different approaches when trying to achieve this goal. Some produce large amounts of offspring in hopes that some of them will survive. Others produce very few offspring but invest a great deal of energy into those offspring giving them a better chance of living to adulthood. Some show parental care while others let offspring fend for themselves. Interestingly, we are finding more and more non-genetic defense mechanisms that are passed on to offspring mainly through maternal transfer into the eggs. This is mostly in the form of chemical defenses that make progeny less palatable to predators.
This type of chemical transfer between mother and offspring is much more commonly studied in invertebrates however; research into vertebrate mechanisms for maternal transfer of toxins is increasing. Some organisms actually produce the toxins themselves while others sequester the toxins from their environment, usually through their diet. When we think about toxic vertebrates the group that comes to mind is amphibians, specifically poison dart frogs, but toxic mammals, birds, fish, and reptiles do exist.
Being toxic, or poisonous, is not necessarily the same as being venomous. Toxins are naturally produced poisonous proteins that are usually used in defense of predators while venoms are toxins usually produced within a gland that is attached to a mechanism for delivery into an organism. The slight difference here is that to be considered venomous the organism must possess a system for delivering the venom. This is usually through the use of fangs, teeth, spines or stingers and the venom is primarily used for subduing prey. On the other hand, to be considered poisonous an organism needs no such delivery systems as the toxin is usually administered through ingestion. In short, venom is injected while poison is ingested.
Interestingly enough, there exists a certain species of snake that is both venomous and toxic. The tiger keelback snake (Rhabdophis tigrinus) is a venomous snake found in eastern Asia that produces venom within specialized glands called Duvernoy’s glands and injects this venom using fangs located in the rear of its mouth. Duvernoy’s glands are specific to colubrid snakes and distinguished from venom glands found in vipers and elapids. The peculiar thing about this snake is that it has another set of glands called nuchal glands where it houses a powerful toxin called bufadeinolides. Bufadeinolides get their name from the toads in the genus Bufo that produce them. These snakes almost exclusively prey on amphibians and a new study provides evidence that female tiger keelbacks may selectively forage for toads when gravid. The reason behind this is that those females are able to pass on the toxins that they acquire from the toads to their offspring potentially providing the young with an extra line of defense against predators.
This type of maternal transfer of chemicals for defense purposes is well documented in amphibians. Rough skinned newts (Taricha granulosa) and harlequin frogs (Atelopus chiriquiensis) pass tetrodotoxin to their eggs to protect them from predators. Cane toads (Rhinella marina) invest bufadeinolides into their eggs, which makes the eggs as well as the tadpoles toxic to predators. Many other amphibian species provision their eggs with chemical defenses however, in most case these defenses decrease as the tadpoles develop. A recent study showed that strawberry poison frogs (Oophaga pumilio) actually provide their young with chemical defenses after they hatch into tadpoles. O. pumilio are obligate egg feeders meaning they produce unfertilized eggs to feed to their tadpoles. No toxins were found within fertilized eggs but they were detected in the unfertilized eggs that females were feeding their offspring suggesting that they were providing tadpoles with a chemical defense from predators after hatching. This is the first instance of an organism providing chemical defenses to young post hatching.
What makes the tiger keelback so interesting is that it actively seeks out toads that produce bufadeinolides while gravid. This is quite different from other organisms such as the aforementioned amphibians whose diet primarily consists of prey that provides them with chemical defenses. Tiger keelbacks, on the other hand, primarily feed on non-toxic frogs throughout the year but switch to toads during reproduction. They even switch from foraging in grassland habitats to forging in forested habitats causing them to travel longer distances to find food. This could actually put the females in greater risk of predation but it shows the great lengths that some mothers will go to provide their offspring with a better chance of survival.
At the individual level, the greatest accomplishment of any organism is surviving long enough to reproduce. Most organisms seem satisfied with merely the chance that their genes will make it into the next generation. However, the simple act of fertilization does not guarantee that traits will survive for long especially in organisms whose offspring have few defense mechanisms against predators. Some mothers are turning the tide in favor of their offspring by arming them with powerful chemical weapons in hopes that they will have a greater chance of surviving to adulthood.
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