Amphibians: A Very Short Introduction – T. S. Kemp

Thoughts: A decent introduction to the subject of amphibians.

Instead of making notes with references to specific pages for this book, I tried asking ChatGPT to come up with questions for me to answer as I read it; I also came up with a few of my own questions. You can read the how and the why of these questions at these booknotes’ accompanying blog post.

Kemp, T. S. 2021. Amphibians: A Very Short Introduction. Oxford UP.

Questions

what are amphibians - what features do all amphibians share, and what distinguishes them from other animals? Where do they fit in the tree of life?

• answer before reading the book:
  • they grow from eggs laid in water, become larvae (i.e., tadpoles), and then metamorphose into adult forms which can be terrestrial or (occasionally) aquatic. Most have four limbs with five digits each, though some have only vestigal limbs. They have lungs, but can also breathe through their skin.
  • They share a common ancestor with other tetrapods
• after having read more:
  • amphibians are monophyletic - they all descended from a single ancestor, an ancestor that is different from those of the amniotes (tetrapods that lay eggs on land, the group to which reptiles and mammals belong)
  • Indeed! Amphibians do grow from eggs laid in water, become larvae (i.e., tadpoles), and then metamorphose into adult forms which can be terrestrial or (occasionally) aquatic. Most have four limbs with five digits each, though some have only vestigal limbs. They have lungs, but can also breathe through their skin. They share a common ancestor with other tetrapods

how diverse are amphibians - how many species are there? genera? families? where, in a global sense, are they found, and in what types of ecosystems?

• answer before reading the book:
  • there are 8000 identified species, and they are grouped into 3 orders: frogs/toads, salamanders/newts, and another caecilians(?), which have vestigal limbs.
  • while some amphibians spend some time hunting in tide pools and can survive in brackish water for an extended period of time, amphibians are found in all ecosystems on earth other than the oceans. They are most prevalent in hot, humid biomes such as tropical forest biomes. Especially with amphibians that burrow beneath the ground, they can be found in ecosystems as extreme as deserts and cold alpine streams
  • amphibians are rarely found on isolated islands separated from other land by salt water
• after having read more:
  • 8000 identified species, grouped in 3 orders:
    • Anura (frogs/toads)
      • short, tailless bodies, wide mouths, long back legs. 7000 living species
    • Urodela (salamanders)
      • 700 living species. long, narrow body with tail, short legs
    • Caecilia
      • 200 living species, but a great many probably not discovered yet. long, cylindrical body with no limbs. Look a lot like large earthworms, burrow in the soil

what ecological role(s) do amphibians play in ecosystems?

• answer before reading the book:
  • they predate on insects/insect larvae during their tadpole phase, and eat mostly invertebrates during their adult lives
  • they serve as food for larger animals including birds, mammals, lizards
• after having read more:
  • many larval amphibians are filter feeders, feeding on algae and small invertebrates. Some feed by scraping algal growths from the bottoms of streams or ponds.

what specific challenges do they face wrt ecosystem disruption, climate change, etc.?

• answer before reading the book:
  • amphibians are particularly sensitive to environmental changes, so they are threatened by deforestation, and by climate change when it leads to drier conditions. Since their skin is so porous, they are more affected than other organisms by chemical pollution. Also, amphibians are affected by fungal diseases which are becoming more prevalent with rising temperatures, and are inadvertently spread throughout the world by humans
• after having read more:
  • many amphibians are threatened with extinction (or already extinct) due to habitat fragmentation and destruction, the introduction of invasive species, and changing climatic conditions (especially where these changes lead to a less humid environment, such as in Costa Rica, where a warming local climate has led to a decrease in the fogs that historically have permeated the forests)

how has what we know about amphibians changed over time? how did we come to acquire our current knowledge about amphibians?

• answer before reading the book:
  • guessing the ancient Greeks, e.g., knew about how they metamorphosed. Guessing it was during the period of European colonial expansion, with renewed interest in the earth’s life, that they were first scientifically described and categorized, and that a big leap in understanding has been achieved following the introduction of DNA sequencing techniques
• after having read more:

What defining traits distinguish amphibians from other vertebrates, and how do these traits reflect their evolutionary history and ecological roles? Think about anatomy, physiology (e.g. skin, respiration), and their transitional place between aquatic and terrestrial life.

• answer before reading the book:
  • with other quadrupeds, they share four limbs ending with four digits. Whereas reptiles and birds lay eggs on land and mammals give live birth, amphibians lay their eggs in water as most fish do. Their skin allows for oxygen and other gases/chemicals to pass through it, compared to the relatively impervious skin of mammals and scales of reptiles. This works only when amphibians’ skins are wet, which limits most amphibians’ choice of habitats to places where they can stay moist. Amphibians have lungs, which aid in their respiration.
• after having read more:
  • amphibians are adapted to moving between water and dry land. They can breathe through their skins both in and out of water, can breathe through lungs (as well as through the nasal and mouth cavities) while on land, and can absorb oxygen through gills (while tadpoles. Some species never lose their gills when they metamorphose, though) when underwater. Larger species, with a larger volume-to-surface-area ratio, depend more on lungs/gills, while smaller species can get by more by simply respiring through their skin.

How do amphibians reproduce, and in what ways do their life cycles reflect both ancestral aquatic origins and adaptations to land? This will touch on eggs, metamorphosis, reproductive strategies, and developmental stages.

• answer before reading the book:
  • Amphibians lay eggs in water, and these eggs are fertilized in water also. The eggs hatch into legless, taily larvae, which metamorphose into quadrupedal, tailless (in the case of frogs, but not salamanders) adult forms which are able to emerge from the water, where they hunt and eventually return to the water to lay and fertilize eggs.
• after having read more:
  • due to their small size and porous skin, newly hatched amphibians would quickly dry out if they didn’t live in water. Thus, many amphibians lay their eggs in water, though some species lay eggs underground and some actually give live birth
  • frogs use sound in their courtship rituals, and they engage in amplexus, where the male remains attached to the female’s back for a length of time by grasping them with their front legs. Salamander and caecilian courtship involves chemical signals more than sonic signals; male salamanders deposit sperm on the ground and then drag females over the deposited sperm.
  • metamorphosis involves development of lungs and absorption of gills, growth of limbs and absorption of tail, rearrangement of bone structures and changes to mouthparts to switch from filter feeding to hunting of live prey
  • some amphibians are very active in protecting their eggs/tadpoles as they mature, and protection strategies can be quite varied. Eggs can be affixed to legs or to backs, and some species swallow their eggs following fertilization to vomit them up again after they have hatched into tadpoles.

In what ways are amphibians adapted to their environments in terms of movement, feeding, sensory perception, and communication? Look for variety across species and connections between form, function, and habitat.

• answer before reading the book:
  • I imagine they sense auditorily, through touch and vibration, and (to a lesser extent) by sight, as well as by sensing chemicals in the environment (smell? but possibly involving all of their skin rather than just their noses). They communicate largely through chemical and audible (in the case of frogs) signals.
• after having read more:
  • tadpoles come in different shapes, with mouths oriented differently, depending on whether larvae of the species stay at the bottoms of water bodies, stay attached to the surface of the water, or swim through the water column
  • in general, larval stages are better adapted to life in the water, and adult forms better adapted to life out of the water. Some species that live entirely aquatic lives retain features of juveniles (e.g. gills), and numerous salamander species remain in the water in their juvenile forms unless stressed (by lack of food, or by being crowded due to dry conditions), in which case they fully metamorphose and emerge to live on land
  • tadpoles move by undulating from side to side, like fish.
    • Salamanders retain this basic motion, supplementing it with their legs, which allow them to lift their heads/bellies off the ground, and afford greater purchase on the ground. Some salamanders will revert to simply slithering, snake-like, when startled on land. (this kind of addresses the “ancestral origins” part of the reproduction question)
    • frogs are adapted for locomotion by jumping, with long legs and toes that can be extended quickly and powerfully to launch themselves forwards. This basic mechanism, however, has been adapted in some species for swimming, burrowing and even aerial gliding.
    • adult caecilians are adapted for burrowing, using what is called the “internal concertina” mechanism of locomotion, gripping the walls of a burrow with their skin while pushing their head forward. In the water or on the land’s surface, caecilians can also get around by using lateral undulation
  • adult amphibians are exclusively carnivores
    • among salamanders:
      • most have teeth, but these are mostly useful for making sure prey doesn’t escape once it’s in the mouth, rather than for chewing
      • when feeding underwater, they tend to use suction feeding
      • when feeding on land, most salamanders use a sticky tongue that is shot from the mouth to catch prey and draw it into the mouth, though some have strong jaws for catching and eating larger prey
    • among frogs:
      • few frogs use suction feeding when underwater
      • frogs use a long sticky tongue that is flung from the mouth, similar to salamanders, but the mechanism is different (think of a yo-yo being thrown, rather than the rail gun–style mechanism of salamanders) and evolved separately
    • caecilians capture prey using their strong jaws
  • larval amphibians have lateral lines, which can detect vibrations in water. Adults (except those that retain juvenile characteristics) lose these when they metamorphose. Frogs have acute vision that can be binocular when needed, are trichromats, and some species have two types of rod cells, allowing them to see certain colours even in low light conditions. They also have ear drums. Salamanders have less acute vision, less acute hearing, and depend much more on pheromones to communicate.

How have amphibians interacted with humans culturally, scientifically, and economically—and what impact have those interactions had on amphibian populations? Reflect on amphibians in myth, medicine, research, and as environmental indicators.

• answer before reading the book:
  • Salamanders are named after a fiery being in Greek myth. In Europe, at least, they were used in medieval medicine, being associated with witchcraft.
  • compounds they produce (think poison dart frogs) have been used in the hunt for new drugs.
  • amphibians are indicator species, and their population levels and diversity can be used as a proxy for ecosystem health
• after having read more:
  • in Europe, amphibians, particularly toads, were associated with witchcraft. In ancient Egypt, they were associated with fertility. In China, it was said that robes made of salamander skin rendered the wearer fireproof
  • amphibians, mainly frogs and toads, are eaten around the world. They’re fairly nutritious (j: follow-on question: how sustainable a source of protein and/or calories are they, measured against carbon emissions and/or land use?)
  • amphibians have been used in folk medicine around the world. Of interest for modern medicine, the secretions of frogs, in particular, contain many biologically active compounds, and the author lists as painkillers, antibiotics, and cancer treatments as potential uses for such compounds
  • amphibians are kept as pets - the collection of amphibians as pets if often conducted illegally
  • cane toads, native to North America but released in Australia, are a prime example of the economic and environmental damage caused by the human translocation of animals - due to their toxicity and lack of natural predators, their numbers have expanded dramatically
  • the study of amphibians has led to numerous scientific breakthroughs. Their skin is illustrative of how protein pumps work, and their internal organs are easily accessible, leading to discoveries about, e.g., the function of kidneys.

What are the primary threats facing amphibians today, and what conservation strategies are being used or proposed to ensure their survival? This question will help you connect the biological with the political and environmental realities amphibians face.

• answer before reading the book:
  • deforestation leads to loss of habitat, and chemical pollution interferes with their reproduction and growth.
  • Rising temperatures lead to changes in habitat, often deleterious, and contribute to the spread of novel fungal diseases.
  • Humans work to preserve amphibian habitats and work to control pollution, and in the case of some endangered species, they are bred in laboratories in order to preserve them, possibly for eventual release into the wild.
• after having read more:
  • the author identifies a few major causes of amphibian population decline: habitat fragmentation and destruction, changing climate (to which amphibians are somewhat more sensitive than other groups, because disruptions to both their aquatic and terrestrial habitats can impact them), predation by introduced species, pathogens (of which the fungus Batrachochytrium dendrobatidis is the most notable example, while other fungi and viruses are lesser players), and human hunting/trapping for scientific and medical needs as well as the exotic pets trade.
  • the author notes that the two actions most necessary for reversing the decline of amphibians are the creation of large protected natural areas and reducing the extent of climate change
  • amphibians are generally easy to breed in labs, so certain populations nearing extinction can be preserved in breeding programs in labs. The author lists several institutions in the United States that are already carrying out this work. There is also hope that certain extinct species may be resurrectable through cloning based on preserved tissue samples