Chapter 6 - Amphibians
1. Tetrapods
a. Arose in the Devonian
b. Oldest skeletal remains of terrestrial vertebrates come from upper Devonian ~
360 mya
c. Conditions during Devonian
i. From Devonian to Permian, continents coalescing (will ultimately form
supercontinent of Pangea)
ii. Most land mass between equator and South pole
iii. Fresh-water basins preserved; shallow fresh-water swamps appear
iv. Major period of marine reef-building; highly diverse marine communities
v. Plants invading, at least in wetlands, helping create cool, moist
microclimates on land
vi. Terrestrial invertebrates include detritivores, arachnids (forming potential
food resource)
2. Life on land presents very different challenges from life on water; requires major
changes in anatomy and physiology
a. Dryness is most obvious change
i. Vertebrates (like most organisms) are mostly in water
ii. Regulating salt/water balance on land is very different from in water
iii. Requires investment in water-conservation mechanisms, including switch
to productions of urea and uric acid
b. Gravity becomes impediment to support and locomotion
i. Do not have buoyancy of water to support body
ii. Requires major changes to musculoskeletal system to support body,
enable movement
c. Gas exchange with air vs. water
i. Good news is that oxygen availability is much better: higher O2, more
rapid diffusion, cheap to ventilate
ii. Bad news is that expose thin, permeable gas exchange surface to dry air
means potential for water loss
d. Suction feeding no longer an option: can not “suck in” food with water, so need
new feeding mechanisms
3. How does a land animal evolve in water?
a. The basic puzzle
i. Current (very extensive) fossil evidence clearly indicates that earliest
tetrapods were still aquatic, not terrestrial
ii. But characteristics that make life on land possible can not evolve “in
advance” – selection won’t favor traits that will only be beneficial in the
future
iii. So hypotheses for the evolution of life on land must explain how traits that
would permit life on land would be adaptive in the essentially aquatic
environments of the Devonian
b. Some ideas
i. Aquatic environments during Devonian featured complex communities,
with many potential competitors and predators
ii. With invasion of land by plants and invertebrates, terrestrial habitats (at
least on margins of streams, swamps, etc.) would offer potential benefits
1. Plants and soils mean microclimate still relatively cool, moist (so
less danger of water loss)
2. Invertebrates are major food source
3. No competitors or predators
iii. Many of the changes that permit life on land would be adaptive for animal
living in relatively shallow, anoxic water at edges of swamps and streams
1. These habitats would provide refuge from competitors, predators
and access to terrestrial food sources
2. Shallow water would not provide much buoyancy, so modifications
of musculoskeletal system would be adaptive for support and
locomotion
3. Shallow water tends to be warm, anoxic – so ability to breathe air
would be adaptive
4. Feeding along the margins of stream banks would favor changes in
the feeding apparatus
iv. So one reasonable hypothesis is that characteristics that permitted
invasion of terrestrial habitats arose originally as adaptations for life in
shallow water, with selection continuing to favor those that permitted more
independence from water
4. Changes involved in the transition to life on land
a. Note that these changes did not take place in a linear, “progressive” sequence;
rather, they appear mosaic-fashion (i.e., at different rates and degrees in different
groups)
b. Musculoskeletal changes easiest to document from fossils
i. Loss of bones connecting pectoral girdle to skull = development of neck
1. Specialized vertebrae include modification of first two vertebrae
(atlas and axis) to support head
2. Allows head to move independently of rest of body for feeding
3. Allows head to remain stationary while animal is walking
ii. Articulated, weight-bearing vertebral column
1. Vertebrae develop process that help “lock” them in place
2. Allows vertebral column to act like suspension bridge, bearing the
weight of the animal and transferring it to the limbs
3. Ribs are enlarged, giving rigidity to the trunk
4. Modified sacral ribs provide bony connection of hind limbs to
vertebral column
a. Enhances weight-bearing
b. Lets pelvic girdle transmit “propulsion” from legs to trunk
5. Articulated, weight-bearing limbs allow animal to hold body off
ground and walk
a. Tetrapod limb characterized by jointed limbs with digitbearing hands and feet
b. Note “push-up” posture: limbs held out at right angles from
body
c. Means muscles being used to support body weight during
locomotion
d. Walking still basically by lateral undulation, using limbs
primarily to anchor body to ground and provide “pivot points”
6. Large, flat, wide head with relatively long snout
a. Shape seems to be important for gular pumping (ventilating
lungs)
b. Muscular tongue moves food from mouth to esophagus
iii. Other changes include:
1. Differentiated abdominal muscles to support viscera, contract lungs
2. Loss (eventually) of lateral line system
3. Addition of tympanic ear in some (“old” hyomandibular becomes
stapes, transmitting air-born vibrations to inner ear)
4. Addition of urinary bladder to store urine, water; acts as water
reservoir
5. Systematics, taxonomy, and early radiations
a. Taxonomy is in some dispute, with differences among experts about which
groups should be included in formal Tetrapoda
i. Historically, we equated two events: origin of limbs bearing digits and
origin of terrestrial vertebrates – so “Tetrapoda” referred simultaneously to
all vertebrates with limbs and all terrestrial vertebrates
ii. Now, of course, we know that limbs arose well before life on land
iii. We also have a wealth of fossil “intermediates” showing mixtures of
ancestral and derived characteristics
b. Early tetrapod radiations and diversity
i. Some dates
1. Earliest fossil fragments of modern amphibians date to Permian
2. Earliest complete fossil = early Triassic Triadobatrachus
3. Modern families date to Jurassic
a. Fossils very like modern forms of all three groups
b. Indicates long period of independent evolution from each
other
ii. Earliest tetrapod radiations were those of non-amniotes (Batrachomorpha)
1. Spread over ~ 200 my, from Late Devonian to Early Cretaceous
2. Batachomorpha includes most likely ancestor of modern
amphibians
3. Precursors to reptiles probably arose within ~ 20 my of first
tetrapods
iii. Notes on Batrachomorpha
1. Wide range of body sizes (from <10 cm - > 1m) and forms suggests
radiation into numerous ecological niches, including:
a. Large terrestrial predators
b. Legless burrowers
c. Secondarily aquatic forms
2. These animals were very different from modern amphibians
a. Many were very large
b. Many retained dermal armor (heavy bony scales)
c. Likely did not have thin, permeable skin modified for gas
exchange
d. Reached peak taxonomic diversity in Late
Carboniferous/Early Permian; most lineages extinct by midPermian
6. Systematics, taxonomy, diversity
a. The relationships among the three major lineages (frogs, salamanders,
caecilians) are still unresolved
i. Problem is that frogs and caecilians are very highly specialized
ii. So traits are either shared among all three groups or unique to one – few,
if any, shared, derived characters can be identified
b. Three groups (usually considered orders within the class Amphibia)
i. Urodela = Caudata (“tailed ones”): salamanders
1. 10 families, 415 species
2. Found primarily in North America, with peak diversity in Central
America
3. Some legless forms, but most are terrestrial tetrapods as adults
ii. Anura (“no tail”): frogs
1. 27 families, 4300 species
2. Cosmopolitan distribution (except Antarctica)
3. Body modified for jumping
a. Hindlimbs form lever system
b. Tibia and fibula are fused
c. Powerful pelvis fused to vertebral column
d. Posterior vertebrae fused into urostyle
e. Forelimbs and flexible pectoral girdle for shock absorption
iii. Gymnophiona: caecilians – tropical, burrowing forms; very poorly known
(not even number of families, species)
7. General characteristics of Amphibia
a. Note that this group is highly modified relative to earlier forms – do not think of
them as “primitive”
b. General characteristics include
i. Skin thin, permeable, highly modified for gas exchange (derived character)
ii. All forms are carnivorous, at least as adults
iii. Anamniote egg (like fish)
1. No extraembryonic membranes
2. Eggs dependent on external moisture to avoid desiccation
iv. Hyomandibular modified to form columella (= stapes) to transmit sound to
inner ear
v. Lungs present in all terrestrial forms except Plethodontid salamanders
1. Lungs relatively simple, with relatively small surface area
2. Ventilated by buccal pumping (like lung fish)
vi. Remnants of lateral line persist in larval forms and in some secondarily
aquatic adult forms
vii. Temperature regulation depends on habitat
1. Aquatic forms poikilothermic (but can be adapted to remain active
under ice!)
2. Terrestrial forms somewhat homeothermic via ectothermy
a. Can decrease TB by evaporative cooling
b. Can increase TB by basking
viii. Circulatory system/heart different from fish
1. Patterns of circulation can be highly variable depending on site of
gas exchange (lung, skin, gills, etc.)
2. Heart consists of 3 chambers: two atria, one ventricle
3. Circulation is “dual circuit pump”
a. Pulmonary circuit = oxygen-depleted blood leaves heart,
goes to site of gas exchange, returns to heart
b. Systemic circuit = oxygenated blood from heart to tissues,
then back to heart
c. Complex internal structure of ventricle keeps blood streams
well-separated (so little or no mixing of oxygenated and
deoxygenated blood)
d. Complex arrangement of heart and major blood vessels
permits extreme flexibility in circulatory patterns
i. Can switch among gas exchange modes (e.g., lungs
vs. skin)
ii. Can alter pattern depending on whether animal is in
water or on land
ix. Urinary bladder important for water regulation
1. Amphibians produce dilute urine, so good source of water for
reabsorption
2. Terrestrial anurans can store 20-30% of body mass as water in
urinary bladder
x. Generally have two-phase life history
1. Aquatic tadpole
2. Terrestrial adult
3. Involves remarkable transformation that approaches lepidopteran
metamorphosis in its complexity
8. Cutaneous respiration
a. Skin highly modified for gas exchange
i. Ancestral forms had dermal scales; modern amphibians have very thin,
permeable skin with extensive blood supply
1. Thin to decrease the distance gasses must diffuse
2. Poorly keratinized to increase permeability
3. Highly vascularized so blood in close contact with skin
ii. Poor keratinization means skin more easily damaged; reduces protection
from predators
iii. Extensive mucus glands provide protection from damage, predation,
desiccation
iv. Even partial reliance on cutaneous gas exchange constrains amphibian
behavior, ecology, etc
1. Constrains size: must be small to maintain favorable SA/V (only
relatively small animals will have sufficient skin surface to supply
volume)
2. Constrains habitats: permeable to oxygen = permeable to water, so
always at some desiccation risk
v. Feeding specializations of plethodontid salamanders illustrate evolutionary
tradeoffs
1. Plethodontidae = largest family of salamanders
2. All species use modified, protrusible tongue to capture prey
a. Requires modification of hyoid apparatus that make hyoid
apparatus less useful for “normal” functions
i. Buccal pumping to ventilate lungs in adults not
possible
ii. Suction feeding in aquatic tadpoles not possible
b. Solution has been to:
i. Eliminate lungs completely
ii. In most highly derived group (Bolitoglossinae),
eliminate aquatic larval stage as well (so “adult”
specializations appear during embryonic development
rather than after metamorphosis)
9. Predator defenses in amphibians
a. Skin in most amphibian species is toxic at some level (bufotoxins in toads,
alkaloids in poison arrow frogs) and is imbedded within the mucus that covers
skin
b. In addition to chemical defense, mucus can contain antibacterial property
c. In some cases, it has an adhesive property and serves as physical defense
i. Many salamanders have sticky material and toxins on tail (slimy
salamander in east)
ii. When attacked, bend tail upward toward predator
iii. Causes debris to adhere to predators nose and predator may temporarily
let go
iv. Mucus glands and poison glands are two different structures in skin
d. Poison glands produce a huge variety of toxins
i. Protein and alkaloid; many with pharmacological uses
ii. Range from deadly to just making animals taste badly
iii. Poison often advertised with aposematic coloration (with consequent
mimicry as well)
iv. In 3 species, poison made more effective by unique delivery mode: ribs
pierce poison gland, then body wall when animal grabbed by predator – so
literally stab predators with poisoned ribs!
v. Where do toxins originate?
1. Many toxins in poison arrow frogs are similar to those found in
insects
2. Ants, millipedes, and beetles found in leaf littler with frogs share
toxins
3. Perhaps obtain from prey
4. Fed captive Panamanian dart-poison frogs strict diet
a. Ones that lived in glass cages with no insects had no toxins
b. Ones fed fruit flies and ants in sterilized leaf litter had four
toxins, all similar to ants that had eaten
c. Ones fed natural food in fresh leaf litter had 16 different toxic
alkaloids from native insects
vi. Aposematic coloration seen in many of these species
vii. See Batesian mimicry, where unpalatable species looks similar to a
poisonous or dangerous model to fool a predator
viii. Also see Mullerian mimicry in poison-arrow frogs
1. Many species of unpalatable species look very similar
2. Have a group model, or group defense
3. If predator tries one of the frogs, all others are protected
10. Other interesting advancement relates to regeneration
a. Salamanders are primarily first vertebrate group where we can see regeneration
of major structures
b. Tails and legs can be removed and will begin to grow back within few days
c. Often have breaking points with cells at junction that are stem-cell like for
regrowth
d. Requires enormous amounts of energy
e. Since tail is often storehouse for fat and other storage energy, loss of tail can be
critical to survival
f. But if have energy stores, able to grow back
g. Advantages
i. If grabbed by a predator, can lose body part without dying
ii. So if predator grabs tail, better to lose tail than to die
iii. Also see in lizards (Class Reptilia)
iv. Some species actually have different colored tails that are more
conspicuous that rest of body so predators will focus attention there
11. Reproduction
a. Life history variation (around general “two-stage” theme)
b. First, consider why a “two-stage” life cycle might have arisen and been
maintained by selection
i. In the earliest tetrapods, terrestrial adults were probably favored because
land represented a favorable environment (many resources, little
competition)
ii. But, anamniotic egg requires wet environment for development
iii. For whatever reason, pattern seems to be maintained because it works
1. Meets the constraints imposed by anamniotic egg
2. Also allows flexibility in resource use: aquatic habitats for some
activities and terrestrial for others
c. Caecilians
i. Fertilization is internal with an intromittent organ protruded from the cloaca
(shared opening)
ii. Some lay eggs, and female may coil around them
iii. But most (75%) are viviparous
iv. Initially the fetus uses yolk to grow
v. But gone well before born, so feed by scraping the mother’s oviduct walls
vi. “Uterine milk” secreted by walls
d. Salamanders
i. Most have internal fertilization, but not by organ
ii. Male transfers packet of sperm (spermatophore) to female
1. Shape differs among species
2. Always is sperm on top of a gelatinous base
iii. Can either deliver it to her and put on her skin, or leave it for her to pick up
iv. Have extensive courtship
e. Frogs
i. Can have either explosive or prolonged breeding period
1. Explosive
a. In ephemeral sites (desert or temporary vernal ponds)
b. Last only a few days
c. Way to take advantage of water, since won’t last long
d. Males and females arrive at same time
e. Mating success for males is constant
2. Prolonged
a. Males arrive before females
b. Establish territories for very long periods of time, defending
against approach of other males
c. Females arrive and choose males for mating
i. Often based on calls
ii. Larger males (may provide better genes) have deeper
calls (in deeper and cooler water) and usually
selected by females
iii. Smaller males (younger) often left out of breeding
iv. Form satellite population around the periphery of the
action
v. When females approach larger males, usually have to
swim through the satellite population
vi. Satellite males often get quick mating as females
move through to larger males
ii. Frogs use vocalizations to attract mates
1. Cost is relatively high
2. Expense of making call
3. Potentially higher predation since conspicuous
4. Since females use calls to locate a male, predators can also use
iii. Fertilization is usually external, but some species have internal
fertilization, and viviparity
iv. Great variety in placement of eggs if hold them
1. Some carry around and have high levels of parental care
a. In stomach
b. On back
c. In vocal sacs
d. In pockets in skin
2. Aquatic usually form into larval form (tadpoles), and terrestrial often
born as miniature adults
a. Some poison arrow frogs lay eggs on ground, hatch into
tadpoles, which male carries to water
b. Many place them in protected spot, some covered in large
amount of material to keep them together and protect
f. If general life history pattern is maintained, at least in part, by selection favoring
use of different habitats when those habitats are favorable, then we should
expect to see variation in that pattern when different habitats become “risky” (the
benefits of using them are less than the costs)
i. Variations in life history
1. Tropical frogs minimize the use of water when risks are high
a. In many tropical areas, aquatic habitats have large numbers
of potential predators, competitors – so may be adaptive to
avoid them
b. Tropical frogs exhibit a range of adaptations to minimize
reliance on water
i. Lay eggs in bromeliads in trees, on land near shore
ii. Lay eggs on floating “foam” nests; tadpoles drop into
water when they hatch (presumably at less risk than
eggs because they’re bigger, more mobile)
iii. Lay eggs on leaves overhanging water; tadopoles
drop when they hatch (as above, only more so!)
iv. Eliminate aquatic stage entirely: have direct
development from egg to “froglet” with no larval stage
1. Occurs in 20% of frog species
2. Often accompanied by parental care of eggs
(to prevent desiccation), froglets
3. Examples include
a. Male Darwin frog carries eggs in vocal
sacs, “burps” them out when developed
b. Female Surinam toad carries eggs in
specialized depressions/pouches on her
back
c. Female gastric brooding frog (Australia)
carries eggs in stomach – with
concomitant modification of stomach
lining, etc.
2. Paedomorphic salamanders reduce reliance on terrestrial habitats
(the opposite of the frogs) by remaining aquatic as adults
a. Heterochrony = change in the timing of development
b. Neoteny = appearance/retention of juvenile traits in adults
c. Paedomorphosis = process that produces neoteny
i. In the case of salamanders = retention of gills and
other traits associated with remaining aquatic as
adults
ii. Phenomenon in salamanders is best studied in
members of genus Ambystoma – within this genus,
get whole range of variation
1. Some species have “normal” life history
2. Some have facultative paedomorphosis
a. Normal metamorphosis in some
populations under some conditions
b. Paedomorphosis in others under other
conditions
3. Some are obligate paedomorphs: virtually
never metamorphose (except possibly under
extreme laboratory conditions)
iii. General pattern: as aquatic habitats become more
predictable and terrestrial habitats more hostile and
unpredictable, paedomorphosis becomes more
common
iv. Examples
1. Ambystoma gracile
a. Increased frequency of paedomorphosis
in cold Rocky Mountain ponds (few fish,
permanent water)
b. Increased frequency of “normal”
metamorphosis in lowland habitats
(ponds more likely to dry up; more
predators present)
2. A. tigrinum
a. Normal metamorphosis in temporary
ponds in eastern part of range (NJ)
b. Increased frequency of paedomorphosis
in permanent ponds in western U.S.
12. Body shape in anurans
a. Frogs have been able to invade a variety of habitats
i. Many species are well adapted to aquatic environment
1. See in true frogs of family Ranidae
2. Have extensive webbing between toes, very long hind legs for
jumping
3. Also very streamlined for aquatic environment
ii. Terrestrial forms in several families
1. See in toads (family Bufonidae)
2. Heavy, stout bodies
3. Blunt heads and short legs
4. May be adaptations to limit loss of water
iii. Arboreal frogs
1. See in family Hylidae
2. Large heads and eyes
3. Many move by walking rather than jumping
4. Toe pads have peg-like projections separated by spaces or canals
5. Usually pads have mucus glands that secrete viscous solution
6. Use wet adhesion to stick to smooth surfaces
7. When walk, peel toes off starting from rear of toe
8. Able to stick past the vertical
9. But must rest with toes facing upward
a. Weight causes toe pads to peel off surface
b. So always orient body upward
c. So if must scale down a tree, walk backward rather than
forward
b. Foraging behavior related to environment
i. Aquatic engulf food through suction
ii. Semiaquatic and terrestrial have sticky tongues that flip outward to trap
prey
13. Conservation status
a. Two major phenomena: amphibian declines and malformations
b. Major declines of species worldwide over the past 20 years
ii. Multiple causes
1. Frogs are very susceptible because of life history – aquatic stage
2. Likely causes
a. Habitat loss
i. Wetlands declined by 50% since 1900
b.
c.
d.
e.
f.
ii. So human encroachment of breeding areas is major
factor
Habitat degradation
i. Changes in forest changes the microclimate
ii. Removal of overstory trees (by logging) changes the
microclimate from cool and moist to dry and warmer
iii. Anything that changes the forest floor will have an
effect (especially on salamanders)
iv. Colonization of new sites is more difficult with habitat
degradation
1. Not good at crossing barriers
2. Any non-forested habitat
Cattle
i. Affect wetlands and field ponds
ii. Amphibians attracted to ponds for breeding
iii. Dry up faster with cattle use
iv. Cattle tromp through mud as dry up, creating
unnavigable surface for amphibians
Acid precipitation
i. Major factor
ii. Especially seen in northeast and Midwest
iii. Nitric and Sulfuric acids are formed when nitrogen
and sulfur oxidizes
iv. From burning of fossil fuels
v. Most amphibians are killed at pH of 5 or less
vi. If not killed, they attain smaller size
UV radiation
i. Destruction of ozone in stratosphere
ii. Usually by chemical pollutants, such as found in
aerosol canisters
iii. Much lower use now than 10 years ago
iv. Seen mostly near poles
1. Hole in ozone is larger now than ever before in
history
2. Appears to be slowly shrinking
v. Kills eggs and larvae
vi. Also see effect based on experimental evidence
vii. But UV rays are blocked by turbidity and overhanging
vegetation
viii. Also, some species more susceptible than others
based on differences in sensitivity
ix. Worst at high elevations where atmosphere is the
thinnest
Disease
i. Always been a factor affecting amphibians
ii. So unlikely that it is a factor leading to declines
iii. May be more of an effect in combination with other
factors
iv. See in Tiger Salamanders
1. Can produce two different larval forms
2. Normal and cannibalistic
3. In populations that are infected with an
iridovirus, don’t see any cannibals in
populations
4. Likely that increase the load of disease if ingest
infected individuals
c. Notes about amphibian conservation
i. Causes may be different for declines and malformations, may not – no
one really knows yet
ii. Almost certainly multiple causes for both phenomena – very unlikely that a
single process or phenomenon could be affecting populations in the same
way all over the world
iii. The same things that make amphibians susceptible make them good
“miner’s canaries” – declines may be “early warning” of subtle
environmental problems that, if left unchecked, could have more
widespread effect