Reproductive Modes and Terrestriality

Question 1

3 major clades of amphibians

Answer 1

caecilians
caudates
frogs (anura) (90%)

Question 2

Life on land with permeable skin
habitat constraints and staying hydrated

Answer 2

amphibians have highly permeable skin - at risk of desiccation
T/F most constrained to habitats and activity periods which reduce the chances of dehydration, such as living in tropical rainforests (high humidity), in water, under leaf little and cover objects (which maintain humid microclimates), or being active at night (when temp = lower and humidity = often higher than during the day)

Question 3

Life on land with permeable skin
Morphology - what do the morphological changes allow for? exmaple?

Answer 3

(particularly those form more arid habitats) - evolved morphological adaptations that enable them to collect or absorb water effectively (e.g. pelvic patches and associated vascular networks - Roth 1973), and to reduce rates of water loss through the skin

Question 4

Life on land with permeable skin
Behaviour - what do body postures benefit? Choice of habitat? other behavioural benefits?

Answer 4

use body posture to minimise sa exposed to air to minimise the area available for evaporative water loss

some frogs live in burrows (humidity = higher) + (in arid environments) can create waterproof ‘cocoons’ by repeated shedding of skin until many layers overlap each other and envelop the frog to retain water

another beh strategy = secretion and active anointing of skin w/ specialise wax that reduces water loss rates by up to 95%

Question 5

Evolution of non-permeable skin
- who has evolved non-permeable skin?
- how is non-permeable skin formed?

Answer 5

tetrapod groups (amniotes) have evolved non-permeable skin

formed from layers of the protein ‘keratin’ (present in amphibian skin but only small amounts)
Keratin strands wind round each other and overlap to impart strength, flexibility, and a tight barrier to water

Question 6

Water as a reproductive constraint and the amniotic egg
- mammal development?
- what did amniotic egg allow for?

Answer 6

• even tho no egg shell for mammals - still essentially within an amniotic egg
• amniotic egg = key development that enabled amniotes to greatly reduce their dependence on standing water for reproduction (making their own miniature water bodies for the embryos to develop in, a key function of the amniotic sac)

Question 7

Reproductive Mode Diversity in Amphibians
- how have amphibians relaxed constraints imposed by terrestriality?
- what are the 4 broad categories?

Answer 7

• relaxed constraints imposed by terrestriality via their diversity of reproductive modes
• in context of terrestriality - 4 broad categories defined by whether the eggs are laid in water or on land, and whether there is a tadpole stage or not
• can be both ovoviviparous (holding eggs in the mother’s body until they hatch out) and viviparous (the mother produces secretions in the oviduct, which the developing offspring eat while still in her reproductive system before being born as fully developed froglets)

Question 8

Dependence on Water Bodies
- Natterjack toads
- Eastern newts

Answer 8

although still dependent on water bodies, some amphibians have evolved life cycles which limit the duration that water is needed

Natterjack toads
- use ephemeral ‘scrapes’ (shallow pools) to breed + have much higher rate of development than sympatric species like common toads
- allows them to develop w/o competition from other species but also reproduce w/o need for permanent water bodies

Eastern Newt
- added entirely newt stage to their life cycle which allows them to spend part of their time out of water during the terrestrial ‘red eft’ stage (between aquatic larvae and aquatic adults), reducing dependency + allowing them to move between water bodies to seek better quality ones

these examples = still highly dependent on water for reproduction, but have some adaptations to ease the constraints of needing permanent water bodies

Question 9

Reducing dependence on water
- how have amphibians reduced dependence on water bodies?
- ‘foam nests’?
- poison dart frog

Answer 9

• reduced dependence on water bodies via various forms of parental care

many frogs build ‘foam nests’ from secretions during mating, + eggs are laid in nest (keeping them in semi-aquatic foam state) which is left on vegetation or the ground near water bodies
- when hatch, tadpoles wriggle out of foam to drop from vegetation into water bodies below or the parent may dig a channel between the water body + nest on ground to allow tadpoles to swim into water + resume development - Polypedates leucomystax

• poison dart frog - need only tiny water bodies in form of water collected in leaves of bromeliads, and rainfall = sufficient to keep microhabitats in existence - females stick eggs onto vegetation + when hatch, male transport tadpoles on back to bromeliad ‘pools’; have little to no food source in pools, males monitor tadpoles and ensure that females lay (infertile) eggs into the bromeliads, on which tadpoles feed - Oophaga pumilio

dependence on water is present
reproductive modes like these greatly reduce the constraints to requiring only highly ephemeral water bodies or tiny pools collected in vegetation

Question 10

Eliminating Dependence on Water
- ‘nests’?
- extension of ‘foam nests’

Answer 10

some species evolved reproductive modes that eliminate requirement of water body to reproduce

two forms of viviparity mentioned previously

some species lay eggs in ‘nests’ on humid forest floor (maybe cover with leaves to retain humidity) + have direct development, such as froglets hatch out of egg with no need for water bodies

• foam nesters - some species produce so much foam that as the nest disassembles it simply creates a ‘puddle’ of more liquid foam that serves as aquatic medium for development of tadpoles

some species have entirely removed any requirement for standing water bodies and have seemingly mitigated the evolutionary constraints of being a terrestrial anamniote entirely

Question 11

Parental Care and Terrestriality

Answer 11

• both have evolved many times in frogs
• most forms of parental care = rare in aquatic frogs
• conc. of parental care amongst terrestrial lineages = likely related to need for active strategies to mitigate the constraints of reproduction in terrestrial systems (Vági et al. 2019)
• shows fundamental importance of reproduction modes in the evolution of terrestriality and how many strategies can successfully overcome evolutionary constraints