Geoscience Reference
In-Depth Information
at.colder.temperatures..While.the.mechanisms.for.these.observations.are.unknown,.
there.are.a.number.of.possible.explanations..For.example,.cold.temperatures.could.
alter.larval.amphibian.behavior,.which.could.expose.them.to.higher.levels.of.UV-B.
or. increase. their. exposure. time.. Alternatively,. cold. temperatures. could. reduce. the.
activity.of.repair.enzymes,.such.as.photolyase,.that.are.essential.in.repairing.DNA.
damage.after.exposure.to.UV-B..Because.many.high-elevation.sites.have.high.levels.
of.UV-B.in.combination.with.cold.temperatures,.amphibians.inhabiting.these.loca-
tions. may. be. particularly. at. risk. from. UV-B.. Global. warming. is. likely. to. change.
temperatures.in.many.habitats,.which.will.alter.the.threat.of.UV-B.for.amphibians.
and.make.the.risk.of.negative.effects.from.UV-B.exposure.hard.to.predict.
Like. other. animals,. over. evolutionary. time. numerous. selection. pressures. have.
inluenced.amphibians.in.a.variety.of.ways..Some.selection.pressures.have.shaped.
life-history.characteristics.and.behaviors.that.relate.to.how.amphibians.are.exposed.
to.sunlight..Amphibians.seek.sunlight.for.thermoregulation.and.to.maximize.their.
growth.and.development..Especially.in.temperate.regions,.larvae.often.seek.shallow,.
warm.water.that.ultimately.results.in.an.increase.in.their.growth.rate.(Wollmuth.et.
al..1987).
For. example,. Cascades. frog. (
R. cascadae
). tadpoles. are. frequently. observed. in.
warmer,.sunlit.areas.in.the.afternoon.(Wollmuth.et.al..1987)..Mountain.yellow-legged.
frog. (
Rana muscosa
). tadpoles. concentrate. where. water. temperatures. are. highest.
(near.shore.during.the.day,.deeper.in.the.late.afternoon.and.evening;.Bradford.1984)..
In. one. study,. all. but. the. latest-stage. bullfrog. (
Lithobates catesbeianus
). tadpoles.
selected.the.warmest.microhabitats.(Wollmuth.and.Crawshaw.1988).
.Many.frog.species.bask.in.sunlight.for.prolonged.periods.(Hutchison.and.Dupré.
1992)..For.example,.Lillywhite.(1970).found.that.more.than.70%.of.bullfrogs.present.
in.a.pond.were.basking.from.1300.to.1700.hours.on.a.sunny.day,.compared.to.less.
than.20%.on.a.cloudy.day..An.observational.study.of.
R. muscosa
.showed.that.eight.
frogs.present.in.a.pond.had.from.50%.to.80%.of.their.body.exposed.to.full.sunlight.
for.half.the.sunlight.hours.(Bradford.1984)..Juvenile.western.toads.(
A. boreas
).often.
bask.in.the.thousands.along.the.shoreline.(Blaustein.et.al..2005).
Many.amphibian.species.lay.their.eggs.in.shallow.open.water.in.direct.sunlight.
(Behler.and.King.1979;.Ashton.and.Ashton.1988;.Nussbaum.et.al..1983;.Stebbins.
and.Cohen.1995)..Eggs.are.often.laid.in.shallow.water.or.even.loating.on.the.water.
surface. to. develop. in. a. more. oxygenated. environment. and. probably. in. areas. with.
lower. risk. of. predator. or. parasite. attacks.. Oxygenation. of. eggs. is. critical. to. their.
development.(Duellman.and.Trueb.1986).
A.recent.comprehensive.study.incorporating.ield.transect.surveys.and.laboratory.
and. ield. experiments. illustrates. that. even. though. UV-B. radiation. may. be. harm-
ful.to.amphibians,.the.selection.pressures.related.to.thermoregulation,.growth,.and.
development.are.overriding.and.amphibians.seek.sunlit.microhabitats.(Bancroft.et.
al..2008b)..This.was.the.case.even.in.experimental-choice.tests.
In.summary,.amphibians.living.today.often.seek.sunlight.because,.over.evolution-
ary.time,.seeking.sunlight.was.beneicial.for.thermoregulation,.growth,.and.devel-
opment.and.perhaps.for.other.reasons..Yet.today's.sunlight.exposes.amphibians.to.
doses.of.UV-B.radiation.that.can.kill.or.signiicantly.damage.them.
