Squirrels versus Rattlesnakes: the Evolution of Unique Antipredator Behavior Barbara Clucas, PhD...

Squirrels versus Squirrels versus Rattlesnakes: Rattlesnakes:

the Evolution of Unique the Evolution of Unique Antipredator BehaviorAntipredator Behavior

Barbara Clucas, PhDCollege of the EnvironmentUniversity of Washington

Animal BehaviorAnimal Behavior

• The study of how animals use behavior to survive and reproduce

• How and why behavior evolves

• Social, reproductive, movement, antipredator

Animal BehaviorAnimal Behavior

• The study of how animals use behavior to survive and reproduce

• How and why behavior evolves

• Social, reproductive, movement, antipredator

Antipredator BehaviorAntipredator Behavior

• Reduce the risk of predation

• Most animals are prey

• Evolution of a vast array of antipredator behavior

Antipredator BehaviorAntipredator Behavior

Ground Squirrels Ground Squirrels ((SpermophilusSpermophilus))

• Diverse genus of species

• Worldwide distribution (except Australia and Antarctica)

• Live in burrows in the ground

• Species vary in habitats and sociality

Ground squirrel predatorsGround squirrel predators

Ground squirrel predatorsGround squirrel predators

Rattlesnakes Rattlesnakes ((CrotalusCrotalus))

• Warning rattle• Venomous• Skilled rodent predators

– Lethal venom– Acute sense of sight and

smell– Pit organs can sense

temperature changes

rattle

Ground squirrel defensesGround squirrel defenses

• Venom resistant • Harass, attack

rattlesnakes• Tail-flagging

– visual and infrared signal

Rattlesnakes are still predators…Rattlesnakes are still predators…

• Ground squirrel pups– Not large enough to be

venom resistant– Anti-snake behavior not

fully developed– Depend on adults for

protection (especially their mothers)

Recent discoveryRecent discovery

• Another unique snake-related behavior found in certain species of ground squirrels

• “Snake scent application”

Snake Scent ApplicationSnake Scent Application

Snake Scent Application (SSA)Snake Scent Application (SSA)

1. Why are squirrels applying rattlesnake scent?• Test 3 functional hypotheses

2. Evolutionary history• Phylogenetic comparative methods

Functional hypotheses of SSAFunctional hypotheses of SSA

1. Antipredator

2. Social

3. Ectoparasitic defense

1. Antipredator1. Antipredator

• SSA disguises squirrel odor– Rattlesnakes

may bypass burrows with snake-scented squirrels

2. Social2. Social

• Conspecific deterrence

• SSA deters rivals– Snake-scented

squirrels win more aggressive encounters

3. Ectoparasite defense3. Ectoparasite defense

• SSA reduces fleas

– Flea host-finding behavior affected by snake scent

Testing hypotheses of functionTesting hypotheses of function

• Study 1: Time spent applying snake scent

– Which squirrels apply more?

• Study 2: Series of experiments directly testing targets

– What are the effects of snake scent?

Study 1: Study 1: Which squirrels SSA more?Which squirrels SSA more?

Study species

• California ground squirrel, (S. beecheyi)

– Winters, California

• Rock squirrel, (S. variegatus) – Caballo, New Mexico

• Trapped and marked squirrels

• Recorded:– sex– age– flea load

Study 1: Study 1: Which squirrels SSA more?Which squirrels SSA more?

• Staked out shed rattlesnake skins

• Filmed individual squirrels

• Recorded duration of SSA

Quantifying application behaviorQuantifying application behavior

PredictionsPredictions

1. Antipredator• adult females & pups > adult males

2. Conspecific deterrence• adult males > adult females & pups

3. Ectoparasite defense• time spent related to flea load• pups > adults

Adult females & pups > adult malesAdult females & pups > adult males

*P < 0.005; Error bars = SE Clucas et al. 2008, Anim Behav

SSA not related to flea loadSSA not related to flea load

None Low Med High

Spearman rank correlation: rs: -0.033, N=45, P=0.829 Clucas et al. 2008, Anim Behav

Study 1:Study 1: Antipredator hypothesis supportedAntipredator hypothesis supported

• Pups most susceptible to predation, adult females share burrows with and protect their pups

• No support for alternative hypotheses– squirrels with more fleas do not apply more– most aggressive squirrels (adult males) do

not SSA the most

Study 2: What are the effects of snake scent?Study 2: What are the effects of snake scent?

• Experiment 1: Rattlesnake foraging behavior

• Experiment 2: Squirrel behavior before and after

applying

• Experiment 3: Flea host choice

Rattlesnake Foraging BehaviorRattlesnake Foraging Behavior

Experiment 1Experiment 1

N = 8

C. oreganus oreganus

• 3 scent-type trials1. Ground squirrel2. Ground squirrel

+Rattlesnake3. Rattlesnake

• Water control

Rattlesnake Foraging BehaviorRattlesnake Foraging Behavior

• Behavior scored– Time spent over– Tongue-flicking

Experiment 1Experiment 1

N = 8

C. oreganus oreganus

Repeated measures GLM: F2,14=4.667, P = 0.028; planned

comparisons: all P<0.05

Experiment 1Experiment 1

Spent more time over ‘squirrel’Spent more time over ‘squirrel’

888 888N =

Scent Type

Squirrel+RattlesnakeRattlesnakeSquirrel

Tim

e O

ver

(sec

on

ds)

+/-

SE

250

200

150

100

50

0

- 50

Scent

Water

Clucas et al. 2008, PRSL

Repeated measures GLM: F2,14=4.478, P = 0.031;

planned comparisons: Sq>R P=0.03, Sq>S+R P=0.07

Experiment 1Experiment 1

Tongue flicked more over ‘squirrel’Tongue flicked more over ‘squirrel’

888 888N =

Scent Type

Squirrel+RattlesnakeRattlesnakeSquirrel

Ton

gue

Flic

ks +

/- S

E

100

80

60

40

20

0

- 20

Scent

Water

Clucas et al. 2008, PRSL

Experiment 2Experiment 2

Before and After SSA behaviorBefore and After SSA behavior

SCENTED

Pre-trial SSA trial Post-trial

CONTROLS

Pre-trial No SSA trial Post-trial

24-48 hours 24-48 hours

24-48 hours 24-48 hours

Experiment 2Experiment 2

Before and After SSA behaviorBefore and After SSA behavior

• Recorded:– Social interactions (aggressive or tolerant)

CONTROLS

* No differences between before and after

Experiment 2Experiment 2

Social InteractionsSocial Interactions

SCENTED

* No differences between before and after

Repeated Measures GLM; P>0.05 Clucas et al. 2008, PRSL

Flea host choiceFlea host choice

• Juvenile ground squirrels as hosts

• Fleas – Removed from

ground squirrels

Experiment 3Experiment 3

Control Flea SSA Squirrel starting Squirrel point

Flea host choiceFlea host choice

• Flea behavior recorded– Choice– Latency to move– Choice latency

Experiment 3Experiment 3

Control Flea SSA Squirrel starting Squirrel point

??

Fleas not affected by snake scentFleas not affected by snake scent

• No significant difference in choice (2

1=0.455, N=56, P=0.500)

• Latencies did not differ by choice

– Latency to move: t53=0.661, P=0.512

– Choice latency: t53=-0.030, P=0.976

Experiment 3Experiment 3

==

Clucas et al. 2008, PRSL

Study 2: Antipredator hypothesis supportedStudy 2: Antipredator hypothesis supported

• Rattlesnake foraging behavior affected by snake scent

• No support for alternative hypotheses– Neither conspecific behavior nor flea

behavior affected by snake scent

Function of Applying Function of Applying Snake ScentSnake Scent

• All evidence supports an antipredator function

• Olfactory camouflage– Snakes did not avoid

rattlesnake scent, rather showed low foraging behavior

Evolutionary history Evolutionary history

• Explore the origins of applying snake scent

– When did it evolve?

– What caused it to evolve?

Studying evolutionary historyStudying evolutionary history

• Phylogenetic comparative methods

• Phylogenetic tree

Evolutionary history Evolutionary history

?Common Ancestor

Evolutionary history Evolutionary history

?

Ground Ground squirrel squirrel

phylogenyphylogeny

• Molecular (cytochrome b)

• Divergence times– Time (in

million of years) when species diverged

Comparative studyComparative study

• Tested multiple ground squirrel and chipmunk species with rattlesnake scent

• Recorded presence/absence of application behavior

When did scent application originate?When did scent application originate?

• Ancestor state reconstruction

– estimate whether squirrel ancestors possessed the scent application trait using maximum likelihood analysis

Ancestral State Reconstruction

• Common ancestor likely had behavior

• Behavior lost several times

What caused SSA to evolve?What caused SSA to evolve?

• Is rattlesnake presence related to scent application?– Test with correlated

trait evolution analysis

SSA Correlated with rattlesnake presenceSSA Correlated with rattlesnake presence

Correlated Trait Evolution Correlated Trait Evolution Snake Scent Application (SSA)Snake Scent Application (SSA)

Transition qij Independent model

Dependent model

No Pred, No SSA to No Pred, SSA q12 0.06681 0.07035 Gain SSA Pred, No SSA to Pred, SSA q34 5.46777

No Pred, SSA to Pred, SSA q24 12.26164 Retain SSA Pred, SSA to No Pred, SSA q42 0.000002

No Pred, SSA to No Pred, No SSA q21 0.04732 1.36796 Lose SSA Pred, SSA to Pred, No SSA q43 0.06255

Pred, No SSA to No Pred, No SSA q31 0.04732 14.32849 No SSA No Pred, No SSA to Pred, No SSA q13 0.06681 0.0000003

L(I)

L(D)

L(R)

-11.5970 -5.1190 12.95 p<0.02

However…However…

• Current predator presence

• What about historical co-occurrence?

Historical predator presenceHistorical predator presence

• Fossil records – established

squirrel and rattlesnake co-occurrence in the past

Historical predator presenceHistorical predator presence

• First squirrel fossil about 30 mya

• First squirrel-rattlesnake co-occurrence about 15 mya

Squirrel and rattlesnake ancestors

• Behavior evolved before co-occurrence

Original sources of selectionOriginal sources of selection

• Snake scent application evolved at least 28 mya– Rattlesnake ancestor not

present until 15 mya

• Original source of selection may have been older snake species (e.g., Boavus spp.)

More Recent Past: 10-400 More Recent Past: 10-400 thousand years agothousand years ago

• Presently existing squirrel species

– Species that do not SSA did not historically co-occur with rattlesnakes

– Species that do SSA did historically co-occur with rattlesnakes

Past and PresentPast and Present

• Typically species had both historic and present co-occurrence with rattlesnakes

• However, there were several exceptions…

Interesting exceptions…Interesting exceptions…

• California ground squirrels in Davis, CA– Historically had

rattlesnakes – Ended about 9000 years

ago

• Do not apply snake scent

• Behavior rapidly lost

Interesting exceptions…Interesting exceptions…

• Belding’s ground squirrels in MWR, OR– Did not have rattlesnakes

historically

– Currently do co-occur • Do not apply snake scent• Behavior not regainable?

Final ConclusionsFinal Conclusions

• Squirrels apply predator scents to reduce predation risk

• Predator scent application is an evolutionarily ancient trait in squirrels

• Original source of selection unknown• Recent past, behavior maintained by

rattlesnake presence, dependent on historic co-occurrence

Antipredator behavior: Antipredator behavior: applications for conservationapplications for conservation

• Captive breeding programs– Will individuals in captivity maintain

antipredator behavior?

• Reintroductions of predators– Will individuals from predator-naïve

populations be able to defend themselves?

Black tailed prairie dogs Black tailed prairie dogs

• 98% decline in North America

• Candidate species for Endangered Species Act listing

• Translocating individuals to boost small or extirpated populations

• Low survival rates after translocations

Prairie Dog Prairie Dog Antipredator BehaviorAntipredator Behavior

• Alarm calls denote certain predators – Mammalian (e.g., coyotes)– Hawks– Snakes

• Different alarm calls refer to different response behavior and urgency

Prairie Dog Prairie Dog Antipredator BehaviorAntipredator Behavior

• Pre-release predator training for captive-born juveniles– Paired presentation of

predators with appropriate alarm calls

• Enhanced antipredator behavior and increased post-release survival

Shier & Owings 2006

Predator ReintroductionPredator Reintroduction

• Wolves reintroduced in areas in Wyoming after 30-year absence

• Moose calf death rate increased

• But… tested moose that lost calves to wolf predation and showed hypersensitivity to wolf vocalizations

Berger et al. 2001

Animal Behavior and Animal Behavior and ConservationConservation

• Understanding behavior can lead to better conservation of wildlife

• Taking historic information into account may be important

AcknowledgementsAcknowledgementsDon OwingsMatt RoweTim Caro

Jamie CorneliusAnnie LeonardTerry OrdGeorge & Maria ClucasDick CossDoug DineroTom HahnAnn HedrickPeter MarlerLori MiyasatoLarry RabinAaron Rundus

ABGG students 2002-2008, Pat & Roy Arrowood, Stan Bursten, Marian Bilheimer, Jenn DeBose, Taylor Chapple, John Hammond, Tyson Schmidt, Aysha Taff, 2008 Bodega Phylogenetics Workshop (especially Brian O’Meara), Fred Armstrong, Gwen Bachman, Gretchen Baker, Duane Davis, Karen Hughes, Michael Magnuson, Phillip McClelland, Sonia Navarro Perez, Richard Roy, Donna Stovall, Renee West, Sebastian, Batman, Sugar, the Celtic soccer team, and the countless people who donated shed snake skins

NSF UC Davis Animal Behavior

Graduate GroupAnimal Behaviour SocietyAmerican Society of

MammalogyUCMexusUC Davis College of

Agricultural & Environmental Sciences