Squirrels versus Rattlesnakes: the Evolution of Unique Antipredator Behavior Barbara Clucas, PhD College of the Environment University of Washington

Animal Behavior The study of how animals use behavior to survive and reproduce How and why behavior evolves Social, reproductive, movement, antipredator

Animal Behavior The study of how animals use behavior to survive and reproduce How and why behavior evolves Social, reproductive, movement, antipredator

Antipredator Behavior Reduce the risk of predation Most animals are prey Evolution of a vast array of antipredator behavior

Antipredator Behavior

Ground Squirrels (Spermophilus) Diverse genus of species Worldwide distribution (except Australia and Antarctica) Live in burrows in the ground Species vary in habitats and sociality

Ground squirrel predators

Rattlesnakes (Crotalus) Warning rattle Venomous Skilled rodent predators –Lethal venom –Acute sense of sight and smell –Pit organs can sense temperature changes rattle

Ground squirrel defenses Venom resistant Harass, attack rattlesnakes Tail-flagging –visual and infrared signal

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 discovery Another unique snake-related behavior found in certain species of ground squirrels “Snake scent application”

Snake Scent Application

Snake Scent Application (SSA) 1.Why are squirrels applying rattlesnake scent? Test 3 functional hypotheses 2.Evolutionary history Phylogenetic comparative methods

Functional hypotheses of SSA 1.Antipredator 2.Social 3.Ectoparasitic defense

1. Antipredator SSA disguises squirrel odor –Rattlesnakes may bypass burrows with snake-scented squirrels

2. Social Conspecific deterrence SSA deters rivals –Snake-scented squirrels win more aggressive encounters

3. Ectoparasite defense SSA reduces fleas –Flea host-finding behavior affected by snake scent

Testing 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: 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: Which squirrels SSA more?

Staked out shed rattlesnake skins Filmed individual squirrels Recorded duration of SSA Quantifying application behavior

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

SSA not related to flea load None Low Med High Spearman rank correlation: r s : , N=45, P=0.829 Clucas et al. 2008, Anim Behav

Study 1: Antipredator 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? Experiment 1: Rattlesnake foraging behavior Experiment 2: Squirrel behavior before and after applying Experiment 3: Flea host choice

Rattlesnake Foraging Behavior Experiment 1 N = 8 C. oreganus oreganus 3 scent-type trials 1.Ground squirrel 2.Ground squirrel +Rattlesnake 3.Rattlesnake Water control

Rattlesnake Foraging Behavior Behavior scored – Time spent over – Tongue-flicking Experiment 1 N = 8 C. oreganus oreganus

Repeated measures GLM: F 2,14 =4.667, P = 0.028; planned comparisons: all P<0.05 Experiment 1 Spent more time over ‘squirrel’ Clucas et al. 2008, PRSL

Repeated measures GLM: F 2,14 =4.478, P = 0.031; planned comparisons: Sq>R P=0.03, Sq>S+R P=0.07 Experiment 1 Tongue flicked more over ‘squirrel’ Clucas et al. 2008, PRSL

Experiment 2 Before and After SSA behavior SCENTED Pre-trial SSA trial Post-trial CONTROLS Pre-trial No SSA trial Post-trial hours

Experiment 2 Before and After SSA behavior Recorded: – Social interactions (aggressive or tolerant)

CONTROLS * No differences between before and after Experiment 2 Social Interactions SCENTED * No differences between before and after Repeated Measures GLM; P>0.05 Clucas et al. 2008, PRSL

Flea host choice Juvenile ground squirrels as hosts Fleas –Removed from ground squirrels Experiment 3 Control Flea SSA Squirrel starting Squirrel point

Flea host choice Flea behavior recorded –Choice –Latency to move –Choice latency Experiment 3 Control Flea SSA Squirrel starting Squirrel point ??

Fleas 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: t 53 =0.661, P=0.512 –Choice latency: t 53 =-0.030, P=0.976 Experiment 3 == Clucas et al. 2008, PRSL

Study 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 Snake Scent All evidence supports an antipredator function Olfactory camouflage –Snakes did not avoid rattlesnake scent, rather showed low foraging behavior

Evolutionary history Explore the origins of applying snake scent –When did it evolve? –What caused it to evolve?

Studying evolutionary history Phylogenetic comparative methods Phylogenetic tree

Evolutionary history ? Common Ancestor

Evolutionary history ?

Ground squirrel phylogeny Molecular (cytochrome b) Divergence times –Time (in million of years) when species diverged

Comparative study Tested multiple ground squirrel and chipmunk species with rattlesnake scent Recorded presence/absence of application behavior

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? Is rattlesnake presence related to scent application? –Test with correlated trait evolution analysis

SSA Correlated with rattlesnake presence

Correlated Trait Evolution Snake Scent Application (SSA)

However… Current predator presence What about historical co-occurrence?

Historical predator presence Fossil records –established squirrel and rattlesnake co-occurrence in the past

Historical 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 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: thousand 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 Present Typically species had both historic and present co-occurrence with rattlesnakes However, there were several 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… 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 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: applications 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 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 Antipredator Behavior Alarm calls denote certain predators –Mammalian (e.g., coyotes) –Hawks –Snakes Different alarm calls refer to different response behavior and urgency

Prairie Dog Antipredator 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 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 Conservation Understanding behavior can lead to better conservation of wildlife Taking historic information into account may be important

Acknowledgements Don Owings Matt Rowe Tim Caro Jamie Cornelius Annie Leonard Terry Ord George & Maria Clucas Dick Coss Doug Dinero Tom Hahn Ann Hedrick Peter Marler Lori Miyasato Larry Rabin Aaron Rundus ABGG students , 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 Group Animal Behaviour Society American Society of Mammalogy UCMexus UC Davis College of Agricultural & Environmental Sciences