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Hoogland and Brown, Electronic Supplementary Materials

  1. Taxonomy of Wyoming ground squirrel

Before designation as Urocitellus elegans in 2000 [29], the Wyoming ground squirrel was known as Spermophilus richardsonii in the 1970s [24,27] and as Spermophilus elegans in the 1980s and 1990s [28].

  1. Animals observed to prey on ground squirrels

Assistants and JLH observed 220 predations on ground squirrels by 15 non-prairie dog species during our research at ANWR from 2007 through 2012 (Table ESM1).

  1. Avian scavengers observed to pick up and fly away with freshly killed ground squirrels

For 35 of 101 interspecific killings (IKs) of ground squirrels by prairie dogs that we observed, an avian scavenger picked up the fresh carcass and flew away with it, usually within 120 min—and often within 10 min—afterthe killing (Table ESM2). We inferred IKs for 58 other cases when we saw an avian scavenger pick up a fresh carcass and then fly away with it, but when we did not see a killing before the takeaway (Table ESM2).

  1. Frequencies of IKs and predations at the study-colony

For 3 of 6 years, IKs (observed and inferred) at the study-colony were more common than the cumulative number of predations on ground squirrels by American badgers, Swainson’s hawks, and thirteen other species of predators (Fig. ESM1).

  1. Dates of IKs of ground squirrelsby prairie dogs, and other associated behaviors

IKs occurred as early as 6 April, and as late as 25 June; 78% (127/163) of IKs occurred from 16 May through 15 June, when weaned juvenile ground squirrelswere first appearing aboveground from their natal burrows.

Sometimes the prairie dog killer either took the ground squirrel victim into a burrow within 2 min after killing it (12/101 = 12%), or partially buried it within 5 min after the victim had died (13/101 = 13%). Following 15% (16/105) of IKs, a ground squirrel adult or juvenile began to cannibalize the carcass on the same day as, or the day after, the killing.

  1. Sex and reproductive condition of killers, age of victims, and site of IKs

Most IKs involved a lactating prairie dog female killing a juvenile ground squirrel.Specifically, adult female prairie dogs were about 1.7 times more common than adult males each spring, but IKs by females was almost five times more common than IKs by males (n= 84 IKs versus n= 17, χ21 = 27.0, P < 0.001). Further, 62% (136/220) of female prairie dogs weaned a litter from 2007 through 2012, but 95% (80/84) of observed IKs by a female involved a lactating female (χ21 = 33.0, P < 0.001). Probably because juvenile ground squirrels cannot run as fast as adult ground squirrelsand are easier to overpower, 96% (156/163) of the ground squirrel victims were juveniles. For 96% of IKs (97/101), the ground squirrel victim was in the area near (<20 m from) the prairie dog killer’s home-burrow.

Table ESM1. Animals observed to prey on ground squirrels at Arapaho National Wildlife Refuge from 2007 through 2012. IKs of ground squirrels by prairie dogs are not shown here.

Predator (most common to least common) / Number of predations of ground squirrels by this predator observed from 2007 through 2012
American badger, Taxidea taxus / 92
Swainson’s hawk, Buteo swainsoni / 49
Long-tailed weasel, Mustela frenata / 24
American crow, Corvus brachyrhynchos / 15
Prairie falcon, Falco mexicanus / 10
Coyote, Canis latrans / 8
California gull, Larus californicus / 6
Common raven, Corvus corax / 5
Short-eared owl, Asio flammeus / 3
Golden eagle, Aquila chrysaetos / 2
Northern harrier, Circus cyaneus / 2
American kestrel, Falco sparverius / 1
Black-billed magpie, Pica hudsonia / 1
Harris’s hawk, Parabuteo unicinctus / 1
Sharp-shinned hawk, Accipiter striatus / 1
Total / 220

Table ESM2. Scavenging of ground squirrel carcasses after observed and inferred IKs by prairie dogs.

Avian scavenger / Scavenger picked up dead carcass after IK was observed / Scavenger picked up dead carcass when IK was inferred, but not observed
American crow / 23 / 33
California gull / 11 / 24
Common raven / 1 / 1
Totals / 35 / 58

Table ESM3.Mixed model investigating predictor variables (modeled as fixed effects) for the number of ground squirrels a female prairie dog killed each year.Female identity was treated as a random effect.Beginning (global) model included all predictors, and the step at which each variable was sequentially removed is shown.Variables that remained in the final model are indicated by F. β, SE, df, t, and P are shown for the step at which the variable was removed.

Variable / β / SE / df / t / P / Step
Female’s rate of aggression to ground squirrels / 1.568 / 0.220 / 38 / 9.79 / < 0.001 / F
Female’s body mass / 0.000 / 0.001 / 22 / 0.14 / 0.89 / 1
Clan size / 0.006 / 0.060 / 33 / 0.10 / 0.92 / 2
Female’s age / -0.013 / 0.094 / 34 / -0.14 / 0.89 / 3
Ward size / -0.007 / 0.028 / 35 / -0.26 / 0.80 / 4
Female’s rate of aggression to prairie dogs / -0.085 / 0.204 / 36 / -0.42 / 0.68 / 5
Study-colony’s annual rate of aggression / -0.028 / 0.028 / 37 / -0.99 / 0.33 / 6

Table ESM4.Mixed model investigating predictor variables (modeled as fixed effects) for annual fitness of female prairie dogs.Female identity was treated as a random effect.Beginning (global) model included all predictors, and the step at which each variable was sequentially removed is shown.Variables that remained in the final model are indicated by F. β, SE, df, t, and P are shown for the step at which the variable was removed.

Variable / β / SE / df / t / P / Step
Annual number of ground squirrels a female prairie dog killed / 0.181 / 0.050 / 38 / 3.57 / 0.001 / F
Clan size / -0.026 / 0.044 / 22 / -0.61 / 0.55 / 1
Female’s body mass / 0.000 / 0.001 / 23 / 0.55 / 0.59 / 2
Female’s rate of aggression to prairie dogs / -0.043 / 0.136 / 34 / -0.32 / 0.75 / 3
Ward size / -0.006 / 0.019 / 35 / -0.30 / 0.77 / 4
Study-colony’s annual rate of aggression / 0.008 / 0.020 / 36 / 0.41 / 0.69 / 5
Female’s age / -0.055 / 0.064 / 37 / -0.86 / 0.39 / 6

Table ESM5.Mixed model investigating predictor variables (modeled as fixed effects) for litter size.Female identity was treated as a random effect.Beginning (global) model included all predictors, and the step at which each variable was sequentially removed is shown.Variables that remained in the final model are indicated by F. β, SE, df, t, and P are shown for the step at which the variable was removed.

Variable / β / SE / df / t / P / Step
Annual number of ground squirrels a female prairie dog killed / 0.704 / 0.194 / 26 / 3.61 / 0.001 / F
Female’s body mass / 0.007 / 0.002 / 26 / 2.75 / 0.010 / F
Female’s rate of aggression to prairie dogs / -1.197 / 0.489 / 26 / -2.45 / 0.021 / F
Ward size / 0.009 / 0.080 / 22 / 0.12 / 0.91 / 1
Study-colony’s annual rate of aggression / 0.014 / 0.086 / 23 / 0.17 / 0.87 / 2
Clan size / -0.036 / 0.173 / 24 / -0.21 / 0.84 / 3
Female’s age / 0.109 / 0.260 / 25 / 0.42 / 0.68 / 4

Table ESM6.Mixed model investigating predictor variables (modeled as fixed effects) for whether a female prairie dog survived to the next mating season.Female identity was treated as a random effect.Beginning (global) model included all predictors, and the step at which each variable was sequentially removed is shown.Variables that remained in the final model are indicated by F. β, SE, df, t, and P are shown for the step at which the variable was removed.

Variable / β / SE / df / t / P / Step
Annual number of ground squirrels a female prairie dog killed / -0.540 / 0.271 / 38 / -1.99 / 0.054 / F
Clan size / 0.000 / 0.161 / 22 / 0.00 / 0.99 / 1
Female’s age / -0.045 / 0.246 / 23 / -0.18 / 0.86 / 2
Ward size / -0.028 / 0.074 / 24 / -0.38 / 0.70 / 3
Female’s body mass / -0.001 / 0.002 / 25 / -0.42 / 0.68 / 4
Study-colony’s annual rate of aggression / 0.020 / 0.065 / 36 / 0.31 / 0.76 / 5
Female’s rate of aggression to prairie dogs / -0.132 / 0.449 / 37 / -0.29 / 0.77 / 6

Table ESM7.Mixed model investigating predictor variables (modeled as fixed effects) for lifetime fitness of female prairie dogs.Female identity was treated as a random effect.Beginning (global) model included all predictors, and the step at which each variable was sequentially removed is shown.Variables that remained in the final model are indicated by F. β, SE, df, t, and P are shown for the step at which the variable was removed.

Variable / β / SE / df / t / P / Step
Lifetime number of ground squirrels a female prairie dog killed / 0.418 / 0.093 / 51 / 4.50 / < 0.001 / F
Study-colony’s annual rate of aggression / -0.020 / 0.061 / 41 / -0.34 / 0.74 / 1
Ward size / 0.031 / 0.074 / 42 / 0.34 / 0.68 / 2
Female’s rate of aggression to prairie dogs / -0.332 / 0.645 / 43 / -0.52 / 0.61 / 3
Clan size / 0.058 / 0.104 / 49 / 0.56 / 0.58 / 4
Female’s lifespan / 0.217 / 0.172 / 50 / 1.26 / 0.21 / 5

Fig. ESM1. Frequency by year of 163 IKs of ground squirrels (shaded bars, n = 101 observed, n = 62 inferred) and 220 predations of ground squirrels (open bars) at the study-colony from 2007 through 2012.