Richardson's Ground Squirrels Research Articles

Spontaneous Cutaneous Mast Cell Tumor with Lymph Node Metastasis in a Richardson's Ground Squirrel (Spermophilus Richardsonii)

A 4-year-old female Richardson's ground squirrel (Spermophilus richardsonii) presented with multicentric nodules arising from the skin of the middle of the tail and lumbosacral regions. Histologically, the nodules were composed of a proliferation of spindloid to pleomorphic cells that sometimes formed sheets and fascicular to storiform patterns. Diffuse infiltration of eosinophils was also noted. The results of immunohistochemistry indicated positive labeling for vimentin, mast cell tryptase, c-kit, and Ki-67. Toluidine blue stain revealed fine, metachromatic, cytoplasmic granules. The histologic diagnosis was mast cell tumor. The neoplasm recurred and metastasized to the right lumbar lymph node 1 month later.

Signaler and Receiver Ages Do Not Affect Responses to Richardson's Ground Squirrel Alarm Calls

Abstract Alarm calls alert receivers to the presence and/or nature of a predatory threat. Studies of alarm communication in Richardson's ground squirrels (Spermophilus richardsonii) have focused on juvenile signalers and receivers; however, adult and juvenile receivers may tailor their response to alarm calls based on different underlying signal parameters and attend differentially to alarm calls broadcast by adult versus juvenile signalers. To examine the potential influence of signaler and receiver age on the perception of response urgency, we presented free-living juvenile and adult S. richardsonii with alarm calls produced by juveniles and adults. Behavioral responses of call recipients were videotaped and analyzed to determine the influence of signaler and receiver age-class. No differences in vigilance response after juvenile- and adult-produced calls were detected for either adult or juvenile receivers. At the proximate level, the absence of any detectable difference in the acoustic attributes of j...

Demography of Ferruginous Hawks Breeding in Western Canada

Abstract: We assessed age‐dependent survival, site‐fidelity, and, together with data on prey and reproduction, trends in the population of ferruginous hawks (Buteo regalis) breeding in western Canada. Our analysis included 7,129 ferruginous hawks banded near Hanna, Alberta, and Kindersley‐Alsask, Saskatchewan, from 1972 to 2003. We estimated annual adult survival rate to be 0.708 (SE = 0.024) and first year survival for nestlings was 0.545 (SE = 0.147). Resighting probability was modeled as a constant for nestlings (0.009, SE = 0.010), but it varied among years for adults consistent with our sampling efforts. Band reporting rate was at 0.022 (SE = 0.007) for both nestlings and adults. Fidelity to the study site was 1.00 (SE = 0.000) for adults and 0.035 (SE = 0.014) for nestlings. Nesting density ranged from 3.1 to 14.0 pairs/100 km2 and averaged 9.8 pairs/100 km2. We observed an average clutch size of 3.2 (SE=0.06) and brood sizes of 2.71 (SE=0.07) near Hanna and 2.79 (SE = 0.99) at Kindersley‐Alsask. Richardson's ground squirrels (Spermophilus richardsonii) made up 95% of ferruginous hawk prey. Prey availability was positively correlated with number of offspring near Hanna and Kindersley‐Alsask. We believe the lower than expected adult survival did not result in population decrease; rather, declines in reproduction resulting from declines in the abundance of ground squirrels better explain an observed 4.5‐fold decline in nesting densities during the study. The results suggest that ferruginous hawk management should address prey in addition to habitat management, and that management needs are regional in scope with particular emphasis on the breeding range within the northern Great Plains.

Skeletal muscle hexokinase: regulation in mammalian hibernation

Skeletal muscle hexokinase (HK) from Richardson's ground squirrels was analyzed to determine how the enzyme is regulated during hibernation, a state of cold torpor. The HK II isozyme dominated in muscle and ~15% of total HK was bound to the insoluble fraction. HK maximum activity was 33% lower in hibernator muscle and the enzyme showed a significantly higher K ( m ) ATP (by 80%) and a lower K ( i ) for glucose-6-P (by 40%) than euthermic HK (assayed at 22 degrees C). However, 5 degrees C assay significantly reduced K ( m ) glucose of hibernator HK. Stimulation of AMP-dependent protein kinase (AMPK) in hibernator extracts elevated the HK activity and reduced K ( m ) ATP, but did not affect euthermic HK. Stimulation of protein phosphatases significantly lowered the HK activity in both situations. AMPK-dependent phosphorylation was confirmed by immunopreciptiation of (32)P-labeled HK. DEAE-Sephadex ion exchange chromatography revealed two peaks of HK in hibernator muscle extracts (low and high phosphate forms), whereas only a single peak of phospho-HK was present in euthermic muscle. We conclude that differential control of muscle HK in euthermic versus hibernating states is derived from two main regulatory influences, reversible protein phosphorylation and temperature effects on kinetic properties.

The first cut is the deepest: primary syllables of Richardson's ground squirrel, Spermophilus richardsonii, repeated calls alert receivers

Richardson's ground squirrels produce alarm calls that warn conspecifics of potential predatory threat. Variation in the acoustic structure of individual syllables within a call carries information regarding the extent of threat. Given this variation, along with the production of multiple syllables within a repeated call, the syntactical structure of the call could also encode meaningful information. To test for evidence of syntax in Richardson's ground squirrel alarm communication, we presented adult and juvenile subjects with repeated alarm calls with unaltered syllable order and those same calls with syllable order randomized. There was limited evidence that syllable order affected receiver response, but squirrels were more responsive to primary syllables of repeated calls even when these syllables were embedded within randomized calls. This suggests that primary syllables serve a general alerting function, possibly priming receivers for information that follows. Distinctive structural elements of primary syllables, including reduced syllable duration and reduced harmonic durations relative to the fundamental frequency, were correlated with the heightened vigilance responses that these syllables evoked.

Regulation of Akt during hibernation in Richardson's ground squirrels

Akt (or protein kinase B) plays a central role in coordinating growth, survival and anti-apoptotic responses in cells and we hypothesized that changes in Akt activity and properties would aid the reprioritization of metabolic functions that occurs during mammalian hibernation. Akt was analyzed in skeletal muscle and liver of Richardson's ground squirrels, Spermophilus richardsonii, comparing the enzyme from euthermic and hibernating states. Akt activity, measured with a synthetic peptide substrate, decreased by 60–65% in both organs during hibernation. Western blotting showed that total Akt protein did not change in hibernation but active, phosphorylated Akt (Ser 473) was reduced by 40% in muscle compared with euthermic controls and was almost undetectable in liver. Kinetic analysis of muscle Akt showed that S0.5 values for Akt peptide were 28% lower during hibernation, compared with the euthermic enzyme, whereas S0.5 ATP increased by 330%. Assay at 10 °C also elevated S0.5 ATP of euthermic Akt by 350%. Changes in ATP affinity would limit Akt function in the hibernator since the muscle adenylate pool size is also strongly suppressed during cold torpor. Other parameters of euthermic and hibernator Akt were the same including activation energy calculated from Arrhenius plots and sensitivity to urea denaturation. DEAE Sephadex chromatography of muscle extracts revealed three peaks of Akt activity in euthermia but only two during hibernation suggesting isozymes are differentially dephosphorylated during torpor. Altered enzyme properties and suppression of Akt activity would contribute to the coordinated suppression of energy-expensive anabolic and growth processes that is needed to maintain viability during over weeks of winter torpor.

Trypanosoma (Herpetosoma) kuseli sp. n. (Protozoa: Kinetoplastida) in Siberian flying squirrels (Pteromys volans)

All trypanosome species classified in the subgenus Herpetosoma in sciurid hosts have been recorded from ground and tree squirrels to date, but not from any flying squirrels. We describe in this paper a novel trypanosome species, Trypanosoma (Herpetosoma) kuseli sp. n., from Siberian flying squirrels (Pteromys volans) imported from China, and compare it with T. (H.) otospermophili in Richardson's ground squirrels (Spermophilus richardsonii) and Columbian ground squirrels (Spermophilus columbianus) from the USA. Due to a short free flagellum, the new species appeared stumpy compared with T. otospermophili (length of free flagellum 7.0 +/- 0.8 microm, total length 32.1 +/- 0.8 microm, n = 13 and length of free flagellum 15.5 +/- 1.6 microm, total length 35.9 +/- 1.0 microm, n = 13, respectively). Another conspicuous morphological feature of the new species was an anteriorly positioned kinetoplast, found approximately at the midpoint between the nucleus and the posterior end. These characters have not been recorded from any squirrel Herpetosoma trypanosome species. Comparison of the nucleotide sequences of the small and large subunit rRNA genes indicated that T. kuseli sp. n. was more homologous to T. otospermophili than murid Herpetosoma species, such as T. grosi, T. lewisi, T. musculi, T. microti and T. evotomys.

VARIATION IN LITTER SIZE: A TEST OF HYPOTHESES IN RICHARDSON'S GROUND SQUIRRELS

We studied litter size variation in a population of Richardson's ground squirrels (Spermophilus richardsonii) in Alberta, Canada, from 1987 to 2004. Litter size at first emergence of juveniles from the natal burrow ranged from 1 to 14; the most common litter sizes, collectively accounting for 41.0% of 999 litters, were 6 and 7. The number of offspring surviving to adulthood (attained on emergence from hibernation as yearlings) increased with increasing litter size, a result that was not predicted by Lack's "optimal litter size" hypothesis, Mountford's "cliff-edge" effect, or the "bad-years" effect. Contrary to the negative effects predicted by the "cost of reproduction" hypothesis, litter size had no significant influence on survival of mothers to the subsequent year or on the size of the subsequent litter. Rather, our results best fit the predictions of the "individual optimization" hypothesis, which suggests that litter size is determined by the body condition and environmental circumstances of each mother. Supporting this hypothesis, survival of individual offspring was not significantly associated with litter size. Additionally, year-to-year changes in maternal body mass at mating were positively associated with concurrent changes in litter size (r = 0.56), suggesting that litter size depends on the body condition of the mother. Because the mean number of recruits to adulthood increased as litter size increased (r2 = 0.96) and litter size increased with maternal condition, offspring productivity was greater for mothers in better body condition.

Spontaneous Adenocarcinoma Immunoreactive to Cyclooxygenase-2 and Transforming Growth Factor-.BETA.1 in the Buccal Salivary Gland of a Richardson's Ground Squirrel (Spermophilus richardsonii)

The ground squirrel is used as an experimental animal because of its unique biological nature. A 3-year-old female Richardson's ground squirrel developed a mass, 1.5 cm in diameter, in the buccal mucosa. The mass consisted of neoplastic epithelial cells showing acinar, ductular, intraductal papillary, solid, and lobular growth patterns; the cells were immunoreactive to cytokeratin, cyclooxygenase-2 (a marker of malignancy) and TGF-beta1. After resection, the tumor recurred with increased area having a solid or lobular pattern with little differentiation. This tumor was diagnosed as an adenocarcinoma arising from the buccal gland, the first case reported in the ground squirrel. A prominent desmoplastic reaction was present. The interstitial cells reacted to alpha-smooth muscle actin and vimentin, indicating a myofibroblastic nature, presumably induced by epithelial TGF-beta1.

Diversity ofBartonellaGenotypes in Richardson's Ground Squirrel Populations

The diversity and dynamics of Bartonella genotypes found in wild Richardson's ground squirrels (RGS), Spermophilus richardsonii were monitored at multiple sites in Saskatchewan, Canada from 2002 to 2004. Based on sequence analysis of a portion of the Bartonella citrate synthase (gltA) gene, four different genotypes were detected in 233 isolates from 176 animals. The majority (87%) of sequences were identified as genotype H, with genotypes I, J, and K accounting for 8%, 4%, and 1% of sequences, respectively. Only one animal was concurrently infected with multiple Bartonella genotypes. Of 23 animals sampled four times or more, 26% were never infected with Bartonella. Of 32 RGS infected with Bartonella at first capture and then sampled again the following month, 50% were infected with the same Bartonella genotype, 41% were no longer infected, and 9% were infected with a different Bartonella genotype in the subsequent sample. The diversity of Bartonella genotypes varied among sites. At one site almost all RGS were infected with genotype H in September, and up to 60% of the same population was infected with genotype I the following spring. We compare our results with previous studies of Bartonella infections in rodents and discuss possible explanations for the observed differences.

EFFECT OF EXPERIMENTAL ECTOPARASITE CONTROL ON BARTONELLA INFECTIONS IN WILD RICHARDSON'S GROUND SQUIRRELS

The purpose of this study was to investigate the role of ectoparasites in transmitting Bartonella infections in wild Richardson's ground squirrels (Spermophilus richardsonii). Richardson's ground squirrels were trapped, examined for fleas, and tested for Bartonella bacteremia once monthly, at six sites, from April to September 2004. After the initial trapping session in April, burrows at three sites were treated with deltamethrin insecticide. Richardson's ground squirrels trapped on treated sites were less likely to have fleas and had fewer fleas than squirrels on control sites in all months following treatment. We found no difference in the prevalence of Bartonella infections on control and treated sites in May, immediately following treatment; however, significantly fewer squirrels were infected with Bartonella on treated sites in June and July. We conclude that ectoparasites are a main route of transmission for Bartonella infections in Richardson's ground squirrels.

DEMOGRAPHIC FEATURES OF BARTONELLA INFECTIONS IN RICHARDSON'S GROUND SQUIRRELS (SPERMOPHILUS RICHARDSONII)

The epidemiology of Bartonella infections in Richardson's ground squirrels (Spermophilus richardsonii) was studied at multiple sites in Saskatchewan, Canada, from 2002 to 2004. The overall prevalence of Bartonella infection was 48%. Juvenile squirrels were significantly more likely to be infected with Bartonella than were adults (58% and 37%, respectively), and juvenile animals also were significantly more likely to have high levels of bacteremia compared to adult animals. Prevalence of Bartonella infection appeared to decrease with age; only 24% of animals known to be > or = 2 yr old were infected with Bartonella. Prevalence of infection was lowest in May (27%) and highest in late summer and early autumn (71%). The prevalence of fleas also varied seasonally, and animals were more likely to have fleas in the late summer and early autumn than in early summer. We found no relationship between Bartonella prevalence and host density or flea prevalence.

Adult Richardson's Ground Squirrels (Spermophilus richardsonii) Ignore Rate Changes in Juvenile Alarm Calls: Age-Differential Response Urgency Perception?

Juvenile Richardson's ground squirrels (RGS; Spermophilus richardsonii) communicate response urgency by modulating the rate of syllable production in repetitive alarm calls, although longer call bouts do not promote more pronounced or longer-lasting (tonic) vigilance in juvenile call recipients. We exposed free-living adult RGS to playbacks of alarm calls differing in rate and length to determine whether adult receivers respond to the same alarm parameters as juveniles. Adult squirrels did not respond differentially to differences in call rate or length, suggesting that adult RGS do not attend to call rate as do juveniles. This difference in response may be attributable to a developmental change in the perceptual mechanisms by which individuals extract information regarding response urgency, but could also be a product of adult receivers devaluing information encoded in alarm calls emitted by relatively inexperienced juvenile signalers.

Use of Playback Alarm Calls to Detect and Quantify Habitat Use by Richardson's Ground Squirrels

As part of a larger study on prairie ecosystems, we evaluated the use of playback alarm calls as a monitoring protocol to obtain more representative measures of the detection rate and abundance of Richardson's ground squirrels (Spermophilus richardsonii). Using these data we quantified habitat preferences of Richardson's ground squirrels by comparing availability and use of native pastures and cultivated lands. Lastly, we determined whether grass height influenced Richardson's ground squirrel habitat use. Results from surveys indicated that the playback of alarm calls increased levels of detection of Richardson's ground squirrels in 2003 (t24 = −6.82, P < 0.001) and 2004 (t32 = −5.91, P < 0.001) and estimates of abundance in 2003 (t24 = −4.35, P < 0.001) and 2004 (t32 = −6.82, P < 0.001). Our habitat comparisons showed that Richardson's ground squirrels preferred native pasture and selected against cultivated lands when compared to habitat type availability (X21 = 45.22, P < 0.001). Ground squirrels selected native pasture with grass heights between 0–30 cm and selected against areas with tall grass (>l30 cm; X21 = 7.69, P = 0.0056). Our results suggest that playback alarm calls may constitute an important monitoring protocol to determine Richardson's ground squirrel abundance and habitat use.

Age‐specific resource investment strategies: evidence from female Richardson's ground squirrels (Spermophilus richardsonii)

AbstractTo avoid a possible cost to their future survival and/or reproduction, individuals must balance their somatic and reproductive investments. The van Noordwijk and De Jong model of resource investment predicts that investments into reproduction and soma can vary among individuals of a population based on the variation in the total amount of energy that individuals acquire. With principal components analysis (PCA), we created two axes of life history for female Richardson's ground squirrels Spermophilus richardsonii: an index of total energy investment (PC1) and an index of investment tactic (PC2). Using these indices, we examined patterns of resource allocation to reproductive and somatic investments. Because yearling female Richardson's ground squirrels complete growth to adult size during pregnancy and early lactation, their somatic needs exceed those of older, fully grown females. Therefore, we predicted that yearlings would show more evidence of a tradeoff between reproductive and somatic investments compared with older females. Both yearling and older females invested four to five times more mass into their litters than into their own body mass. With increasing total investment, yearling females increased investment in both reproduction and themselves, whereas older females invested relatively more in reproduction than themselves. Regardless of age, females that emerged heavier from hibernation invested fewer resources into themselves and more into their litters. Variation in total energy investment and investment tactic indices was similar for yearling and older females. Contrary to our prediction, however, yearling females showed positive associations between reproductive and somatic investments, whereas older females exhibited showed no significant association between reproductive and somatic investments.

Increased cholinergic contractions of jejunal smooth muscle caused by a high cholesterol diet are prevented by the 5-HT4agonist – tegaserod

BackgroundExcess cholesterol in bile and in blood is a major risk factor for the respective development of gallbladder disease and atherosclerosis. This lipid in excess negatively impacts the functioning of other smooth muscles, including the intestine. Serotonin is an important mediator of the contractile responses of the small intestine. Drugs targeting the serotonin receptor are used as prokinetic agents to manage intestinal motor disorders, in particular irritable bowel syndrome. Thus, tegaserod, acting on 5-HT4 receptor, ideally should obviate detrimental effects of excessive cholesterol on gastrointestinal smooth muscle. In this study we examined the effect of tegaserod on cholesterol-induced changes in the contractile responses of intestinal smooth muscle.MethodsThe effects of a high cholesterol (1%) diet on the in vitro contractile responses of jejunal longitudinal smooth muscle from Richardson ground squirrels to the cholinergic agonist carbachol were examined in the presence or absence of tetrodrodotoxin (TTX). Two groups of animals, fed either low (0.03%) or high cholesterol rat chow diet, were further divided into two subgroups and treated for 28 days with either vehicle or tegaserod.ResultsThe high cholesterol diet increased, by nearly 2-fold, contractions of the jejunal longitudinal smooth muscle elicited by carbachol. These cholinergic contractions were mediated by muscarinic receptors since they were blocked by scopolamine, a muscarinic receptor antagonist, but not by the nicotinic receptor antagonist, hexamethonium. Tegaserod treatment, which did not affect cholinergic contractions of tissues from low cholesterol fed animals, abrogated the increase caused by the high cholesterol diet. With low cholesterol diet TTX enhanced carbachol-evoked contractions, whereas this action potential blocker did not affect the augmented cholinergic contractions seen with tissues from animals on the high cholesterol diet. Tegaserod-treatment removed the effects of a high cholesterol diet on neuronal muscarinic receptors, as the potentiating effect of TTX on carbachol-elicited contractions was maintained in these animals.ConclusionA high cholesterol diet causes significant changes to cholinergic neurotransmission in the enteric nerves of the jejunum. The mechanisms by which these effects of cholesterol are reversed by tegaserod are unknown, but relate to removal of an inhibitory effect of cholesterol on enteric nerves.

Carcasses of Shot Richardson's Ground Squirrels May Pose Lead Hazards to Scavenging Hawks

Shooting and using poison baits (e.g., strychnine, zinc phosphide) are current management options for controlling Richardson's ground squirrels (Spermophilus richardsonii; RGS). Bullets used for shooting RGS contain lead, fragment upon impact, and RGS carcasses are not usually recovered after being shot. For these reasons, we hypothesized that scavenging birds of prey may be at risk of lead poisoning. To test this, we took radiographs of 15 shot RGS and analyzed the area around the path of the bullet for lead. Lead levels ranged from 0.01 to 17.21 mg/carcass (median = 3.23 mg), and fragments appeared as dust. Two common scavenging hawks (Swainson's and ferruginous hawks [Buteo swainsonii, B. regalis]) consume eviscerated RGS carcasses and would consequently ingest this amount of lead per feeding. In a previous study, an estimated 5.71 mg/kg of lead, eroded in vivo from ingested lead shot, was lethal to bald eagles (Haliaeetus leukocephalus). Fitting the residue values to a normal distribution and based on the mass of an average raptor, we determined that roughly 1 in 5 RGS carcasses had lead levels that exceeded this value. Based on the average amount of lead in carcasses, and assuming that uptake of lead from the carcass is as high as that of eroded lead, we suggest that hawks would have to eat roughly 6.5 carcasses, taking an average of 23 days of feeding on an uninterrupted supply of shot carcasses, to attain a lethal dose of lead. Uncertainties remain, but shot RGS carcasses appear to be an appreciable source of lead that could prove fatal to scavenging hawks. This hazard could be avoided with the collection and disposal of shot carcasses and with the use of (green) ammunition.

Effects of tegaserod on bile composition and hepatic secretion in Richardson ground squirrels on an enriched cholesterol diet

BackgroundTegaserod is effective in treating IBS patients with constipation, and does not alter gallbladder motility in healthy individuals or in patients with IBS. However, it is not known if tegaserod affects the biliary tract in gallstone disease, so to this end the effects of tegaserod on bile composition and hepatic secretion of Richardson ground squirrels maintained on an enriched cholesterol diet were examined.ResultsAnimals were fed either a control (0.03%) or enriched (1%) cholesterol diet for 28 days, and treated s.c. with tegaserod (0.1 mg/kg BID) or vehicle. Bile flow, bile acid, phospholipids and cholesterol secretion were measured with standard methods. Tegaserod treatment or enriched cholesterol diet, alone or combination, did not alter body or liver weights. The enriched cholesterol diet increased cholesterol saturation index (CSI), cholesterol concentrations in gallbladder and hepatic duct bile by ~50% and decreased bile acids in gallbladder bile by 17%. Tegaserod treatment reversed these cholesterol-induced changes. None of the treatments, drug or diet, altered fasting gallbladder volume, bile flow and bile salts or phospholipid secretion in normal diet and cholesterol-fed animals. However, tegaserod treatment prevented the decreases in bile acid pool size and cycling frequency caused by the enriched cholesterol diet, consequent to re-establishing normal bile acid to concentrations in the gall bladder. Tegaserod had no effect on these parameters with normal diet animals.ConclusionTegaserod treatment results in increased enterohepatic cycling and lowers cholesterol saturation in the bile of cholesterol-fed animals. These effects would decrease conditions favorable to cholesterol gallstone formation.

Rodent-AssociatedBartonellain Saskatchewan, Canada

Six species of wild rodents were sampled at 10 sites in 2002 and 2003 to determine the prevalence of Bartonella infections in rodent communities near Saskatoon, Saskatchewan, Canada. Isolates were characterized genotypically and compared with isolates found at other locations. Of 104 wild rodents examined, 57% were infected with Bartonella and prevalence within species varied from 49% for Richardson's ground squirrels (Spermophilus richardsonii) to 90% for Franklin's ground squirrels (S. franklinii). Infected rodents were found at all sites. Sequencing of a 379-bp portion of the citrate synthase gene was performed on 54 isolates and revealed 13 distinct genotypes, eight of which had not been described previously. The most common genotype detected in red-backed voles (Clethrionomys gapperi) was 99.1% similar to B. grahamii, a known human pathogen. Two of 10 Franklin's ground squirrels were concurrently infected with multiple Bartonella genotypes. All genotypes, with the exception of one detected in both Franklin's and thirteen-lined ground squirrels (S. tridecemlineatus), were found in only one host, and all genotypes from each species, with the exception of genotypes detected in red-backed voles, clustered together within the same relatedness group, suggesting that at least some Bartonella genotypes are specific to some rodent hosts.

Functional morphology of Richardson's ground squirrel, Spermophilus richardsonii, alarm calls: the meaning of chirps, whistles and chucks

Repetitive alarm vocalizations of Richardson's ground squirrels vary in terms of the acoustic structure of their primary syllables and the inclusion of brief, lower-amplitude, frequency-modulated trailing elements we term ‘chucks.’ Chucks are included in calls of both males and females and increase in prevalence with the proximity of the caller to the alarm-evoking stimulus. Furthermore, chuck presence is not independent of primary syllable type: chucks follow primary syllables that have constant frequency and diminishing amplitude (‘whistle’), but do not trail primary syllables with diminishing frequency and nondescending amplitude spectra (‘chirps’). Playbacks to free-living squirrels of repeated alarm calls having whistle- or chirp-like primary syllables factorially combined with chuck presence or absence revealed that chirp-like syllables elicited greater vigilance from call recipients during signal propagation. The addition of chucks to the end of primary syllables of either type, however, increased initial vigilance duration and the proportion of time devoted to vigilance during and after signal reception. Chucks thus promote increased and lasting vigilance on the part of call recipients. Beyond enhancing vigilance, however, the inclusion of frequency-modulated chucks and chirps facilitates the orientation of receivers to the signaller. Multiple acoustic parameters of Richardson's ground squirrel alarm vocalizations thus interact to communicate information regarding several aspects of a predator encounter. Receivers use this information to their advantage, affording greater attention to calls that would be more readily located by predators, and hence are more costly for signallers to produce.