Kinship does not affect vigilance in Columbian ground squirrels (Urocitellus columbianus).(Report)

Introduction

Vigilance, scanning the surroundings, is a widespread antipredator behaviour of prey species (Lima and Dill 1990). Columbian ground squirrels (Urocitellus columbianus (Ord, 1815), formerly Spermophilus columbianus (Ord, 1815); (Helgen et al. 2009)) show a decrease in vigilance with an increase in group size (Fairbanks and Dobson 2007). This phenomenon is commonly found in gregarious prey species and is called the "group-size effect" on vigilance (Lima and Dill 1990). The group-size effect in Columbian ground squirrels is due to the detection or many-eyes effect (Fairbanks and Dobson 2007), which is a decrease in individual vigilance owing to an increase in the number of individuals scanning for predators (Pulliam 1973). Because females are philopatric (King 1989a), an individual in a larger group is likely to have more kin present than an individual in a smaller group. Therefore, the group-size effect on vigilance could be the result of active or passive cooperation among kin for mutual benefit, rather than dependence on group mates in general. Female kin cooperate via greater overlap of territory (King 1989b), so shared vigilance for predators might be expected. The presence of nearby kin is a rarely studied factor that has been shown to affect vigilance in some species (Black and Owen 1989; Griesser 2003). We studied the effect of kinship on vigilance in Columbian ground squirrels in an effort to ascertain whether the group-size effect on vigilance is influenced by kinship.

We tested two hypothesized effects of kinship on vigilance. The first possible effect is kin cooperation, a mutual benefit or reciprocity of vigilance among kin, where kin depend on one another for vigilance. This would be indicated by individuals with living kin having lower vigilance than those without kin. The second possibility is unidirectional nepotism, a one-way favouritism within certain types of kinships. Columbian ground squirrel mothers show nepotism by producing risky alarm calls for young of the year when terrestrial predators are present (MacWhirter 1992). There are no studies of Columbian ground squirrels that show whether this same nepotism in alarm calling exists for adult offspring, but many other ground-dwelling sciurids exhibit nepotism toward adult offspring (see Sherman 1977; Hoogland 1995). To differentiate the nepotism hypothesis from mutual cooperation, we expected nepotism to be unidirectional and predicted that mothers should be relatively more vigilant when adult offspring are present and that offspring should be less vigilant. In this case of nepotism, the increased vigilance of the mother would not be an effect of kin selection (indirect fitness), but of natural selection (direct fitness) through increased offspring survival.

Naturally, cooperation and nepotism might both be benefits that individuals receive from different types of kin, and both might thus contribute to the lowered mean vigilance of the group-size effect. Associations that result in mutual benefit (cooperation) and unidirectional benefit (nepotism) should be discernable by examining rates of vigilance for different types and numbers of kin. However, the group-size effect might operate without the influence of kinship at all, lending support to the idea that cooperation among Columbian ground squirrels is generally independent of kinship (Hare and Murie 2007).

Precise levels of kinship were difficult to estimate because Columbian ground squirrels exhibit multiple paternity (Murie 1995; Hare and Murie 1996; Hare et al. 2004), so we were able to assess maternal kinship but not paternal kinship, such as half-siblings within a single litter or half-siblings in different litters. However, these ground squirrels behave towards one another on the basis of "uterine kinship" (mother-daughter and litter-mate siblings), whereas nonuterine kin (siblings or half-siblings from a different litter) are generally treated as …