Premating behavioral tactics of Columbian ground squirrels

Sexes in sync: phenotypic plasticity, sexual selection and phenological synchrony between the sexes in a wild hibernator

Desynchrony of phenological responses to climate change is a major concern for ecological communities. Potential uncoupling between one of the most fundamental divisions within populations, males and females, has not been well studied. To address this gap, we examined sex-specific plasticity in hibernation phenology in two populations of Columbian ground squirrels (Urocitellus columbianus). We find that both sexes display similar phenological plasticity to spring snowmelt dates in their timing of torpor termination and behavioural emergence from hibernation. As a result of this plasticity, the degree of protandry (i.e. males' emergences from hibernation preceding those of females) did not change significantly over the 27-year study. Earlier male behavioural emergence, relative to females, improved the likelihood of securing a breeding territory and increased annual reproductive success. Sexual selection favouring earlier male emergence from hibernation may maintain protandry in this population, but did not contribute to further advances in male phenology. Together, our results provide evidence that the sexes should remain synchronized, at least in response to the weather variation investigated here, and further support the role of sexual selection in the evolution of protandry in sexually reproducing organisms.

Social organization in a North African ground squirrel

Research on sociality in temperate ground-dwelling squirrels has focused on female philopatry and other life history trade-offs, which are influenced by constraints in the duration of the active growing season. Temperate ground-dwelling squirrels that experience high predation pressure, are large in body size, and have a short active season, show a more complex social organization. In contrast, African ground squirrels are active year-round, suggesting that instead of a short active season, distinct selective pressures influence their social organization. We examined the social organization of Barbary ground squirrels, Atlantoxerus getulus, and compared the social organization of temperate and African ground-dwelling sciurids. Anecdotal accounts on Barbary ground squirrels’ social organization suggested that they were either solitary or gregarious, or live in small family groups. We recorded the group size, composition, cohesion, and genetic relatedness, of the population on the arid island of Fuerteventura, Spain. Our data indicate that females live in small (1–8) all-female kin groups separate from adult males, and that unrelated adult males share sleeping burrows with immature individuals of either sex. We observed sex-biased dispersal with males primarily the dispersing sex and females primarily philopatric. Females sleep solitarily during gestation and lactation and nest either communally or singly after juvenile emergence. During the day, males and females can be active in the same area. Barbary ground squirrels are social because the squirrels share sleeping burrows and show spatiotemporal overlap. Barbary ground squirrels’ social organization resembles that of the closely related Cape ground squirrel rather than that of the temperate ground-dwelling sciurids, although the former are more temperate, seasonal breeders. In addition to describing the social organization of a previously unstudied species, this paper sheds light on the ecological drivers of sociality, and the evolution of distinct social organizations in ground-dwelling sciurids.

Paternity and male mating strategies of a ground squirrel ( Ictidomys parvidens ) with an extended mating season

Animal mating systems are driven by the temporal and spatial distribution of sexually receptive females. In mammals, ground-dwelling squirrels represent an ideal clade for testing predictions regarding the effects of these parameters on male reproductive strategies. While the majority of ground squirrel species have a short, highly synchronous annual breeding season that occurs immediately after females emerge from hibernation, the Mexican or Rio Grande ground squirrel ( Ictidomys parvidens ) differs markedly in having an extended mating season (2 months) and a long delay between emergence from hibernation and female receptivity (1–2 months). Both traits are expected to favor polygyny by increasing the chances that a male can secure matings with multiple females (e.g., females that come into estrus on different days). To test this prediction, we used microsatellite markers to characterize the mating system of a population of Rio Grande ground squirrels from Carlsbad, New Mexico. Our analyses indicated a high frequency of multiple paternity of litters in this population. Paternity was not related to spatial overlap between known mothers and assigned fathers, suggesting that territory defense is unlikely to be an effective male reproductive strategy in the study population. Dominance interactions among males were frequent, with heavier males typically winning dyadic interactions. Surprisingly, however, males with lower dominance scores appeared to have higher reproductive success, as did males that were active over a greater extent of the study site. Collectively, these results suggest that the mating system of the Rio Grande ground squirrel is best described as scramble competition polygyny, with the primary male reproductive strategy consisting of searching for estrous females. Similar patterns of male–male competition have been reported for a few other ground squirrel species, providing potentially important opportunities for comparative studies of the factors favoring this form of male reproductive strategy.