Activity, overlap of range areas, and sharing of resting locations in the moon-toothed degu,Octodon lunatus

Multi-year assessment of variability in spatial and social relationships in a subterranean rodent, the highland tuco-tuco (Ctenomys opimus)

Abstract

In some species, populations routinely contain a mixture of lone and group-living individuals. Such facultative sociality may reflect individual differences in behavior as well as adaptive responses to variation in local environmental conditions. To explore interactions between individual- and population-level variabilities in behavior in a species provisionally described as facultatively social, we examined spatial and social relationships within a population of highland tuco-tucos (Ctenomys opimus) at Laguna de los Pozuelos, Jujuy Province, Argentina. Using data collected over 5 consecutive years, we sought to (1) confirm the regular occurrence of both lone and group-living individuals and (2) characterize the temporal consistency of individual social relationships. Our analyses revealed that although the study population typically contained lone as well as group-living animals, individual spatial and social relationships varied markedly over time. Specifically, the extent to which individuals remained resident in the same location across years varied, as did the number of conspecifics with which an animal lived, with an overall tendency for individuals to live in larger groups over successive years. Collectively, these analyses indicate that population-level patterns of behavior in C. opimus are consistent with facultative sociality but that this variation does not arise due to persistent differences in individual behavior (i.e., living alone versus with conspecifics). Instead, based on changes in spatial and social relationships across years, we suggest that variation in the tendency to live in groups is shaped primarily by local ecological and demographic conditions.

Significance statement

Characterizing variation in conspecific relationships is critical to understanding the adaptive bases for social behavior. Using data collected over 5 successive years, we examined temporal variation in spatial and social relationships within a population of highland tuco-tucos (C. opimus) from northern Argentina. In addition to providing the first multi-year assessment of the behavior and demography of this species, our analyses generate important insights into relationships between individual behavior and population-level patterns of social organization. The behavioral variability evident in our study population suggests that C. opimus is an ideal system in which to explore the causes and consequences of individual differences in social behavior.

Dynamic spatial overlap in a solitary subterranean rodent: the Anillaco tuco-tuco (Ctenomys sp.)

Despite striking diversity in mammalian social behavior, studies of social organization have often dichotomized species by identifying them as either solitary or social (i.e., group living). This tendency has been particularly pronounced for subterranean rodents, the majority of which have long been assumed to be solitary. As a result, variation in social organization has likely been underestimated for these animals, particularly for species in which patterns of space use suggest limited or temporally dynamic opportunities for interactions among conspecifics. Here, we characterize patterns of space use in a population of tuco-tucos (Ctenomys sp.) from Anillaco, La Rioja Province, Argentina. Although these animals have been the subject of extensive research regarding circadian patterns of activity, spatial and social relationships among free-living individuals have not been documented. Analyses of radiotelemetry data from 17 individuals monitored during the breeding season (December 2015) revealed that partial overlap of individual home ranges was common, occurring between male–female as well as female–female pairs of animals. Spatial relationships, however, were dynamic, with both home range sizes and overlap changing on a daily basis. Although members of the study population did not meet the criteria typically used to identify group living in subterranean species, they were not completely solitary. Instead, the animals displayed an intermediate form of social organization characterized by persistent partial overlap of the areas occupied by different adults. These data add to the growing comparative picture of social variation in Ctenomys and suggest that further studies of these animals should contribute to improved understanding of the factors underlying differences in mammalian social systems.

Can social behaviour drive accessory olfactory bulb asymmetries? Sister species of caviomorph rodents as a case in point

In mammals, the accessory olfactory or vomeronasal system exhibits a wide variety of anatomical arrangements. In caviomorph rodents, the accessory olfactory bulb (AOB) exhibits a dichotomic conformation, in which two subdomains, the anterior (aAOB) and the posterior (pAOB), can be readily distinguished. Interestingly, different species of this group exhibit bias of different sign between the AOB subdomains (aAOB larger than pAOB or vice versa). Such species-specific biases have been related with contrasting differences in the habitat of the different species (e.g. arid vs. humid environments). Aiming to deepen these observations, we performed a morphometric comparison of the AOB subdomains between two sister species of octodontid rodents, Octodon lunatus and Octodon degus. These species are interesting for comparative purposes, as they inhabit similar landscapes but exhibit contrasting social habits. Previous reports have shown that O. degus, a highly social species, exhibits a greatly asymmetric AOB, in which the aAOB has twice the size of the pAOB and features more and larger glomeruli in its glomerular layer (GL). We found that the same as in O. degus, the far less social O. lunatus also exhibits a bias, albeit less pronounced, to a larger aAOB. In both species, this bias was also evident for the mitral/tufted cells number. But unlike in O. degus, in O. lunatus this bias was not present at the GL. In comparison with O. degus, in O. lunatus the aAOB GL was significantly reduced in volume, while the pAOB GL displayed a similar volume. We conclude that these sister species exhibit a very sharp difference in the anatomical conformation of the AOB, namely, the relative size of the GL of the aAOB subdomain, which is larger in O. degus than in O. lunatus. We discuss these results in the context of the differences in the lifestyle of these species, highlighting the differences in social behaviour as a possible factor driving to distinct AOB morphometries.

Spatial relationships among free-living cururos (Spalacopus cyanus) demonstrate burrow sharing and communal nesting

Spatial relationships among conspecifics can provide insights into numerous aspects of social behavior. Spatial data may be particularly important for characterizing the behavior of difficult-to-study species such as subterranean rodents, direct observations of which are challenging. To characterize the social organization of the cururo (Spalacopus cyanus), a subterranean species in the rodent family Octodontidae, we used radiotelemetry to quantify spatial relationships within populations of this species located in Parque Nacional Bosque Fray Jorge and Santuario de la Naturaleza Yerba Loca, Chile. Specifically, we sought to determine if adults in this diurnal species share burrows and subterranean nests, the two criteria typically used to identify subterranean rodents as social. Analyses of radio fixes collected during February–March 2003 revealed that cururos at both Fray Jorge and Yerba Loca shared nighttime nest sites; cluster analyses of these data identified multiple spatially distinct subsets of adults in each population. Overlap of minimum convex polygons constructed from radio fixes collected during daylight hours suggested burrow sharing by animals in both populations. Cluster analyses of overlap values revealed the same spatially distinct groups of individuals identified from analyses of nest sharing; in addition, these analyses revealed one cluster of animals in each population that was not evident from analyses of nighttime data. Collectively, these results confirm that cururos are social, with adults in both study populations sharing burrow systems and communal nests. Our findings add to the growing understanding of social organization in octodontid rodents and reveal a new system for comparative studies of the ecology and evolution of behavioral variation in burrow-dwelling mammals.

Effects of Habitat and Social Complexity on Brain Size, Brain Asymmetry and Dentate Gyrus Morphology in Two Octodontid Rodents

Navigational and social challenges due to habitat conditions and sociality are known to influence dentate gyrus (DG) morphology, yet the relative importance of these factors remains unclear. Thus, we studied three natural populations of O. lunatus (Los Molles) and Octodon degus (El Salitre and Rinconada), two caviomorph species that differ in the extent of sociality and with contrasting vegetation cover of habitat used. The brains and DG of male and female breeding degus with simultaneous information on their physical and social environments were examined. The extent of sociality was quantified from total group size and range area overlap. O. degus at El Salitre was more social than at Rinconada and than O. lunatus from Los Molles. The use of transects to quantify cover of vegetation (and other physical objects in the habitat) and measures of the spatial behavior of animals indicated animal navigation based on unique cues or global landmarks is more cognitively challenging to O. lunatus. During lactation, female O. lunatus had larger brains than males. Relative DG volume was similar across sexes and populations. The right hemisphere of male and female O. lunatus had more cells than the left hemisphere, with DG directional asymmetry not found in O. degus. Degu population differences in brain size and DG cell number seemed more responsive to differences in habitat than to differences in sociality. Yet, large-sized O. degus (but not O. lunatus) that ranged over larger areas and were members of larger social groups had more DG cells per hemisphere. Thus, within-population variation in DG cell number by hemisphere was consistent with a joint influence of habitat and sociality in O. degus at El Salitre.