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Octodontoidea is the most species-rich clade among hystricomorph rodents, and has a fossil record going back to at least the late Oligocene. Affinities of fossils previous to the late Miocene differentiation of the extant families Abrocomidae, Echimyidae and Octodontidae are controversial, essentially because these fossils may share few apomorphies with modern species. In fact, pre-late Miocene representatives of Abrocomidae had not been recognised until very recently. Here we revise the early Miocene genusAcarechimys, originally assigned to Echimyidae, and alternatively to stem Octodontoidea or to Octodontidae. A systematic and parsimony-based phylogenetic analysis of the species traditionally included inAcarechimysshowed that this genus is part of stem Abrocomidae. These results are primarily supported by morphology of the mandible and lower molars.Acarechimysis here restricted to three species,A. minutus,A. pulchellusandAcarechimys pascualisp. nov., while another species,A. constans, is here transferred to a new abrocomid genus. The remaining species were nested within Octodontidae. According to these results, Abrocomidae might have been as diverse as its sister clade Octodontidae-Echimyidae during the late Oligocene–early Miocene. Extinction of this diversity would have resulted in marked loss of evolutionary history, with extant abrocomids being currently restricted to late-diverged euhypsodont representatives. [ABSTRACT FROM PUBLISHER]

Within South American rodents, the Acaremyidae is an independent fossil lineage of octodontoids represented in the late Oligocene–middle Miocene of Patagonia.Acaremysis represented by six species recorded in the early Miocene, which have not been re-studied since their original description. Morphological and phylogenetic analyses suggest thatAcaremysis paraphyletic. Three species are valid,Acaremysmurinus,Acaremysmajor, andAcaremysmessor.Acaremyskaraikensisis a junior synonym ofAcaremysmurinus. ‘Acaremys’tricarinatusis excluded from the genus being closely related toSciamys. The new species,Pseudoacaremys kramarzi, is closely related to ‘Acaremys’tricarinatusandSciamys. ‘Acaremys’preminutusis excluded from the family being closely related to the early MioceneProtacaremys prior, or living octodontoids. The phylogenetic analysis demonstrated that Acaremyidae includesPlatypittamys,Galileomys,Acaremys,PseudoacaremysandSciamys. The new acaremyid increases the diversity of extinct octodontoids and added a new evolutionary lineage within Acaremyidae. The evolutionary history of the superfamily suggests that the hypsodonty and the consequently occlusal simplification evolved twice within Octodontoidea: in Acaremyidae and in Octodontidae. In addition, the cladistic analysis confirmed that most character ambiguities are due to missing data, and hence, it is essential to find better remains to elucidate the relationships among acaremyids. [ABSTRACT FROM AUTHOR]

Daily rhythms in different biochemical and hematological variables have been widely described in either diurnal or nocturnal species, but so far no studies in the rhythms of these variables have been conducted in a dual-phasing species such as the degus. The Octodon degus is a rodent that has the ability to switch from diurnal to nocturnal activity under laboratory conditions in response to wheel-running availability. This species may help us discover whether a complete temporal order inversion occurs parallel to the inversion that has been observed in this rodent's activity pattern. The aim of the present study is to determine the phase relationships among 26 variables, including behavioral, physiological, biochemical, and hematological variables, during the day and at night, in diurnal and nocturnal degus chronotypes induced under controlled laboratory conditions through the availability of wheel running. A total of 39 male degus were individually housed under a 12:12 light-dark (LD) cycle, with free wheel-running access. Wheel-running activity (WRA) and body temperature (Tb) rhythms were recorded throughout the experiment. Melatonin, hematological, and biochemical variables were determined by means of blood samples obtained every 6 h (ZT1, ZT7, ZT13, and ZT19). In spite of great differences in WRA and Tb rhythms between nocturnal and diurnal degus, no such differences were observed in the temporal patterns of most of the biological variables analyzed for the two chronotypes. Variation was only found in plasma urea level and lymphocyte number. A slight delay in the phase of the melatonin rhythm was also observed. This study shows the internal temporal order of a dual-phasing mammal does not show a complete inversion in accordance with its activity and body temperature pattern; it would appear that the switching mechanism involved in the degu's nocturnalism is located downstream from the pacemaker. (Author correspondence: ). [ABSTRACT FROM AUTHOR]

Octodon degus is a primarily diurnal rodent that presents great variation in its circadian chronotypes due to the interaction between two phase angles of entrainment, diurnal and nocturnal, and the graded masking effects of environmental light and temperature. The aim of this study was to test whether the circadian system of this diurnal rodent can be internally dissociated by imposing cycles shorter and longer than 24 h, and to determine the influence of degus chronotypes and wheel-running availability on such dissociation. To this end, wheel-running activity and body temperature rhythms were studied in degus subjected to symmetrical light-dark (LD) cycles of T28h and T21h. The results show that both T-cycles dissociate the degus circadian system in two different components: one light-dependent component (LDC) that is influenced by the presence of light, and a second non–light-dependent component (NLDC) that free-runs with a period different from the external lighting cycle. The LDC was more evident in the nocturnal than diurnal chronotype, and also when wheel running was available. Our results show that, in addition to rats and mice, degus must be added to the list of species that show an internal dissociation in their circadian rhythms when exposed to forced desynchronization protocols. The existence of a multioscillatory circadian system having two groups of oscillators with low coupling strength may explain the flexibility of degus chronotypes. (Author correspondence: ) [ABSTRACT FROM AUTHOR]

Octodon degus, a mainly diurnal rodent, characterized by its ability to shift to a nocturnal locomotor pattern under laboratory conditions, was studied to determine whether restricted food access during the scotophase could induce nocturnalism. To address this question, wheel running activity, feeding, and body temperature rhythms were analyzed for diurnal degus housed with a wheel and subjected to either long (12 h) or short (2 h) food availability periods, in the latter case with random or scheduled food access times. The results show that allowing nocturnal feeding for 2 h, but not 12 h, can shift a previous diurnal phase preference for wheel running activity and body temperature to the scotophase, with random feeding being more effective than scheduled food availability. However, this behavioral inversion proved to be unstable, as the degus returned to the diurnal phase within only a few days after the restricted feeding was discontinued. In addition, the negative masking effect induced by light, which is characteristic of the degus' nocturnal chronotype, was not observed when the animals were forced to feed at night. Thus, neither long, short, random, nor scheduled food-availability during the scotophase was able to induce all the characteristics of the nocturnal chronotype in Octodon degus. (Author correspondence: ) [ABSTRACT FROM AUTHOR]

Diurnal and nocturnal species have different patterns of general activity, sleep-wake rhythms, and endocrine rhythms, but the mechanisms underlying these differences are not clear. Here, we tested the hypothesis that rhythms in immediate early gene (IEG) products in the suprachiasmatic nucleus (SCN) of a diurnal rodent (Octodon degus) reflect their diurnal chronotype. We also compared male and female degus with respect to temporal patterns of expression of one of these gene products, Fos-related Antigen (FRA). Animals were killed across the light:dark cycle, brains were collected, and sections through the SCN were stained for FRA (females and males) or c-Fos and Calbindin (males only) using immunocytochemistry. An additional set of females placed in constant darkness was pulsed with light (or not) during the subjective day or night. Labeled cells in the SCN were counted in all animals. In males, c-Fos/FRA positive (+) cell counts in the dorsomedial SCN were low after lights-on and peaked around the time of lights-off (ZT 13, ZT 0=lights-on). Effect size analyses indicated that females have a 24 h rhythm in FRA expression that is shifted relative to that of males. Light pulses in the subjective day and night produced area-specific changes in IEG expression in females that differs from that reported for males. Overall, these studies reveal the pattern of immediate early gene expression in the SCN of degus is not the same in males and females, and that it differs from the one seen in other species, both nocturnal and diurnal. [ABSTRACT FROM AUTHOR]

The Octodon degus, or degu, is an excellent animal model for studying the theoretical and neural underpinnings of diurnality. The power of this model comes from their unique evolutionary lineage, long lives, and relative ease of care in the laboratory for a non-domesticated species. We have summarized the field and laboratory data indicating the critical variables that influence the degus' phase preference and the possible mechanisms for the phase flexibility observed in the field and laboratory. We also review studies examining the physiology and anatomy of light and non-photic inputs to the degu circadian system and studies of the circadian pacemaker itself, with particular emphasis placed on characteristics that appear to be convergent adaptations to a diurnal niche. Finally, we begin to seek the origin for the diurnally-phased activity output of the degu, although we conclude that significant work remains to be done. [ABSTRACT FROM AUTHOR]