Comparative Feeding Ecology of Sympatric <Emphasis Type="Italic">Microcebus berthae and M. murinus</Emphasis>

Most Malagasy primate communities harbor a diverse assemblage of omnivorous species. The mechanisms allowing the coexistence of closely related species are poorly understood, partly because only preliminary data on the feeding ecology of most species are available. We provide an exemplary feeding ecology data set to illuminate coexistence mechanisms between sympatric gray and Madame Berthe’s mouse lemurs (Microcebus murinus, M. berthae). We studied their feeding ecology in Kirindy Forest/CFPF, a highly seasonal dry deciduous forest in western Madagascar. Between August 2002 and December 2007, we regularly (re-)captured, marked, and radiotracked females of both species. A combination of direct behavioral observations and fecal analyses revealed that both Microcebus species used fruit, arthropods, gum, insect secretions, and small vertebrates as food sources. However, Microcebus berthae and M. murinus differed in both composition and seasonal variation of their diets. Whereas the diet of Microcebus murinus varied seasonally and was generally more diverse, M. berthae relied mainly on insect secretions supplemented by animal matter. The differences were also reflected in a very narrow feeding niche of Microcebus berthae and a comparatively broad feeding niche of M. murinus. Resource use patterns of Madame Berthe’s and more so of opportunistic gray mouse lemurs broadly followed resource availability within the strongly seasonal dry forest. Feeding niche overlap between the 2 sympatric species was high, indicating that food resource usage patterns did not reflect niche partitioning, but can instead be explained by constraints due to food availability.     

Relationship Between Microhabitat Structure and Distribution of Mouse Lemurs (Microcebus spp.) in Northwestern Madagascar

Understanding processes affecting the distribution and abundance of organisms is a central issue in ecology and conservation biology. In northwestern Madagascar, we found an uneven distribution pattern of the golden-brown mouse lemur (Microcebus ravelobensis) and the grey mouse lemur (M. murinus). In one area (JBA) the two species lived sympatrically, whereas in another forest area (JBB) Microcebus ravelobensis occurred exclusively. To investigate whether differences in forest structure may explain this uneven distribution, we conducted a microhabitat analysis and related it to specific distribution. In JBA the habitat of Microcebus ravelobensis was characterized by a higher percentage of trees with many lianas and a higher cover of the herb layer, whereas that of M. murinus had a higher number of trees with a diameter at breast height (DBH) >10 cm. The comparison of the forest structure of the microhabitats of the two species between JBA and JBB revealed further differences. The cover of the overstory, the percentage of trees without lianas and the number of larger trees (DBH >10 cm) among the microhabitats were higher for Microcebus murinus in JBA than for M. ravelobensis in JBB whereas the microhabitats of M. ravelobensis at the two sites did not differ concerning these vegetation structures. Differences between the two species coincide with those of resources important for survival. Our results indicate the importance of microhabitat analyses for the understanding of distribution patterns of species and for successful conservation planning.     

The use of Vocalizations of the Sambirano Mouse Lemur (<Emphasis Type="Italic">Microcebus sambiranensis</Emphasis>) in an Acoustic Survey of Habitat Preference

Primate vocalizations convey a variety of information to conspecifics. The acoustic traits of these vocalizations are an effective vocal fingerprint to discriminate between sibling species for taxonomic diagnosis. However, the vocal behavior of nocturnal primates has been poorly studied and there are few studies of their vocal repertoires. We compiled a vocal repertoire for the Endangered Sambirano mouse lemur, Microcebus sambiranensis, an unstudied nocturnal primate of northwestern Madagascar, and compared the acoustic properties of one of their call types to those of M. murinus and M. rufus. We recorded vocalizations from radio-collared individuals using handheld recorders over 3 months. We also conducted an acoustic survey to measure the vocal activity of M. sambiranensis in four forest habitat types at the study site. We identified and classified five vocalization types in M. sambiranensis. The vocal repertoires of the three Microcebus species contain very similar call types but have different acoustic properties, with one loud call type, the whistle, having significantly different acoustic properties between species. Our acoustic survey detected more calls of M. sambiranensis in secondary forest, riparian forest, and forest edge habitats, suggesting that individuals may prefer these habitat types over primary forest. Our results suggest interspecific differences in the vocal repertoire of mouse lemurs, and that these differences can be used to investigate habitat preference via acoustic surveys.     

Sleeping and Ranging Behavior of the Sambirano Mouse Lemur, <Emphasis Type="Italic">Microcebus sambiranensis</Emphasis>

Primates require secure sleeping sites for periods of rest, but despite their importance, the characteristics of desired sleeping sites are poorly known. Here we investigated the sleeping ecology of a radio-collared population of the Sambirano mouse lemur, Microcebus sambiranensis, during the nonreproductive season in the Anabohazo forest, northwestern Madagascar. We also investigated their ranging behavior and examined the spatial distribution of sleeping sites within the home ranges of the collared individuals. We took measurements of the sleeping tree’s physical characteristics and recorded the number of collared individuals using each sleeping site. We found that M. sambiranensis generally use foliage sleeping sites more frequently than tree holes and individuals slept more frequently in densely foliated trees than in sparsely foliated trees, often alone. We observed no significant differences in home range size or nightly travel distance between males and females; however, home ranges were smaller than those described for other mouse lemur species. Finally, we found that M. sambiranensis sleep peripherally and forage centrally within their home ranges, a behavior not previously described for mouse lemurs. Our results indicate profound differences in the social organization between M. sambiranensis and other mouse lemur species described in the literature, suggesting species-specificity in mouse lemur ecology. Understanding the sleeping ecology and ranging behavior of mouse lemurs is of great importance to their conservation, as these data facilitate the planning of long-term reforestation, habitat management, and population assessment.     

Sequence analysis of the grey mouse lemur (<Emphasis Type="Italic">Microcebus murinus</Emphasis>) <Emphasis Type="Italic">MHC class II DQ</Emphasis> and <Emphasis Type="Italic">DR</Emphasis> region

We here report the genomic organisation of the grey mouse lemur (Microcebus murinus) MHC class II DQ and DR region based on BAC clone analysis. The sequenced Mimu-MHC haplotype spans 343 kb and encompasses the genes TAP2, DOB, DQB, DQA, DRB, DRA, BTNL2 and a further BTNL gene. The DQ and DR genes of this haplotype are not duplicated. Mimu-DOB is not transcribed and represents a pseudogene due to deletions and premature stop codons. Analysis of BAC clone DNA, a cDNA sample and eight genomic DNA samples suggests that Mimu-DRB, Mimu-DQA and Mimu-DQB are highly polymorphic with the majority of peptide-binding residues being affected by polymorphisms. In contrast, Mimu-DRA is moderately polymorphic, and the variable amino acid positions are not part of the peptide-binding region. Phylogenetic analysis of Mimu-DQA and Mimu-DQB and other primate DQA and DQB genes indicates that duplication of DQA and DQB loci occurred in Anthropoidea after the split from Strepsirrhini.     

Hybrid disadvantage in the larval foraging behaviour of the two neotropical species of <Emphasis Type="Italic">Drosophila pavani</Emphasis> and <Emphasis Type="Italic">Drosophila gaucha</Emphasis>

Flies from two populations of the Chilean endemic neotropical species Drosophila pavani and two populations of its sibling species Drosophila gaucha were crossed reciprocally to obtain intra- and interspecific hybrids. The developmental pathways of locomotor activity and feeding rate were analysed for eleven of twelve possible genotype groups. The hybrids showed reduced fitness indicated by a decrease in the measured traits. Hybrid disadvantage was strongest in interspecific hybrids, especially with respect to feeding behaviour. This evidence supports the contention that D. pavani and D. gaucha have evolved different coadapted gene pools controlling the developmental pathways for behavioural traits expressed during larval foraging; but genetic divergence affecting these behaviours has also taken place between locally adapted populations within each species.     

Spatial distribution of <Emphasis Type="Italic">Marmota baibacina</Emphasis> and <Emphasis Type="Italic">M. sibirica</Emphasis> (<Emphasis Type="Italic">Marmota</Emphasis>, Sciuridae,Rodentia) in a zone of sympatry in Mongolian Altai: Bioacoustic analysis

The spatial distribution of two marmot species Marmota baibacina and M. sibirica in a zone of coexistence was studied by using their alarm call as a diagnostic trait. It was found that M. baibacina prefers to inhabit bouldery screes, whereas M. sibirica inhabits all suitable biotopes. The difference in biotopic distribution of these species could be explained by M. sibirica forcing M. baibacina out of optimum habitats. Cases of coexistence of both species in one family group sites were registered, which might contribute to the appearance of hybrids.     

Genetic Differentiation and Demographic History of the Northern Rufous Mouse Lemur (<Emphasis Type="Italic">Microcebus tavaratra</Emphasis>) Across a Fragmented Landscape in Northern Madagascar

Phylogeographic barriers, together with habitat loss and fragmentation, contribute to the evolution of a species’ genetic diversity by limiting gene flow and increasing genetic differentiation among populations. Changes in connectivity can thus affect the genetic diversity of populations, which may influence the evolutionary potential of species and the survival of populations in the long term. We studied the genetic diversity of the little known Northern rufous mouse lemur (Microcebus tavaratra), endemic to Northern Madagascar. We focused on the population of M. tavaratra in the Loky–Manambato region, Northern Madagascar, a region delimited by two permanent rivers and characterized by a mosaic of fragmented forests. We genotyped 148 individuals at three mitochondrial loci (D-loop, cytb, and cox2) in all the major forests of the study region. Our analyses suggest that M. tavaratra holds average genetic diversity when compared to other mouse lemur species, and we identified two to four genetic clusters in the study region, a pattern similar to that observed in another lemur endemic to the region (Propithecus tattersalli). The main cluster involved samples from the two mountain forests in the study region, which were connected until recently. However, the river crossing the study region does not appear to be a strict barrier to gene flow in M. tavaratra. Finally, the inferred demographic history of M. tavaratra suggests no detectable departure from stationarity over the last millennia. Comparisons with codistributed species (P. tattersalli and two endemic rodents, Eliurus spp.) suggest both differences and similarities in the genetic clusters identified (i.e., barriers to species dispersal) and in the inferred demographic history. These comparisons suggest that studies of codistributed species are important to understand the effects of landscape features on species and to reconstruct the history of habitat changes in a region.     

Foraging,Food Choice,and Food Processing by Sympatric Ripe-Fruit Specialists: <Emphasis Type="Italic">Lagothrix lagotricha poeppigii</Emphasis> and <Emphasis Type="Italic">Ateles belzebuth belzebuth</Emphasis>

Studies of interspecific competition and niche separation have formed some of the seminal works of ecology. I conducted an 18-mo study comparing the feeding ecologies of 2 sympatric, closely-related ripe-fruit specialists, Humboldt's woolly monkeys (Lagothrix lagotricha poeppigii), and the white-bellied spider monkeys (Ateles belzebuth belzebuth) in Amazonian Ecuador. Woolly monkeys in the terra firme forest live at roughly triple the density of spider monkeys (31 versus 11.5 animals/km²). Woolly monkeys spend 17% of their time foraging, while spider monkeys spend only 1% of their time foraging. Spider monkeys alone fed on soil and termitaria, which are rich in phosphorus. Woolly monkeys are not hard-fruit specialists. Their fruit diet is significantly more diverse than that of spider monkeys. Dietary overlap between the 2 species is high, yet each specializes to some degree on a different set of fruit resources. Woolly monkeys visit more food sources per unit of time, feed lower in the canopy, visit more small food patches, and prey on more seeds. Spider monkeys feed on fewer, richer food sources and are more than twice as likely to return to a particular fruit source than woolly monkeys are. Spider monkeys maximize fruit pulp intake, carrying more intact seeds in their guts, while woolly monkeys minimize seed bulk swallowed through more careful food processing. Surprisingly, several preferred spider monkey foods with high fat content and large seeds are avoided by woolly monkeys. I outline the different ecological dimensions involved in niche separation between the 2 species and discuss the possible impetus for their evolutionary divergence.     

Vegetation Thresholds for the Occurrence and Dispersal of <Emphasis Type="Italic">Microcebus griseorufus</Emphasis> in Southwestern Madagascar

Primate populations are declining the world over due to anthropogenic threats, including habitat loss and degradation. This raises the important question of how much habitat degradation a species can cope with. Habitat degradation is pronounced in Madagascar, where most of the human population depends on the direct exploitation of natural resources. We aimed to identify the response of Microcebus griseorufus (the gray-brown mouse lemur) to forest degradation and to define the structural traits of the vegetation that might be crucial for the species’ occurrence in anthropogenic landscapes. We documented the occurrence of Microcebus griseorufus in relation to vegetation structures along a gradient of forest degradation, at the edge of and west of Tsimanampetsotsa National Park in April and May 2007 and from October to December 2015. We confirmed the occurrence of Microcebus griseorufus using trapping and visual surveys, and measured vegetation structure. Logistic regression models showed that Microcebus griseorufus has a threshold response to tree density and the diameter of thick trees. The thresholds of occurrence were at 10–15% of the tree density recorded in intact forest and a mean diameter of trees with a diameter at breast height of >10 cm of 14.3 cm. The definition of such thresholds might help to maintain suitable habitat for this species and other primates living in anthropogenic landscapes, providing connectivity between isolated protected areas and allowing dispersal between populations.     

Gray-brown Mouse Lemurs (<Emphasis Type="Italic">Microcebus griseorufus</Emphasis>) as an Example of Distributional Constraints through Increasing Desertification

Climate change will impose new constraints on the distribution of species through desertification. Small-scale endemists common in biodiversity hotspots such as Madagascar are especially threatened. Among them are the gray-brown mouse lemurs (Microcebus griseorufus), which occupy the driest habitats in Madagascar of all Microcebus spp. We studied impacts of aridity on this species to identify critical factors for distributional limits. Accordingly, we compared populations of 2 adjacent habitats that differ in their humidity levels. We found that the more humid habitat provided more high-quality food and maintained a higher population density of Microcebus griseorufus, with individuals in better condition compared to the drier habitat. At the end of the wet, but not in the dry season, Microcebus griseorufus adjusted its home range size to local food plant density, which indicates that individuals optimize food intake in the wet season to prepare for the dry season. We found a negative exponential relationship between food plant density and home range size, which suggests an upper limit for the size of home ranges. According to this relationship, individuals from the drier habitat could not compensate for reduced food availability by enlarging their home range beyond this threshold. Thus, in case of declining food availability during the wet season due to a generally drier climate, individuals will not be able to extend their home ranges to include more food resources, and hence to accumulate enough fat reserves for the dry season. In consequence, they will have to migrate toward more mesic refugia. Migration, however, requires habitat connectivity, which is scarce in Madagascar’s largely anthropogenic and heavily fragmented landscape. Our data suggest that upper limits in home range sizes can limit a species’ ability to adapt to increasing aridity.     

Fecundity and feeding of <Emphasis Type="Italic">Galerucella calmariensis</Emphasis> and <Emphasis Type="Italic">G. pusilla</Emphasis> on <Emphasis Type="Italic">Lythrum salicaria</Emphasis>

Fecundity and feeding of two introduced sibling biological control species, Galerucella calmariensis and G. pusilla (Coleoptera: Chrysomelidae) on purple loosestrife, Lythrum salicaria L. (Lythraceae) were compared at constant temperatures of 12.5, 15, 20, 25, and 27.5 °C. Larval feeding was also carried out at 30 °C, but at this temperature, larvae developed only to the L2 stage and none pupated. Thus, data for this temperature were not used in the analysis. There were significant species × temperature interactions in fecundity. Of the two species, Galerucella pusilla laid more eggs. Although egg production of both species was lowest at 12.5 °C and increased to 20 °C, at higher temperatures, the two species reacted differently. From 25 to 27.5 °C, egg production decreased for G. pusilla, but G. calmariensis fecundity peaked at 27.5 °C. Significant temperature × species × life-stage interactions were also observed in feeding. For each species, the amount of feeding varied with temperature and stage of development. Galerucella pusilla adults consumed more foliage at 15, 20, and 27.5 °C. However, at 12.5 °C G. calmariensis adults fed more than G. pusilla. G. pusilla larvae consumed an average of 25% less foliage than G. calmariensis. The lower larval consumption of G. pusilla suggests that when food is limited, G. pusilla larvae may have a higher survival rate because of its ability to complete larval development with less food and produce more progeny due to its greater fecundity. When food is not limited neither species would have a competitive advantage and both species could coexist temporally and spatially. However, since G. calmariensis larvae consumed more leaf material, the larval stage of this species would have a greater impact on purple loosestrife than G. pusilla.     

MtDNA reveals strong genetic differentiation among geographically isolated populations of the golden brown mouse lemur, <Emphasis Type="Italic">Microcebus ravelobensis</Emphasis>

Microcebus ravelobensis is an endangered nocturnal primate endemic to northwestern Madagascar. This part of the island is subject to extensive human intervention leading to massive habitat destruction and fragmentation. We investigated the degree of genetic differentiation among remaining populations using mitochondrial control region sequences (479–482 bases). Nine populations were sampled from the hypothesized geographic range. The region is composed of three inter-river systems (IRSs). Samples were collected in three areas of continuous forests (CFs) and six isolated forest fragments (IFFs) of different sizes. We identified 27 haplotypes in 114 animals, with CFs and IFFs harbouring 5–6 and 1–3 haplotypes, respectively. All IFFs were significantly differentiated from each other with high Φ_ST values and sets of unique haplotypes. The rivers constitute significant dispersal barriers with over 82% of the molecular variation being attributed to the divergence among the IRSs. The data suggest a deep and so far unknown split within the rufous mouse lemurs of northwestern Madagascar. The limited data base and the lack of ecological and morphological data do not allow definite taxonomic classification at this stage. However, the results clearly indicate that M. ravelobensis consists of three evolutionary significant units, possibly cryptic species, which warrant urgent and separate conservation efforts.     

Local coexistence and niche differences between the Lusitanian and Mediterranean pine voles (<Emphasis Type="Italic">Microtus lusitanicus</Emphasis> and <Emphasis Type="Italic">M. duodecimcostatus</Emphasis>)

In the present study, we analyzed the coexistence pattern of the Lusitanian pine vole (Microtus lusitanicus) and the Mediterranean pine vole (Microtus duodecimcostatus) in a potential area of sympatry in a Mediterranean landscape (Portugal). We also determined the relative contribution of local, landscape, and spatial factors explaining the differences in the distribution patterns of the two species in the region. Using a kriging interpolation method, we obtained a map of sympatric and allopatric areas of species occurrence. The estimated sympatry area corresponded to a northwest–southeast belt representing 11.3% of the study area. Habitat niche differences were assessed with binomial GLMs followed by a variance partitioning. At a local scale, higher altitude, higher cover of shrubs, lower clay content in the soil, and lower cover of tree canopy were the most important factors distinguishing M. lusitanicus presence sites from those with M. duodecimcostatus. At a larger scale, the presence of forest landscape units and the low abundance of “montado” units were the most influencing landscape factors in the identification of M. lusitanicus occurrence sites when compared to M. duodecimcostatus. Our results suggested that local coexistence of M. lusitanicus and M. duodecimcostatus in the field is a rare event. The differences in distribution patterns of the two pine vole species were mostly explained by fine-scale environmental factors and by shared spatial effects.     

Trophic ecology of two savanna grazers,blue wildebeest <Emphasis Type="Italic">Connochaetes taurinus</Emphasis> and black wildebeest <Emphasis Type="Italic">Connochaetes gnou</Emphasis>

The feeding niches and trophic ecology of two South African grazers, blue wildebeest Connochaetes taurinus and black wildebeest Connochaetes gnou, are compared using stable carbon and nitrogen isotope data from feces and tooth dentine collagen. As sympatric, closely related taxa predicted to occupy similar trophic positions, the blue and black wildebeest provide a good model for studying the mechanisms of coexistence and macroevolution in mammals. Data from feces collected from a single reserve in the Free State Province reveal different trophic behaviors between two herds of blue wildebeest and between both compared with a single herd of black wildebeest. These data suggest that sympatric coexistence of blue and black wildebeest is facilitated by differential niche occupation at family group or herd levels, rather than between species. However, such separation does not occur over longer time scales: results from dentine collagen support the hypothesis that the two species are indistinct in terms of trophic behavior, although blue wildebeest show more feeding flexibility, probably because of their wider habitat tolerance range. Similarities in premaxillary width of males and females of both species also suggest that both species are adapted to similar feeding styles. Thus, it is unlikely that changes in trophic behavior provided the trigger for divergence of the black from the blue wildebeest lineage in the Middle Pleistocene. We argue that the case of these two species represents an example of speciation that was not driven by resource competition, as is often assumed for many turnover events in mammalian evolution. We briefly discuss a previous suggestion that links black wildebeest evolution to their more territorial breeding behavior associated with Middle-to-Late Pleistocene landscape changes in southern Africa.     

Sensory Basis of Food Detection in Wild <Emphasis Type="BoldItalic">Microcebus murinus</Emphasis>

Very little is known about how nocturnal primates find their food. Here we studied the sensory basis of food perception in wild-caught gray mouse lemurs (Microcebus murinus) in Madagascar. Mouse lemurs feed primarily on fruit and arthropods. We established a set of behavioral experiments to assess food detection in wild-born, field-experienced mouse lemurs in short-term captivity. Specifically, we investigated whether they use visual, auditory, and motion cues to find and to localize prey arthropods and further whether olfactory cues are sufficient for finding fruit. Visual cues from motionless arthropod dummies were not sufficient to allow reliable detection of prey in choice experiments, nor did they trigger prey capture behavior when presented on the feeding platform. In contrast, visual motion cues from moving prey dummies attracted their attention. Behavioral observations and experiments with live and recorded insect rustling sounds indicated that the lemurs make use of prey-generated acoustic cues for foraging. Both visual motion cues and acoustic prey stimuli on their own were sufficient to trigger approach and capture behavior in the mouse lemurs. For the detection of fruit, choice experiments showed that olfactory information was sufficient for mouse lemurs to find a piece of banana. Our study provides the first experimental data on the sensory ecology of food detection in mouse lemurs. Further research is necessary to address the role of sensory ecology for food selection and possibly for niche differentiation between sympatric Microcebus species.     

Characterization of 21 microsatellite marker loci in the ring-tailed lemur (<Emphasis Type="Italic">Lemur catta</Emphasis>)

Ring-tailed lemur (Lemur catta) is the only species in the Genus Lemur, distributed in the deciduous and spiny forests of southwestern Madagascar. This species is listed as endangered due to the loss and fragmentation of its natural habitat, a consequence of deforestation. Twenty-one nuclear microsatellite loci were isolated from a genomic DNA derived from a free-ranging ring-tailed lemur population from the Tsimanampetsotsa National Park, Madagascar. We report various parameter estimates and measures to establish the utility of this marker suite as screened among individuals this single forest fragment.     

Subchondral Bone Apparent Density and Locomotor Behavior in Extant Primates and Subfossil Lemurs <Emphasis Type="Italic">Hadropithecus</Emphasis> and <Emphasis Type="Italic">Pachylemur</Emphasis>

We analyze patterns of subchondral bone apparent density in the distal femur of extant primates to reconstruct differences in knee posture, discriminate among extant species with different locomotor preferences, and investigate the knee postures used by subfossil lemur species Hadropithecus stenognathus and Pachylemur insignis. We obtained computed tomographic scans for 164 femora belonging to 39 primate species. We grouped species by locomotor preference into knuckle-walking, arboreal quadruped, terrestrial quadruped, quadrupedal leaper, suspensory and vertical clinging, and leaping categories. We reconstructed knee posture using an experimentally validated procedure of determining the anterior extent of the region of maximal subchondral bone apparent density on a median slice through the medial femoral condyle. We compared subchondral apparent density magnitudes between subfossil and extant specimens to ensure that fossils did not display substantial mineralization or degradation. Subfossil and extant specimens were found to have similar magnitudes of subchondral apparent density, thereby permitting comparisons of the density patterns. We observed significant differences in the position of maximum subchondral apparent density between leaping and nonleaping extant primates, with leaping primates appearing to use much more flexed knee postures than nonleaping species. The anterior placement of the regions of maximum subchondral bone apparent density in the subfossil specimens of Hadropithecus and Pachylemur suggests that both species differed from leaping primates and included in their broad range of knee postures rather extended postures. For Hadropithecus, this result is consistent with other evidence for terrestrial locomotion. Pachylemur, reconstructed on the basis of other evidence as a committed arboreal quadruped, likely employed extended knee postures in other activities such as hindlimb suspension, in addition to occasional terrestrial locomotion.     

Population dynamics of red-backed voles (<Emphasis Type="Italic">Myodes</Emphasis>) in North America

We review the population dynamics of red-backed voles (Myodes species) in North America, the main deciduous and coniferous forest-dwelling microtines on this continent, and compare and contrast their pattern with that of the same or similar species in Eurasia. We identify 7 long-term studies of population changes in Myodes in North America. Using autoregressive and spectral analysis, we found that only 2 of the 7 show 3- to 5-year cycles like those found in some Eurasian populations. There was no relationship between latitude and cycling. The general lack of cyclicity is associated with two key aspects of their demography that act in tandem: first, poor overwinter survival in most years; second, chronically low densities, with irregular outbreak years. Eight factors might explain why some Myodes populations fluctuate in cycles and others fluctuate irregularly, and we review the evidence for each factor: food supplies, nutrients, predation, interspecific competition, disease, weather, spacing behavior and interactive effects. Of these eight, only food supplies appear to be sufficient to explain the differences between cyclic and non-cyclic populations. Irregular fluctuations are the result of pulsed food supplies in the form of berry crops (M. rutilus) or tree seeds (M. gapperi) linked to weather patterns. We argue that, to understand the cause for the patterns in the respective hemispheres, we must know the mechanism(s) driving population change and this must be linked to rigorous field tests. We suggest that a large-scale, year-round feeding experiment should improve overwintering survival, increase standing densities, and flip non-cyclic Myodes populations into cyclic dynamics that would mimic the patterns seen in the cyclic populations found in parts of Eurasia.