Chitty Effect Research Articles

Biometric analysis of the cranial features of common voles

Although cranial bones reliably identify the features of mammals, these traits can be affected by sex, population size and seasonality. This paper biometrically analyses the cranial features of the common vole. The research sample consisted of adult (n = 134), subadult (n = 57), of western Slovakia. Overall, software based on the highlighted marks measured 29 cranial features. Significant sexual dimorphism was observed for three features in adult individuals (width of the maxillary tooth row – right side “WMTRR”, maximum height of the mandibula excluding coronoid process “AMdm” and coronoid height of the mandibula “AMd”) and four features in subadults (length of the tooth row in the maxilla “LOSD”, “WMTRR”, length of the molar “LoM” and alveolar length of the lower molars “ALLM”). Results showed a significant positive relationship between body weight and cranial size in 18 of the 29 features. They also suggested a weak but significant positive effect of common vole abundance on cranium size (Chitty effect) likewise in 6 of the 29 features. In several cases, this effect affects, in addition to the abundance, the onset of sexual maturity of individuals.

Extreme Body Condition Index Values in Small Mammals.

The body condition index (BCI) values in small mammals are important in understanding their survival and reproduction. The upper values could be related to the Chitty effect (presence of very heavy individuals), while the minimum ones are little known. In this study, we analyzed extremes of BCI in 12 small mammal species, snap-trapped in Lithuania between 1980 and 2023, with respect to species, animal age, sex, and participation in reproduction. The proportion of small mammals with extreme body condition indices was negligible (1.33% with a BCI < 2 and 0.52% with a BCI > 5) when considering the total number of individuals processed (n = 27,073). When compared to the expected proportions, insectivores and herbivores were overrepresented, while granivores and omnivores were underrepresented among underfit animals. The proportions of granivores and insectivores were higher, while those of omnivores and herbivores were lower than expected in overfit animals. In several species, the proportions of age groups in underfit and overfit individuals differed from that expected. The male-female ratio was not expressed, with the exception of Sorex araneus. The highest proportion of overfit and absence of underfit individuals was found in Micromys minutus. The observation that individuals with the highest body mass are not among those with the highest BCI contributes to the interpretation of the Chitty effect. For the first time in mid-latitudes, we report individuals of very high body mass in three shrew species.

Habitat type modulates sharp body mass oscillations in cyclic common vole populations

Cyclic rodent populations exhibit pronounced changes in body mass associated with the population cycle phase, long-known as Chitty effect. Although Chitty effect is a common epiphenomenon in both America and Europe, there is still incomplete evidence about the generality of these patterns across the entire range of most species. Moreover, despite decades of research, the underlying factors driving Chitty effect remains poorly understood. Here, we examined the influence of intrinsic and extrinsic factors that may underlie observed patterns in vole size variation in the Iberian common vole Microtus arvalis asturianus. We weighed and measured 2816 adult voles that were captured during 6 trapping periods. Vole numbers and body mass showed strong period- and phase-related variation both in females and males, demonstrating marked Chitty effect in the studied population. Body mass of adult males correlated with body length, evidencing that heavier males are also structurally larger. Statistical models showed that probability of occurrence of large-sized vole (> 37 g) was significantly more likely in reproductive males, during increase and peak phases, and it was modulated by habitat, with crop fields and field margins between crops showing an increased likelihood. We suggest an effect of the habitat on vole body mass mediated by predation.

Population Cycles and the Chitty Syndrome

Data are presented on the long-term population dynamics of small mammals (1982–2019) and variation in the body weight and length of the bank vole, a dominant species, in southern taiga forests of the Middle Urals. Spontaneous change in the pattern of dynamics observed in our previous study—transition from regular 3-year cycles to noncyclic regime—provided the possibility to estimate the influence of different factors on the Chitty effect (increase in body size at high population numbers) under different regimes. This effect was clearly manifested in the bank vole population with cyclic dynamics: the average body weight and length of the animals were greater at the phases of population growth and peak than at the trough (low) phase, while the same parameters of voles from the noncyclic regime had intermediate values. The body size of animals depends not so much on population density as on factors determining their physiological status (including reproductive status).

Is There a Relationship between the Chitty Effect and the Types of Population Dynamics?

Thirty-year studies of populations of eight shrew species in the Middle Yenisei taiga have shown that at the late 20th century the population dynamics had a pronounced cyclic character, but in the 21st century cyclic dynamics changed with fluctuating one. The analysis of the dependence of the animal body weight on the population density have revealed that under cyclic dynamics at the peaks of the population, the animal's body weight is significantly higher than that at population decline, i.e., the corollary from the Chitty hypothesis (the Chitty effect) is observed. In populations with fluctuating dynamics such regularity has not been observed.

Density-related pattern of variation in body growth, body size and annual productivity in the common hamster

Body size is one of the most influential life history traits, often covarying with population density. While decreasing in some organisms, such as large herbivores, body size may increase with population density in small ones, such as voles. Unlike small voles, the common hamster (Cricetus cricetus L.), the endangered Eurasian rodent, does not exhibit cyclic population dynamics but it does have large variation in population numbers and high reproductive capacity; yet, its density-related processes which affect morphological traits are still poorly understood. We analysed density-related changes in body growth rates and body size (length and weight) collected in a natural population over 9 years, separately for each sex and age category (subadult/adult). Annual population density, the maximum Jolly–Seber estimate of population size per hectare reached in that year, was found to increase linearly with productivity index and with the length of the breeding season. Body length growth rates increased with population density in adults, but not in subadults. In adults, body length was found to increase with population density; however, we found no relationship between body weight and population density. The evidence for changes in subadults’ body size with population density was weak. These results suggest that, in years of peak numbers, adult hamsters grow faster, are longer and breed for longer time periods, thus having higher productivity index. Body size is therefore positively related to variation in population density, commonly referred to as the Chitty effect. Based on our findings, we conclude that variation in individual’s body growth rate and size drives the variation in productivity which in turn causes the changes in hamster population density.

Trait-demography relationships underlying small mammal population fluctuations.

Large-scale fluctuations in abundance are a common feature of small mammal populations and have been the subject of extensive research. These demographic fluctuations are often associated with concurrent changes in the average body mass of individuals, sometimes referred to as the 'Chitty effect'. Despite the long-standing recognition of this phenomenon, an empirical investigation of the underlying coupled dynamics of body mass and population growth has been lacking. Using long-term life-history data combined with a trait-based demographic approach, we examined the relationship between body mass and demography in a small mammal population that exhibits non-cyclic, large-scale fluctuations in abundance. We used data from the male segment of a 25-year study of the monogamous prairie vole, Microtus ochrogaster, in Illinois, USA. Specifically, we investigated how trait-demography relationships and trait distributions changed between different phases of population fluctuations, and the consequences of these changes for both trait and population dynamics. We observed phase-specific changes in male adult body mass distribution in this population of prairie voles. Our analyses revealed that these changes were driven by variation in ontogenetic growth, rather than selection acting on the trait. The resulting changes in body mass influenced most life-history processes, and these effects varied among phases of population fluctuation. However, these changes did not propagate to affect the population growth rate due to the small effect of body mass on vital rates, compared to the overall differences in vital rates between phases. The increase phase of the fluctuations was initiated by enhanced survival, particularly of juveniles and fecundity, whereas the decline phase was driven by an overall reduction in fecundity, survival and maturation rates. Our study provides empirical support, as well as a potential mechanism, underlying the observed trait changes accompanying population fluctuations. Body size dynamics and population fluctuations resulted from different life-history processes. Therefore, we conclude that body size dynamics in our population do not drive the observed population dynamics. This more in-depth understanding of different components of small mammal population fluctuations will help us to better identify the mechanistic drivers of this interesting phenomenon.

Individual growth rates in natural field vole, Microtus agrestis, populations exhibiting cyclic population dynamics

Rodents that have multi-annual cycles of density are known to have flexible growth strategies, and the "Chitty effect", whereby adults in the high-density phase of the cycle exhibit larger average body mass than during the low phase, is a well-documented feature of cyclic populations. Despite this, there have been no studies that have repeatedly monitored individual vole growth over time from all phases of a density cycle, in order to evaluate whether such variation in body size is due to differences in juvenile growth rates, differences in growth periods, or differential survival of particularly large or small voles. This study compares growth trajectories from voles during the peak, increase and crash phases of the cycle in order to evaluate whether voles are exhibiting fast or slow growth strategies. We found that although voles reach highest asymptotic weights in the peak phase and lowest asymptotes during the crash, initial growth rates were not significantly different. This suggests that voles attain larger body size during the peak phase as a result of growing for longer.

Changes in individual quality during a 3-year population cycle of voles.

In small mammal populations with multiannual oscillations in density, the occurrence of large individuals in the peak phase (the "Chitty effect") is a typical feature, but mechanisms behind this phenomenon have remained unclear. We analysed long-term data sets collected in western Finland between 1984 and 1992 to: (1) find out how the body size and body condition of voles (Microtus agrestis, M. rossiaemeridionalis, Clethrionomys glareolus) and shrews (Sorex araneus) was associated with the 3-year population cycle of voles, and (2) relate the quality (body condition) of the individuals to changes in the biotic environment in order to detect how the different hypotheses about the mechanisms behind the Chitty effect can explain the observed variation. In the 3-year cycle studied, the mean body size and quality were strongly related to density oscillations in voles but not in sympatric shrews. Voles were lean in the decline phase but very stocky in the summer of the peak phase. This pattern appeared to be mainly caused by changes in body condition or body shape rather than mere size (body length). The quality of voles appeared to be delayed density dependent, especially in autumn when the dominant time lag was 12months. Previous vole density was strongly related to changes in the environment (activity of specialist predators, production of hay until early summer). We suggest that the previous density of voles mainly affects the quality of voles indirectly through changes in the biotic environment, and that the proximate cause behind the Chitty effect is the combined effect of changes in predation pressure and availability of food.

The Chitty Effect: A Consequence of Dynamic Energy Allocation in a Fluctuating Environment

An important biological feature of cyclic populations of voles and lemmings is phase-related changes in average body mass, with adults in high-density phases being 20–30% heavier than those in low-density phases of a cycle. This observation, called the “Chitty effect,” is considered to be a ubiquitous feature of cyclic populations. It has been argued that understanding the Chitty effect is fundamental to unraveling the enigma of population cycles. However, there exists no agreement among biologists regarding the causes of the Chitty effect. Here, I propose a simple hypothesis to explain the Chitty effect, based on phase-related, dynamic allocation of energy between reproductive and somatic effort. The essence of the hypothesis is that: (1) reproduction is suppressed in animals born or raised in the later part of the increase phase by environmental factors, including social influences; (2) suppression of reproduction limits the amount of energy that is diverted for reproductive effort, and forces a disproportionately greater amount of surplus power (the energy left after the energetic costs of standard and active metabolism are met) to be allocated for somatic effort; (3) the surplus energy, above and beyond what is required for routine biological activities, will allow continuous growth and deposition of additional body mass, which causes an increase in body mass; and (4) animals grow to a larger size as a population enters the peak density phase, causing an increase in the average body mass. The Chitty effect is predicted to be most pronounced at the late increase or peak phase of a population cycle. Possible causes of reproductive suppression include direct or indirect influences of the environmental factors. The Chitty effect may be a consequence, not a cause, of population cycles in small mammals.

Body mass dynamics in the Mediterranean pine vole Microtus duodecimcostatus

The occurrence of large individuals in fluctuating populations of microtine rodents (the Chitty effect) has received considerable attention. Recent studies showed that there are no causal relationships between the Chitty effect and the multi-annual population cycles of voles and lemmings. We studied five populations of Microtus duodecimcostatus in southern France with capture-recapture methods. Mean body mass was constant in all populations. Maximum observed body mass was more variable, and slightly positively correlated with density, except in the most fluctuating population where the correlation was strong. A careful study of the occurrence of large voles in these populations showed that most were pregnant females, and that the few large males were present when density was high and sample sizes were large. Comparison with published studies indicates that the Chitty effect, though not restricted to microtines, is not a general phenomenon in small mammal populations. Our field results and laboratory data on the same species suggest that M. duodecimcostatus has a determinate body growth, contrary to some Microtus species which have an indeterminate growth.

Body mass dynamics in the Mediterranean pine vole Microtus duodecimcostatus

AbstractThe occurrence of large individuals in fluctuating populations of microtine rodents (the Chitty effect) has received considerable attention. Recent studies showed that there are no causal relationships between the Chitty effect and the multi‐annual population cycles of voles and lemmings. We studied five populations of Microtus duodecimcostatus in southern France with capture‐recapture methods. Mean body mass was constant in all populations. Maximum observed body mass was more variable, and slightly positively correlated with density, except in the most fluctuating population where the correlation was strong. A careful study of the occurrence of large voles in these populations showed that most were pregnant females, and that the few large males were present when density was high and sample sizes were large. Comparison with published studies indicates that the Chitty effect, though not restricted to microtines, is not a general phenomenon in small mammal populations. Our field results and laboratory data on the same species suggest that M. duodecimcostatus has a determinate body growth, contrary to some Microtus species which have an indeterminate growth.

Does the "Chitty Effect" Occur in Peromyscus?

I tested for the “Chitty Effect” in white-footed mice, Peromyscus leucopus , by looking for the presence of a large-bodied, dominant, genetic morph in peak populations. Large-sized animals were present primarily during high, but not low, densities. However, large-sized males nested with females less often than did small males and they were not dominant to small males. Body mass of offspring was not correlated with that of their mothers, and thus, large size was not a unique genetic morph. Large-sized animals were older than small animals having survived through two breeding seasons under conditions of high availability of food. The occurrence of large-sized animals in this fluctuating population of Peromyscus was similar to that proposed by Chitty (1958, Cold Springs Harbor Symposium on Quantitative Biology, 22:99-113) for microtines, but whether it was analogous or not may be conjectural until comparable studies are conducted on Microtus .

Extra-Large Body Size in California Voles: Causes and Fitness Consequences

Extra-large body size in microtine rodents is a ubiquitous feature of peak population densities, and it has been hypothesized that these giant individuals represent a genetically based morphotype that has high fitness under increasing and high densities, and may represent a key element of a genetic polymorphism driving multi-annual cycles (Chitty/Krebs model). We examine this large-size phenomenon (Chitty Effect) in the California vole utilizing three approaches : analyzing the weight distribution in a non-cyclic population over a 13 yr period with comparisons to cyclic populations, analyzing body composition, especially fat content, as a function of body size, and observing the microhabitat distribution of extra-large males in two populations, one of which cycles

Morphometric variation in Clethrionomys gapperi: are all voles created equal?

A general feature of population cycles in microtine rodents is the Chitty effect: large body size in peak populations. This phenomenon does not occur in a subarctic population of Clethrionomys gapperi that has been studied extensively for 15 years. Voles from various phases of the population cycle differ primarily in terms of complex patterns of "shape" rather than "size." These patterns are strongly correlated with environmental conditions during periods of growth or thermal stress. Increasing populations are nevertheless characterized by changes in the relationships between body size and skull size. Voles larger than predicted on the basis of their skull size are captured in springs characterized by an increase in density from the previous spring. Hence the Chitty effect is present in this population but it is expressed in a different way. Qualitative changes in some weakly cycling microtine populations may therefore be relatively difficult to detect without a better understanding of the mechanisms underlying this morphometric, behavioural, genetic complex.

Viability of Large‐ and Small‐Sized Adults in Fluctuating Vole Populations

Peak populations of many species of lemmings and voles contain adults 20—30% larger than adults found in low populations (the Chitty effect). What is the adaptive advantage of being large? We try to answer this question by analyzing the survival rates of large— and small—size adults of 4 Microtus species. Two alternative hypotheses are proposed to explain the adaptive value of large size. Large adults may be r—selected genotypes at an adaptive advantage in increasing populations. Microtus pennsylvanicus and male M. townsendii fit this alternative because the survival advantage of large size is positively related to population growth rate and negatively related to density. Alternatively, large adults may be α—selected genotypes at an adaptive advantage in high density populations when aggression is important. Microtus californicus and M. ochrogaster fit this hypothesis because in these species the survival advantage of large size is positively related to density and negatively related to population growth rate. Thus large body size may be adaptive for quite contrary reasons in different species of cycle voles and lemmings.