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Featured researches published by John S. Millar.


Ecology | 2005

RESTITUTION OF MASS–SIZE RESIDUALS: VALIDATING BODY CONDITION INDICES

Albrecht I. Schulte-Hostedde; Bertram Zinner; John S. Millar; Graham J. Hickling

Body condition can have important fitness consequences, but measuring body condition of live animals from wild populations has been the subject of much recent debate. Using the residuals from a regression of body mass on a linear measure of body size is one of the most common methods of measuring condition and has been used in many vertebrate taxa. Recently, the use of this method has been criticized because assumptions are likely violated. We tested several assumptions regarding the use of this method with body composition and morphometric data from five species of small mammals and with statistical simulations. We tested the assumptions that the relationship between body mass and body size is linear, and that the proportion of mass associated with energy reserves is independent of body size. In addition, we tested whether the residuals from reduced major axis (RMA) regression or major axis (MA) regression performed better than the residuals from ordinary least squares (OLS) regression as indices of body condition. We found no evidence of nonlinear relationships between body mass and body size. Relative energy reserves (fat and lean dry mass) were generally independent or weakly dependent on body size. Residuals from MA and RMA regression consistently explained less variation in body composition than OLS regression. Using statistical simulations, we compared the effects of violations of the assumption that true condition and residual indices are independent of body size on the OLS, MA, and RMA procedures and found that OLS performed better than the RMA and MA procedures. Despite recent criticisms of residuals from mass-size OLS regressions, these indices of body condition appear to satisfy critical assumptions. Although some caution is warranted when using residuals, especially when both inter- individual variation in body size and measurement error are high, we found no reason to reject OLS residuals as legitimate indices of body condition.


Evolution | 1977

ADAPTIVE FEATURES OF MAMMALIAN REPRODUCTION

John S. Millar

Considerable progress in our understanding of reproductive rates and other life history phenomena has been made through comparative studies of major ecological groups (e.g. Thorson, 1950; Lack, 1968; Tinkle et al., 1970) as well as through specific studies involving few species (e.g. Price, 1973; Menge, 1974; Forsythe and Robertson, 1975) and theoretical considerations (e.g. Cole, 1954; Cody, 1966; Pianka, 1970; Smith and Fretwell, 1974). Recent considerations have centered on the concept of r and K selection, and a number of correlates of r and K selection have been suggested (Pianka, 1972). Presumably, animals in unpredictable environments (r selected) should allocate a greater proportion of their resources to reproduction than animals living in predictable environments (K selected). Reproductive effort (the proportion of total energy budget allocated to reproduction) should be a measure of an animals position along an r-K continuum. However, studies designed to examine the relationship between reproductive effort and environmental or demographic parameters (e.g. Menge, 1974; Tinkle and Hadley, 1975) have not completely supported these views. Perhaps the consideration of general selective forces (environmental predictability or demography) obscures important parameters in the evolution of life history parameters (Wilbur, Tinkle, and Collins, 1974). A fruitful approach might be to consider the specific features that contribute to reproductive effort, namely the size and number of offspring and the duration of the breeding effort. The purpose of this paper is to compare reproductive and developmental characteristics among mammals and to relate them to a consideration of mammalian reproductive rates. To date, mammals have not been dealt with in any general way, except from a mechanistic point of view (e.g. Asdell, 1966; Sadleir, 1969; Weir and Rowlands, 1973). No attempts have been made to define the ecologically important parameters of mammalian reproduction, but the following points seem appropriate. First, the energy demands of mammalian offspring increase throughout pregnancy and lactation (Hadjipieris and Holmes, 1966; Kaczmarski, 1966; Millar, 1975) so that late lactation is the energetically critical period of the breeding cycle. Pregnancy is energetically much less demanding than lactation and provides the offspring with a high degree of protection and environmental stability. Second, adult body size is related to mobility and home range (McNab, 1963), feeding strategy (McNab, 1971) and energy requirements (Kleiber, 1961). In general, the size of an animal is one measure of its niche so that the size of a mammal at weaning, or independence, reflects the extent to which it is prepared for coping with the adult niche. Third, reproductive effort is usually measured as the cumulative amount of energy committed to offspring (e.g. Menge, 1974; Tinkle and Hadley, 1975). This measure may not be appropriate for mammals. Mammals are homeotherms, a condition which permits a very rapid physiological response to environmental changes. Females do not appear to accumulate much energy prior to breeding, but support their offspring primarily through increased feeding when demands are greatest (Millar, 1975). Some


Ecology | 1978

Energetics of Reproduction in Peromyscus Leucopus: The Cost of Lactation

John S. Millar

Captive Peromyscus leucopus exhibited a positive relationship between weight of @V and number and size of nestlings. Size of neonates, but not size of post—neonatal young was negatively related to litter size. Ingested energy related to offspring was linearly related to litter size and varied from 899 kJ (74% of nonbreeding requirements) for 2 offspring to 2100 kJ (146% of nonbreeding requirements) for 6 offspring. Despite these differences between litter sizes, mean litter size was not different (° = 4.60 and 4.94) respectively between captive and wild females, and varied little seasonally and geographically. The relatively constant commitment to offspring may be related to the fact that conception occurs long before the period of peak energy demand. See full-text article at JSTOR


Ecology | 1983

Life Histories of Mammals: An Analysis of Life Tables

John S. Millar; Richard M. Zammuto

We quantitatively test theoretical predictions concerning mammalian life histories, using published data on survival, reproduction, and body mass for 29 eutherian mammals. Larger mammals have a greater age at maturity, greater generation length, greater life expectancy, lower reproductive value at maturity, and smaller litters than do smaller mammals. Residual reproductive value at maturity is not correlated with adult body mass or survival. Litter size varies inversely with generation length and adult survival. Age at maturity is positively correlated with life expectancy. Twenty-seven of 29 mammals display a generation length longer than their life expectancy at birth, and the same proportion shows a greater life expectancy at maturity than at birth. A fairly high proportion (76-82%) of the variation in these dependent variables is attributable to adult mass. Many life table characteristics of mammals are interrelated, although not necessarily in the ways predicted by theory. Design constraints may preclude significant differences in life history patterns among mammals, so that the life table characteristics of only a few species may depict the pattern of life table evolution in most eutherian mammals.


Behavioral Ecology and Sociobiology | 2004

Intraspecific variation of testis size and sperm length in the yellow-pine chipmunk (Tamias amoenus): implications for sperm competition and reproductive success

Albrecht I. Schulte-Hostedde; John S. Millar

Comparative analyses have found that relative testis size is a strong predictor of the prevalence of sperm competition for many taxa, including mammals, yet underlying this pattern is the assumption that intraspecific variation in testis size is related to individual fitness. Because intraspecific variation in ejaculate investment underlies interspecific patterns, it is critical to understand the causes and consequences of intraspecific variation in ejaculate investment. We examined relationships between ejaculate investment (testis size and sperm length) and reproductive success, body size and condition in the yellow-pine chipmunk (Tamias amoenus), a small ground squirrel in which sperm competition occurs. We examined genetic estimates of male reproductive success from a wild population of yellow-pine chipmunks and determined that males with large testes had higher annual reproductive success than males with small testes. This result provides empirical support for the numerous comparative studies that indicate testis size is associated with the intensity of sperm competition. In addition, males in good condition had relatively larger testes than males in poor condition, but there was no evidence of sperm length being dependent on condition. Finally, contrary to many predictions, males that invested more in sperm production (relatively heavy testes) produced shorter sperm, not longer sperm, than males that invested less.


Evolution | 1981

PRE-PARTUM REPRODUCTIVE CHARACTERISTICS OF EUTHERIAN MAMMALS

John S. Millar

A striking feature of reproduction in eutherian mammals is its limited diversity. All have internal fertilization, viviparity, and lactation. Most reproductive traits, including gestation time (Gunther and Guerra, 1955; Kilstrom, 1972; Sacher and Staffeldt, 1976), birth weight (Leutenegger, 1976; Millar, 1977; Robbins and Robbins, 1979), litter weight (Leitch et al., 1959), weight at weaning (Millar, 1977), and post natal growth (Gunther and Guerra, 1955; Millar, 1977; Case, 1978) are highly predictable from adult size. Only offspring number and duration of lactation appear poorly related to adult size (Millar, 1977). A general explanation for these relationships is that they are related to morphological and physiological constraints. Numerous physiological variables, including metabolic rate, are related to adult size (Gunther and Guerra, 1955; Stahl, 1967; Calder, 1974; Gunther, 1975; SchmidtNielsen, 1975). Structural traits are related to mechanical requirements so that they also are predictable from adult size (Huxley, 1932; Bonner, 1965; Gould, 1966). Together, these morphological and physiological patterns appear to influence ecological and behavioral patterns (Sacher, 1959; McNab, 1963; Lindstedt and Calder, 1976; Blueweiss et al., 1978; Harestad and Bunnell, 1979). The existence of a strong design constraint system in mammals presents problems when single traits are examined in relation to environmental conditions; foremost among these problems is that very few traits are independent of other traits. Characteristics not considered in any comparison can lead to biased results. In other words, changes in one trait may occur simply because another trait responds to environmental conditions. Another problem is that very few traits are linearly related to other traits so that the relative magnitudes of traits also vary predictably. Simple ratios cannot be used to compensate for these nonlinear relationships. These and other problems (considered in detail by Gould [1966] and Clutton-Brock and Harvey [1979]) have not always been recognized in studies of mammalian reproductive traits. The purpose of this study was to examine the interrelationships among reproductive traits of mammals in order to define the system of morphological and physiological constraints, and to examine the residual variation in this system in relation to taxonomic groups and ecological habits.


Ecology | 1985

Environmental Predictability, Variability, and Spermophilus columbianus Life History over an Elevational Gradient

Richard M. Zammuto; John S. Millar

We quantitatively tested various aspects of the theory of r— and K—selection for six populations of Columbian ground squirrels from Alberta, Canada. Three measures of environmental predictability (maximum and minimum temperatures, precipitation) and a presumed measure of food resource levels supported the prediction that environments at lower elevations were less predictable, and had greater, more variable food resource levels than environments at higher elevations. Columbian ground squirrels in more predictable environments (i.e., at higher elevations) had higher adult survival rates, later ages at maturity, and possibly lower genetic variabilities than did squirrels in less predictable environments (at lower elevations). Body mass was greater at lower elevations than at higher elevations. Litter size showed no trend with respect to elevation, but it tended to be most variable in unpredictable environments at lower elevations. Although they were more predictable, the daily minimum temperatures at higher elevations tended (P = .06) to show wider variation than the minimum temperatures at lower elevations. Previous apparent problems with r—K theory may be attributable in part to the assumption that predictability and stability should covary. The major difference between the pattern emerging from our study and that predicted by traditional theory is that predictability of environments, and concomitantly the occurrence of K—strategists, was found to increase with movement up an elevational gradient. Portions of r—K theory may be found to be useful once all of its parameters are consistently measured.


Evolution | 2002

FEMALE‐BIASED SEXUAL SIZE DIMORPHISM IN THE YELLOW‐PINE CHIPMUNK (TAMIAS AMOENUS): SEX‐SPECIFIC PATTERNS OF ANNUAL REPRODUCTIVE SUCCESS AND SURVIVAL

Albrecht I. Schulte-Hostedde; John S. Millar; H. Lisle Gibbs

Abstract.— Sexual size dimorphism is ultimately the result of independent, sex‐specific selection on body size. In mammals, male‐biased sexual size dimorphism is the predominant pattern, and it is usually attributed to the polygynous mating system prevalent in most mammals. This sole explanation is unsatisfying because selection acts on both sexes simultaneously, therefore any explanation of sexual size dimorphism should explain why one sex is relatively large and the other is small. Using mark‐recapture techniques and DNA microsatellite loci to assign parentage, we examined sex‐specific patterns of annual reproductive success and survival in the yellow‐pine chipmunk (Tamias amoenus), a small mammal with female‐biased sexual size dimorphism, to test the hypothesis that the dimorphism was related to sex differences in the relationship between body size and fitness. Chipmunks were monitored and body size components measured over three years in the Kananaskis Valley, Alberta, Canada. Male reproductive success was independent of body size perhaps due to trade‐offs in body size associated with behavioral components of male mating success: dominance and running speed. Male survival was consistent with stabilizing selection for overall body size and body size components. The relationship between reproductive success and female body size fluctuated. In two of three years the relationship was positive, whereas in one year the relationship was negative. This may have been the result of differences in environmental conditions among years. Large females require more energy to maintain their soma than small females and may be unable to maintain lactation in the face of challenging environmental conditions. Female survival was positively related to body size, with little evidence for stabilizing selection. Sex differences in the relationship between body size and fitness (reproductive success and survival) were the result of different processes, but were ultimately consistent with female‐biased sexual size dimorphism evident in this species.


Behavioral Ecology and Sociobiology | 1994

Philopatry and mother-daughter associations in bushy-tailed woodrats: space use and reproductive success

Richard A. Moses; John S. Millar

Social groups in mammals are often based on overlapping generations of philopatric females, yet few researchers have examined fitness consequences of associations between females and their daughters. We examined survival and reproductive consequences of sharing a natal site with ones mother or daughter in wild bushy-tailed woodrats (Neotoma cinerea). Bushy-tailed woodrats reside on discrete rocky outcrops, and females tend to spend their entire reproductive life on their natal outcrop. Mothers and daughters that co-existed on the natal outcrop were closely associated in space. Juvenile females were more likely to survive their first winter to become breeding yearlings at their natal site if their mother was also present at the natal site during the winter. Juveniles survived equally well with or without their mother where densities of adult females were experimentally reduced, suggesting that females enhance their daughters survival by facilitating their access to limited resources. Yearling females exhibited no reproductive costs from association with their mothers at the natal site during the breeding season; in fact, yearlings that bred at the natal site simultaneously with their adult mothers tended (P = 0.07) to experience fewer reproductive failures than did those that bred in the absence of their mothers. Mothers did not experience any detectable costs when sharing a natal site with a daughter. We conclude that bushy-tailed woodrats exhibit stable, fitness-enhancing associations among mothers and their philopatric daughters.


Ecoscience | 1996

The mating system of northern populations of Peromyscus maniculatus as revealed by radiotelemetry and DNA fingerprinting

David O. Ribble; John S. Millar

We studied the social organization and mating system of Peromyscus maniculatus in the Kananaskis Valley of Alberta by investigating both home range use and mating success. Radiotelemetry indicated ...

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Duncan G. L. Innes

University of Western Ontario

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Nikhil Lobo

University of Western Ontario

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Graham J. Hickling

University of Western Ontario

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Fred J. Longstaffe

University of Western Ontario

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Michael G. Topping

University of Western Ontario

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Monika A. Havelka

University of Western Ontario

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Richard A. Moses

University of Western Ontario

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