Importance Of Body Size Research Articles

Mate Choice and Spawning Success in the Fighting FishBetta splendens: the Importance of Body Size, Display Behavior and Nest Size

AbstractCourtship displays should be exaggerated enough to attract mates and yet tempered so as not to deter them. We tested this hypothesis in the fighting fishBetta splendensby studying courtship displays and body size and their relationships with male parental quality and female fecundity, as well as the effects of display behavior and body size on mate choice decisions and spawning success. Because of their high degree of parental investment, males are expected to be discriminating in their choice of mates. Males who displayed more frequently built larger nests, a measure of parental quality, but larger males did not. When females were paired with males with high display rates, however, the pair had fewer eggs in their nest, even when accounting for female body mass. In a mate choice test using computer‐generated male stimuli that differed only in display behavior, females showed no preferences for displaying males vs. non‐displaying males, or for males with higher display rates vs. lower display rates. In similar tests in which the computer‐generated males differed only in size, females preferred larger males, but also preferred males that differed with respect to body size (negative assortative mating). Males preferred computer‐generated females that performed courtship displays over non‐displaying females, but showed no preferences for female body size. Neither a female's body size nor her display behavior was a significant predictor of her fecundity as estimated by the number of eggs released during spawning. Thus, our results suggest that femaleB. splendensmust balance male parental quality (nest size) with the risk of potentially disruptive or dangerous behavior during spawning, and that females may minimize these risks through negative size‐assortative mating. Female display behavior, while unrelated to fecundity in our study, may attract males because it indicates reproductive readiness or serves a species‐recognition function.

Behavioural trade‐offs between growth and mortality explain evolution of submaximal growth rates

1. The importance of body size and growth rate in ecological interactions is widely recognized, and both are frequently used as surrogates for fitness. However, if there are significant costs associated with rapid growth rates then its fitness benefits may be questioned. 2. In replicated whole-lake experiments, we show that a domestic strain of rainbow trout (artificially selected for maximum intrinsic growth rate) use productive but risky habitats more than wild trout. Consequently, domestic trout grow faster in all situations, experience greater survival in the absence of predators, but have lower survival in the presence of predators. Therefore, rapid growth rates are selected against due to increased foraging effort (or conversely, lower antipredator behaviour) that increases vulnerability to predators. In other words, there is a behaviourally mediated trade-off between growth and mortality rates. 3. Whereas rapid growth is beneficial in many ecological interactions, our results show the mortality costs of achieving it are large in the presence of predators, which can help explain the absence of an average phenotype with maximized growth rates in nature.

Sequential size assessment and multicomponent decision rules mediate aerial wasp contests

In aerial conflicts among territorial insects, injury costs are not obviously high and contestants cannot physically ‘force the issue’. Resource-holding potential in these cases usually relates to morphological and/or biophysical determinants of flight performance and endurance rather than traditional parameters such as size and strength. However, success is sometimes related to body size. Males of the landmark-defending wasp Hemipepsis ustulata compete via elaborate noncontact aerial duels in which large individuals enjoy an advantage. We evaluated the hypothesis that size is important because of a correlation with high-performance flight. We used a residency manipulation to establish 92 escalated contests in which some individuals had the outer 16–18% of their forewings removed to reduce flight performance (independently of body size). Initial residents won most (78%) contests, but logistic analysis indicated that no other parameter, including size, wing treatment and age, could explain the pattern of contest outcome. Survival regression analysis implicated size, age and site location, but not wing treatment, as significant predictors of nonresident contest persistence. We also found evidence of assessment of relative body size. Since our wing treatment had no measurable effect, the importance of body size does not appear as an emergent property of a system in which contest ability is determined primarily by flight performance dynamics. We conclude that aerial contests in H. ustulata are mediated by complex multicomponent decision rules, in which contest role, relative body size, age and perception of resource value collectively determine a contestant's persistence time.

Importance of Body Size in Determining Dominance Hierarchies among Diverse Tropical Frugivores1

ABSTRACTMost studies examining dominance hierarchies have focused at the intraspecific level. While some examples of interspecific hierarchies have been noted, these have usually been limited to a few species in the same taxonomic group that utilize resources in similar ways. Here, we examine evidence for dominance interference competition among vertebrates comprising a diverse frugivore community, including 19 species of birds, squirrels, and primates in a mature Central African rainforest. A total of 38 fruiting trees from 18 species were observed for 2058 h to record dominance interactions between foraging vertebrates. We show that interference competition occurs within and between taxonomically diverse species of vertebrates at fruiting trees. The resulting cross‐taxonomic dominance hierarchy includes larger vertebrates, such as primates and hornbills, as well as smaller ones, such as squirrels and parrots. Within this hierarchy, the dominance rank of each species is highly correlated with body mass, and is shown to significantly affect the number of fruits removed from a given tree. Because a majority of tropical tree species depend on vertebrates to disperse their seeds, and particular vertebrates may preferentially disperse the seeds of specific tree species, results may have important conservation implications for the maintenance of tree diversity in regions where populations of larger frugivores have been depressed or extirpated.

Ecological context and the importance of body and gnathopod size for pairing success in two amphipod ecomorphs

Ecological context generates interspecific variation in mating behavior by imposing differential constraints on the action of sexual selection, but operation of these constraints in nature is not well understood. We used field and laboratory studies to examine the importance of body size and size of sexually dimorphic appendages, the gnathopods, for pairing success in two freshwater amphipod species within the Hyalella azteca species complex. We focused on a large-bodied species found in habitats where ecological factors tend to favor large body size, and a small-bodied species in habitats where small body size is favored by size-selective predation. A field study indicated that although male pairing success was greater for larger males in both species, pairing success increased throughout the range of variation in male size in the large species, whereas, in the small species, pairing success was low for smaller individuals, but roughly constant across intermediate to larger sizes. A laboratory mate choice experiment was consistent with the field study, finding that females of the large species exhibited a preference for larger males that was independent of absolute male size, but females of the small species were indifferent when choosing between males of intermediate to larger size. Species also differed in the direction of sexual size dimorphism in the field, with males being the larger sex in the large species but the smaller sex in the small species, a pattern consistent with the species differences in mate preference. Large gnathopod size relative to body size was associated with enhanced pairing success across all body sizes for the large species, but, in the small species, large gnathopod size enhanced pairing success only in smaller males. Species differences in mating behavior appear consistent with change driven by differing forms of the interaction between sexual and natural selection.

Body size and mating strategies in the simultaneous hermaphroditeOxynoe olivacea(Mollusca, Opisthobranchia, Sacoglossa)

SummaryTo better understand the role and importance of body size in hermaphroditic mating system theory, the mating behaviour of the shelled sacoglossanOxynoe olivaceawas studied. This simultaneous hermaphrodite exhibits bilateral and unilateral sperm transfer and thus it is particularly suitable for studies on hermaphrodite sexual conflict.In this study three hypotheses on body size were tested: thatO. olivaceapartner size has an effect on (i) mating mode choice, (ii) duration of mating modes, and (iii) choice of sexual role. Furthermore, we tested Charnov's hypothesis that inO. olivacea, like many simultaneous hermaphrodites, a sexual conflict exists and the male role is preferred.A laboratory experiment was done to test the aforementioned hypothesis and to observe the general mating behaviour ofO. olivacea. Pairs of similar (S) and different (D) sizes were considered for a total of five different treatments (S1, S2, S3, D1, D2).During the five copulatory treatmentsO. olivaceamated up to 93 times. Specifically, pairs of the same size mated with bilateral sperm transfer while unilateral copulation was common in pairs of slightly differing size. Contrary to predictions, animals of small body size started mating in the female role.O. olivaceashowed an active alternation of sexual roles and thus did not specialize in one particular sexual role.This study produced evidence of a relationship between mating mode and body size inO. olivaceaand highlighted the need for new studies on this simultaneously hermaphroditic species.

The relative importance of body size, phylogeny, locomotion, and diet in the evolution of forelimb dexterity in fissiped carnivores (Carnivora)

Using a new rating scale of forelimb dexterity that separates the contribution of proximal components (shoulder, upper forelimb, and lower forelimb) from distal components (forepaw), we examined the relationship between functional demands and phylogeny and forelimb dexterity in 45 species of fissiped carnivores (Carnivora). Specifically, we examined the effects of body size, phylogenetic relatedness, diet (vertebrate predation), and locomotion (arboreality) on the differential evolution of forelimb dexterity. Regression analyses indicate that, although body size does appear to be positively correlated with the dexterity of the proximal components, the inclusion of phylogenetic information results in a nonsignificant relationship. Phylogenetic relatedness was found to account for a significant amount of interspecific variation in proximal, distal, and total (proximal + distal) dexterity. When phylogenetic effects were incorporated, arboreality was not significantly correlated with any of the dexterity scor...

Female sand gobies prefer good fathers over dominant males

Female preference for males successful in male–male competition is generally assumed to result in mating with high quality males. Here I report results from an experiment disentangling the effects of intra– and intersexual selection in the sand goby, Pomatoschistus minutus , a marine fish that exhibits paternal care. I show that large males are successful in male–male competition, but contrary to what one would expect, dominants are not preferred by females and are not better at taking care of the eggs. Female preference, however, correlated with the subsequent hatching success of the eggs. Thus, female choice selects for good parenting. Hence, direct benefits in the form of superior paternal care can explain female choice in this species, supporting a good parent process of sexual selection. However, choosing on the outcome of male–male competition does not enable females to mate with the ‘best’ males.

Vertebral number in adders,Vipera berus: direct and indirect effects on growth

Given the importance of body size, and thereby growth rate, for many reproductive parameters in snakes, morphological traits conferring an advantage in terms of growth may be important targets of selection. Studies have demonstrated effects of vertebral number of growth rate in garter snakes. In this study effects of total number of body vertebrae and of number of abnormal body vertebrae (obtained by counting number of ventral scutes and number of abnormal scutes) on growth rate in free-ranging male and female adders,Vipera berus(L.), are examined by calculating directional performance gradients (estimating linear effects) and stabilizing performance gradients (estimating curvilinear effects). After controlling for body size (SVL) female adders demonstrated a significant positive directional gradient for vertebral number, and a significant interaction between body size and vertebral number, showing that females with more vertebrae have higher size-specific growth rates, and that this effect is strongest among small, fast growing individuals. Females also showed a weak stabilizing effect of abnormal vertebrae. Males, on the other hand, showed a positive directional gradient for number of abnormal vertebrae, whereas no effect of vertebral number was observed. Indirect effects of the same variables were evaluated by use of path analysis. Generally, indirect effects were weak and did not substantially increase the amount of explained variance in growth rate. Field data showed that the correlation between vertebral number and growth rate in females was stronger in years with higher overall growth rate. To evaluate whether vertebral number and food availability show an interactive effect I used captive born juvenile adders in an experiment with two different food levels. The experiment confirmed the field data. No relationship between vertebral number and growth was observed in the low food level group, whereas in the high food level group a significant positive correlation was demonstrated. Finally, the heritability of vertebral number was examined using a mother-offspring regression and a full-sib analysis. The estimated heritabilities were 0·30 and 0·39, respectively. From these results it is concluded that both vertebral number and abnormal vertebral number may significantly affect growth in adders, but that this effect may differ between sexes and among years.

Vertebral number in adders, Vipera berus: direct and indirect effects on growth

Abstract Given the importance of body size, and thereby growth rate, for many reproductive parameters in snakes, morphological traits conferring an advantage in terms of growth may be important targets of selection. Studies have demonstrated effects of vertebral number of growth rate in garter snakes. In this study effects of total number of body vertebrae and of number of abnormal body vertebrae (obtained by counting number of ventral scutes and number of abnormal scutes) on growth rate in free-ranging male and female adders, Vipera berus (L.), are examined by calculating directional performance gradients (estimating linear effects) and stabilizing performance gradients (estimating curvilinear effects). After controlling for body size (SVL) female adders demonstrated a significant positive directional gradient for vertebral number, and a significant interaction between body size and vertebral number, showing that females with more vertebrae have higher size-specific growth rates, and that this effect is strongest among small, fast growing individuals. Females also showed a weak stabilizing effect of abnormal vertebrae. Males, on the other hand, showed a positive directional gradient for number of abnormal vertebrae, whereas no effect of vertebral number was observed. Indirect effects of the same variables were evaluated by use of path analysis. Generally, indirect effects were weak and did not substantially increase the amount of explained variance in growth rate. Field data showed that the correlation between vertebral number and growth rate in females was stronger in years with higher overall growth rate. To evaluate whether vertebral number and food availability show an interactive effect I used captive born juvenile adders in an experiment with two different food levels. The experiment confirmed the field data. No relationship between vertebral number and growth was observed in the low food level group, whereas in the high food level group a significant positive correlation was demonstrated. Finally, the heritability of vertebral number was examined using a mother-offspring regression and a full-sib analysis. The estimated heritabilities were 0·30 and 0·39, respectively. From these results it is concluded that both vertebral number and abnormal vertebral number may significantly affect growth in adders, but that this effect may differ between sexes and among years.

The scaling of the size and stiffness of primary flight feathers

Birds encompass a large range of body sizes, yet the importance of body size on feather morphology and mechanical properties has not been characterized. In this study, I examined the scaling relationships of primary flight feathers within a phylogenetically diverse sample of avian species varying in body size by nearly three orders of magnitude. I measured the scaling relationships between body mass and feather linear dimensions as well as feather flexural stiffness. The resnlts of an independent contrasts analysis to test the effects of phylogenetic history on the characters measured had no effect on the scaling relationships observed. There was slight, but not significant, positive allometry in the scaling of shaft diameter with respect to feather length across a range of body masses. The scaling of feather length and diameter against body mass was not significantly different from isometry. Flexural stiffness, however, exhibited strong negative allometry. Therefore, larger birds have relatively more flexible feathers than smaller birds. The more flexible primary feathers of large birds may reduce stresses on the wing skeleton during take‐off and landing and also make these feathers less susceptible to mechanical failure. Conversely, the greater flexibility of these feathers may also reduce their capacity to generate aerodynamic lift.

Variation in use of green flushes following burns among African ungulate species: the importance of body size

SummaryIn order to determine if use of green flushes following fire varied among ungulate species, six common species of the Serengeti Ecosystem were censused on burned and in adjacent unburned sites. Thomson's gazelle (Gazella thomsonii Günther), impala (Aepyceros melampus (Lichtenstein)), Grant's gazelle (Gazella granti Brooke) and wildebeest (Connochaetes taurinus Thomas) were more abundant in green flush sites compared to unburned controls. No significant difference was found in topi (Damaliscus korrigum (Matschie)) and zebra (Equus burchelli Gray) between burned and unburned sites. Preference (differential use) for burned sites was negatively related to average ungulate body size across species. It is hypothesized that larger species (e.g. topi and zebra) try to maximize their energy and nutrient intake by feeding on both burned sites where there is a low quantity of high‐quality forage, and unburned sites where forage quantity is high but forage quality is low. Smaller species (e.g. gazelles and impala) feed exclusively in burned sites (where there is higher quality forage) to fulfil the high metabolic requirements associated with small body‐sized animals.RésuméPour déterminer si l'utilization de la verdure rase qui repousse après les feux varie selon les espèces d'ongulés, on a relevé six espèces communes de l'écosystème du Serengeti dans les endroits brûlés et dans les zones adjacentes. La gazelle de Thomson (Gazella thomsoni Günther), l'impala (Aepyceros melampus (Lichtenstein)), la gazelle de Grant (Gazell granti Brooke) et le gnou (Connochaetes taurinus Thomas) étaient plus abondants aux endroits de repousse qu'à ceux qui n'avaient pas brûlé. On ne trouvait pas de différence significative pour le topi (Damaliscus korrigum (Matschie)), ni pour le zèbre (Equus burchelli Gray), entre les endroits brûlés ou non. La préférence (la différence d'utilisation) pour les sites brûlés était inversément liés à la taille corporelle moyenne de l'espèce. On émet l'hypothèse que les plus grandes espèces (par ex. le topi et le zèbre) essaient de maximaliser leur prise d'énergie et de nutriments en se nourrissant aux endroits brûlés, où il y une quantité réduite de fourrage de haute qualité, et aux endroits non brûlés, où la quantité de fourrage est plus grande mais de moinsbonne qualité. Les plus petites espèces (par ex. les gazelles et l'impala) se nourrissent exclusivement sur les sites brûlés (où le fourrage est de meilleure qualité) pour satisfaire les exigences métaboliques élevées associées aux animaux de petite taille.

Organ and cell allometry in Hawaiian Drosophila: how to make a big fly.

The importance of body size in predicting many aspects of an animal's biology has become well established in recent years. However, little is known about how body size evolves at the cellular level. Some published data suggest that it is cell number and not cell size that accompanies changes in organ and body size across taxa. We examined organ and cell allometry in the wing, eye and basitarsus of adult Hawaiian Drosophila, ranging in body length from 0.2 mm to 0.8 mm. Linear measurements of all three structures exhibit a positive allometry with body length. Exponents of the allometric equation were 0.96, 0.55 and 1.50 for wing, eye and basitarsus, respectively. Surface markers were used to quantify cell size of each organ. The allometric exponents for cell size as a function of organ size were 0.53, 0.68 and 0.33 for wing, eye and basitarsus, respectively. In contrast to reports in the literature on other systems, our results for Hawaiian Drosophila indicate that cell size may contribute between one third and two thirds to evolutionary changes in organ and body size.

Warm-Up Rates and Body Temperatures in Bees: The Importance of Body Size, Thermal Regime and Phylogeny

ABSTRACT We assess the importance of body mass and the minimum ambient temperature at which foraging occurs in determining the warm-up rates and thoracic temperatures in flight at an air temperature of 22 °C of 55 species of bee (Hymenoptera: Apoidea) from six families adapted to a variety of thermal environments. To control for the effects of taxonomic differences in the relationships between these variables, we use multiple regression incorporated in the phylogenetic regression method developed by Grafen (1989). The prediction made by May (1976) that for very small heterotherms warmup rate will correlate positively with body mass is confirmed when the effects of phylogeny and the thermal environment to which the bee is adapted have been controlled for. The relationship between warm-up rate and body mass within the Apoidea is thus not an extension to lower body masses of the relationship found for heterothermic vertebrates. Having controlled for the effects of body mass in our analyses, we demonstrate that bees able to fly at lower ambient temperatures have higher thoracic temperatures and warm-up rates than bees adapted to warmer environments. There is some suggestion that kleptoparasitic bees, being freed from the need to forage in order to provision cells, have lower warm-up rates than provisioning species. The significance of these relationships in the ecology of bees is discussed in relation to studies of body temperatures and warm-up rates in bees and other insects.

Importance of Body-Size for Cadmium Accumulation By Forest Litter Arthropods

To determine the role of soil arthropods in the cycling of cadmium, the influence of body weight on cadmium accumulation was studied using nine species of arthropods sampled for four seasons from a metal contaminated forest soil. The relationship between weight and body burden was described by a power function, the parameters of which were estimated by functional regression methods according to Ricker. A clear relationship between weight and cadmium burden has been found in 26 out of 39 samples. The slopes of the regression lines varied between 0.57 and 5.97. Variation within a species occurred throughout the year, but the differences were significant in a few cases only. Age, growth and activity appear to be the main factors determining seasonal variation in slope coefficient. Slopes calculated for single life stages appeared to be steeper than slopes calculated for the whole population. This phenomenon may be attributed to the ability of species to loose cadmium between the life stages. The analysis shows that the relationship between body-size and cadmium level cannot be interpreted as a simple accumulation phenomenon, since there are opposite effects of age, weight and season. These factors must be taken into account when calculating the annual flow of cadmium through a soil arthropod community.

Predator Avoidance by Cricotopus: Cyclomorphosis and the Importance of Being Big and Hairy

In laboratory experiments, we tested in importance of body size and long abdominal hairs in determining susceptibility of two aquatic midges, Cricotopus sylvestris and C. bicinctus, (Diptera: Chironomidae), to predation by Hydra and the damselfly Ischnura verticalis. The chironomids co—occur on dense macrophytes in the Yahara River, Dane County, Wisconsin, USA, and differ structurally in that C. sylvestris has long hairs and C. bicinctus has short hairs. We also monitored Hydra density and C. sylvestris hair length for 2 yr. Large C. bicinctus were less susceptible to predation by both Hydra and small Ischnura, but prey size had no effect on susceptibility to large Ischnura. Long hairs offered no protection from Ischnura, but did protect C. sylvestris from Hydra, relative to short—haired C. bicinctus or C. sylvestris with experimentally shortened hairs. Hair length is negatively allometric relative to body size in ontogeny; hair absolute length remains constant as the larvae increase in size. Hair length was also cyclomorphic; successive generations of C. sylvestris showed a seasonal pattern of hair length. Hydra density also varied seasonally. When Hydra density was high, hairs on C. sylvestris were longest; when Hydra density was low, hairs were shortest. The observed cyclomorphic and allometric patterns of hair length were both beneficial characteristics in the presence of Hydra, and the benefits were additive.

Body Composition and Anthropometric Correlates of Isokinetic Leg Extension Strength of Young Adult Males

Abstract The present study examined the relative importance of body size and composition as determinants of individual differences in isokinetic leg extensor strength in young adult males performing at slow, moderate, and fast speeds. The subjects were 31 males between the ages of 19 and 29 years, who were not participating in weight training programs. Low to moderate correlations were found between isokinetic strength and body size or body composition measurements. At each speed, similar proportions of the variance in peak torque values were accounted for by lean body weight (23-30%) and thigh volume (20-37%). However, body weight, often suggested as the best reference standard for equalizing strength scores, also accounted for only 13-24% of the variance in peak torque values at each of the speeds tested. The moderate correlations reported in this study do not support the use of body size and composition measurements as a means of adjusting strength values.

Importance of Body Size to the Recruitment Process in Largemouth Bass Populations

Abstract Results of pond studies indicated that the number of young-of-the-year (age-0) largemouth bass Micropterus salmoides that grow to 15 cm or longer depends on the presence of suitable prey such as bluegills Lepomis macrochirus and gizzard shad Dorosoma cepedianum. Prey availability, which influences growth and length distributions of young largemouth bass, may be a major determinant of recruitment to adult stock. On the premise that large age-0 fish have lower mortality rates than small members of the same cohort, we applied a model of size structure to the problem of forecasting eventual recruitment from cohorts of young largemouth bass. The model confirmed that length distributions of age-0 fish can have important influences on subsequent recruitment; good first-summer growth may mitigate, in part, such detrimental events as water-level fluctuations. The model may be a more refined predictor of recruitment than density of age-0 fish. Received March 20, 1984 Accepted February 23, 1985

Relative Allocation of Energy to Growth and Development of Homeothermy in the Eastern Wood Rat (Neotoma floridana) and Hispid Cotton Rat (Sigmodon hispidus)

Allocation of energy to growth and heat production was measured by constructing energy budgets of male and female Sigmodon hispidus (x Adult body mass = ABM = 113 g ♂ 105 g ♀) and Neotoma floridana (ABM = 270 g ♂ 184 g ♀) from birth to maturity. Energy ingested and oxygen consumption per gram of body mass of post—weaning animals were not significantly different between sexes within species, but did differ significantly between the species. In all animals the relation between post—weaning ingestion (per gram body mass) and body mass follows a negative power function. Post—weaning metabolic rate as a function of body mass follows a different negative power function. The pattern of change in metabolic rate prior to weaning is different in the two species. In Sigmodon, metabolic rate increases linearly from birth to about 20 g (or about 20% of ABM). The rate at 20 g is about 5 cm3°g—1°h—1. In Neotoma metabolic rate remains constant at 1.7 cm3°g—1°h—1 from birth to about 26 g (or about 11% of ABM), then increases linearly to about 2.2 cm3°g—1°h—1 at 65 g (29% ABM). In both species, the peak in metabolic rate and nutritional weaning coincide with development of homeothermy, (measured as ability to maintain adult—level body temperatures at an ambient temperature of 20°C). Young Sigmodon become efficient homeotherms by 10—12 d of age whereas Neotoma require 19—22 d. Growth of each sex and species can be described by Gompertz equations. Growth rate constants, K, and asymptotic mass, A, differ between sexes within species and between species. Gompertz equations do not accurately describe growth of pre—weaning Neotoma which was linear from birth to 21—22 d of age. No such period of linearity is seen in Sigmodon growth. Analysis of litter—size dependence of growth of pre—weaning animals indicates that the early linear growth of Neotoma may not be a result of limited milk production, as it seems to be in Sigmodon. The relative total investment of energy in growth for a litter of Sigmodon (° = 4.8 young) between birth and 12 d of age was 416.7 kJ compared to 770.7 kJ for the first 24 d of life of the average Neotoma (x = 2.6 young). Over these same times, a litter of Sigmodon expended 1350.6 kJ on respiration compared with 1296.2 kJ in Neotoma. Thus the total metabolizable energy requirements of 1767.7 kJ and 2066.2 kJ are similar in the two species; however, a litter of Sigmodon requires about twice as much metabolizable energy per day each day they remain with their mother. Neotoma daily requirements are lower but protracted in time. During the pre—weaning period, Neotoma are investing proportionally more of their metabolizable energy in growth, whereas Sigmodon spend more on respiration, perhaps because they depend on their own heat production for thermoregulation while Neotoma rely on their mother's heat. These results are compared with data from the literature on growth and development of homothermy in a variety of small rodents. The hypothesis is advanced that, in general, larger species defer onset of active thermoregulation to larger body masses when lower metabolic rates occur, perhaps as an adaptation to permit more efficient early growth. Small species sacrifice growth efficiency in favor of rapid attainment of early independence and they pay the high energy costs of that speed. The different developmental physiologies of Neotoma and Sigmodon are explainable by inference from current ideas on the evolution of life histories, but much of the difference is also predictable simply from their difference in body size. This may indicate the importance of body size in the complex of life history traits.

Respiratory metabolism of temperature acclimated Fundulus heteroclitus (L.): Zones of compensation and dependence

The oxygen consumption of temperature acclimated mummichogs, Fundulus heteroclitus (L.) weighing ≈0.1–10.0 g, was measured at 5, 13, 21, and 29 C. Between 13 and 21°C and 21 and 29°C, the values of Q 10 were 1.55 and 1.04, respectively, indicating relative thermal independence of respiratory metabolic rate over this 16°C range ( Q 10 = 1.27). This range encompasses the normal late spring, summer, and early fall range of habitat temperature in Maine estuaries, so that mummichogs are able to grow and reproduce relatively independent of environmental temperature. Between 5 and 13°C, respiratory metabolism is very temperature sensitive ( Q 10 = 4.42) indicating a substantial reduction of metabolic processes at low temperatures. This enables mummichogs to conserve any metabolic reserves during the coldest months. The regression of log weight-specific oxygen consumption on log body weight was determined at each experimental temperature. All had significantly negative slopes indicating the importance of body size in mummichog respiration.