Developmental and acclimatory contributions to water loss in a desert rodent: investigating the time course of adaptive change.

Understanding the evolution of physiological traits requires considering three nonexclusive mechanisms that underlie phenotypes and cause their change over different time scales: acclimation, developmental plasticity, and natural selection for genetically fixed traits. Physiological adjustments to changes in the desiccating potential of the environment were investigated with one subspecies of common desert rodent, Dipodomys merriami merriami (Merriam's kangaroo rat). We raised young whose parents originated from environments that differ in both temperature and humidity. These young were raised under either desiccating or water-abundant conditions, and their water loss was measured at a series of temperatures to determine the effect developmental conditions have on resistance to desiccation. We then determined the contribution of acclimation to desiccation resistance by keeping the differentially raised young in conditions opposite to those during their development and again measuring water loss. We found that developmental plasticity and acclimation can completely account for the existing intraspecific variability in desiccation resistance under certain conditions. In fact, developmental and acclimatory changes can equal genetically based differences of the populations. This phenotypic plasticity can operate relatively quickly and therefore may attenuate the actions of natural selection. Understanding the extent and nature of such flexibility is critical to our understanding intraspecific variability and the consequences of changing climate.

Unappreciated tolerance to high ambient temperatures in a widely distributed desert rodent, Dipodomys merriami.

A long-held assertion has been that nocturnality is an escape mechanism for many nocturnal desert rodents because of limited tolerances to heat. To test this claim, we used a treadmill to examine the tolerances to high ambient temperatures (T(a)'s) of one subspecies of desert rodent, Merriam's kangaroo rat, Dipodomys merriami merriami, from contrasting environments. We simultaneously measured body temperature (T(b)), evaporative water loss, and metabolic rates at an ecologically relevant speed (0.6 km h(-1)) at different ambient temperatures (Ta=25 degrees -42.5 degrees C). We hypothesized that kangaroo rats from a more xeric site would have greater abilities to remain active and maintain stable T(b) than those from a more mesic site, but mesic- and xeric-site animals had comparable tolerances and were active until Tb=42 degrees C. At Ta=42.5 degrees C, however, T(b) of mesic-site animals increased more quickly than in xeric-site animals. Although most animals could not run more than 18 min at Ta=42.5 degrees C, most could run at Ta=40 degrees C for at least 30 min. Benefits of nocturnality for this species may reside more in purposes of water conservation and avoidance of predation and less on the direct regulation of T(b), as T(b) is more labile than commonly thought.

Prevalence of cutaneous evaporation in Merriam's kangaroo rat and its adaptive variation at the subspecific level.

Previous estimates suggested that ventilatory evaporation constitutes the major source of water loss in kangaroo rats (Dipodomys spp.). We quantified rates of water loss in Merriam's kangaroo rat (Dipodomys merriami) and demonstrate the degree to which acclimation to a particular thermal and hydric environment plays a role in the intraspecific variation in water loss evident in this species. We draw the following conclusions: (1) that water loss varies intraspecifically in Merriam's kangaroo rat, in association with habitats of contrasting aridity and temperature; (2) that animals from more xeric locations have lower water loss rates than those from more mesic sites; (3) that most water loss is cutaneous, with ventilatory evaporative water loss contributing, at most, only 44% to total evaporative water loss; and (4) that intraspecific differences in rates of water loss are not acclimatory, but fixed. After acclimating under the same conditions, xeric-site animals still show a 33% lower rate of evaporative water loss than mesic-site animals.

Do metabolic responses to solar radiation scale directly with intensity of irradiance?

Endotherms exposed to air temperatures below thermal neutrality reduce their metabolic heat production when exposed to sunlight. The physiological effects of this additional source of heat gain from the environment usually are assumed to be proportional to the intensity of irradiance if other factors are held constant. We test this assumption by measuring changes in metabolic heat production produced by exposing a small mammal, the Siberian hamster (Phodopus sungorus) to four intensities of simulated solar radiation (0 W m-2, 317 W m-2, 634 W m-2 and 950 W m-2). In the absence of solar radiation, metabolic heat production is inversely correlated with air temperature over the measured range of 3-27 degrees C. The respiratory quotient varies significantly with ambient temperature, indicating that the catabolic substrate and the thermal equivalent of oxygen consumed or carbon dioxide produced also vary with temperature. The depression of metabolic heat production resulting from exposure to simulated solar radiation is not simply a multiple of the intensity of irradiance. Rather, metabolic responses to higher levels of irradiance are blunted by 14-29% compared with those expected on the basis of the response to less intense irradiance. Because changes in irradiance levels do not have simple linear effects upon the animal's metabolic heat production, even in a simplified situation, significant errors may accumulate in biophysical analyses in which an animal's responses to a restricted set of radiative conditions are measured and the results are extrapolated to a wider range observed in nature.

Mitochondrial transcription initiation: promoter structures and RNA polymerases.

A diversity of promoter structures. It is evident that tremendous diversity exists between the modes of mitochondrial transcription initiation in the different eukaryotic kingdoms, at least in terms of promoter structures. Within vertebrates, a single promoter for each strand exists, which may be unidirectional or bidirectional. In fungi and plants, multiple promoters are found, and in each case, both the extent and the primary sequences of promoters are distinct. Promoter multiplicity in fungi, plants and trypanosomes reflects the larger genome size and scattering of genes relative to animals. However, the dual roles of certain promoters in transcription and replication, at least in yeast, raises the interesting question of how the relative amounts of RNA versus DNA synthesis are regulated, possibly via cis-elements downstream from the promoters. Mitochondrial RNA polymerases. With respect to mitochondrial RNA polymerases, characterization of human, mouse, Xenopus and yeast enzymes suggests a marked degree of conservation in their behavior and protein composition. In general, these systems consist of a relatively non-selective core enzyme, which itself is unable to recognize promoters, and at least one dissociable specificity factor, which confers selectivity to the core subunit. In most of these systems, components of the RNA polymerase have been shown to induce a conformational change in their respective promoters and have also been assigned the role of a primase in the replication of mtDNA. While studies of the yeast RNA polymerase have suggested it has both eubacterial (mtTFB) and bacteriophage (RPO41) origins, it is not yet clear whether these characteristics will be conserved in the mitochondrial RNA polymerases of all eukaryotes. mtTFA-mtTFB; conserved but dissimilar functions. With respect to transcription factors, mtTFA has been found in both vertebrates and yeast, and may be a ubiquitous protein in mitochondria. However, the divergence in non-HMG portions of the proteins, combined with differences in promoter structure, has apparently relegated mtTFA to alternative, or at least non-identical, physiological roles in vertebrates and fungi. The relative ease with which mtTFA can be purified (Fisher et al. 1991) suggests that, where present, it should be facile to detect. mtTFB may represent a eubacterial sigma factor adapted for interaction with the mitochondrial RNA polymerase.(ABSTRACT TRUNCATED AT 400 WORDS)

Four themes in fertility research.

This study is concerned with four main themes that have been combined to describe U.S. fertility trends since 1920. These themes are identified as a time adjustment mechanism, income, the wife's labor force activities, and some intergenerational factor. "In order to obtain an unambiguous picture each theme will be represented by only one variable. After selecting the variables and estimating a model, an examination of the changing role these variables have had in explaining changes in fertility (the total fertility rate) throughout much of this century will be made. Finally, the implications these themes have for future fertility will be examined." The authors conclude that "in interpreting historical U.S. fertility rates, the results seem to indicate that much of the early decline in U.S. fertility was due to falling infant mortality. After the Second World War, fertility rose sharply as the age-structure variable declined and income rose. Eventually both women's labor force participation rates and the age-structure variable rose and, consequently, fertility fell." They also suggest that the age structure variable may cause a temporary upward swing in fertility in the near future.

Maternal affectionate behavior and infant-mother attachment patterns.

Four categories of affectionate behavior were identified in the 23 mothers of Ainsworth's longitudinal sample whose infants were observed in the strange situation at age 12 months. Mothers of babies classified as showing pattern A (anxiously attached and avoidant) in terms of their strange situation behavior were found to emphasize kissing proportionally more than the other mothers, and hugging/cuddling proportionally less. This is congruent with the previous finding that such mothers are averse to close bodily contact. Since mothers of pattern-A babies had also been found to be more rejecting, the findings suggest that rejection does not necessarily imply an absence of affectionate behavior but a different mode of expression.

Effects of maternal interference on the attachment and exploratory behavior of one-year-olds.

Effects of maternal interference on social behavior toward mother and exploratory play were examined in a laboratory experimental paradigm. Subjects were 40 1-year-olds and their mothers. Mothers of the 20 interference-group infants were instructed periodically to physically interfere with their child's independent object play during the first half of the observation session. A postinterference free-play period immediately followed. The 20 control-group infants were permitted by mother to play freely throughout the session. Groups were matched for exposure to play materials. Despite its aversiveness, interference had no subsequent effect on infant social initiatives to mother, responsiveness to mother's social bids, or exploratory play.

Infant approach behavior as related to attachment.

The chief purpose of this study was to identify characteristics that distinguish approach as an attachment behavior from approach serving other behavioral systems. Locomotor approaches of 16 male and 10 female white middle-class infants to an attachment figure (the mother) and to a nonattachment figure (the visitor-observer) were examined under naturalistic conditions at home. Observations were made during 4-hour home visits at 3-week intervals; those from 26 to 54 weeks were used. When approach was examined in a free-choice situation and without regard to behavioral context, infants approached the mother proportionally (though slightly) more often than the visitor (p less than .05). In 2 behavioral contexts, however, spontaneous infant approaches were sharply differential to the mother; approaches accompanied by crying and approaches terminating in a pickup appeal were directed almost exclusively to the mother (p less than .0001 for both). Touching upon completion of the approach was not differential, except in the context of a pickup appeal. Object-oriented approaches were more often directed to the visitor than to the mother (p less than .002) and may be either exploratory or affiliative. It is suggested that neither approach nor touching can be assumed to serve the attachment system without consideration of context- both environmental and behavioral.