The nitrogen content of a plant is only one of the many plant characteristics that are vitally important to herbivores. However, because of its central role in all metabolic processes as well as in cellular structure and genetic coding, nitrogen is a critical element in the growth of all organisms. Supplementary N often elicits enhanced health, growth, reproduction, and survival in many organisms. This suggests that N is a limiting factor. Since N makes up a large portion of the earth's atmosphere (about 78%), the problem is not an absolute but a relative shortage-that is, a scarcity of usable or metaboliza­ ble N during critical growth periods (159, 328). Plants encounter shortages of inorganic nitrogen (nitrate and/or ammonium ions); animals experience shortages of organic nitrogen (specific proteins and/or amino acids). This article reviews and examines the evidence (a) that N is scarce and perhaps a limiting nutrient for many herbivores, and (b) that in response to this selection pressure, many herbivores have evolved specific behavioral, morphological, physiological, and other adaptations to cope with and uti­ lize the ambient N levels of their normal haunts. McNeill & Southwood (201) and White (328) have also reviewed these general questions. There­ fore, this review explores additional evidence and further develops the fundamental arguments. The review is organized into three major divisions. The first focuses on important sources of variation in plant N (seasonal and ontogenetic trends, different tissues and species, etc) because such variation may be the basis

Herbivory in relation to plant nitrogen content

Animals from flies to humans are able to distinguish subtle gradations in temperature and show strong temperature preferences. Animals move to environments of optimal temperature and some manipulate the temperature of their surroundings, as humans do using clothing and shelter. Despite the ubiquitous influence of environmental temperature on animal behaviour, the neural circuits and strategies through which animals select a preferred temperature remain largely unknown. Here we identify a small set of warmth-activated anterior cell (AC) neurons located in the Drosophila brain, the function of which is critical for preferred temperature selection. AC neuron activation occurs just above the fly's preferred temperature and depends on dTrpA1, an ion channel that functions as a molecular sensor of warmth. Flies that selectively express dTrpA1 in the AC neurons select normal temperatures, whereas flies in which dTrpA1 function is reduced or eliminated choose warmer temperatures. This internal warmth-sensing pathway promotes avoidance of slightly elevated temperatures and acts together with a distinct pathway for cold avoidance to set the fly's preferred temperature. Thus, flies select a preferred temperature by using a thermal sensing pathway tuned to trigger avoidance of temperatures that deviate even slightly from the preferred temperature. This provides a potentially general strategy for robustly selecting a narrow temperature range optimal for survival.

/pdf/an-internal-thermal-sensor-controlling-temperature-3ryarpq1sf.pdf

An internal thermal sensor controlling temperature preference in Drosophila

Question: Associations of body size and of body temperature with fitness have complex relationships for ectotherms, but three general patterns are known. Bigger is better: Larger body size is frequently associated with greater fitness within populations. Hotter is smaller: Smaller adult body sizes typically result from development at higher temperatures. Hotter is better: Greater maximal performance at the optimal temperature is frequently associated with higher optimal temperatures. How do we – or even can we – reconcile these three apparently conflicting empirical patterns about temperature, size, and fitness of ectotherms? Methods: We summarize available evidence supporting or contradicting these three rules. We present a conceptual framework that describes how developmental and adult body temperatures affect causal connections among size, performance, and key components of fitness. Findings: There is strong empirical support for Bigger is better and Hotter is smaller (≥ 79% of studies/estimates), primarily for terrestrial insects, reptiles, and annual plants. Evidence regarding Hotter is better is still limited (and primarily from terrestrial insects), but most available information supports the rule. Analyses of counterexamples are particularly instructive. The rules operate at different levels. Bigger is better describes phenotypic variation within populations. Hotter is smaller describes phenotypic plasticity of a genotype. Hotter is better describes evolved variation in reaction norms among genotypes or between species. Conclusions: We unify these three rules into a path diagram that describes how temperature impacts critical rate processes throughout the life cycle. Adult body size and development time are key traits that are not only consequences of temperature-dependent processes, but also are causes of variation in fitness. An unresolved issue involves how to determine the appropriate fitness metric for a particular ecological context (population and environment). For example, the intrinsic rate of population increase (r) is strongly influenced by generation time (and development time), whereas net reproductive rate (R0) is strongly influenced by fecundity (and size). Because the relative strengths of different paths contributing to fitness change differ for these fitness metrics, the choice of metric can affect whether Hotter is better is ‘better’ than Bigger is better.

https://faculty.washington.edu/hueyrb/pdfs/KingsolverHuey08.pdf

Size, temperature, and fitness: Three rules

3 – Food Consumption and Utilization

Body temperature (T(b)) profoundly affects the fitness of ectotherms. Many ectotherms use behavior to control T(b) within narrow levels. These temperatures are assumed to be optimal and therefore to match body temperatures (Trmax) that maximize fitness (r). We develop an optimality model and find that optimal body temperature (T(o)) should not be centered at Trmax but shifted to a lower temperature. This finding seems paradoxical but results from two considerations relating to Jensen's inequality, which deals with how variance and skew influence integrals of nonlinear functions. First, ectotherms are not perfect thermoregulators and so experience a range of T(b). Second, temperature-fitness curves are asymmetric, such that a T(b) higher than Trmax depresses fitness more than will a T(b) displaced an equivalent amount below Trmax. Our model makes several predictions. The magnitude of the optimal shift (Trmax - To) should increase with the degree of asymmetry of temperature-fitness curves and with T(b) variance. Deviations should be relatively large for thermal specialists but insensitive to whether fitness increases with Trmax ("hotter is better"). Asymmetric (left-skewed) T(b) distributions reduce the magnitude of the optimal shift but do not eliminate it. Comparative data (insects, lizards) support key predictions. Thus, "suboptimal" is optimal.

https://faculty.washington.edu/hueyrb/pdfs/MartinHuey08.pdf

Why “Suboptimal” Is Optimal: Jensen’s Inequality and Ectotherm Thermal Preferences

Alternating temperatures resulted in higher values of the innate capacity for increase (rm) of Drosophila melanogaster Meigen than mean constant temperatures within the range of temperature favorable for growth and reproduction. This difference resulted from slightly faster development and earlier attainment of maximum fecundity at alternating temperatures.

Preliminary mathematical models relating rm to constant and alternating temperatures are derived. These models are:

rm = 0.587-0.0228(7.80e0.00012(30.3-t°C)2) for constant temperatures, and

rm = 0.587-0.0228(9.96e0.0137(25.3-t°C)2) for alternating temperatures. Average deviation between empirical and computed values is 2.3%. The efficacy of these models is restricted to temperatures favorable for reproduction.

Population Growth of Drosophila melanogaster (Diptera: Drosophilidae) at Constant and Alternating Temperatures

A laboratory study of energy flow in different-aged pea aphids was accom- plished by the balance sheet method. Growth, reproduction, molted exoskeletons, oxygen consumption, and honeydew production were monitored daily. Absolute amounts of energy expended on reproduction, growth, maintenance, and rejecta all varied with age of the aphid. Efficiencies of production and respiration also varied with the life history stage. Assimilation efficiency did not vary with age. Gross production efficiency (x = 49%) and assimilation efficiency (x = 83%) are higher in this insect than in most other insects reported. The rea- son for such high efficiency may be the unusually high amino acid content of pea phloem sap. Rates of exudation of amino acids and sugars from pea phloem through severed aphid stylets are greater than twice the total daily energy requirement of continuously feeding aphids. This would be true even for passively feeding aphids. Under the study conditions the daily ingestion of one average adult aphid is approximately 8% of the daily net primary production of a 0.15 g (dry wt) pea seedling.

Age‐Specific Energetics of the Pea Aphid, Acrythosiphon pisum

Pea phloem sap was collected from excised pea aphid stylets and analysed chemically. The only sugar found was sucrose, at an average concentration of 5.6±0.82% w/w. Total amino acid content was 4.5% w/w, 98.9% as free amino acid. Phloem sap of young pea plants apparently has lower sugar content and higher total amino acid content than woody perennials and most other herbs. These amounts may be related to the developmental stage of the pea plants and may change with age.

Average rate of exudation from cut stylets was 0.41 mm3/h. At this rate, total potential energy available to an adult aphid is 4.6 cal/day. Pressure within the plant necessary to force sap through the stylets at this rate was calculated to be 16 atm.

Quality and Quantity of Plant Sap Available to the Pea Aphid

Innate capacity for increase (rm) of the Mediterranean Hour moth, Anagasta kuehniella (Zeller), was higher at constant than at alternating temperatures because of shorter developmental times, earlier attainment of maximum fecundity, and greater survival of immature stages at constant temperatures.

Increased amplitude of alternating temperatures decreased survival of immature stages, rates of development, and fecundity, and thus reduced rate of population growth.

Preliminary mathematical models relating innate capacity for increase to temperature are developed. These are:

rm=0.1583—0.00128 [33.33 e0.003048 (35.5-t°C)2]

for constant temperatures and

rm=0.1583—0.00128 [36.76 e0.003002 (34.6-t°C)2]

for alternating temperatures. The average deviation of empirical from computed values is 3%. The efficacy of these models is restricted to temperatures favorable for reproduction.

Population Growth of Anagasta kuehniella (Lepidoptera: Pyralidae) at Constant and Alternating Temperatures

Viviparous, parthenogenetic populations of 1st instars and prereproductive or teneral pea aphids, Acyrthosiphon pisum (Harris), were exposed to 39°C or −12°C for varying times. Intrinsic rates of increase for surviving populations (Gen 1) and their progeny (Gen 2) were calculated. Rate of population growth decreased with increased duration of exposure for all Gen 1 populations. This decrease, which was more pronounced in populations exposed in the teneral stage, was due to decreased fecundity and longevity and increased developmental time. These effects were carried over into Gen 2 populations, particularly after exposure in the teneral stage. It is concluded that temporary exposure of populations to extreme temperature may decrease rates of growth of surviving populations. This effect may extend over subsequent generations and the age-structure of the population at the time of exposure may affect its subsequent growth rate.

C. A. Barlow

Papers

Population Growth of Drosophila melanogaster (Diptera: Drosophilidae) at Constant and Alternating Temperatures

Age‐Specific Energetics of the Pea Aphid, Acrythosiphon pisum

Quality and Quantity of Plant Sap Available to the Pea Aphid

Population Growth of Anagasta kuehniella (Lepidoptera: Pyralidae) at Constant and Alternating Temperatures

Population-Growth of the Pea Aphid, Acyrthosiphon pisum (Homoptera: Aphididae) after Exposure to Extreme Temperatures