Physiological Zero Research Articles

INFLUENCE OF THE AMOUNT OF COMPRESSION ON VENOUS AND ARTERIAL BLOOD FLOW VELOCITY AND SKIN MICROCIRCULATION OF THE LOWER EXTREMITY.

The aim: To determine the effect of compression on the venous and arterial velocity of the main blood flow of the lower limb and the skin microcirculation of the rear part of the foot. Materials and methods: 20 healthy subjects participated in this study: 11 men 11 (55%) men and 9 (45%) girls. The ankle brachial index (ABI), femoropop¬liteal index, femoral arterial blood flow velocity (AFV), venous blood flow velocity (VFV), transcutaneous oxygen pressure (tcPO2) and carbon dioxide pressure (tcPCO2) were measured. After the general measurements were taken, the tire was inflated to 10 mm Hg under general basic conditions and kept for three minutes. The experiment ended when no decrease in tcPO2 was observed between two consecutive pressure levels. Results: The average tire pressure to reach physiological zero was 80 mm Hg. for all participants. At 10 mmHg significant changes in indicators were found by 19% (p=0.0001). tcPCO2 values increased significantly at 10 mmHg (p=0.0319) and continued to increase until the end of the study. It was established that its values increased by 14% compared to the input data (p=0.0005). Conclusions: At the maximum compression of 60 mmHg the arterial blood flow rate decreased by 5.5 times (p=0.0001). TcPCO2 increases significantly when compressed by 10 mm Hg also in parallel with the decrease in the regional perfusion index, which begins at an external compression of 40 mm Hg, which is evidence of the deterioration of the perfusion of the skin of the anterior part of the foot in healthy subjects.

Deep learning analysis of nest camera video recordings reveals temperature-sensitive incubation behavior in the purple martin (Progne subis)

Incubation is a key life history stage for birds, and incubation attentiveness can have significant fitness consequences for both parents and offspring. Incubation is, however, a challenging phenomenon to observe and studies generally either measure some proxy of the target behavior, or risk disturbing birds through direct observation. More recently, nest cameras have provided a non-intrusive way to directly observe incubation, but analysis of these data is time-consuming. Here, we use the results of the first deep learning model which automated analysis of nest camera video recordings from eight purple martin (Progne subis) nests over the entire incubation period at a 1-s resolution. We mathematically define the initiation of incubation, characterize the change in nest attentiveness during incubation, and analyze the factors determining nest attentiveness and on- and off-bout duration during the incubation process. A random forest regression model identified the most important predictors of nest attentiveness. Attentiveness decreased with increasing temperature, but the strength of this response increased above the presumed physiological zero egg temperature, below which egg development ceases. This implies that the purple martins are able to adjust their incubation behavior in a complex, multiple-state manner to an extrinsic stimulus. Our study highlights the value of high-resolution datasets created using artificial intelligence for the analysis of nest camera video recordings of animal behavior. The use of artificial intelligence for image classification tasks is becoming commonplace in society. This technology is beginning to be used to automate the analysis of video recordings of wildlife behavior. Here, we use the results of the first such classification from nest camera video recordings of the purple martin (Progne subis) to determine the factors affecting incubation attentiveness (the proportion of time that the adults spend in contact with eggs). Incubation attentiveness is important because it can affect hatch rate and have carry-over effects both for the condition of the incubating adults and the quality of the resulting offspring. Our analysis found that attentiveness was mainly affected by ambient temperature, with incubating adults reducing their efforts as ambient temperature reaches the minimum threshold for egg development.

Do extended incubation recesses carry fitness costs in two cavity-nesting birds?

Because extended incubation recesses, where incubating songbirds are away from nests for periods much longer than usual, occur infrequently, they have been treated as outliers in most previous studies and thus overlooked. However, egg temperatures can potentially fall below the physiological zero temperature during extended recesses, potentially affecting developing embryos. As such, evaluating extended recesses in an ecological context and identifying their possible fitness effects are important. With this aim, we used iButton data loggers to monitor the incubation behavior of female Blue Tits (Cyanistes caeruleus) and Great Tits (Parus major) during two breeding seasons in central Spain. We classified incubation recesses as extended if they were more than four times the mean recess duration for each species. Extended incubation recesses occurred more frequently in 2012 when females exhibited poorer body condition. Female Blue Tits had more extended incubation recesses than female Great Tits and, for both species, more extended recesses occurred at the beginning of the breeding season. Both nest attentiveness and average minimum nest temperature decreased when at least one extended recess occurred. Incubation periods averaged 4 d longer for nests where females had at least one extended recess, potentially increasing predation risk and resulting in lower-quality nestlings. Overall, our results suggest that extended recesses may be more common among songbirds than previously thought and that, due to their effects on egg temperatures and attentiveness, they could impose fitness costs.

Unusual incubation behavior and embryonic tolerance of hypothermia in the Sichuan Partridge (Arborophila rufipectus)

Incubation is an energetically demanding phase of avian reproduction. Single-sex incubators are often faced with a direct conflict between incubation and foraging. We report an unusual incubation behavior of the Sichuan Partridge (Arborophila rufipectus), with long recesses of females and embryonic hypothermia, at Laojunshan National Nature Reserve, southwest China. The Sichuan Partridge is a globally endangered species that inhabits subtropical evergreen and deciduous broadleaf forests at 1100–2250 m a.s.l. Female Sichuan Partridges incubated eggs alone, and typically took one long incubation recess of 4.5 ± 1.2 h (mean ± SD) day−1. They left the nests at 7:36 a.m. (±50 min) and returned at 12:06 p.m. (±70 min). Nest attentiveness was 81.2 ± 5.2% of the entire incubation period. Females prolonged the recess duration and decreased nest attentiveness significantly in response to high disturbance risk and bad weather. Although eggs experienced ~4.2 h below 26 °C (the physiological zero temperature) for each recess, the total hatching rate was 88.4%, suggesting that embryonic hypothermia had minimal negative effect on the hatching rate. For Sichuan Partridge, the ability to withstand embryonic hypothermia may be an adaptation to cold montane environments.

Phenotypic developmental plasticity induced by preincubation egg storage in chicken embryos (Gallus gallus domesticus).

The developing chicken blastoderm can be temporarily maintained in dormancy below physiological zero temperature. However, prolonged preincubation egg storage impairs normal morphological and physiological development of embryos in a potential example of fetal programming (in this case, “embryonic programming”). We investigated how preincubation egg storage conditions (temperature, duration, hypoxia, and hypercapnia) affects viability, body mass, and physiological variables and functions in day 15 chicken embryos. Embryo viability was impaired in eggs stored for 2 and 3 weeks, with the effects greater at 22°C compared to 15°C. However, embryo size was reduced in eggs stored at 15°C compared with 22°C. Phenotypic change resulting from embryonic programming was evident in the fact that preincubation storage at 15°C diminished hematocrit (Hct), red blood cell concentration ([RBC]), and hemoglobin concentration ([Hb]). Storage duration at 15°C more severely affected the time course (2, 6, and 24 h) responses of Hct, [RBC], and [Hb] to progressive hypoxia and hypercapnia induced by submersion compared with storage duration at 22°C. The time‐specific regulation of acid–base balance was changed progressively with storage duration at both 22 and 15°C preincubation storages. Consequently, preincubation egg storage at 22°C resulted in poor viability compared with eggs stored at 15°C, but size and physiological functions of embryos in eggs stored for 1–2 weeks were worse in eggs stored in the cooler than stored under room conditions. Avian eggs thus prove to be useful for examining developmental consequences to physiology of altered preincubation thermal environment in very early stages of development (embryonic programming).

Features of transition of perennial grasses with adaptive Triticum-type proteins into anabiosis

During 1–7-day stages of ontogenesis (for example, in winter rye), biochemical changes in the aerial part of perennial grasses with adaptive Triticum-type proteins in the process of entering into the state of anabiosis were studied. It was shown that, at this point, biomass of the aerial part of plant was much higher than of the root system, and it accumulated raffinose, sucrose, glucose, and half-cystine. Synthesis of raffinose and accumulation of half-cystine continued until the stabilization of negative temperatures; as for sucrose and glucose, it ceased upon reaching the average temperature of the physiological zero, then the content of half-cystine and raffinose increased sharply, and the content of sucrose and glucose decreased at the same time. In the aerial parts of plants, fructose and phenylalanine were found, and their content increased upon reaching freezing temperatures. It was concluded that the polymerization of water-soluble proteins occurred in perennial grasses in a state of anabiosis. These proteins, along with raffinose, serve as cryoprotectants; sucrose and glucose are substrates for respiration; fructose is a substrate for respiration and protective metabolite; and phenylalanine is a substrate for the future synthesis of antioxidants.

Partial incubation and its function in great tits (Parus major)—an experimental test

In many bird species, females incubate the eggs for short periods before clutch completion. The role of such partial incubation and the effective egg temperatures attained are controversial. Possible functions depend on whether temperatures reach the critical values required for embryogenesis and include the protection of the clutch from predators, brood parasites, and environmental factors; inhibition of bacterial growth on eggs; control of hatching asynchrony; or energy saving by parents. In this study, in a natural population of great tits, we first investigated the occurrence and extent of partial incubation, and second, we evaluated some potential functions by experimental simulation of the observed patterns of partial incubation. We found that egg temperatures during partial incubation were clearly raised above the physiological zero temperature for embryo development. Simulation of partial incubation did not affect any breeding parameters or nestling morphological traits compared with controls. It suggests that partial incubation with temperatures above the minimum temperature for embryo development does not serve to increase the variation in nestling morphology and thus competitiveness, in particular, does not increase hatching asynchrony within brood as one of its currently most discussed functions. Key words: egg temperatures, hatching asynchrony, maternal effects, nestling morphology, Parus major, phenotypic variation. [Behav Ecol]

Potential Influences of Climate and Nest Structure on Spotted Owl Reproductive Success: A Biophysical Approach

Many bird species do not make their own nests; therefore, selection of existing sites that provide adequate microclimates is critical. This is particularly true for owls in north temperate climates that often nest early in the year when inclement weather is common. Spotted owls use three main types of nest structures, each of which are structurally distinct and may provide varying levels of protection to the eggs or young. We tested the hypothesis that spotted owl nest configuration influences nest microclimate using both experimental and observational data. We used a wind tunnel to estimate the convective heat transfer coefficient (hc) of eggs in 25 potential nest configurations that mimicked 2 nest types (top-cavity and platform nests), at 3 different wind speeds. We then used the estimates of hc in a biophysical heat transfer model to estimate how long it would take unattended eggs to cool from incubation temperature (∼36°C) to physiological zero temperature (PZT; ∼26°C) under natural environmental conditions. Our results indicated that the structural configuration of nests influences the cooling time of the eggs inside those nests, and hence, influences the nest microclimate. Estimates of time to PZT ranged from 10.6 minutes to 33.3 minutes. Nest configurations that were most similar to platform nests always had the fastest egg cooling times, suggesting that platform nests were the least protective of those nests we tested. Our field data coupled with our experimental results suggested that nest choice is important for the reproductive success of owls during years of inclement weather or in regions characterized by inclement weather during the nesting season.

Reaction of triticeae cereals (spring and winter wheat and winter rye) to a change in ambient temperature in the fall-winter period of ontogeny

The state of overwintering cereals during their transition into anabiosis is studied. It is shown that by the time growth processes associated with a decrease of temperature below physiological zero ceases, free amino acids are utilized, the quantitative ratio of protein amino acids changes, and raffinose, sucrose, and phenylalanine accumulate in the aboveground part of the plants. It is concluded that the transition of cereals to a state of anabiosis occurs upon maximum binding of intracellular water by the protein-carbohydrateamino acid complex after stabilization of negative ambient temperatures. The results of this work and a number of previous publications permitted formulating an idea about the structure of the physiological and biochemical mechanism of adaptation to negative temperatures acquired by perennial cereals during evolution.

Effects of incubation delay on viability and microbial growth of Wood Duck (Aix sponsa) eggs

Egg viability in birds declines with increasing length of the delay in the start of incubation (i.e., incubation delay) and may be influenced by microbial infection and exposure of eggs to temperatures above physiological zero (>24°C). Onset of incubation during egg laying results in developmental asynchrony of embryos but may help to maintain viability of early-laid eggs. We manipulated incubation delay of Wood Duck (Aix sponsa) eggs and tested for effects of incubation delay and ambient temperature on egg viability and microbial infection. We also examined onset of incubation by egg-laying females. Hatching success declined slowly with increasing length of incubation delay and was very weakly affected by increasing exposure to temperatures >24°C. On average, females began incubation at night 7 days after nest initiation, but egg viability had declined only 8.6% (95% confidence interval: 4.3–15.2) by day 7. Microbial infection of eggshells was not related to length of incubation delay or mean daily temperature of the exposure period. Clutch size declined as the breeding season progressed, and nocturnal incubation began earlier in the laying sequence with declining clutch sizes. Laying females, regardless of clutch size, incubated for 3 nights before starting full incubation. We suggest that early onset of incubation may be more important in reducing incubation period and predation risk than in maintaining viability of early-laid eggs.

Microbial and environmental effects on avian egg viability: Do tropical mechanisms act in a temperate environment?

The viability of freshly laid avian eggs declines after several days of exposure to ambient temperatures above physiological zero, and declines occur faster in tropical than temperate ecosystems. Microbial infection during preincubation exposure has recently been shown as a second cause of egg viability decline in the tropics, but whether microbial processes influence the viability of wild bird eggs in temperate ecosystems is unknown. We determined the microbial load on eggshells, the incidence of microbial penetration of egg contents, and changes in the viability of wild bird eggs (Sialia mexicana, Tachycineta bicolor, Tachycineta thalassina) experimentally exposed to temperate-zone ambient conditions in situ in a mediterranean climate in northern California. Initial microbial loads on eggshells were generally low, although they were significantly higher on eggs laid in old boxes than in new boxes. Eggshell microbial loads did not increase with exposure to ambient conditions, were not reduced by twice-daily disinfection with alcohol, and were unaffected by parental incubation. The rate of microbial penetration into egg contents was low and unaffected by the duration of exposure. Nevertheless, egg viability declined very gradually and significantly with exposure duration, and the rate of decline differed among species. In contrast to studies performed in the tropics, we found little evidence that temperature or microbial mechanisms of egg viability decline were important at our temperate-zone site; neither temperatures above physiological zero nor alcohol disinfection was significantly related to hatching success. Delaying the onset of incubation until the penultimate or last egg of a clutch at our study site may maintain hatching synchrony without a large trade-off in egg viability. These results provide insight into the environmental mechanisms that may be responsible for large-scale latitudinal patterns in avian clutch size and hatching asynchrony.

Development of Incubation Temperature and Behavior in Thrushes Nesting at High Altitude

Abstract Onset of incubation was studied in three Hermit Thrushes (Catharus guttatus) and one American Robin (Turdus migratorius), all with four-egg clutches, at a high altitude site in the Sierra Nevada, California, USA. Behavior of laying females at the nest was measured from continuous recordings of internal egg temperatures of first-laid eggs. Full nocturnal nest attentiveness began immediately with the first egg. Daytime attentiveness increased steadily during laying as foraging time away from the nest decreased. On-off bouts by tending females in daytime increased in frequency and decreased in duration until the last egg was laid. Time on the nest could not be directly equated to occurrence of incubation because eggs were not uniformly warmed to exceed the temperature threshold required for embryonic development (physiological zero). Incubation began, both day and night, after laying of the second egg. It increased steadily thereafter with percentage of daytime devoted to incubation lagging well beh...

Egg Laying, Egg Temperature, Attentiveness, and Incubation in the Western Bluebird

We present the first detailed information on egg laying, egg temperature (Tegg), and development of attentiveness and incubation in Western Bluebirds (Sialia mexicana). We used miniature infrared CCD video cameras to record egg laying at three nest boxes and validate attentiveness measurements derived from Tegg in seven other nests. Females entered the nest box on egg-laying days between 0522 and 1037 hrs PST and laid an egg on average 11.4 6 4.3 min later (n 5 11). Warm weather early in egg laying often elevated Tegg above physiological zero (27u C), despite low parental attendance. Females often began roosting in the nest box 2-3 nights before clutch completion, and most birds began steady night-time incubation with the penultimate egg. Attentiveness, mean Tegg, and number of minutes Tegg exceeded 27u C reached their highest values 1 day or night after clutch completion. Western Bluebirds exhibit gradual onset to incubation that is attributable to vagaries of weather and varying patterns of parental attentiveness. Received 8 September 2008. Accepted 10 February 2009.

Causes of reduced clutch size in a tidal marsh endemic

We tested three hypotheses of clutch size variation in two subspecies of the swamp sparrow (Melospiza georgiana georgiana and M. g. nigrescens). Swamp sparrows follow the pattern of other estuarine endemics, where clutch size is smaller among tidal salt marsh populations (M. g. nigrescens) than their closest inland relatives (M. g. georgiana). Our results support predation risk and temperature, but not adult survival, as explanations of this pattern in swamp sparrows. Coastal nests were twice as likely to fail as inland nests, and parental activity around the nest site was positively related to clutch size at both sites. When brood size was controlled for, coastal adults visited nests less often and females vocalized less frequently during visits than inland birds, which may decrease nest detectability to predators. Coastal parents waited longer than inland birds to feed offspring in the presence of a model nest predator, but there was no difference in their response to models of predators of adults, as would be expected if coastal birds possessed increased longevity. Additionally, coastal females laid more eggs than inland females over a single season, following a within-season bet-hedging strategy rather than reducing within-season investment. Coastal territories experienced ambient air temperatures above the physiological zero of egg development more often, and higher temperatures during laying correlated with smaller clutches and increased egg inviability among coastal birds. Similar effects were not seen among inland nests, where laying temperatures were generally below physiological zero. Both subspecies showed an increase in hatching asynchrony and a decrease in apparent incubation length under high temperatures. Coastal individuals, however, showed less hatching asynchrony overall despite higher temperatures. Both air temperatures during laying and predation risk could potentially explain reduced clutch size in not only coastal plain swamp sparrows, but also other tidal marsh endemics.

Warm temperatures lead to early onset of incubation, shorter incubation periods and greater hatching asynchrony in tree swallows Tachycineta bicolor at the extremes of their range

The onset of incubation varies in birds, with many species beginning incubation prior to clutch completion. Here we examine whether early onset is more likely to occur during high temperatures, a critical prediction of the egg‐viability hypothesis, which suggest that birds begin incubation prior to clutch completion in order to maintain egg‐viability. We examined onset of incubation in tree swallows Tachycineta bicolor at two locations at the extremes of their breeding range, Alaska and Tennessee. A majority of individuals (68%) began incubation prior to clutch completion. While females in Tennessee were more likely to begin incubation early, there was no difference between sites when differences in temperatures inside nestboxes were controlled in analyses. Rather, early onset of incubation was predicted by the proportion of daily temperatures above physiological zero during laying, a critical prediction of the egg viability hypothesis. Both warm weather and early onset led to shorter incubation periods and increased levels of hatching asynchrony. We found no effect of timing of nesting, female body condition index or clutch size on the probability of beginning incubation prior to clutch completion. Our results are consistent with the egg viability hypothesis, not consistent with a threshold clutch size rule, and do not support the hurry‐up hypothesis, that individuals breeding later in the season would begin incubation early to reduce the time spent nesting. Overall, our results suggest that broad scale geographic differences in incubation behaviour may be explained by individual‐level responses to environmental conditions.

Evolution of passerine incubation behavior: influence of food, temperature, and nest predation.

Incubation behavior is one component of reproductive effort and thus influences the evolution of life-history strategies. We examined the relative importance of body mass, frequency of mate feeding, food, nest predation, and ambient temperature to explain interspecific variation in incubation behavior (nest attentiveness, on- and off-bout durations, and nest trips per hour) using comparative analyses for North American passerines in which only females incubate. Body mass and frequency of mate feeding explained little variation in incubation behavior. We were also unable to detect any influence of foot; diet and foraging strategy explained little interspecific variation in incubation behavior. However, the typical temperature encountered during reproduction explained significant variation in incubation behavior: Species breeding in colder environments take shorter bouts off the nest, which prevents eggs from cooling to temperatures below the physiological zero temperature. These species must compensate for shorter off-bouts by taking more of them (thus shorter on-bouts) to obtain needed energy for incubation. Nest predation also explains significant variation in incubation behavior among passerines: Species that endure high nest predation have evolved an incubation strategy (long on- and off-bouts) that minimizes activity that could attract predators. Nest substrate explained additional variation in incubation behavior (cavity-nesting birds have shorter on-bouts and make more frequent nest trips), presumably because nest predation and/or temperature varies among nest substrates. Thus, nest predation can influence reproductive effort in a way previously not demonstrated--by placing a constraint on parental activity at the nest. Incubating birds face an ecological cost associated with reproductive effort (predation of entire brood) that should be considered in future attempts to explain avian life-history evolution.

EVOLUTION OF PASSERINE INCUBATION BEHAVIOR: INFLUENCE OF FOOD, TEMPERATURE, AND NEST PREDATION

Incubation behavior is one component of reproductive effort and thus influences the evolution of life-history strategies. We examined the relative importance of body mass, frequency of mate feeding, food, nest predation, and ambient temperature to explain interspecific variation in incubation behavior (nest attentiveness, on- and off-bout durations, and nest trips per hour) using comparative analyses for North American passerines in which only females incubate. Body mass and frequency of mate feeding explained little variation in incubation behavior. We were also unable to detect any influence of food; diet and foraging strategy explained little interspecific variation in incubation behavior. However, the typical temperature encountered during reproduction explained significant variation in incubation behavior: Species breeding in colder environments take shorter bouts off the nest, which prevents eggs from cooling to temperatures below the physiological zero temperature. These species must compensate for shorter off-bouts by taking more of them (thus shorter on-bouts) to obtain needed energy for incubation. Nest predation also explains significant variation in incubation behavior among passerines: Species that endure high nest predation have evolved an incubation strategy (long on- and off-bouts) that minimizes activity that could attract predators. Nest substrate explained additional variation in incubation behavior (cavity-nesting birds have shorter on-bouts and make more frequent nest trips), presumably because nest predation and/or temperature varies among nest substrates. Thus, nest predation can influence reproductive effort in a way previously not demonstrated—by placing a constraint on parental activity at the nest. Incubating birds face an ecological cost associated with reproductive effort (predation of entire brood) that should be considered in future attempts to explain avian life-history evolution.Corresponding Editor: E. Ketterson

Egg viability as a constraint on hatching synchrony at high ambient temperatures

Summary 1. We tested experimentally the effects of exposure to high ambient temperatures, for periods of 1–9 days, on the viability of eggs of the green‐rumped parrotlet. 2. The hatchability of 534 newly laid parrotlet eggs declined after exposure of 3 or more days. 3. The probability of hatching was not significantly affected by the duration or proportion of time exposed to temperatures below physiological zero or above normal incubation range. 4. Incubation periods were negatively correlated with exposure time, suggesting some embryo development occurred prior to incubation. 5. Results presented here corroborate similar studies of temperate zone birds, and suggest that the decline in egg viability may be particularly extreme in hot climates, such as tropical lowlands. 6. We suggest that the relationship between ambient temperature and egg viability could contribute to both seasonal and latitudinal trends in clutch size, hatching success and hatching asynchrony.

Forearm muscle oxygenation decreases with low levels of voluntary contraction

The purpose of our investigation was to determine if the near infrared spectroscopy technique was sensitive to changes in tissue oxygenation at low levels of isometric contraction in the extensor carpi radialis brevis muscle. Nine subjects were seated with the right arm abducted to 45 degrees, elbow flexed to 85 degrees, forearm pronated 45 degrees, and wrist and forearm supported on an armrest throughout the protocol. Altered tissue oxygenation was measured noninvasively with near infrared spectroscopy. The near infrared spectroscopy probe was placed over the extensor carpi radialis brevis of the subject's right forearm and secured with an elastic wrap. After 1 minute of baseline measurements taken with the muscle relaxed, four different loads were applied just proximal to the metacarpophalangeal joint such that the subjects isometrically contracted the extensor carpi radialis brevis at 5, 10, 15, and 50% of the maximum voluntary contraction for 1 minute each. A 3-minute recovery period followed each level of contraction. At the end of the protocol, with the probe still in place, a value for ischemic tissue oxygenation was obtained for each subject. This value was considered the physiological zero and hence 0% tissue oxygenation. Mean tissue oxygenation (+/-SE) decreased from resting baseline (100% tissue oxygenation) to 89 +/- 4, 81 +/- 8, 78 +/- 8, and 47 +/- 8% at 5, 10, 15, and 50% of the maximum voluntary contraction, respectively. Tissue oxygenation levels at 10, 15, and 50% of the maximum voluntary contraction were significantly lower (p < 0.05) than the baseline value. Our results indicate that tissue oxygenation significantly decreases during brief, low levels of static muscle contraction and that near infrared spectroscopy is a sensitive technique for detecting deoxygenation noninvasively at low levels of forearm muscle contraction. Our findings have important implications in occupational medicine because oxygen depletion induced by low levels of muscle contraction may be directly linked to muscle fatigue.

Egg handling and storage

The temperature and relative humidity of storage, as well as the gaseous environment, interact with the fertile egg over time during storage in such a way as to affect the success of incubation either negatively or positively. This interaction occurs both above and below the “physiological zero”, at which embryonic metabolism is minimal. This interaction below physiological zero implies that certain physical aspects of the egg must be affected by the environmental conditions. As the eggshell is a relatively fixed component, changes in albumen, shell membranes, cuticle, yolk, or embryo proper must account for these time- and environment-related effects. It is concluded that the major contributor is the albumen, as it is obviously the most dynamic component below physiological zero and is strategically positioned.