Mean Incubation Temperature Research Articles

Discovery of the northernmost loggerhead sea turtle ( Caretta caretta) nest

A loggerhead turtle nest was accidentally discovered during the raking of a beach at St Tropez, France, on 18 July 2006. This is the northernmost loggerhead nest so far in the Mediterranean and in the world. The clutch was deposited close to the sea in a sand-soil-pebbles mixture and became partially inundated in September. In total there were 141 eggs, none of which hatched. Only 24% of the eggs contained embryos, while 107 eggs contained yolk but no visible embryo. The failure to hatch was probably the result of the non-suitable nest environment, the low mean incubation temperatures and the inundation. Further observations of beaches are recommended to establish other nesting occurrences of loggerhead turtles in this region.

Nesting lizards (Bassiana duperreyi) compensate partly, but not completely, for climate change

Species in which ambient temperatures directly determine offspring sex may be at particular risk as global climates change. Whether or not climate change affects sex ratio depends upon the effectiveness of buffering mechanisms that link ambient regimes to actual nest temperatures. For example, females may simply lay nests earlier in the season, or in more shaded areas, such that incubation thermal regimes are unchanged despite massive ambient fluctuation. Based on eight years of monitoring nests over a 10-year period in the field at an alpine site in southeastern Australia, we show that, even though lizards (Bassiana duperreyi, Scincidae) have adjusted both nest depth and seasonal timing of oviposition in response to rising ambient temperatures, they have been unable to compensate entirely for climate change. That inability stems from the fact that the seasonal progression of soil temperatures, and thus, the degree to which thermal regimes at the time of laying predict subsequent conditions during incubation, also has shifted with climate change. As a result, mean incubation temperatures in natural nests now have crossed the thermal threshold at which incubation temperature directly affects offspring sex in this population.

Maternal nest-site choice in a lizard with temperature-dependent sex determination

In oviparous species, mothers can optimize the incubation environments of their offspring by selecting nest sites with conditions conducive to embryonic development. Hence, maternal nest-site selection can enhance egg survival and offspring fitness. However, the challenge to a nesting female is complex because of trade-offs among relevant parameters (e.g. hotter nests may be drier) and seasonal shifts in ambient conditions. Moreover, the influence of nest conditions on offspring sex ratios adds another level of complexity for species with environmental sex determination. To elucidate these issues, we need field data on nesting behaviour that incorporate the multiple influences that operate under natural conditions. We radiotracked gravid jacky dragons, Amphibolurus muricatus, in open forest habitats of southeastern Australia to document the lizards' criteria for nest-site choice. Females selected sites with lower than average canopy cover, resulting in relatively warm nests throughout the prolonged nesting season. Although soil moisture levels decreased over this period and were strongly correlated with mean temperatures, female lizards were able to locate nesting sites that provided consistently moist (and increasingly warm) incubation conditions as the season progressed. Such sites apparently became more difficult to find later in the season, resulting in a higher incidence of test holes prior to nest excavation. Seasonal shifts in diel maximum temperatures also pushed the daily time of digging activity into the evening. Because jacky dragons have temperature-dependent sex determination, the seasonal shift in mean incubation temperatures of natural nests would generate seasonal shifts in offspring sex ratios in a direction likely to enhance maternal fitness.

Incubation temperatures and sex ratios in Australian brush‐turkey (Alectura lathami) mounds

Abstract Megapodes are exceptional among birds because they use external heat sources for incubating their eggs. In Australian brush‐turkeys (Alectura lathami), this source is the heat produced by microbial decomposition in mounds of leaf litter. A recent laboratory study showed that artificial incubation of eggs at different temperatures affects the sex ratio of brush‐turkey hatchlings. Here, this phenomenon is confirmed for eggs incubated in natural incubation mounds. Eggs from which females hatched were found at significantly higher incubation temperatures in the mounds (mean 33.7°C) than those from which males hatched (mean 32.9°C). Also, sex ratios of chicks from individual mounds were significantly correlated with mean incubation temperatures in those mounds. Furthermore, incubation temperatures differed significantly between incubation mounds during the same month of the year. Within some mounds, incubation temperatures differed up to 9°C between eggs, in others this difference was only up to 1°C. These latter results show that males differ in their ability to maintain stable incubation temperatures. While the effect of incubation temperatures on sex ratio is a novel discovery for any bird, it was previously suggested that incubation temperatures in megapodes also affect embryo mortality and chick survival. These combined effects of incubation temperature, and the limited ability of males to provide optimal temperatures, seem to provide answers to the question why so few birds have adopted this unusual mode of reproduction – a question asked by many behavioural ecologists and evolutionary biologists.

MOUND AND MATE CHOICE IN A POLYANDROUS MEGAPODE: FEMALES LAY MORE AND LARGER EGGS IN NESTING MOUNDS WITH THE BEST INCUBATION TEMPERATURES

Abstract Males of the polyandrous Australian Brush-turkey (Alectura lathami) build mounds of leaf litter, in which incubation heat is produced by microbial decomposition. Females lay eggs singly, at intervals of several days, over many months. For each egg, they select an incubation mound in which to lay, and they then typically copulate with the male mound-owner; mound choice is, thus, usually equal to mate choice. Freed from incubation and maternal care, these females can invest considerable time and energy in assessing and choosing their potential mates. Low or high incubation temperatures inside the mounds have negative effects on embryos and chicks, and temperatures vary considerably both between and within mounds. Here, I show that mounds with mean incubation temperatures ranging from approximately 32°C to 35°C received more eggs than cooler or warmer mounds. Similarly, when the size of each egg was compared with its temperature when found in the mound, larger eggs were deposited mainly in mound mat...

Mound and Mate Choice in a Polyandrous Megapode: Females Lay More and Larger Eggs in Nesting Mounds With the Best Incubation Temperatures

AbstractMales of the polyandrous Australian Brush-turkey (Alectura lathami) build mounds of leaf litter, in which incubation heat is produced by microbial decomposition. Females lay eggs singly, at intervals of several days, over many months. For each egg, they select an incubation mound in which to lay, and they then typically copulate with the male mound-owner; mound choice is, thus, usually equal to mate choice. Freed from incubation and maternal care, these females can invest considerable time and energy in assessing and choosing their potential mates. Low or high incubation temperatures inside the mounds have negative effects on embryos and chicks, and temperatures vary considerably both between and within mounds. Here, I show that mounds with mean incubation temperatures ranging from approximately 32°C to 35°C received more eggs than cooler or warmer mounds. Similarly, when the size of each egg was compared with its temperature when found in the mound, larger eggs were deposited mainly in mound material at temperatures between approximately 32°C and 35°C, whereas smaller eggs were laid at temperatures above and below this range. Egg size was included as a factor describing female mate choice, because egg size in relation to body size is exceptionally large, which indicates considerable female investment. Overall, these results suggest that the temperature in the males’ incubation mounds considerably affects female mate choice.Selección del Montículo y de la Pareja en un Megápodo Poliándrico: Las Hembras Ponen Más y Mayores Huevos en los Montículos de Nidificación con las Mejores Temperaturas de Incubación

Effects of nest temperature and moisture on phenotypic traits of hatchling snakes (Tropidonophis mairii, Colubridae) from tropical Australia

Previous research on developmentally plastic responses by reptile embryos has paid relatively little attention to tropical species, or to possible interactions between the effects of thermal and hydric regimes. In the present study, eggs of keelback snakes (Tropidonophis mairii), from a tropical area with strong temporal and spatial variation in soil temperatures and moisture levels, were incubated. The phenotypic traits of hatchling snakes (body size, shape, muscular strength) were affected by moisture content of the incubation medium (vermiculite plus 100% vs. 50% water by mass), by mean incubation temperatures (25.7 vs. 27.9 °C) and by diel thermal variation (diel range 6.0 vs. 8.4 °C). Interactions between these factors were negligible. Cooler, more thermostable, moister conditions resulted in larger offspring, a trait under strong selection in this population. Thermal and hydric conditions covary in potential nest-sites (e.g. deeper nests are more thermostable as well as moister). This covariation may influence the evolution of reaction norms for embryogenesis. For example, if moister nests enhance offspring fitness and are cooler, then selection will favour the ability to develop in cool as well as moist conditions. Thus, the evolution of optimal incubation conditions with respect to one variable (e.g. temperature) may be driven by patterns of association with another variable (e.g. soil moisture) among natural nest-sites. Perhaps for this reason, the thermal optimum for incubation is surprisingly low in this tropical species.

Effect of temperature on hatching and nest site selection in the Green lynx spider, Peucetia viridans (Araneae:Oxyopidae)

1. Thirty-seven Peucetia viridans egg sacs were incubated at 15, 17.5, 20, 25, 30, 32.5 or 35 °C. Hatching time was inversely proportional to temperature, and no hatching occurred at the extremes (15, 17.5 and 35 °C). 2. Mean incubation temperature of egg sacs in natural field habitats was 20.0±0.6 °C and mean time until hatching was 30.8±0.8 days. 3. Physical models of egg sacs placed in the field indicated natural incubation sites could vary by as much as 10 °C during the daytime and as little as 1 °C at night. As expected, incubation sites were warmest near the ground and the coolest at the top of vegetation. 4. Regardless of egg sac model placement, predicted hatching time only varied by 2–3 days throughout the incubation period. 5. Natural egg sac placement sites of P. viridans occurred near the top of vegetation indicating females were not selecting the warmest available microhabitats for egg incubation.

The effects of environmental factors on the embryonic survival of the Patagonian squid Loligo gahi

Seawater temperature and salinity are environmental variables that impose physiological limits for the embryonic development of marine invertebrates. For cephalopod species, these limits have rarely been established. This work presents experimental results on the embryonic survival of the Patagonian squid Loligo gahi, which is the last decades' most important loliginid species in terms of volume of commercial catches worldwide, as a function of seawater temperature and salinity. Reference magnitudes of surface seawater temperature and salinity within the area of distribution of the species were explored by analysis of satellite databases and published information. Embryos were incubated under eight constant regimes of temperature within 4–22 °C and four constant salinity regimes within 20–34.33‰ (12 °C). Also, to determine the effects of sudden temperature changes on embryonic survival, embryos were incubated at four variable regimes of temperature, with thermal shifts (6-day long 2-°C magnitude alterations of the incubation temperature) applied both at early and late stages of embryonic development. Embryonic survival was zero in incubations at constant temperature regimes ≤5° and at 22 °C, low at 6 °C, and high within 8–20 °C. A function was fitted by nonlinear regression to relate embryonic survival and mean incubation temperature. Thermal shifts applied in incubations at 20–22–20 °C variable regime of temperature provoked low embryonic survival compared to that observed in incubation at 20 °C constant regime. Embryonic survival was zero in incubations conducted at 20.0‰ and 34.3‰ salinity, and high at 26.4‰ and 32.8‰ salinities.

Incubation regimes of cold-climate reptiles: the thermal consequences of nest-site choice, viviparity and maternal basking

Cold-climate reptiles show three kinds of adaptation to provide warmer incubation regimes for their developing embryos: maternal selection of hot nest sites; prolonged uterine retention of eggs; and increased maternal basking during pregnancy. These traits may evolve sequentially as an oviparous lineage invades colder climates. To compare the thermal consequences of these adaptations, I measured microhabitat temperatures of potential nest sites and actual nests of oviparous scincid lizards (Bassiana duperreyi), and body temperatures of pregnant and non-pregnant viviparous scincid lizards (Eulamprus heatwolei). These comparisons were made at a site where both species occur, but close to the upper elevational limit for oviparous reptiles in south-eastern Australia. Viviparity and maternal basking effort had less effect on mean incubation temperature than did maternal nest-site selection. Eggs retained in utero experienced bimodal rather than unimodal diel thermal distributions, but similar mean incubation temperatures. Often the published literature emphasizes the ability of heliothermic (basking) reptiles to maintain high body temperatures despite unfavourable ambient weather conditions; this putative ability is central to many hypotheses on selective forces for the evolution of viviparity. In cold climates, however, opportunities for maternal thermoregulation to elevate mean body temperatures (and thus, incubation temperatures) above ambient levels may be severely limited. Hence, at least over the broad elevational range in which oviparous and viviparous species live in sympatry, maternal selection of ‘hot’ nests may be as effective as is viviparity in providing favourable incubation regimes. © 2004 The Linnean Society of London, Biological Journal of the Linnean Society, 2004, 83, 145–155.

Seasonal shifts in nest temperature can modify the phenotypes of hatchling lizards, regardless of overall mean incubation temperature

Summary The thermal regimes experienced by a reptilian egg can influence phenotypic traits (size, sex, shape, locomotor performance, etc.) of the hatchling that emerges from that egg. Natural nests of the oviparous scincid lizard Bassiana duperreyi in the Brindabella Range of south‐eastern Australia show strong seasonal shifts in temperature: first and last weeks of incubation often differ by >5 °C. Eggs of B. duperreyi were incubated under thermal regimes with identical overall mean values for average temperature and diel range, but differing in the sequence of temperatures. Some eggs were kept at 18 ± 5 °C throughout incubation; others went gradually from cool (16 ± 5 °C) to warm (20 ± 5 °C); and others from warm (20 ± 5 °C) to cool (16 ± 5 °C). These treatments significantly modified not only incubation periods (stable mean temperatures delayed hatching), but also hatchling traits: progressively decreasing temperatures yielded hatchlings with a higher incidence of deformities, smaller body size, relatively longer tails, and reduced locomotor performance than siblings from increasing temperatures. Seasonal shifts in incubation temperatures are widespread, and may generate important variation in hatchling phenotypes. Sensitivity to such shifts may influence phenomena such as nest‐site selection, the seasonal timing of nesting, and the evolution of viviparity.

DOES VIVIPARITY EVOLVE IN COLD CLIMATE REPTILES BECAUSE PREGNANT FEMALES MAINTAIN STABLE (NOT HIGH) BODY TEMPERATURES?

Viviparity (live bearing) has evolved from egg laying (oviparity) in many lineages of lizards and snakes, apparently in response to occupancy of cold climates. Explanations for this pattern have focused on the idea that behaviorally thermoregulating (sun-basking) pregnant female reptiles can maintain higher incubation temperatures for their embryos than would be available in nests under the soil surface. This is certainly true at very high elevations, where only viviparous species occur. However, comparisons of nest and lizard temperatures at sites close to the upper elevational limit for oviparous reptiles (presumably, the selective environment where the transition from oviparity to viviparity actually occurs) suggest that reproductive mode has less effect on mean incubation temperatures than on the diel distribution of those temperatures. Nests of the oviparous scincid lizard Bassiana duperreyi showed smooth diel cycles of heating and cooling. In contrast, body temperatures of the viviparous scincid Eulamprus heatwolei rose abruptly in the morning, were high and stable during daylight hours, and fell abruptly at night. Laboratory incubation experiments mimicking these patterns showed that developmental rates of eggs and phenotypic traits of hatchling B. duperreyi were sensitive to this type of thermal variance as well as to mean temperature. Hence, diel distributions as well as mean incubation temperatures may have played an important role in the selective forces for viviparity. More generally, variances as well as mean values of abiotic factors may constitute significant selective forces on life-history evolution.

Temperature during Incubation as One Factor Affecting the Distribution of Snake River Fall Chinook Salmon Spawning Areas

AbstractSpawning areas of ocean‐type fall Chinook salmon Oncorhynchus tshawytscha in the Pacific Northwest are largely restricted to relatively warm coastal rivers. Fall Chinook salmon spawners in the Snake River basin clearly favor the main‐stem Snake River and the relatively warm lower reaches of its tributaries. In this note, we describe how mean temperature during incubation affects fry emergence date and influences the distribution of fall Chinook salmon spawning areas in the Snake River basin. Using laboratory results and field data, we estimated fry emergence dates for three brood years of fish in seven areas that are consistently used by spawners and in five areas that are rarely used by spawners. Mean incubation temperature was significantly (P < 0.0001) higher in consistently used areas (5.6 ± 0.1°C) than in rarely used areas (4.5 ± 0.1°C). The mean estimated emergence date for the consistently used areas was 12 May (± 20 d), compared with 26 June (± 20 d) for rarely used areas. A logistic discriminant rule fitted from the mean incubation temperatures of the 12 areas discriminated between consistently and rarely used areas with 97% accuracy. We conclude that temperature is one factor that influences the distribution of fall Chinook salmon spawning areas in the Snake River basin.

When to be born? Prolonged pregnancy or incubation enhances locomotor performance in neonatal lizards (Scincidae).

The degree of offspring development at hatching (or birth) varies among species within most major vertebrate lineages; altricial vs. precocial birds offer the clearest example of a trade-off between early hatching and the degree of locomotor development of the hatchling. No such diversity has been reported for reptiles, but we suggest that natural selection may fine-tune the time of hatching (in oviparous species) or birth (in viviparous species) to optimize offspring phenotypes and hence, maximize fitness. This hypothesis predicts enhanced neonatal performance after more prolonged incubation or gestation, within as well as among populations. Both published and original data on Australian scincid lizards support this prediction. In a field study, viviparous alpine skinks (Niveoscincus microlepidotus) that gave birth later in the season had faster-running offspring, that had a higher probability of surviving through the first year of life. The enhanced performance and survival were not secondary results of larger offspring size. After controlling for effects of mean incubation temperature, prolonged development also correlated with enhanced locomotor performance in hatchlings from eggs of an oviparous skink (Bassiana duperreyi) incubated at warm temperatures (> 20 degrees C) but not at cooler temperatures (< 20 degrees C). We suggest that embryonic reptiles control their date of hatching or birth and thus, their stage of development at this critical life-history transition.

Sunny side up: lethally high, not low, nest temperatures may prevent oviparous reptiles from reproducing at high elevations

Oviparous (egg-laying) lizards and snakes generally inhabit warmer climates than do related viviparous (live-bearing) taxa. This pattern is widely attributed to the failure of oviparous reproduction in cold climates, but the thermal regimes of potential nest-sites above and below the elevational cut-off for oviparous reproduction have never been quantified. We studied oviparous (Bassiana duperreyi) and viviparous (Eulamprus heatwolei) scincid lizards at such a site in the Brindabella Range of south-eastern Australia. Miniature data-loggers monitored temperatures of nest-sites and lizards in midsummer, partway through the incubation period of eggs in natural nests. Our results contradict the simplistic notion that mean nest temperatures determine this elevational limit for oviparity. Instead, potential nest-sites with average temperatures suitable for embryogenesis in Bassiana are available well above the threshold elevation. However, thermal minima decrease consistently with elevation and thus the maximum temperature needed for any given mean incubation temperature increases rapidly with elevation. Potential nest-sites above the elevational threshold can only attain mean temperatures high enough to sustain embryogenesis by having lethally high thermal maxima. Such nest-sites are available close to the soil surface, but cannot support development. In contrast, behavioural thermoregulation allows viviparous lizards to maintain high mean body temperatures concurrently with relatively low maximum temperatures, regardless of elevation. Paradoxically, oviparous reptiles may be restricted to low elevations not because nests that provide appropriate mean incubation temperatures are unavailable further up the mountain, but because eggs laid in such shallow nests would overheat. © 2003 The Linnean Society of London, Biological Journal of the Linnean Society, 2003, 78, 325–334.

Energetic Consequences of Clutch Temperature and Clutch Size for a Uniparental Intermittent Incubator: The Starling

AbstractIn uniparental intermittent incubators, incubating parents must simultaneously regulate both the temperature of the clutch and their own energy level. To examine energetic consequences of providing different thermal environments for clutches of different sizes, a dynamic model was constructed in which energy level of an incubating European Starling (Sturnus vulgaris) and temperature of its clutch were simultaneously described. Adult energy balance after a day of incubation decreased as mean clutch temperature increased, such that the debt accrued while maintaining a clutch within the optimal developmental temperature range (36–39°C) was predicted to be prohibitively high, despite the prediction that the mean basal metabolic rate (BMR) required to maintain this temperature was &lt;2 BMR. Thus, this model can explain our observation that starlings maintained their eggs at 32–33°C, well below the developmental optimum. Consistent with empirical studies, our model predicted that the metabolic demand of incubation increases with clutch size. That increase was predicted to affect adult energy balance and hence cost of incubation when starlings maintained their clutches at optimal temperatures, but not at the lower temperatures actually observed. Hence, clutch-size-dependent variation in incubation demands may be unlikely to influence optimal number of eggs that a starling should lay. However, exact relationship between clutch size and adult energy debt depended on the nature of the relationship between clutch size and clutch thermal properties and on mean incubation temperature. Thus, consequences of clutch size for the cost of incubation are not clearly predictable, and caution may be required when using experimental clutch enlargements to manipulate reproductive costs.

Thermal conditions in nests of loggerhead turtles: further evidence suggesting female skewed sex ratios of hatchling production in the Mediterranean

Temperature was recorded in 23 nests of the loggerhead turtle ( Caretta caretta) and control sites of nest depth at Alagadi (35°33′N, 33°47′E), Northern Cyprus, eastern Mediterranean. Control site sand temperature was found to be highly correlated with mean daily air temperature and mean nest temperature. Mean temperature in nests ranged from 29.5°C to 33.2°C, with mean temperature in the middle third of incubation ranging from 29.3°C to 33.7°C. Hatching success was significantly correlated with incubation temperature, with nests experiencing very high temperatures exhibiting low hatching success. All nests demonstrated regular diel variation in temperature with mean daily fluctuations ranging from 0.3°C to 1.4°C. Increase in temperature above that of the prevailing sand temperature attributed to metabolic heating was clearly demonstrated in 14 of 15 clutches, with the mean level of metabolic heating of all nests being 0.4°C. However, the level of metabolic heating varied markedly throughout the incubation period with levels being significantly higher in the final third of incubation. Incubation duration was found to be significantly correlated to both the mean temperature of nests throughout the incubation period and during the middle third of incubation. The relationship between incubation duration and mean incubation temperature was used to estimate mean incubation temperatures at most major nesting sites throughout the Mediterranean from available data on incubation durations, showing that mean incubation temperature is likely to be above 29.0°C at most sites in most seasons.

Effects of temperature on developmental performance, survival and growth of sea lamprey embryos

This study assesses the influence of thermal regime on the development, survival rates and early growth of embryos of sea lamprey Petromyzon marinus incubated at five constant temperatures (7, 11, 15, 19 and 23° C). The time from fertilization to 50% hatching and from hatching to 50% burrowing were inversely related to incubation temperature. All the embryos incubated at 7° C died at very early stages, while those maintained at 11° C did not attain the burrowing stage. Survival from fertilization to hatching was 61, 89, 91 and 89% at 11, 15, 19 and 23° C, decreasing to 58, 70 and 70% from hatching to burrowing at 15, 19 and 23° C, respectively. Larvae reared during the first 3 months of exogenous feeding in a common environment at constant 21° C, revealed maximum survival for an incubation temperature of 15° C (43% of burrowed larvae) decreasing strongly at 19° C (16%) and 23° C (one suvivor among 240 larvae). Body length at the burrowing stage was maximum for embryos incubated at 19° C, but body mass increased in the interval 15–23° C. Mean incubation temperatures experienced by 117 broods during the embryonic development in the source river were estimated in 15·3±2·30° C and 16·7±1·76° C (mean±1 s.d.) for the periods fertilization‐to‐hatching and hatching‐to burrowing, respectively.

Disturbance of incubating African Black Oystercatchers: is heating of exposed eggs a problem?

Adams, N.J., Kerley, G.I.H. & Watson, J.J. 1999. Disturbance of incubating African Black Oystercatchers: is heating of exposed eggs a problem? Ostrich 70(3&4): 225–228. We measured temperatures of incubated and exposed eggs of African Black Oystercatchers Haematopus moquini nesting in coastal dunes, Algoa Bay, South Africa through three of the 10 hottest days of summer during the 1994/95 breeding season. Mean incubation temperature was 34.4 ± 0.7°C (range: 32.2–37.0; n=3). The average maximum temperature attained by exposed dummy eggs, averaged higher at 37.4 ± 2.7°C (n=5). The absolute maximum recorded for exposed eggs was 40.1°C. These temperatures are within lethal limits measured for other birds and suggests that heating of exposed eggs after disturbance of incubating was not a direct factor in explaining variation in breeding success of oystercatchers in the Alexandria Dunefield, Algoa Bay.

Hatchling sex in the marine turtle Caretta caretta is determined by proportion of development at a temperature, not daily duration of exposure

AbstractMean daily temperature in natural nests of freshwater turtles with temperature dependent sex determination is a poor predictor of hatchling sex ratios when nest temperatures fluctuate. To account for this, a mathematical model has been developed on the assumption that hatchling sex depends on the daily proportion of embryonic development that occurs above the threshold temperature for sex determination rather than the proportion of time spent above the threshold. The model predictions are borne out by experiments using the marine turtle Caretta caretta. Average developmental rates, both overall and during the period that sexual differentiation is sensitive to temperature, are unaffected by diel fluctuations about the mean incubation temperature. Sex ratios, on the other hand, were affected by diel fluctuations and ranged from ca. 100% males under regimes 26 ± 0°C and 26 ± 3°C to 100% females for regimes 26 ± 7°C and 26 ± 8°C. These and intermediate sex ratios were in close agreement with model predictions. Demonstration of an impact of temperature on sex, while holding overall developmental rate constant, gives support to hypotheses invoking a direct role for temperature rather than alternative hypotheses invoking overall developmental rate as a more proximal influence on sex. The model explains why mean temperature is a poor predictor of hatchling sex ratios. It urges caution in using “hours above the threshold” for predicting sex ratios, because 1 hr at 1°C above the threshold will not be equivalent to 1 hr at 4°C above the threshold. It provides a general framework for integrating experiments at constant temperatures with those in the field or laboratory using fluctuating regimes. It provides greater scope for exploring how reptiles with temperature dependent sex determination might respond to climatic change or other disturbances to the incubation environment. And it provides an explanation of why secondary factors such a hydric conditions and oxygen potentials might influence hatchling sex, even if temperature acts directly to influence sex ratios rather than through its influence on overall developmental rate. © Wiley‐Liss, Inc.