Is the Evolution of Viviparity Accompanied by a Relative Increase in Maternal Abdomen Size in Lizards?

Female reptiles with viviparous reproduction should leave space for their eggs that reach the maximum mass and volume in the oviducts. Is the evolution of viviparity accompanied by a relative increase in maternal abdomen size, thus allowing viviparous females to increase the amount of space for eggs? To answer this question, we compared morphology and reproductive output between oviparous and viviparous species using three pairs of lizards, which included two Eremias, two Eutropis and two Phrynocephalus species with different reproductive modes. The two lizards in each pair differed morphologically, but were similar in the patterns of sexual dimorphism in abdomen and head sizes and the rates at which reproductive output increased with maternal body and abdomen sizes. Postpartum females were heavier in viviparous species, suggesting that the strategy adopted by females to allocate energy towards competing demands differs between oviparous and viviparous species. Reproductive output was increased in one viviparous species, but decreased in the other two, as compared with congeneric oviparous species. The space requirement for eggs did not differ between oviparous and viviparous females in one species pair, but was greater in viviparous females in the other two pairs greater in relative clutch mass and relative litter mass. In the two Phrynocephalus species, viviparous females produced heavier clutches than did oviparous females not by increasing the relative size of the abdomen, but by being more full of eggs. In none of the three species pairs was the maternal abdomen size greater in the viviparous species after accounting for body size. Our data show that the evolution of viviparity is not accompanied by a relative increase in maternal abdomen size in lizards. Future work could usefully investigate other lineages of lizards to determine whether our results are generalisable to all lizards.     

Is increased maternal basking an adaptation or a pre-adaptation to viviparity in lizards?

Pregnant females modify their thermoregulatory behaviour in many species of viviparous (live-bearing) reptiles, typically maintaining higher and more stable body temperatures at this time. Such modifications often have been interpreted as adaptations to viviparity, functioning to accelerate embryonic development and/or modify phenotypic traits of hatchlings. An alternative possibility is that similar maternal thermophily may be widespread also in oviparous species and if so, would be a pre-adaptation (rather than an adaptation) to viviparity. Because eggs are retained in utero for a significant proportion of development even in oviparous reptiles, maternal thermophily might confer similar advantages in oviparous as in viviparous taxa. Experimental trials on montane oviparous scincid lizards (Bassiana duperreyi) support the pre-adaptation hypothesis. First, captive females (both reproductive and non-reproductive) selected higher temperatures than males. Second, experimentally imposing thermal regimes on pregnant females significantly affected their oviposition dates and the phenotypic traits (body shape, running speed) of their hatchlings. Thus, as for many other behavioural correlates of pregnancy in viviparous reptiles, maternal thermophily likely may have already been present in the ancestral oviparous taxa that gave rise to present-day viviparous forms.     

Seasonal shifts along the oviparity–viviparity continuum in a cold‐climate lizard population

Squamate embryos require weeks of high temperature to complete development, with the result that cool climatic areas are dominated by viviparous taxa (in which gravid females can sun‐bask to keep embryos warm) rather than oviparous taxa (which rely on warm soil to incubate their eggs). How, then, can some oviparous taxa reproduce successfully in cool climates – especially late in summer, when soil temperatures are falling? Near the northern limit of their distribution (in Sweden), sand lizards (Lacerta agilis) shift tactics seasonally, such that the eggs in late clutches complete development more quickly (when incubated at a standard temperature) than do those of early clutches. That acceleration is achieved by a reduction in egg size and by an increase in the duration of uterine retention of eggs (especially, after cool weather). Our results clarify the ability of oviparous reptiles to reproduce successfully in cool climates and suggest a novel advantage to reptilian viviparity in such conditions: by maintaining high body temperatures, viviparous females may escape the need to reduce offspring size in late‐season litters.     

Thermoregulation during gravidity in the children's python (Antaresia childreni): a test of the preadaptation hypothesis for maternal thermophily in snakes

Pregnant squamate reptiles (i.e. lizards and snakes) often maintain higher and more stable body temperatures than their nonpregnant conspecifics, and this maternal thermophily enhances developmental rate and can lead to increased offspring quality. However, it is unclear when this behaviour evolved relative to the evolution of viviparity. A preadaptation hypothesis suggests that maternal thermophily was a preadaptation to viviparity. Oviparous squamates are unique among oviparous reptiles for generally retaining their eggs until the embryos achieve one fourth of their development. As a result, maternal thermophily by gravid squamates may provide the same thermoregulatory benefits, at least during early development, that have been associated with viviparity. Thus, the evolution of viviparity in squamates may reflect an expanded duration of a pre-existing maternal thermoregulatory behaviour. Despite its evolutionary relevance, thermoregulation during gravidity in oviparous squamates has not yet been explored in depth. In the present study, we examined whether gravidity was associated with thermoregulatory changes in the oviparous children's python, Antaresia childreni . First, we discovered that, compared to most snakes, A. childreni is at an advanced stage of embryonic development at oviposition. Second, using surgically implanted temperature loggers, we detected a significant influence of reproductive status on thermoregulation. Reproductive females maintained higher and less variable body temperatures than nonreproductive females and this difference was most pronounced during the last 3 weeks of gravidity. Overall, these results highlight the continuum between oviparity and viviparity in squamate reptiles and emphasize the importance of thermal control of early embryonic development independent of reproductive mode.  © 2008 The Linnean Society of London, Biological Journal of the Linnean Society , 2008, 93 , 499–508.     

Life‐history strategies indicate live‐bearing in Nothosaurus (Sauropterygia)

In Sauropterygia, a diverse group of Mesozoic marine reptiles, fossil evidence of viviparity (live‐bearing) only exists for Pachypleurosauria and Plesiosauria, and was assumed to also be the case for nothosaurs. Previous studies have successfully applied an extant squamate model to sauropterygian life‐history traits. In extant squamates, oviparity and viviparity are associated with differences in life‐history trait combinations. We establish growth curves for Nothosaurus specimens based on their humeral histology. We then analyse life‐history traits derived from these curves and compare inferred traits to those of modern squamates and pachypleurosaurs to assess their reproduction mode. We show that birth to adult size ratios (i.e. birth size divided by the mother's size) provide good estimates of clutch sizes in extant squamates and in viviparous extinct marine reptiles, but these ratios cannot discriminate viviparous and oviparous squamates. Thus, large ratios do not indicate viviparity in fossil taxa to which the extant squamate model is applicable. Applying differences in birth size, age at maturation, and maximum longevity that are observed between extant viviparous and oviparous squamates to our Nothosaurus sample, we identified 7 out of 24 specimens as being potentially viviparous. Conversely, they suggested oviparity for many nothosaurs but also for many pachypleurosaur samples. Under the assumption that the entire clade Pachypleurosauria was viviparous, the majority of nothosaurs would also have been viviparous as they comprised trait combinations similar to those seen in pachypleurosaurs. Overall, this suggests that within nothosaurs and pachypleurosaurs both reproduction modes existed in different taxa.     

Experimental evidence of early costs of reproduction in conspecific viviparous and oviparous lizards

Reproduction entails costs, and disentangling the relative importance of each stage of the reproductive cycle may be important to assess the costs and benefits of different reproductive strategies. We studied the early costs of reproduction in oviparous and viviparous lizard females of the bimodal reproductive species Zootoca vivipara. Egg retention time in oviparous females is approximately one-third of the time in viviparous females. We compared the vitellogenesis and egg retention stages that are common to both reproductive modes. Precisely, we monitored the thermoregulatory behaviour, the weight gain and the immunocompetence of the females. Moreover, we injected an antigen in half of the females (immune challenge) to study the trade-offs between reproductive mode and immune performance and between different components of the immune system. Finally, we experimentally induced parturition in viviparous females at the time of egg laying in oviparous females. Oviparous and viviparous females did not show strong differences in response to the immune challenge. However, viviparous females spent more time thermoregulating while partially hidden and gained more weight than oviparous females. The greater weight gain indicates that the initial period of egg retention is less costly for viviparous than for oviparous females or that viviparous females are able to save and accumulate energy at this period. This energy may be used by viviparous females to cope with the subsequent costs of the last two-third of the gestation. Such an ability to compensate the higher costs of a longer egg retention period may account for the frequent evolution of viviparity in squamate reptiles.     

Allometry of reproduction in two species of gekkonid lizards (Gehyra): effects of body size miniaturization on clutch and egg sizes

In squamate reptiles there is an allometric pattern for small-bodied females to have smaller clutches and proportionally larger eggs than large-bodied females, and this pattern occurs both among and within species. The allometric patterns in two species of the gecko Gehyra were studied to see how evolutionary reductions in adult body size affect fecundity and offspring size among species, and how these changes affect allometric relationships within species. Gehyra dubia has two eggs per clutch (the typical clutch size for gekkonid lizards), whereas the smallerbodied G. variegata has a single egg per clutch. Within both species, egg size increased with female body size. The data are consistent with at least two mechanistic hypotheses: (1) that the width of the pelvis constrains egg size; and (2) in species with invariant clutch sizes, larger females can only allocate additional energy towards egg size and not number. More direct tests of these hypotheses are warranted. Miniaturization of body sizes in Gehyra is correlated with a clutch size reduction of 50% (from two to one), and a large (1.7-fold) compensatory increase in relative egg mass. However, the small-bodied G. variegata (one egg per clutch) had a lower relative clutch mass than did G. dubia. These findings have implications for understanding the influence of evolutionary reductions in body size on reproductive traits, and for allometric trends in squamate reptiles in general.     

Maternal investment in reproduction and its consequences in leatherback turtles

Maternal investment in reproduction by oviparous non-avian reptiles is usually limited to pre-ovipositional allocations to the number and size of eggs and clutches, thus making these species good subjects for testing hypotheses of reproductive optimality models. Because leatherback turtles (Dermochelys coriacea) stand out among oviparous amniotes by having the highest clutch frequency and producing the largest mass of eggs per reproductive season, we quantified maternal investment of 146 female leatherbacks over four nesting seasons (2001–2004) and found high inter- and intra-female variation in several reproductive characteristics. Estimated clutch frequency [coefficient of variation (CV) = 31%] and clutch size (CV = 26%) varied more among females than did egg mass (CV = 9%) and hatchling mass (CV = 7%). Moreover, clutch size had an approximately threefold higher effect on clutch mass than did egg mass. These results generally support predictions of reproductive optimality models in which species that lay several, large clutches per reproductive season should exhibit low variation in egg size and instead maximize egg number (clutch frequency and/or size). The number of hatchlings emerging per nest was positively correlated with clutch size, but fraction of eggs in a clutch yielding hatchlings (emergence success) was not correlated with clutch size and varied highly among females. In addition, seasonal fecundity and seasonal hatchling production increased with the frequency and the size of clutches (in order of effect size). Our results demonstrate that female leatherbacks exhibit high phenotypic variation in reproductive traits, possibly in response to environmental variability and/or resulting from genotypic variability within the population. Furthermore, high seasonal and lifetime fecundity of leatherbacks probably reflect compensation for high and unpredictable mortality during early life history stages in this species.     

Lerista bougainvillii,a case study for the evolution of viviparity in reptiles

Many factors, both environmental and biotic, have been suggested to facilitate or hinder the evolution of viviparity (live-bearing) in reptiles. Viviparity has evolved recently within the Australian scincid lizard Lerista bougainvillii and the species includes oviparous, viviparous, and reproductively intermediate (with prolonged egg retention) populations; thus, it offers an exceptional opportunity to evaluate the validity of these hypotheses. We carried out such tests by (i) comparing environmental conditions over the geographic ranges occupied by oviparous, viviparous, and intermediate populations (to identify possible selective forces for the evolution of viviparity), and (ii) comparing morphological, reproductive and ecological traits of L. bougainvillii with those of other sympatric scincid species (to identify traits that may have predisposed this taxon to the evolution of viviparity). The areas occupied by viviparous L. bougainvillii are significantly colder than those occupied by both their intermediate and oviparous conspecifics, in accord with the “cold-climate” hypothesis for reptilian viviparity. Rainfall is similar over the ranges of the three forms. Climatic unpredictability (as assessed by the magnitude of year-to-year thermal variation) is lower for viviparous animals, in contradiction to published speculations. Comparison with 31 sympatric scincid species showed that L. bougainvillii is not atypical for most of the traits we measured (e.g., body size, clutch size, thermal preferenda and tolerances). However, oviparous L. bougainvillii do display several traits that have been suggested to facilitate the evolution of viviparity. For example, pregnancy does not reduce locomotor ability of females; the lizards are semi-fossorial; even the oviparous females produce only a single clutch of eggs per year; and they ovulate relatively late in summer, so that the time available for incubation is limited.     

Viviparous mothers impose stronger glucocorticoid‐mediated maternal stress effects on their offspring than oviparous mothers

Maternal stress during gestation has the potential to affect offspring development via changes in maternal physiology, such as increases in circulating levels of glucocorticoid hormones that are typical after exposure to a stressor. While the effects of elevated maternal glucocorticoids on offspring phenotype (i.e., “glucocorticoid‐mediated maternal effects”) have been relatively well established in laboratory studies, it remains poorly understood how strong and consistent such effects are in natural populations. Using a meta‐analysis of studies of wild mammals, birds, and reptiles, we investigate the evidence for effects of elevated maternal glucocorticoids on offspring phenotype and investigate key moderators that might influence the strength and direction of these effects. In particular, we investigate the potential importance of reproductive mode (viviparity vs. oviparity). We show that glucocorticoid‐mediated maternal effects are stronger, and likely more deleterious, in mammals and viviparous squamate reptiles compared with birds, turtles, and oviparous squamates. No other moderators (timing and type of manipulation, age at offspring measurement, or type of trait measured) were significant predictors of the strength or direction of the phenotypic effects on offspring. These results provide evidence that the evolution of a prolonged physiological association between embryo and mother sets the stage for maladaptive, or adaptive, prenatal stress effects in vertebrates driven by glucocorticoid elevation.     

Evolution of viviparity: variation between two sceloporine lizards in the ability to extend egg retention

The evolutionary transition between oviparity and viviparity in squamate reptiles presumably occurs via a gradual increase in the duration of egg retention, the production of thinner eggshells, and increases in the vascularity of maternal and embryonic tissues. The 'ease' of this transition may differ among taxa. For example, in the genus Sceloporus , the scalaris species group contains both oviparous and viviparous species, and female Sceloporus scalaris can extend egg retention facultatively in response to the absence of a suitable site for oviposition without impairing embryonic development. In contrast, the undulatus species group contains only oviparous species, and, while female Sceloporus virgatus can extend egg retention, doing so retards embryonic development. I tested several hypotheses that would explain the greater ability of 5. scalaris than S. virgatus to extend egg retention. In this study, female S. scalaris retained eggs for 19 d without affecting the mortality of embryos, total developmental time, or dry mass of hatchlings. In contrast, when female S. virgatus retained eggs for 18 d, embryos had very high mortality and eggs took significantly longer to hatch than control (non-retained) eggs, although the dry mass of hatchlings was not affected. The ability of S. scalaris females to retain eggs with little negative effect on embryonic development was associated with relatively large chorioallantois, relatively thin eggshells, and relatively small clutch masses. These observations suggest that phylogenetic differences in the ability to extend egg retention may facilitate or constrain the evolution of viviparity in some lineages.     

Cytokines in the oviparity/viviparity transition: evidence of the interleukin-1 system in a species with reproductive bimodality, the lizard Lacerta vivipara

Placental viviparity is a reproductive strategy usually attributed to mammals. However, it is also present in other vertebrate species, e.g. in Squamate reptiles. Although the immunological mechanisms that allow the survival of the semi-allogenic embryo in maternal tissues are still largely unknown, cytokines seem to play an important role in mammalian reproduction. Previous studies in our laboratory showed that interleukin-1 (IL-1), a cytokine associated with implantation in mice, is also expressed at the materno-fetal interface of placental viviparous Squamates. In this study, we used the model of Lacerta vivipara, which exhibits reproductive bimodality, that is, the coexistence of oviparous and viviparous populations. By means of immunohistochemistry and anti-human antibodies, we showed that uterine tissues of L. vivipara (seven oviparous and six viviparous animals) expressed the two IL-1 isoforms, IL-1alpha and IL-1beta, and the type I IL-1 receptor (IL-1R tI) both at the pre-ovulatory stage and during gestation, with no significant difference between oviparous and viviparous females. In L. vivipara, as in most oviparous Squamates, an important phase of embryonic development takes place in the mother's oviduct, before egg-laying. Moreover, although thinner than in oviparous females, an eggshell membrane persists throughout gestation in viviparous females also, which develop a very simple type of placenta. The data suggest that immunological mechanisms that allow the survival of the semi-allogenic embryo in maternal tissues are independent of the timing or intimacy of contact between maternal and fetal tissues.     

Extraembryonic membrane development in a reproductively bimodal lizard, Lacerta (Zootoca) vivipara

Reproductive mode has been remarkably labile among squamate reptiles and the evolutionary transition from oviparity to viviparity commonly has been accompanied by a shift in the pattern of embryonic nutrition. Structural specializations for placental transfer of nutrients during intrauterine gestation are highly diverse and many features of the extraembryonic membranes of viviparous species differ markedly from those of oviparous species. However, because of a high degree of evolutionary divergence between the species used for comparisons it is likely that the observed differences arose secondarily to the evolution of viviparity. We studied development of the extraembryonic membranes and placentation in the reproductively bimodal lizard Lacerta vivipara because the influence of reproductive mode on the structural/functional relationship between mothers and embryos can best be understood by studying the most recent evolutionary events. Lecithotrophic viviparity has evolved recently within this species and, although populations with different reproductive modes are allopatric, oviparous and viviparous forms interbreed in the laboratory and share many life history characteristics. In contrast to prior comparisons between oviparous and viviparous species, we found no differences in ontogeny or structure of the extraembryonic membranes between populations with different reproductive modes within L. vivipara. However, we did confirm conclusions from previous studies that the tertiary envelope of the egg, the eggshell, is much reduced in the viviparous population. These conclusions support a widely accepted model for the evolution of squamate placentation. We also found support for work published nearly 80 years ago that the pattern of development of the yolk sac of L. vivipara is unusual and that a function of a unique structure of squamate development, the yolk cleft, is hematopoiesis. The structure of the yolk sac splanchnopleure of L. vivipara is inconsistent with a commonly accepted model for amniote yolk sac function and we suggest that a long standing hypothesis that cells from the yolk cleft participate in yolk digestion requires further study.     

Variation of Reproductive Traits and Female Body Size in the Most Widely-Ranging Terrestrial Reptile: Testing the Effects of Reproductive Mode, Lineage, and Climate

The European common lizard, Zootoca vivipara, is the most widespread terrestrial reptile in the world. It occupies almost the entire Northern Eurasia and includes four viviparous and two oviparous lineages. We analysed how female snout-vent length (SVL), clutch size (CS), hatchling mass (HM), and relative clutch mass (RCM) is associated with the reproductive mode and climate throughout the species range and across the evolutionary lineages within Z. vivipara. The studied variables were scored for 1,280 females and over 3,000 hatchlings from 44 geographically distinct study samples. Across the species range, SVL of reproductive females tends to decrease in less continental climates, whereas CS corrected for female SVL and RCM tend to decrease in climates with cool summer. Both relationships are likely to indicate direct phenotypic responses to climate. For viviparous lineages, the pattern of co-variation between female SVL, CS and HM among populations is similar to that between individual females within populations. Consistent with the hypothesis that female reproductive output is constrained by her body volume, the oviparous clade with shortest retention of eggs in utero showed highest HM, the oviparous clade with longer egg retention showed lower HM, and clades with the longest egg retention (viviparous forms) had lowest HM. Viviparous populations exhibited distinctly lower HM than the other European lacertids of similar female SVL, many of them also displaying unusually high RCM. This pattern is consistent with Winkler and Wallin’s model predicting a negative evolutionary link between the total reproductive investment and allocation to individual offspring.     

Reproductive mode may determine geographic distributions in Australian venomous snakes (Pseudechis,Elapidae)

Summary Why are viviparous squamate reptiles more common in cold climates, and oviparous ones in warmer areas? The usual explanation is that (1) oviparous squamates cannot reproduce successfully in cold areas because soil temperatures are too low for embryonic development; and (2) viviparous squamates experience lower survivorship or reproductive success than oviparous taxa in warmer areas. These hypotheses suggest that the boundaries of geographic distributions of congeneric oviparous and viviparous squamates should be predictable from data on thermal tolerances of embryos, and estimated temperatures of soils and gravid female reptiles throughout the potential geographic range of the taxon. In large venomous Australian snakes of the genus Pseudechis, distributional boundaries of oviparous and viviparous taxa can be accurately predicted from such data. This predictive ability, if substantiated by studies of other reproductively biomodal squamate taxa, would support the putative role of reproductive mode as a direct determinant of reptilian geographic distributions.     

Capital energy drives production of multiple clutches whereas income energy fuels growth in female collared lizards Crotaphytus collaris

Animals invest energy in reproduction that is obtained at two distinct times relative to the reproductive cycle. Energy obtained during egg production is referred to as income energy whereas stored energy acquired prior to reproduction is capital energy. Similar to most ectotherms, squamate reptiles are generally hypothesized to be capital breeders. Nearly all squamates in which income/capital energy investment has been examined thus far produce only one clutch per reproductive season. Although it is likely that squamates producing multiple seasonal clutches fuel first clutches with capital energy, either capital or income energy may be used to produce later clutches. We first monitored female eastern collared lizards over 14 reproductive seasons to confirm that the number of clutches females produce seasonally is a plastic response to variable environmental parameters, and to examine the effects of female body condition at the beginning of the reproductive season on clutch production. Clutch production varied annually and both the size and number of clutches were positively correlated with body condition. We then tested the competing predictions of the income and capital hypotheses experimentally by supplementing the diets of female collared lizards in situ for one season. Diet‐supplementation had no effect on the number of clutches produced but increased growth rates of gravid females. We further tested the competing predictions of these two hypotheses by examining variation in maternal energy investment per clutch using preserved specimens collected near our primary field site. Clutch size was highly correlated with female body size. Together, our results suggest that variation in reproductive output by female collared lizards is linked to stored capital energy rather than income energy, similar to most ectotherms.     

Maternal body volume constrains water uptake by lizard eggs in utero

1. Flexible shelled eggs of squamate reptiles normally take up substantial amounts of water, and swell accordingly, during development. However, most water uptake occurs after oviposition, beginning immediately or soon after oviposition, and water uptake is severely restricted in eggs that are retained in utero past the normal time of oviposition.
2. These observations suggest that some aspect of retention in the oviductal environment limits the amount of water eggs can absorb prior to oviposition.
3. This paper presents evidence, from two species of Sceloporus lizard, supporting the hypothesis that limited space within the mother's body cavity physically constrains the ability of eggs to expand, and thereby their ability to absorb water. When adjusted for maternal body size, the water content of a female's clutch of eggs is negatively correlated with the dry mass of her clutch (the space available in the body cavity is finite, and thus a greater dry mass content leaves less volume for water).
4. If such a constraint on water uptake is widespread, it can have important implications for our understanding of the evolution of viviparity, the costs of reproduction, and the determinants of reproductive output in squamate reptiles.     

Measuring temporal variation in reproductive output reveals optimal resource allocation to reproduction in the northern grass lizard, Takydromus septentrionalis

We measured the reproductive output of Takydromus septentrionalis collected over 5 years between 1997 and 2005 to test the hypothesis that reproductive females should allocate an optimal fraction of accessible resources in a particular clutch and to individual eggs. Females laid 1–7 clutches per breeding season, with large females producing more, as well as larger clutches, than did small females. Clutch size, clutch mass, annual fecundity, and annual reproductive output were all positively related to female size (snout–vent length). Females switched from producing more, but smaller eggs in the first clutch to fewer, but larger eggs in the subsequent clutches. The mass-specific clutch mass was greater in the first clutch than in the subsequent clutches, but it did not differ among the subsequent clutches. Post-oviposition body mass, clutch size, and egg size showed differing degrees of annual variation, but clutch mass of either the first or the second clutch remained unchanged across the sampling years. The regression line describing the size–number trade-off was higher in the subsequent clutch than in the first clutch, but neither the line for first clutch, nor the line for the second clutch varied among years. Reproduction retarded growth more markedly in small females than in large ones. Our data show that: (1) trade-offs between size and number of eggs and between reproduction and growth (and thus, future reproduction) are evident in T. septentrionalis ; (2) females allocate an optimal fraction of accessible resources in current reproduction and to individual eggs; and (3) seasonal shifts in reproductive output and egg size are determined ultimately by natural selection.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 91 , 315–324.     

THE EVOLUTION OF REPRODUCTIVE EFFORT IN SQUAMATE REPTILES: COSTS,TRADE-OFFS,AND ASSUMPTIONS RECONSIDERED

We evaluated Shine and Schwarzkopf's (SS) model of the evolution of reproductive effort (RE) in squamate reptiles. They suggested that fecundity trade-offs were unimportant in the evolution of RE in most squamate reptiles and that only survival trade-offs needed to be considered. However, we show that by assuming no variation in offspring size exists, and that adult mortality is episodic, the results of the SS model are not general. By extension, we argue that conclusions drawn about factors important in the evolution of RE in squamate reptiles are premature. Using a modified version of the SS model, we demonstrate that variation in the form of trade-offs relating offspring size and survival substantially affect relationships among clutch size, relative clutch mass, and lifetime reproductive success. We also demonstrate that the way in which adult mortality is simulated drastically affects conclusions about the potential fecundity trade-offs experienced by populations of squamate reptiles. Finally, we suggest that a complete understanding of the evolution of RE will come from theory that incorporates trade-offs between offspring size and quality, as well as other system-specific constraints on the allocation of energy to growth, maintenance, storage, and reproduction.