The evolution of different maternal investment strategies in two closely related desert vertebrates

‘Unscrambling’ the drivers of egg production in Agassiz’s desert tortoise: climate and individual attributes predict reproductive output

The ‘bet hedging’ life history strategy of long-lived iteroparous species reduces short-term reproductive output to minimize the risk of reproductive failure over a lifetime. For desert-dwelling ectotherms living in variable and unpredictable environments, reproductive output is further influenced by precipitation and temperature via effects on food availability and limits on activity. We assembled multiple (n = 12) data sets on egg production for the threatened Agassiz’s desert tortoise Gopherus agassizii across its range and used these data to build a range-wide predictive model of annual reproductive output as a function of annual weather variation and individual-level attributes (body size and prior-year reproductive status). Climate variables were more robust predictors of reproductive output than individual-level attributes, with overall reproductive output positively related to prior-year precipitation and an earlier start to the spring activity season, and negatively related to spring temperature extremes (monthly temperature range in March-April). Reproductive output was highest for individuals with larger body sizes that reproduced in the previous year. Expected annual reproductive output from 1990-2018 varied from 2-5 to 6-12 eggs female-1 yr-1 , with a weak decline in expected reproductive output over this time (p = 0.02). Climate-driven environmental variation in expected reproductive output was highly correlated across all 5 Recovery Units for this species (Pearson’s r > 0.9). Overall, our model suggests that climate change could strongly impact the reproductive output of Agassiz’s desert tortoise, and could have a negative population-level effect if precipitation is significantly reduced across the species’ range as predicted under some climate models.

Lizard Embryos Prioritize Posthatching Energy Reserves over Increased Hatchling Body Size during Development

Embryonic development in oviparous organisms is fueled by maternally allocated yolk, and many organisms hatch before that energy store is used completely; the resultant leftover (residual) yolk is internalized and may support early posthatching life. However, embryos that use most, or all, of their yolk supply before hatching should hatch at a larger size than those that do not exhaust those energy reserves, which could also have benefits for posthatching growth and survival. To examine the trade-off between residual yolk and offspring size, we experimentally reduced yolk quantity at oviposition in lizard eggs (Amphibolurus muricatus) and then quantified offspring size and the amount of internalized residual yolk. This design enabled us to determine whether embryos (1) exhaust yolk supply during development (thereby maximizing neonatal size) or (2) reduce neonatal size by retaining yolk reserves at hatching. Our data support the latter scenario. Eggs from the yolk-reduced treatment produced smaller offspring with a proportion of residual yolk similar to that of offspring from unmanipulated eggs, suggesting that the fitness benefits of posthatching energy stores outweigh those of larger neonatal size.

Understanding reproductive allometry in turtles: A slippery "slope"

Measures of reproductive output in turtles are generally positively correlated with female body size. However, a full understanding of reproductive allometry in turtles requires logarithmic transformation of reproductive and body size variables prior to regression analyses. This allows for slope comparisons with expected linear or cubic relationships for linear to linear and linear to volumetric variables, respectively. We compiled scaling data using this approach from published and unpublished turtle studies (46 populations of 25 species from eight families) to quantify patterns among taxa. Our results suggest that for log-log comparisons of clutch size, egg width, egg mass, clutch mass, and pelvic aperture width to shell length, all scale hypoallometrically despite theoretical predictions of isometry. Clutch size generally scaled at ~1.7 to 2.0 (compared to an isometric expectation of 3.0), egg width at ~0.5 (compared to an expectation of 1.0), egg mass at ~1.1 to 1.3 (3.0), clutch mass at ~2.5 to 2.8 (3.0), and pelvic aperture width at 0.8-0.9 (1.0). We also found preliminary evidence that scaling may differ across years and clutches even in the same population, as well as across populations of the same species. Future investigators should aspire to collect data on all these reproductive parameters and to report log-log allometric analyses to test our preliminary conclusions regarding reproductive allometry in turtles.

The evolution of different maternal investment strategies in two closely related desert vertebrates

We compared egg size phenotypes and tested several predictions from the optimal egg size (OES) and bet-hedging theories in two North American desert-dwelling sister tortoise taxa, Gopherus agassizii and G. morafkai, that inhabit different climate spaces: relatively unpredictable and more predictable climate spaces, respectively. Observed patterns in both species differed from the predictions of OES in several ways. Mean egg size increased with maternal body size in both species. Mean egg size was inversely related to clutch order in G. agassizii, a strategy more consistent with the within-generation hypothesis arising out of bet-hedging theory or a constraint in egg investment due to resource availability, and contrary to theories of density dependence, which posit that increasing hatchling competition from later season clutches should drive selection for larger eggs. We provide empirical evidence that one species, G. agassizii, employs a bet-hedging strategy that is a combination of two different bet-hedging hypotheses. Additionally, we found some evidence for G. morafkai employing a conservative bet-hedging strategy. (e.g., lack of intra- and interclutch variation in egg size relative to body size). Our novel adaptive hypothesis suggests the possibility that natural selection favors smaller offspring in late-season clutches because they experience a more benign environment or less energetically challenging environmental conditions (i.e., winter) than early clutch progeny, that emerge under harsher and more energetically challenging environmental conditions (i.e., summer). We also discuss alternative hypotheses of sexually antagonistic selection, which arise from the trade-offs of son versus daughter production that might have different optima depending on clutch order and variation in temperature-dependent sex determination (TSD) among clutches. Resolution of these hypotheses will require long-term data on fitness of sons versus daughters as a function of incubation environment, data as yet unavailable for any species with TSD.

Contrasting reproductive strategies in a narrow latitude range: the case of D’Orbigny’s slider

Reproductive traits and the level of parental investment in offspring varies between individuals and species. These are central issues in life history theory and evolutionary biology. Maternal body size plays an important role in reproduction, and we usually observe variable investment in offspring by females. Thus, optimal egg size may not be reached in some populations or species. In this study, we tested if reproductive traits differed between populations of D’Orbigny’s slider in a specific geographical area in Brazil. We evaluated the relationship between reproductive traits to maternal body size and clutch size to egg size to determine possible trade-offs across populations. At the population level, maternal body size and reproductive traits of D’Orbigny’s slider were different even in geographically nearby areas. Maternal body size had a positive effect on clutch size, but not on egg size, except in the Arroio Grande population. Nevertheless, we did not observe a negative correlation between clutch and egg size in any population. Although maternal body size had effects in the different populations explaining most of the variation of clutch size, variation in egg size may be the result of decreased survival chances in unpredictable environments and possibly morphological constraints. The trade-off between egg size and number was not observed and this could be expected if resource availability and reproductive allocation by females vary greatly among individuals.