Optimal egg size in a suboptimal environment: reproductive ecology of female Sonora mud turtles (Kinosternon sonoriense) in central Arizona, USA

2019

Ecology and Evolution

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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 We compiled scaling data using this approach from published and unpublished turtle studies (46 populations of 25 species from eight families) to quantify patterns among taxa.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.

Section: Discussionmentioning

confidence: 99%

Understanding reproductive allometry in turtles: A slippery “slope”

Iverson

Lindeman

2019

Ecology and Evolution

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“…We investigated the presence of an architectural constraint of the pelvic aperture by conducting analyses of covariance (ANCOVA) to test the homogeneity of slopes between the relationships of pelvic aperture width and mean egg width with CL (see Congdon & Gibbons, ). If slopes are parallel and the largest egg width (x‐radiographed egg width) is larger than the smallest pelvic aperture width in the populations, we concluded that there is a constraint on egg size by the pelvic aperture (Iverson & Smith, ; Lovich, Madrak, et al., ). In these analyses, we used full‐factorial, mixed‐effects regression models via functions from the lme4 package (Bates et al., ) with individual tortoise and year as random effects.…”

Section: Methodsmentioning

confidence: 80%

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

Ennen

Averill‐Murray

et al. 2017

Ecology and Evolution

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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.

“…Overall, OES in the Kiawah population appears to represent a combination of scenario 3 (egg size is morphologically constrained until a threshold body size is reached) and scenario 4 (egg size is not morphologically constrained but also not optimized) described in Lovich et al . () because smaller females show evidence of morphological constraint, but larger females do not appear to exhibit canalization around an optimal egg size despite a lack of obvious morphological constraint. OES in the Edisto population appears to best match scenario 4 (egg size is not morphologically constrained but also not optimized) because we did not find evidence for constraint by either aperture, although the variation in egg size could suggest that egg size has not experienced selection pressures sufficiently strong enough to canalize the trait around an optimal value.…”

Section: Discussionmentioning

confidence: 99%

“…C) may account for some deviations from predictions of optimal egg size theory in turtles (Congdon & Gibbons, ; Clark et al ., ). In addition, the diversity of architectures exhibited by the turtle pelvis may also contribute to observed variation (Lovich et al ., ).…”

Section: Introductionmentioning

confidence: 97%

Relationships of maternal body size and morphology with egg and clutch size in the diamondback terrapin,Malaclemys terrapin(Testudines: Emydidae)

Kern

Guzy

et al. 2015

Biol. J. Linn. Soc.

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Because resources are finite, female animals face trade-offs between the size and number of offspring they are able to produce during a single reproductive event. Optimal egg size (OES) theory predicts that any increase in resources allocated to reproduction should increase clutch size with minimal effects on egg size. Variations of OES predict that egg size should be optimized, although not necessarily constant across a population, because optimality is contingent on maternal phenotypes, such as body size and morphology, and recent environmental conditions. We examined the relationships among body size variables (pelvic aperture width, caudal gap height, and plastron length), clutch size, and egg width of diamondback terrapins from separate but proximate populations at Kiawah Island and Edisto Island, South Carolina. We found that terrapins do not meet some of the predictions of OES theory. Both populations exhibited greater variation in egg size among clutches than within, suggesting an absence of optimization except as it may relate to phenotype/habitat matching. We found that egg size appeared to be constrained by more than just pelvic aperture width in Kiawah terrapins but not in the Edisto population. Terrapins at Edisto appeared to exhibit osteokinesis in the caudal region of their shells, which may aid in the oviposition of large eggs.