Different hypotheses have been proposed to explain the adaptive value of matrotrophy, which is the postfertilization maternal provisioning to developing embryos. The Trexler–DeAngelis model proposes that matrotrophy provides fitness advantages when food abundance is high and availability is constant. If food availability is low or unpredictable, prefertilization maternal provisioning (lecithotrophy) should be favored over matrotrophy. In this study, we tested this model in two fish species from the family Poeciliidae, Poeciliopsis gracilis and P. infans, using field and laboratory data. In the field study, we explored the effects of population, season, and food abundance on the degree of matrotrophy. In P. infans, we found evidence that supports this model: In the population where food abundance decreased during the dry season, females reduced the amount of postfertilization provisioning and thus exhibited a more lecithotrophic strategy. In P. gracilis, we observed patterns that were partially consistent with this model: Food abundance decreased during the wet season in three populations of this species, but only in one of these populations, females exhibited less postfertilization nutrient transfer during this season. In the laboratory study, we tested the effects of constant, fluctuating, and low food availability on the relative amounts of pre‐ and postfertilization provisioning of P. infans. Our laboratory results also support the Trexler–DeAngelis model because both low and fluctuating food regimes promoted a more lecithotrophic strategy. Together, our findings indicate that the benefits of matrotrophy are more likely to occur when females have constant access to food sources.
Examination of the spatial and temporal variation in survival rates provides insight on how the action of natural selection varies among populations of single species. In this study, we used mark‐recapture data from seven populations of the viviparous lizard Sceloporus grammicus in Central Mexico and a multi‐model inference framework to examine interpopulation variation in the survival of adult males and females. We aimed to analyze the potential effects of aridity, human‐induced disturbance, and reproductive costs on the survival rates of these lizards. For females in particular, we also searched for a negative relationship between litter size (adjusted for female size) and female survival. Our results demonstrate seasonal changes in survival for males and females. In three out of our seven study sites female survival decreased during the birthing season. In contrast, male survival did not appear to decrease during the mating season. We found an interaction between site‐specific aridity and reproductive season affecting female survival. A decrease in female survival during the birthing season was observed in relatively arid sites. In one of these arid sites we found a negative effect of size‐adjusted litter size on female survival: females producing more offspring than those expected for their size were more likely to die. This result represents evidence of a physiological trade‐off for gravid females occurring in at least one of the studied populations. Interpopulation variation in the degree of human‐induced disturbance could not explain the observed patterns of spatial variation in survival rates. Our results demonstrate wide variation in sex‐specific survival patterns of this viviparous lizard and provide evidence that negative associations between reproduction and survival are highly dependent on the local environmental conditions.
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