Life history tradeoffs and the costs of reproduction in Arizona Bark Scorpions (Centruroides sculpturatus).

摘要

Tradeoffs in life history evolution result from conflicts in the time and energy that can be simultaneously invested in activities such as growth, reproduction, and survival. Reproduction is an energetically costly activity for organisms, and is known to elicit alterations in the daily activity patterns of individuals. I investigated reproductive tradeoffs in the Arizona Bark Scorpion, Centruroides sculpturatus. To better understand the nature of tradeoffs in this species, I examined the influence of reproductive state on the predatory efficiency and thermal preference of reproductive females. Further, I examined the influence of reproductive activities (i.e. gestation) on the ability of reproductive females to conserve water at high temperatures. Finally, I used morphometric analyses to investigate tradeoffs in weapon and ornament allometry in male and female C. sculpturatus. To examine tradeoffs that occurred between reproductive activities and the predatory efficiency of female C. sculpturatus, I conducted prey handling trials of non-gravid and gravid females and compared the time it took them to successfully capture and subdue a prey item (Common House Cricket, Acheta domesticus). Using Survival Analyses, I determined that pregnancy did not significantly reduce the predatory efficiency of gravid female C. sculpturatus, which suggests that gravid females can maintain their predatory abilities despite significant increases in body mass and the physiological costs associated with gestation. However, all female C. sculpturatus exhibiting maternal care (i.e. carrying offspring) were unable to capture prey during the trial period, suggesting that brooding behaviors incur substantial energetic costs for female C. sculpturatus. Thus, female C. sculpturatus experience a tradeoff between caring for current offspring and their ability to acquire resources during the brooding period. Next, to examine the influence of reproductive state on the thermal preference of females, I designed and constructed a thermal gradient, and recorded the body temperatures (Tb) of non-gravid and gravid females over a 24 hour period. Using Profile Analysis, I documented that gravid females selected significantly higher diurnal and nocturnal Tb than non-gravid females. The selection of higher Tb by gravid C. sculpturatus may improve offspring fitness by facilitating favorable temperatures for embryonic development. However, the preferred Tb of gravid females was close to their critical maximum temperature, which may significantly increase their risk of heat-induced mortality. The morphological (increased body mass) and physiological (increased metabolic rate) changes that occur in reproductive females can increase their susceptibility to water loss, compared to non-reproductive females. To compare rates of water loss between non-gravid and gravid females, I measured their water loss rates using flow-through respirometry. Gravid female C. sculpturatus lost water significantly faster than non-gravid females. This is another tradeoff experienced by reproductive C. sculpturatus, because engaging in current reproduction (i.e. gestation) may increase their risk of mortality through desiccation at higher temperatures. Finally, to examine possible tradeoffs in weapon and ornament allometry in C. sculpturatus, I measured the length of several distinct morphological characters: carapace, mesosoma, metasomal segments I-V, telson, chela, and walking legs I and IV. Using Principal Component Analysis, Analysis of Covariance, and a correlation analysis, I compared the relative sizes of traits between male and female C. sculpturatus. There is sexual dimorphism in the size of the chelae and tail in C. sculpturatus, as females had larger chelae than males, and males had significantly longer tails than females. Males exhibited a negative correlation between the relative size of weapons and ornaments (chelae and tail) and total body length (carapace + mesosoma length), because individuals with relatively larger chelae and tails possessed relatively shorter bodies. These data demonstrate the negative influence that weapon development can have on the resources allocated to other body parts. In female C. sculpturatus, increases in the relative size of the mesosoma were negatively correlated with the size of the chelae, carapace, and the length of leg IV. Thus, it appears that the allocation of energetic resources to a structure (the mesosoma) that influences reproductive output may reduce the energy available for weapon development and somatic growth in different regions of the female's body. These intersexual differences in the body allometry of male and female C. sculpturatus likely reflect the influence that sexual selection can exert on particular traits. Understanding the mechanisms that generate these tradeoffs in life history evolution may help elucidate factors that lead to diversity in the behaviors and reproductive strategies of species in nature.