Reproduction Alters Hydration State but Does Not Impact the Positive Effects of Dehydration on Innate Immune Function in Children’s Pythons (Antaresia childreni)

Abstract

Resource availability can impact immune function, with the majority of studies of such influences focusing on the allocation of energy investment into immune versus other physiological functions. When energy is a limited resource, performance trade-offs can result, compromising immunity. Dehydration is also considered a physiological challenge resulting from the limitation of a vital resource, yet previous research has found a positive relationship between dehydration and innate immune performance. However, these studies did not examine the effects of dehydration on immunity when there was another concurrent, substantial physiological challenge. Thus, we examined the impact of reproduction and water deprivation, individually and in combination, on immune performance in Children's pythons (Antaresia childreni). We collected blood samples from free-ranging A. childreni to evaluate osmolality and innate immune function (lysis, agglutination, bacterial growth inhibition) during the austral dry season, when water availability is limited and this species is typically reproducing. To examine how reproduction and water imbalance, both separately and combined, impact immune function, we used a laboratory-based 2 × 2 experiment. Our results demonstrate that A. childreni experience significant dehydration during the dry season and that, overall, osmolality, regardless of the underlying cause (seasonal rainfall, water deprivation, or reproduction), is positively correlated with increased innate immune performance.

Dehydration enhances innate immunity in a semiaquatic snake from the wet‐dry tropics

“…We did not detect any significant increases in agglutination, however. These results are consistent with findings of a positive relationship between hyperosmolality and innate immune performance in other squamates (Gila monsters; Moeller, Butler, & DeNardo, ; rattlesnakes; Brusch & DeNardo, ; Children's pythons; Brusch et al, ). As in those studies, we found that rehydration rapidly returned immune metrics to baseline values (Figure ), suggesting that the innate molecules responsible for the positive influence of dehydration on immune function rapidly disassociate or become ineffective upon rehydration.…”

“…Comparing our data (pooled from our field and laboratory studies) with similarly pooled data from other experiments that used analogous methods on a sympatric species (Children's pythons; Brusch et al, ) and a xeric‐adapted species (western diamond‐backed rattlesnake; Brusch & DeNardo, ), reveal some interesting comparisons (Figure ). Water pythons have the smallest range of plasma osmolality of the three species (279–343 mOsm/kg), Children's pythons are intermediate (279–354 mOsm/kg), and rattlesnakes have the largest range (277–436 mOsm/kg).…”

“…It is important to address an obvious alternative explanation: as the animals dehydrate, their blood volume decreases and, as a result, innate molecules become more concentrated. Previous work to address this possibility showed that diluting plasma samples from dehydrated animals to adjust for the decreased volume, still resulted in higher immune metrics compared to plasma samples from hydrated animals (Brusch & DeNardo, ; Moeller et al, ), suggesting that dehydration leads to an upregulation (either in total number or activity) of innate molecules. Although we did not conduct a similar experiment for our study, we predict that a similar upregulation would be detected in dehydrated water pythons.…”

“…This threshold is low for water pythons (~310 mOsm/kg), intermediate for Children's pythons (~325 mOsm/kg), and high for rattlesnakes (~360 mOsm/kg). Children's pythons occur in the same area as water pythons (the study by Brusch et al, was conducted at Beatrice Hill) but are typically found in dry woodlands and rocky outcrops (Wilson & Swan, ) and may be adapted to longer periods without access to water (compared to semiaquatic water pythons). Similarly, the study by Brusch and DeNardo () used rattlesnakes from the Sonoran Desert, where the snakes often experience no rainfall for >3 months during annual seasonal droughts (and can survive for 16 weeks without water before showing signs of clinical dehydration).…”

“…To cope with water scarcity, many organisms exhibit water‐conserving behavioral, morphological, or physiological adaptations to maintain water balance (Takei, ). Alternatively, other organisms tolerate dramatic fluctuations in plasma osmolality and endure bouts of dehydration (i.e., hyperosmolality) until water becomes available (Brusch & DeNardo, ; Peterson, ). Although the latter strategy is well‐documented, we have only a limited understanding of the behavioral and physiological implications of nonlethal dehydration.…”