Abstract
Biological investments, such as reproduction, are influenced by both biotic and abiotic factors and their interactions. The trade-off between reproduction and survival has been well established. Seasonally breeding species, therefore, may exhibit variations in these trade-offs, but there is a dearth of knowledge concerning this. This study investigated the physiological cost of reproduction (measured through oxidative stress) across seasons in the cooperatively breeding highveld mole-rat (Cryptomys hottentotus pretoriae), one of the few seasonal breeding mole-rats. Oxidative stress indicates elevated reactive oxygen species (ROS) levels, which can overwhelm antioxidant defences resulting in damaged proteins, lipids and DNA, which overall can reduce longevity and compromise reproduction. Oxidative markers such as total oxidant status (TOS-measure of total peroxides present), total antioxidant capacity (TAC), oxidative stress index (OSI), and malondialdehyde (MDA) are utilised to measure oxidative stress. In this study, breeding and non-breeding male (NBM) and female mole-rats were captured during the dry season (breeding period) and wet season (non-breeding period). There was an apparent cost of reproduction in the highveld mole-rat; however, the seasonality pattern to the cost of reproduction varied between the sexes. Breeding females (BFs) had significantly higher MDA during the breeding period/dry season in comparison to the non-breeding period/wet season; this is possibly a consequence of bearing and nursing offspring. Contrastingly, breeding males (BMs) showed increased oxidative damage in the non-breeding/wet season compared to the breeding/dry season, possibly due to increased activities of protecting their mating rights for the next breeding/dry season, but this was not significant. Interestingly, during the non-breeding period/wet season, non-breeding females (NBFs) are released from their reproductive suppression, which resulted in increases in TOS and OSI, which again indicated that just the mere ability to be able to breed results in a cost (oxidative stress). Therefore we can speculate that highveld mole-rats exhibited seasonal variation in redox balance brought about by variation in abiotic variables (e.g., rainfall), physiology and behaviour. We conclude that physiological changes associated with reproduction are sufficient to induce significant acute oxidative stress in the plasma of female highveld mole-rats, which become alleviated following transition to the non-breeding season/wet period suggesting a possible hormetic effect.
Highlights
IntroductionLife history theory is defined by how an animal spreads both the costs of reproduction and survival over its lifespan, which is influenced by abiotic (physical environment) and biotic (living components in the environment) factors and their interactions (Brommer, 2000; Chainy et al, 2016; Withers et al, 2016; Varpe, 2017; Dantas et al, 2021)
Life history theory is defined by how an animal spreads both the costs of reproduction and survival over its lifespan, which is influenced by abiotic and biotic factors and their interactions (Brommer, 2000; Chainy et al, 2016; Withers et al, 2016; Varpe, 2017; Dantas et al, 2021)
Oxidative stress was determined by the total oxidant status (TOS):TAC ratio, which represents the OSI arbitrary unit, which was calculated as follows: OSI = [(TOS, μmol H2O2 equivalent/L)/(TAC, μmol Trolox equivalent/L)] × 100 (Bitiren et al, 2010; Jacobs et al, 2021a,b)
Summary
Life history theory is defined by how an animal spreads both the costs of reproduction and survival over its lifespan, which is influenced by abiotic (physical environment) and biotic (living components in the environment) factors and their interactions (Brommer, 2000; Chainy et al, 2016; Withers et al, 2016; Varpe, 2017; Dantas et al, 2021). The cost of reproduction hypothesis predicts that animals require a trade-off into high investment processes such as those between reproduction, survival and longevity (Speakman et al, 2015; Blount et al, 2016; Costantini, 2016; AlonsoAlvarez et al, 2017; Viblanc et al, 2018). Elevated ROS levels leading to OS was thought to be one of the primary costs to reproduction in the life history trade-off between survival and reproduction (Sainz et al, 2000; Alonso-Alvarez et al, 2004; Costantini, 2008; Stier et al, 2012). Recent findings widely debate an oxidative cost to reproduction, as there is uncertainty whether there is an actual oxidative cost to reproduction (Monaghan et al, 2009; Metcalfe and Monaghan, 2013; Speakman et al, 2015; Costantini, 2019)
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