Rainfall shapes the physiological condition, but not the body size, of an introduced dung beetle in Brazilian pastures

Seasonal rainfall patterns play a crucial role in shaping the biology, ecology, and physiology of animals, particularly in tropical and subtropical ecosystems. Understanding these dynamics is critical under climate change scenarios, as shifts in rainfall regimes can disrupt phenological and physiological adaptations. However, physiological responses of Digitonthophagus gazella (Fabricius, 1787), an African dung beetle introduced worldwide for pasture improvement, to seasonal rainfall variation remain poorly understood. Here, we evaluated the effects of seasonal rainfall on physiological condition of D. gazella collected during the dry and rainy seasons in introduced Brazilian pastures. So, we developed one dataset containing body condition metrics (e.g., body size, body dry mass, lipid mass and muscle mass) of males and females of D. gazella sampled in the dry and rainy season in pasturelands. We found that seasons did not affect D. gazella body size, dry and lipid masses. The dry season had individuals with higher muscle mass compared to the rainy season. These findings suggest that D. gazella individuals tolerate seasonal fluctuations, which may buffer them against seasonal environmental stress. Such plasticity may underlie the species’ resilience and contribute to its successful establishment and ongoing expansion in tropical and subtropical pastures.

Methods
We sampled beetles in December 2023 (rainy) and July 2024 (dry), in two pasturelands sites (Urochloa spp.) in Aquidauana, Mato Grosso do Sul. The sampling was carried out using light traps and directly from cattle fecal excrement in the pastures to capture live beetles. Individuals were individually stored in plastic vials, being posteriorly sacrificed in freezers at -5 °C. They were stored 14 days before the estimation of dung beetles’ physiological condition. From each individual, we measured dry, lipid, and muscle masses as indicators of physiological condition. 

To obtain the body dry mass, the individuals were placed in a desiccator (SolidSteel – SSA; 50 °C) for 72 h and measured to the nearest 0.0001 mg with a precision balance (Gehara AG 200). To extract the lipid mass the beetles were placed individually in falcon tubes containing 3mL of chloroform for 48 h, then the individuals were placed in the desiccator again for 72 h and the difference between the body dry mass and this new mass was considered lipid mass (Lee et al., 2004; Plaistow & Siva-Jothy, 1996). Finally, the muscle of the beetles was extracted using the method of Plaistow & Siva-Jothy (1996) with 0.2 M potassium hydroxide for 48 h, then the individuals were rinsed with water, re-dried, and re-weighed. The difference between the mass without lipid and this new mass was considered muscle mass. Body size was estimated as the linear distance from the clypeus to the pygidium, with a digital caliper with 0.01 mm of precision.