Desert lizards modulate their nutritional responses to match seasonal biological needs

Animals in extreme environments, such as deserts, experience severe seasonal fluctuations in abiotic conditions and resources. Under these conditions, they must obtain macronutrients in optimal amounts and ratios to meet their nutritional requirements, which themselves vary with seasonal changes in physiological and life-history processes. To understand whether nutritional intakes and retention align with key life-history events across seasons, we examined the nutritional ecology of the desert-dwelling lizard Saara hardwickii. We first measured seasonal variation in the quantity and quality of plants available for these herbivorous lizards. By combining field observations of foraging behaviour with elemental analysis, we determined carbon (C) and nitrogen (N) levels in dietary intakes and faecal matter across seasons. Intake C: N was lowest and faecal C: N highest in June, reflecting greater proportional consumption and retention of N, likely meeting increased protein demands during breeding. Conversely, dietary C: N and faecal %N were highest in October, indicating greater consumption and retention of C prior to hibernation. Surprisingly, these putative herbivores consumed insects only around the breeding season, despite their year-round availability. Overall, we show that S. hardwickii employs both behavioural diet choice and post-ingestive physiology to match seasonal nutritional needs by differentially consuming and retaining nutrients in an extreme environment.

Methods
We sampled S. hardwickii from April to October 2022 in Jorbeer-Gadhwala Conservation Reserve, Rajasthan, India. All measurements were repeated in the same 20 plots of 20x20m across four seasons - April, June, August, and October. Since these desert lizards are known to be herbivorous, we estimated the plant species composition and wet biomass by setting up 20 smaller quadrats of 1x1m in each of the larger sampling plots. We carried out behavioural focal observations in a separate sampling plot each day and recorded all the items and their amounts consumed by these lizards throughout the day. At the end of focal observations, we collected samples of all the consumed food items from the same plot to measure the nutritional composition of each dietary component. We also collected fresh faecal pellets from each plot to estimate their elemental composition and infer about retained nutrients. Elemental composition was measured for all the dietary components and faecal matter, and this information was combined with behavioural data to estimate the nutritional composition of diets across seasons (N = 20/season).