Eating under stress? Not for these lizards!

The research emphasises the need to learn from the mechanisms by which animals cope with stressful situations, writes Seshadri K S

Life is difficult’, wrote the American Psychiatrist, Scott Peck, in the book Road Less Traveled. Although the book, published in 1978 explores the challenges of navigating life. Life is difficult for all organisms, not just humans.

Researchers from the Indian Institute of Science (IISc), use the Peninsular rock Agama (Psammophilus dorsalis) as a model organism to understand how it copes with life’s difficulties.

In a surprising find, these common and colourful lizards maintain a consistent dietary intake despite fluctuating stress levels and instead, adjust how they process their food, selectively retaining more carbon and nitrogen when stressed. 

Physiological stress 

Stress, simply put, is any nonspecific response of the body to any demand placed upon it and is therefore a coping mechanism. Free-ranging animals must dodge predators, grow up, attract mates, and leave offspring behind. All this comes at a tradeoff, costing energy and resources while stressing the animal.

Among the Rock Agamas, the stress hormone, corticosterone, was lower during the breeding season in both sexes. High levels of stress can have impacts on offspring, and therefore, these findings are not surprising.

Stressful situations often make humans reach for foods rich in fats and sugar, which can be transformed to energy more easily than proteins, generalised as stress eating.

“What ‘stress’ does is help an animal cope with the challenge, by mobilizing energy to the right places in the body such as the muscles (if the animal needs to run away) or to the brain (if the animals need to solve the problem)” said Maria Thaker, a professor at Centre for Ecological Sciences (CES), IISc.

“If the stressful event is short, then animals can cope effectively without changing their eating. If the stressful condition is chronic, animals are forced into an ‘energetic challenge,” she added.

Prof. Thaker, who heads the Macrophysiology Lab, has studied the evolutionary ecology of reptiles, particularly lizards, for over a decade.

Lizard platter

The Rock Agama is a common lizard found in rocky habitats. They were once common across the city and are now found in newly formed layouts and on compound walls in vacant plots.

The researchers captured the lizards using a noose and flushed their stomachs by pumping clean water. Later, they examined the stomach contents under a microscope and identified the different Orders of prey.

Surprisingly, both sexes of the agama consumed similar prey, comprising nearly eight different arthropods, dominated by ants.

However, there were differences in diet composition over seasons. While diet composition can be insightful, nutritional needs are met by the elemental components of what we eat. Carbon is a primary energy source, and nitrogen is essential for protein synthesis and growth.

Thus, the ratio of Carbon: Nitrogen (C:N) can be informative to determine if there is an imbalance, especially as a stress response. “As the diet compositions were similar, the estimated C:N ratios from the prey items were also remarkably similar between sexes and across seasons,” said Mr. Avik Banerjee, a doctoral student at CES and the lead author of the study. 

The ratio of C: N is widely used as a measure of nutritional composition rather than individual components, as they are theoretically expected to be along a gradient. The ratios are useful measures indicating a balanced diet. It also allows for comparison across individuals, as ratios remove any confounding effects such as body size or body condition of the individual.

“Overall, despite variations in prey availability and stress over seasons or between individuals, the lizards were eating a rather tightly balanced diet - this is much better compared to humans!” said Avik, who spent many days, with his colleagues, eating at erratic times and stressing over data in the rocky outcrops of Bengaluru.

Stress in a changing world

If the lizards do not change their diet or the C:N ratio, how are they coping with high stress? The researchers wondered. Surprisingly, while the C:N ratios of ingested food did not differ, the faecal concentrations were different under stressful conditions, and the lizards reduced the elemental carbon when corticosterone levels were high.

“Determining how they do this was beyond the scope of our study, but several animals, including insects, rodents, and even humans, can do this. For example, slowing the gut down allows more time for nutrients to be absorbed. The composition of microbiota in animal guts can also influence how fast and which nutrients are absorbed.” Prof. Thaker emphasised. 

There is an increasing emphasis on stress across taxa, especially as humans rapidly modify landscapes. Changes to land use can potentially stress animals and push them to resource-poor environments or isolate them. The rock agama seems to be going strong and obtaining the necessary resources, irrespective of seasons or associated stress.

The authors conducted this study during the COVID-19 pandemic, which was an incredibly stressful period for all humankind. The authors opine that their work emphasises the need to go beyond human systems and merely monitoring diet, and study the underlying mechanisms by which animals cope with stressful situations. “Understanding stress helps us gauge resilience—not just in wildlife, but in our own lives,” concludes Prof. Thaker.

(The author is an ecologist and faculty member at ATREE)