During a visit to Harrisburg Academy’s 1st-4th graders, I asked the students if they had ever experienced “stress”. A dozen hands shot up. “I’m stressed all the time!” said one 1st grader. “When my sister comes in my room when I don’t want her to, that stresses me out,” said another. Although a 7 year old’s understanding of “stress” may differ to mine – or anyone else’s! – it was clear from this very switched-on young focus group that the concept of “stress” is ubiquitous.
“Stress” as a response to encounters we don’t want or things we don’t like may be an anthropomorphic term, but in its broadest sense it is a concept that we can extend to non-human vertebrates too. When we experience a “stressor”, from speaking in public to a near-miss accident as we cross the road, our brain triggers a hormone response resulting in the production of a glucocorticoid hormone, cortisol. The function of this hormone product is to mobilise the body’s fight or flight response – generating the immediate production of glucose, for example, and suppressing processes that don’t need energy in that moment of danger, such as reproduction and immune function. This same pathway is similarly triggered in most other vertebrates by encounters with ecological stressors, such as coming across a predator while foraging, or being in an aggressive altercation with a fellow group member, though the hormone produced may differ (rodents and primates generally also produce cortisol; other vertebrates may produce corticosterone).
So, if an organism in the wild is stressed as regularly as a first grader, we can assume that they are experiencing regular spikes of glucocorticoid hormones. While these elevations facilitate the response necessary for escape from predators and moving out of danger and are therefore likely to be selected for by evolution, studies have shown that elevated glucocorticoid levels can also have negative effects – for example, elevated glucocorticoid levels are associated with reduced immune function (McCormick et al. 2014), and lowered body condition (De Vos et al. 1995; Klein, 2015). However, few studies have investigated whether regular short-term increases in glucocorticoids (i.e. as would be expected if an animal was frightened by a predator cue, or got into a fight every day) lead to any reductions in “fitness” – an animal’s ability to pass on its genes through surviving to be able to reproduce.
We set out to test this in the eastern fence lizard which, as I’ve written about before, is likely to be frequently “stressed” given the frequency of its interactions with predators, including invasive fire ants across the lower half of its range. We were interested in the effects of frequently elevated glucocorticoids on two parameters: adult survival, and their reproductive success, which we measured as the proportion of the eggs they laid that actually hatched live babies.
To do this, we brought gravid female lizards from the field in Alabama into the lab at Penn State, and treated them with a low-dose glucocorticoid hormone between capture and laying to mimic a daily short-term spike such as they would experience if they were encountering a predator in the wild. We then monitored their survival over the next weeks, and the success of the eggs that they laid after treatment.
We found that frequent, low-level elevations of glucocorticoid hormone (corticosterone) led to reduced female survival, AND reduced egg hatching success (fewer eggs successfully hatched of those that were laid). Interestingly, we also found that the effect of corticosterone elevations were greater in 2016 compared to 2015. Two years do not a pattern make – but one potential cause for this greater effect could be that the winter between 2015 and 2016 was significantly warmer than that between 2014 and 2015 – I’ve written more about why warmer winters could be bad for reptiles here.
So, why does this matter? Well, going back to our “stressed” 1st graders for a moment – if low level stress is really so ubiquitous, then our results suggest that we may be underestimating its effects on individuals and populations. For example, we have a good understanding of the effects of direct predation on animal populations – predators kill prey animals, resulting in fewer prey animals. But perhaps the “stress” of encountering predators could also lead to reduced survival and reproduction, even in instances where prey animals escape and live to fight (or flight) another day. As animals are exposed to human-induced environmental change, they will face increasing and novel stressors, such as invasive species and climatic warming – considering the effects of the physiological outcomes of these stressors is therefore useful going forward in understanding how they will actually affect individuals and communities.