Stress doesn’t stop male lizards from showing off

(Credit: Tracy Langkilde/Penn State)

The physical traits and behaviors that lizards use to attract potential mates and fend off competitors may be so important that they don’t change in the face of stress.

A new study shows that low levels of stress-associated hormones don’t affect the blue and black badges on the throats and abdomens of male fence lizards—or the signaling behaviors used to show them off.

“Animals in the wild experience stress every day when they flee from predators, fight with others over food, or face extreme weather,” says Kirsty MacLeod, a postdoctoral scholar at Penn State at the time of the research and lead author of the paper, which appears in Scientific Reports.

“But they are facing increasing amounts of stress due to increased interactions with people, a changing climate, and other anthropogenic changes. Because of this, it is rapidly becoming more important to understand the myriad effects of stress on population health.”

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The researchers studied the effects of stress on “secondary sexual traits,” which, like a deer’s antlers or a bird’s brilliant colors, are important in attracting mates or warding off potential rivals, and ultimately contribute to an animal’s ability to reproduce.

Specifically, they studied the blue and black badges that appear on the throats and abdomens of male eastern fence lizards.

“Secondary sexual traits are the billboard ads animals use to advertise their condition,” MacLeod says. “Being more colorful or having bigger ornaments—like antlers—than your neighbor, could mean the difference between mating and passing on your genes, or not mating at all.

“They also help in maintaining a great territory that can provide resources for your offspring and preventing constant attack by rival males. If stress affects these secondary sexual traits, it could affect which individuals are successful in mating or holding territories, which could in turn affect the evolution and persistence of these populations—even though these traits are often not thought of as being central to population health.”

The researchers found that the color of a lizard’s badges is not related to the normal circulating levels of the stress hormone corticosterone in their blood. Additionally, artificially elevating levels of corticosterone, replicating the elevation that occurs when a lizard encounters a stressor like a predator, repeatedly over the course of a few weeks did not affect badge color.

Lizard push-ups

In addition to investigating the physical trait of badge color, the research team also looked at how the trait is displayed through behaviors. Much like a male peacock might raise its colorful tail, fence lizards perform push-ups and bob their heads to show off their badges, warding off other males or attracting potential mates.

“If stress did not affect color but did affect the behavior, for example if lizards stopped doing push-ups, then it wouldn’t matter if their color was the same because it wouldn’t be seen,” MacLeod says. “It would be like having a big flashy billboard lying on the ground.”

Elevating stress hormones did not affect signaling behaviors, including the number of push-ups or head bobs that the lizards performed, the researchers found.

“We know that elevating stress hormones can have important effects in this lizard species, including on immune function and behaviors that allow them to deal with predators, so these results are particularly interesting,” says Tracy Langkilde, professor and head of biology and senior author of the paper.

“It may be that low-level stress is not enough to impact these traits. Or that sexual signaling—to find better mates and maintain better territories—is so important that, when lizards experience stress, more resources are allocated to maintain them.”

Negative results

Next, the researchers plan to investigate whether maintaining these signaling traits under stress has an increased physiological cost on other aspects of lizard health and survival.

Sharing negative results like these, which show that a factor of interest does not have an effect, provide meaningful information and can challenge the status quo, the researchers say.

“It’s always cool to show that something you expected to have an impact on animals, like stress, does so, but it’s equally important to show where there is no impact,” MacLeod says.

“If we only reported results that show stress having an impact, we might over-estimate the effects of stress. Our results suggest that animals are resilient to stress where it matters: in the context of sexual signaling, which is likely to be critical in determining their ability to successfully reproduce.”

MacLeod is currently a research fellow at Lunds University in Sweden. Gail McCormick of Penn State also contributed to the research. The research had funding from the National Science Foundation and the Society of Ichthyologists and Herpetologists.

Source: Penn State