Black Widows and Urban Heat: Can Spiders Cope With Increased Temperatures?

This week’s post comes from Dale Stevens, who tells us about the effects of elevated urban temperatures on black widow spiders. Along with landscape changes, urbanization also leads to a significant increase in temperature.  Human-made surfaces retain heat during the day and stay hot well into the night. Because of this, animals that thrive in urban environments have to deal with higher temperatures at night. These higher night temperatures can result in thermal stress, which can impact growth, development, and behavior. In a recent study published by Chad Johnson, Javier Urcuyo, Claire Moen, and Dale Stevens, the role that urban heat has on the growth and survivorship of black widows was tested to determine just how harmful these increased temperatures can be.

How hot are urban heat islands?

Figure 1. Spiders experience average temperatures on average 6°C greater in urban environments compared to rural environments.

The authors first had to determine just how much hotter the city of Phoenix is than the surrounding desert. To do this, they placed miniature thermometers (aka “iButtons”) in black widow spider webs to measure the temperature differences between desert and urban sites. These measures also showed what temperatures the spiders themselves were exposed to while in their webs. Spider habitats in the city are on average 6° Celsius hotter than those in the surrounding desert (Fig. 1).

The experimental design

To collect spiders for the experiment, researchers went around to back alleys and freeway underpasses. Once they had enough adult females, they fed and housed spiders in their research facility at Arizona State University at the West Campus until an egg sac was produced.

Researchers then split the eggs into two groups: one reared at 27° C, the other reared at 33° C. These temperatures mimic the two extremes measured in urban webs. The spiders’ growth and mortality were recorded as they developed into adults.

 

Spiders develop smaller and slower in the heat

The results of this experiment were quite exciting! Spiders in the 33°C treatment reached their third molt – or the 3rd time spiders develop a new external skeleton – much later than those in the 27°C treatment. Furthermore, the spiders in the 33°C treatment were significantly smaller than those in the 27°C treatment at this growth stage (Fig. 2).

Figure 2. Spiders exposed to warmer temperatures took longer to reach their 3rd molt and were smaller than spiders exposed to cooler temperatures.

When looking at male spiders, those in the 33°C treatment reached the sub-adult phase (near-maturity) much quicker than those in the 27°C treatment but were also significantly smaller when they got there. Sadly, only 2 males were able to reach full maturity in the 33°C treatment and they died the next day. This means that the male spiders exposed to these high temperatures would typically die before they are able to reproduce in the wild!

Figure 3. Males matured quicker at warmer temperatures but were smaller at adult size.

Spider behavior changes for males and females differently

The researchers were also interested in how these different temperature treatments influence behavior. One important behavior for widow spiders is prey capture. For males, those in the 33°C treatment were much faster at attacking prey (fruit flies) than those in the 27°C treatment.

The researchers also asked how these temperatures influenced female spider web-building behavior. Black widow females depend on their webs to capture prey and keep them safe from predators. The way these temperatures influence female web building and prey capture behavior is critical to understand how they are able survive in harsh urban environments. Over the last three stages of development, female spiders exposed to 33°C were significantly less active in building a new web than those at 27°C. However, this effect got smaller and smaller as the widows matured into reproductive adults. Eventually, there was no effect on the spider’s web-building behavior when they reached adulthood.

Figure 4. Female black widows exposed to higher temperatures were less active when building a new web compared to those at the lower temperature, but this effect decreased as the spiders matured.

So what does all of this mean?

It seems clear that these spiders don’t do very well in hotter temperatures. They developed slower and at smaller sizes, and while males reached near-maturity faster, they died before they could reach reproductive maturity. The male spiders are more voracious when they forage, perhaps in an attempt to make up for the stress their bodies are under at the hotter temperature. In contrast, females invest less energy into web-building when young, possibly in an attempt to preserve as much of their resources as possible so that they have the energy to reproduce when they reach maturity. Overall, it seems like urban spiders will struggle with elevated urban temperatures, but will find a way to compensate by being either more aggressive or less active as a means to mitigate the physiological challenges that come with higher temperatures.

Read the full manuscript:

Urban heat island conditions experienced by the Western black widow spider (Latrodectus hesperus): Extreme heat slows development but results in behavioral accommodations

 

Cover image: James Gathany, Black widow spider 9854 lores, marked as public domain, more details on Wikimedia Commons

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