Reference: Nielebeck, C., Kim, S. H., Pepe, A., Himes, L., Miller, Z., Zummo, S., Tang, M., Monzón, J. D. (2023). Climatic stress decreases tick survival but increases rate of host-seeking behavior. Ecosphere, 14(1), e4369. https://doi.org/10.1002/ecs2.4369
Out for Blood
Choosing how to spend limited time, energy, and resources is a daily struggle for all animals. While there might not be a correct solution to this debacle, weighing the options daily can help increase efficiency and productivity. Thus, animals must decide on the optimal strategy that will allow them to survive another day. Even animals as small as ticks walk this tightrope.
Ticks survive by feeding on the blood of humans and animals, called a blood meal. Ticks can pose a health risk to humans, domestic animals, and wildlife due to the numerous diseases they can transfer during a bite. Studying their behavior can provide insight into the circumstances that make them more likely to contact humans, pets, or livestock.
Like humans, water is a vital ingredient to ticks’ survival, but most ticks cannot drink water. They absorb most of their water from the air when the humidity is high (65-100%). In their habitat, that means staying near the wetter soil. However, searching for a blood meal requires climbing up tall grass – far from the moist soil – and outstretching their front legs to snag a passerby, an activity called questing. Enter the debacle: is it better to conserve water and stay near the soil or climb the grass to quest for a nutritive blood meal?
Ticks use information about weather conditions to inform their decision. Under climate change, increased temperature and relative humidity could change tick behavior in unexpected ways. Tick experts at Pepperdine University tested this hypothesis in three common tick species found in North America.
Testing for Questing
Ticks are most likely to attach to humans and animals in open spaces of grasslands, brushland, or forests. Yet warming temperatures due to climate change have allowed ticks to expand into areas they haven’t been found previously. Exotic species are also on the rise as they travel with humans from other parts of the world. As a result, the number of tick-borne diseases has increased and the number of reports of tick-borne illness has doubled in the United States in the last two decades.
The lone star tick (Amblyomma americanum), the American dog tick (Dermacentor variabilis), and the black-legged tick (Ixodes scapularis) are the most medically important ticks in North America. These ticks are known carriers of a list of tick-borne diseases. In particular, a bite from the lone star tick can cause alpha-gal syndrome, an allergic reaction to red meat. Also, the black-legged tick is infamous for carrying the bacteria that causes Lyme disease. Thus, figuring out how changing climatic conditions will affect the tick’s decision to quest has huge implications for human health.
To find out what makes these ticks tick, Caleb Nielebeck and a team of researchers tested the questing behavior of these three species in different temperatures and humidity treatments. Ticks either experienced low (60-75oF) or high (75-95oF) temperatures and low (32%), medium (58%), or high (84%) relative humidity. When in stressful climatic conditions, ticks were more likely to quest but less likely to survive.
Temperatures and Questing on the Rise
All ticks died more rapidly in hot, dry conditions. Ticks generally died when they had lost 27 to 30% of their body mass in water, but they differed in how quickly they lost water. Black-legged ticks lost water the fastest – five times faster than the lone star tick and 11 times faster than the American dog tick. All ticks quested more frequently when they were near death as a last-ditch effort to find a blood meal. Temperature and humidity had different effects on the questing behavior of the three tick species. The American dog tick quested at higher heights in the hot, humid condition. The lone star and black-legged ticks quested more frequently in hotter than colder conditions.
Climate change will likely have different impacts depending on the species of concern. While climate change is expected to reduce the survival of all ticks, it is also suspected to increase the frequency of tick questing. Where more ticks are questing, there is an increased likelihood of them attaching to humans and animals and transferring disease. The Lyme disease-carrying black-legged tick was very vulnerable to changing climates. However, the alpha-gal syndrome-causing lone star tick quested more frequently as things heated up, which could lead to more attachments.
Research that tracks tick behavior through changing environments is worth the time, energy, and resources because it helps to protect humans against tick-borne diseases. For more information, visit the Centers for Disease Control and Prevention’s page on ticks.