Effects of temperature on ecology, behavior, and physiology in desert-dwelling jumping spiders
Temperature affects ectotherms in a variety of ways that are critical to fitness. This work focuses on how temperature impacts the behavior, physiology, species distribution, and ecology of jumping spiders to better understand their evolution and natural history. As my study system, I used spiders in the genus Habronattus, mostly H. clypeatus. I chose to study these animals because they live in the desert, an extremely thermally-variable habitat. Habronattus are also ecologically-important mid-level predators and have striking complex courtship signals.
My first chapter assessed how temperature influences sexual behavior. Temperature has been known to affect behavior in different ectotherm species. The effects on sexual behavior can be especially complex, as different sexes may be affected differently by temperature. I examined this in Habronattus clypeatus. In this species, males court females using visual and vibratory signals. I tested whether key intersexual behaviors would change with temperature in similar, predictable ways across males and females. I first measured temperature and apparent activity of individuals in H. clypeatus habitat across the day. I found that H. clypeatus are active across a wide range of temperatures (11-56 °C) and are most active at times of day when temperature ranges from 13-46 °C. Next, I performed mating experiments across behaviorally relevant temperatures. Females were more likely to allow males to progress to later stages of courtship and had higher mating rates at higher temperatures. Male visual and vibratory courtship behaviors generally became faster, higher-pitched and lower in amplitude at higher temperatures. This relationship between temperature and signal aspects generally attained a roughly curvilinear shape, with an asymptote around 40 °C. Intriguingly, mating rates in the lab were highest at temperatures potentially above those during peak spider activity in the field. My results suggest that temperature’s effects on behavior are complex and can affect males and females differently. This work emphasizes that understanding temperature effects on mating is critical to understanding sexual selection patterns particularly in species which use complex signals.
There is evidence of environmental temperature influencing species distributions. However, despite the importance of a broad understanding of an animal’s thermal biology, few studies incorporate more than one of these metrics of thermal biology. I explored how temperature influences species distributions in six different species of Habronattus distributed along an elevational cline. I measured several different aspects of their thermal biology including thermal limits (CTmin, CTmax), thermal preference, V̇CO2 as proxy for metabolic rate, locomotor behavior, and warming tolerance. I used these data to test whether thermal biology helped explain how species were distributed across elevation. Habronattus had very high CTmax values (~ 52°C), which did not differ among species across the elevational gradient. The highest-elevation species had a lower CTmin than any other species. All species had a strong thermal preference around 37°C. With respect to performance, one of the middle elevation species was significantly less temperature-sensitive in metabolic rate. Differences between species with respect to locomotion (jump distance) were likely driven by differences in mass, with no differences in thermal performance across elevation. I suggest that Habronattus distributions follow Brett’s Rule, a macrophysiological principle that predicts more geographical variation in cold tolerance than heat. Additionally, I suggest that physiological tolerances interact with biotic factors, particularly those related to courtship and mate choice to influence species distributions. Habronattus also had very high warming tolerance values (<20°C, on average). Taken together, these data suggest that Habronattus are resilient in the face of climate-change related shifts in temperature.
The most common studies of thermal biology are undertaken with lab experiments. Far less work has been done to understand natural thermal environments, particularly on the spatial and temporal scales relevant to the animal in question. In my final chapter, I sought to put into context the various laboratory measurements I had taken in the previous two chapters, specifically for H. clypeatus. I conducted a study to assess (1) the variability of thermal environments, (2) the ability of animals to thermoregulate and (3) substrate usage. I used a number of thermal ecology methods, including focal observations and a variety of different habitat temperature measurements (thermal cameras, ambient temperature loggers, and operative temperature models). I first found that males and females differed in their thermal preferences in the lab. Although spiders were able to thermoregulate remarkably well in the field, the differences between males and females disappeared. I suggest that this is primarily because males search for and follow females through the habitat, and females therefore decide where courtship occurs. This was corroborated by data that showed that adult makes move farther than any other age-sex class. Spider thermal habitats are also extremely thermally variable. This is mostly due to variability in substrates, rather than air temperature. Spiders also used these substrates in non-random ways. This could have important implications for thermoregulation and mate choice. Behavioral differences between the sexes hint at additional potential for conflict between the sexes. Females are active earlier in the day than males. These earlier times correspond to habitat temperatures at which females are less receptive. We also found that males spent more time hiding, which possibly relates to tradeoffs associated with greater activity and exposure to predators. Overall, this chapter study suggests that habitat temperature interacts with animals in complex ways, providing the potential for tradeoffs that could be under selection. Understanding thermal biology in the context of natural environments is therefore key to gaining a wholistic view of how animals interact with temperature.
Overall, this work emphasizes the importance of an integrative view of thermal biology. Temperature affects animals on many different levels, and one must reach across disciplines to gain the full picture. These studies also suggest that temperatures’ effects can be subtle. Attention to detail is key, and one must have a thorough understanding of the habitats in which animals live in order to understand how selection acts. In this way, we can make predictions about how individuals, populations, and species might respond in response to changes in climate.