Siblicide in the city: the urban heat island accelerates sibling cannibalism in the black widow spider (Latrodectus hesperus)

Urbanization can compromise biodiversity as cities expand further into native landscapes. The urban heat island (UHI) describes elevated urban temperatures due to heat retained by built structures (e.g. concrete surfaces). Animal behavior may be critical in determining an animal’s ability to thrive in the wake of human disturbance. Yet, we have a relatively poor understanding of how animal behavior is affected by the UHI. We examined sibling cannibalism in urban and desert juvenile black widow spiderling (Latrodectus hesperus) lineages exposed to extreme UHI temperatures (33 °C) or native Sonoran desert temperatures (27 °C). Family of origin had a significant impact on cannibalism, while the effects of temperature and habitat were dependent on spiderling density. Our most pronounced results came at the lower densities of three and two spiderlings. Here habitat and temperature interacted such that spiderlings at 33 °C were consistently more cannibalistic than spiderlings at 27 °C, and this result was consistently stronger for spiderlings from urban families than it was for spiders from desert families. If UHI-induced siblicide promotes the survival and growth of a subset of spiderlings from a clutch, then it might actually foster urban population growth. In contrast, if siblicide in the city simply reduces clutch size, then we might expect the UHI to slow urban infestations. Understanding behavioral phenotypes underlying the explosive population growth of urban pest species will aid in the development of safer and more effective deterrents to infestations, and allow us to better understand the mechanisms shaping urban biodiversity patterns.