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City spiders bigger, more fertile than rural counterparts: Study

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Spiders in cities are larger and produce more offspring than their cousins in the country, according to a new study from Australia.

The humped golden orb-weaving spider, otherwise known as Nephila plumipes, is native to wide areas of the Land Down Under. The eight-legged animal is found in rural areas, as well as urban environments. Despite evolving in the wilderness, the species is thriving in man-made settings.

University of Sydney researchers investigated the spiders, comparing the size and reproductive ability of urban and rural populations. The team found spiders living in cities were larger, and capable of giving birth to more offspring. This last data was obtained by weighing ovaries.

"The effects of urbanization on wildlife are very varied - some do well, others don't. Animals which benefit from urbanization are called urban exploiters, and these species (including the spiders from this study) do better in urban areas than their natural habitats," Elizabeth Lowe, a doctoral candidate at the University of Sydney's School of Biological Sciences and leader of the study, said.

Arachnids, over 200 in all, were collected from urban, suburban and rural areas in and around Sydney, Australia.

Tibias, often used by biologists to measure body size in spiders, averaged 0.42 inches long over all specimens collected. However, spiders from the city had the longest tibias of all, averaging 0.55 inches in length.

The heat island effect is likely responsible for the greater size seen in urban spiders, according to researchers.

"Hard surfaces [like buildings, roads and concrete] retain heat, leading to the urban island-heat effect. This increase in temperatures is likely what is leading to increased growth of the spiders," Lowe stated in a press release.

Heat islands are created when man-made objects like roads, buildings and bridges retain heat more efficiently than natural objects, such as trees and underbrush. This can raise temperatures in urban areas by as much as 5.4 degrees Fahrenheit over surrounding undeveloped areas.

Ovaries of urban spiders weighed in at 39 percent of body weight, the highest ratio of all specimens in the study. This suggests they are more capable then their rural cousins of rapid population growth.

Humped golden orb-weaving spiders in cities may be beneficial to people living in the city. The arachnids trap and consume vast numbers of mosquitoes and other small creatures that can bother humans, and potentially carry disease. The spiders are capable of producing a mildly painful bite, but they are wary of humans, and rarely attack.

Investigation of size and reproductive capabilities of orb-weaving spiders was detailed in the online journal Plos One.

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