Spider signals

31 October 2016 | Research
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Typical courtship behaviour between the male (left) and female spiders (Picture: Caleb Nicholson)

Ultraviolet (UV) reflective markings on male jumping spiders of the Cosmophasis umbratica species have been proven to impact the spiders’ sex recognition and sexual selection.

The study, led by Associate Professor Li Daiqin from NUS Biological Sciences, is the first to investigate the impact of two different types of UV on sexual signalling; UVA — UV light with wavelength between 315 and 400 nanometres, and UVB — UV light with wavelength between 280 and 315 nanometres. Previous similar studies focused only on UVA or UVB.  

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The entire study, led by Assoc Prof Li, took about a year to complete

“Only the male reflects UVA and UVB but the females don’t,” said Assoc Prof Li. “It’s a very important phenomenon called sexual dimorphism in colours.” The markings led the team to hypothesise that the males use the UV reflective markings to attract the females, while the markings on the males act as a factor for the females’ choice of mate and male-male competition.

Filters which selectively block UVA and UVB reflected from the male spiders were used during the experiments and male-male interactions and male-female interactions were observed. Female spiders were shown to spend significantly more time interacting with males when UVB was not blocked as opposed to males with UVB blocked.

However, when the female spiders faced male spiders with UVA blocked, the former displayed agonistic behaviour instead of courtship behaviour. This indicates that they no longer perceive the male as a potential mate, but as a rival. In a similar manner, male spiders would display courtship behaviour to another male spider when UVA is blocked. This indicates that due to lack of UVA reflection, the male spiders no longer view the other male as a rival, but as a potential mate.

The researchers were able to conclude that UVB is a factor in mate choice while UVA is important in sex recognition.

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The UV reflective markings, known as structural colours, can be seen as iridescence on various parts of the spider’s body

These UV reflective markings are structural colours, Assoc Prof Li further explained. Unlike commonly used chemical colours like paint, the colours are not pigments but instead produced by the arrangement of physical structures like scales and feathers. They are seen as iridescence by human eyes and at different angles, can display different colours. This allows the spider to simultaneously camouflage itself and hide from predators while sending courtship signals. Studies into structure colours include biomimicry for use in the military industry.

The findings of this study were published in Animal Behaviour earlier this year.