Female octopuses stretch further

Scientists in Italy developed this method of measuring octopuses' arm extensions

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Female octopuses go to extra lengths when stretching their arms to reach food, a study has found.

Scientists in Italy measured octopuses' arm extensions as they reached up a tube towards tasty bait.

They found for the first time that octopuses' arm elongation ability differed depending on their sex and size.

The findings are published in the Journal of Experimental Marine Biology and Ecology.

Scientists had expected to find that smaller octopuses elongated their arms further than larger animals, but "more surprising was the difference between males and females similar in body size", said research team member Dr Laura Margheri, from the BioRobotics Institute at Scuola Superiore Sant'Anna university in Pisa, Italy.

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In the wild, octopuses employ their flexible appendages in a range of activities such as cleaning, defending themselves, capturing prey and mating.

Their arms make up the majority of their body weight, and contain most of the nervous system.

Researchers wanted to understand more about octopus biomechanics and how this might relate to their behaviour, so set up a novel experiment to measure common octopuses' (Octopus vulgaris) ability to elongate their arms in pursuit of food.

The team set up a tank containing a diagonally-positioned, transparent tube. Octopuses were trained to extend an arm towards a food parcel, which researchers pulled up the tube to encourage the animals to stretch their arm to its maximum length.

The scientists used video cameras to record the octopuses' arm extensions inside the tube. They compared the recorded lengths with the normal length of the animals' arms when swimming with their limbs straight out behind them.

Reaching out

The measurements showed that all 19 octopuses observed could extend their arms more than twice their normal length.

Common octopus Common octopuses can be found off the UK's south and south-west coasts

Females and smaller-sized octopuses achieved higher elongation percentages than males and larger animals.

"Greater elongations by smaller animals to reach a food target... may be explained by a greater need of nutrients, higher agility and energetic metabolism," explained Dr Margheri.

But she said that the "influencing behavioural differences between females and males are less clear".

While the octopuses preferred to use their longest arm to reach for the food, the sex divide in stretching ability was greatest when males used an adapted arm called the "hectoctylus".

Males use this specialised limb during mating to insert sperm into the female.

Dr Margheri described the differing elongation of this particular arm as "an interesting observation".

"[It] could be explained as a preservation and defence mechanism of the reproductively modified arm because an injury might preclude any future mating," she said.

She added that previous studies have observed male octopuses holding their hectocotyli close to the body when foraging in the wild.

"It is possible to assume that males would extend [the arm] only for the purpose of mating," she told BBC Nature.

Scientists suggest that studying the biomechanics of octopuses' arms could help them understand how mechanical differences might influence the animals' behaviour such as hunting, exploring and mating.

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