NatureWrap: Remora on a whale of a ride

Remora are famous for hitching a ride on the bodies of sharks and whales, using the suction in their front dorsal fins to seemingly hang on for dear life.

Looks can be deceiving, however. By observing camera-tag video data, researchers from the US and Spain have discovered that they in fact move around to feed and socialise. And that’s all possible if they pick the right spot on their transport. 

“Our work provides strong evidence that remora preferentially select regions offering lower drag and, in particular, regions near separation of flow caused by surface features and in the wakes that develop downstream of those regions,” the authors write in the Journal of Experimental Biology.

Complicated computer calculations found that by sheltering in these regions the remora (also known as sharksuckers or suckerfish) experienced only 20% of the drag that they would if swimming freely. A few individuals were spotted in the wrong regions – but usually moved to the right ones.

In the right places they were able to lift off to feed before returning, manoeuvring with ease in the slow-moving cushion of water carried along by whales.

“Positional changes were often accomplished by a skimming behaviour in which a remora maintained a distance of a couple of centimetres or less above the body of the whale, swimming in a slow and controlled fashion,” the authors write. 

Gorillas stay friends – on their terms

Mountain gorilla groups are friendly to familiar neighbours provided they stay out of core parts of their territory, new research shows.

Gorillas live in tight-knit groups, foraging, resting and sleeping together around a core home range and a wider peripheral range, but these groups sometimes split permanently, separating gorillas that may have lived together for years and be closely related.

The study, based on 16 years of data on 17 groups in Volcanoes National Park, Rwanda, shows that groups that were previously united are more than four times as likely to be friendly to each other when they meet, even if they had split over a decade earlier. The findings are published in the Journal of Animal Ecology.

Humans have a recognised capacity for cooperation based on the friendships that extend beyond our immediate group, and one theory for the evolutionary benefit of these wider friendships is that they allow shared access to space and resources with a reduced risk of aggression.

The new study – by researchers from the Dian Fossey Gorilla Fund and the University of Exeter, UK – is the first to test this theory beyond humans and the findings suggest, they say, that gorillas may benefit in the same way from maintaining “friendships” between groups.

Ogre-faced but not hard of hearing

The rather harshly named ogre-faced spider (Deinopis spinosa) has great vision thanks to its massive eyes, and surprisingly good hearing despite having no ears.

Studies show that it uses hairs and joint receptors on its legs to pick up sounds from at least two metres away, suggesting, the researchers say, that spiders can hear low-frequency sounds from insect prey as well as higher frequency sounds from bird predators.

“I think many spiders can actually hear, but everybody takes it for granted that spiders have a sticky web to catch prey, so they’re only good at detecting close vibrations,” says Ron Hoy from Cornell University, US, senior author of a paper in Current Biology.

“Vibration detection works for sensing shaking of the web or ground, but detecting those airborne disturbances at a distance is the province of hearing, which is what we do and what spiders do too, but they do it with specialised receptors, not eardrums.”

While it uses its keen night vision to catch prey on the ground, D. spinosa (also known as the net-casting spider) can catch insects in the air by performing an elaborately choreographed backwards strike, which does not seem to rely on vision.

What Hoy and colleagues want to know is whether that also means it has directional hearing.

Whales eat the same but different

Australian researchers have used biochemical tracers in blubber to track the diet of humpback whales over 10 years.

There have been recent observations of whales feeding on multiple food sources when migrating along the south-east coast of Australia, and the team from Griffith University and CSIRO wanted to know more.

Each food source leaves a distinct chemical signature in body tissue, and by following free swimming whales off Queensland’s North Stradbroke Island the researchers were able to extract those signatures, or fatty acid profiles, and identify any variability in diet over time.

And the results surprised them. The whales don’t seem to alter their diet from year to year, but their principal prey – Antarctic krill – does.

“Krill migrate both vertically and across regions and as their diet depends on the organic matter they encounter, their changing chemical profiles from year to year likely reflects climate induced changes in the phytoplankton assemblages they’re eating, as well as changes in where they are feeding,” says Griffith’s Jasmin Groß, corresponding author of a paper Scientific Reports.

And that, she says, may be a positive. “The fact that Antarctic krill, the humpback’s primary food and a crucial link in marine ecosystems, have a more varied diet could make krill populations more resilient to changing environmental conditions.”

The dangers of microplastics

Less positive is a study, led by Australia’s ARC Centre of Excellence for Coral Reef Studies, which suggests that microplastic exposure may have a greater impact on fish survival and behaviour than the degradation of their coral reef habitat.

The researchers raised damselfish in microplastic polluted waters then placed them on live or dead/degraded coral patches. They found those reared on microplastics or released into dead corals were bolder an more active, and had lower survival than controls.

The effects of plastic exposure were also more pronounced than reef degradation alone. The authors suggest, in their paper in Proceedings of the Royal Society B, that efforts to address microplastic ingestion should go hand-in-hand with efforts to restore reefs.