If you follow along with dolphin science news, you probably know about a recent study showing that whale and dolphin (cetacean) pelvic bones, which were once considered 'evolutionary leftovers', are now thought to be a vital component of their sexual reproduction equipment. Scientists discovered that bigger pelvic bones in cetaceans correspond to bigger testicles, and also allow for more agile penises by acting as a stable base to anchor the penis muscles. Yes, dolphins have penis muscles. A bit more penile control coupled with large testicles (which produced more sperm) gave males of some species of cetacean a mating advantage.
Old news, right?
But this list of dolphin news might not be. Below are a few dolphin science discoveries that seemed to have slipped under the media’s radar over the past few months.
Octopuses attack dolphins in New Zealand, hijinks ensue
Kaikoura in New Zealand is home to a group of wild dusky dolphins that are well-loved by both tourists and dolphin researchers. But they're less well-loved by the octopuses that also live in the area. Dolphins have been observed playing with innocent sea creatures like octopuses in other parts of the world – possibly passing them around in a game of cephalopod keep-away (like they do with seaweed). The dusky dolphins in New Zealand engage in similar hijinks. But the octopuses are having none of this nonsense, and were observed on two separate occasions in a recent study having firmly attached themselves to the faces, flanks and nether regions of the dolphins. Scientists suspect the octopuses were trying to avoid being chewed on by the dolphins, who were none too pleased to have a suckered hitchhiker – and tried to shake them off by swimming erratically and just generally freaking out. Strangely enough, the dusky dolphins, who are famous for their acrobatic leaps, didn’t try to jump out of the water to dislodge the octopuses (which calls into question one of the proposed functions of dolphin leaping).
Dolphins crash a fishing tournament, steal all the bait
After fishermen approached a conservation biologist to complain about dolphins stealing their bait during the Luanda Sailfish Classic off the coast of Angola, a mini-investigation was launched to determine how big of a problem this dolphins-stealing-bait thing (officially called depredation) was. A survey was given to the fishermen who had taken part in the tournament, with a number of boats reporting groups of rough-toothed dolphins – numbering in the hundreds – stealing bait from their fishing lines. All attempts to outrun the marauding dolphin groups were futile, it seems. A handful of fishermen threw in the towel on their fishing exploits and instead hung out with the dolphins, tossing fish to them and even jumping in the water for an impromptu dolphin-swim encounter. From a conservation standpoint, this behaviour is bad news: dolphins that are fed from boats or interact with fishing gear are likely to get entangled, or receive injuries from boat propellers. Similar incidents involving commercial fishing industries sometimes result in frustrated fishermen taking revenge on dolphins – shooting them, poisoning them, or even trying to 'persuade' them to leave the area by dropping explosives in the water. (You can learn more about the risks that the fishing industry poses to dolphins and other animals at the website of the Consortium for Wildlife Bycatch Reduction.)
Further evidence that dolphin signature whistles function like names
Scientists have uncovered more evidence that dolphins call out each other’s signature whistles to get each other’s attention, and that this is a friendly gesture. Signature whistles are unique whistles that many species of dolphins use as a way to broadcast their identity to their social partners. Each individual dolphin will have its own whistle that functions a bit like a name. Dolphins occasionally use the whistle of their close social partners, which scientists suspected is a way for them to get their friends’ attention. A recent study showed that when a dolphin hears their own signature whistle being used by a friend, they usually respond by immediately repeating their own whistle back again. Unlike many bird species that will imitate each other’s songs as an act of aggression when defending their territory, it seems that dolphins use their social partners' whistles like this in a friendly manner.
Dolphins (finally) pass the invisible displacement test
Dolphins are skilled at understanding how to manipulate objects, whether it’s using marine sponges as tools or playing complex games with bubble rings. The language-trained dolphins at the Kewalo Basin Marine Mammal Laboratory were even able to 'tell' researchers whether an object was present or absent. So it was a bit of a shock when dolphins failed parts of the 'object permanence test'. The test measures whether an animal understands that an object continues to exist even when it’s no longer visible. In the final stage of the test (called the invisible displacement test) an object like a ball is placed in a container and the container is covered with something like a sheet. The ball is then surreptitiously dumped out, and the now-empty container is pulled from behind the sheet and shown to the animal. For some reason, tests like this seem to flummox dolphins, who don’t understand that the ball is now behind the sheet. But in a recent study, researchers tweaked the experimental design and showed that dolphins can figure out invisible displacement after all. Instead of using things like balls and containers, the researchers projected 2D shapes onto a screen, with the target object disappearing behind a series of large blocks. In this test, the dolphins were easily able to track the movements of the target object. Researchers suspect it was something about the idea of an object disappearing into a container that was stumping the dolphins in previous experiments (after all, there are very few container-like objects in a dolphin's natural environment).
Top header image: jE norton, Flickr