During penis fencing, each flatworm tries to pierce the skin of the other using one of its penises. The first to succeed becomes the de facto male, delivering its sperm into the other, the de facto female. For the flatworms, this contest is serious business. Mating is a fight because the worm that assumes the female role then must expend considerable energy caring for the developing eggs.

Chimpanzees were observed jabbing the spears into hollow trunks or branches, over and over again. After the chimp removed the tool, it would frequently smell or lick it.

In the vast majority of cases, the chimps used the tools in the manner of a spear, not as probes. The researchers say they were using enough force to injure an animal that may have been hiding inside.

However, they did not photograph the behaviour, or capture it on film.

In one case, Pruetz and Bertolani, from the Leverhulme Centre for Human Evolutionary Studies in Cambridge, UK, witnessed a chimpanzee extract a bushbaby with a spear.

They set up a careful experiment to allow the birds to cache food in a certain way if they were indeed planning, and found the birds were up to the task.

Their study, published in the journal Nature, adds to several others that show animals such as great apes and certain birds can plan ahead in much the same way as people do.

"Knowledge of and planning for the future is a complex skill that is considered by many to be uniquely human," Nicola Clayton and colleagues at the University of Cambridge wrote.

"We show that the jays make provision for a future need, both by preferentially caching food in a place in which they have learnt that they will be hungry the following morning and by differentially storing a particular food in a place in which that type of food will not be available the next morning," they added.

Whether it's a blowout argument or a dinner-table disagreement, a spat with your lover can be trying. Humans have of course devised ways of making up, including tight hugs and the customary apology flowers.

Killer whales have their own tricks for mending relations, a new study finds. Rather than a bouquet, however, they might opt for an intimate swim.

Studies have shown that chimpanzees kiss and hug after a dispute, and other primates such as bonobos resort to sexual activity to resolve conflicts. Until now, reconciliatory behavior had not been shown in any marine mammal.

Orcas, the largest members of the dolphin family, can reach swimming speeds at sea of 30 miles per hour (50 kilometers per hour) for short stints.

After the mother chased the father for several minutes, each zipped away to separate aquatic quarters to cool off for about 10 minutes. Then, the mates smoothed over their clash with side-by-side swimming, called echelon swimming