Kissing is a very enjoyable aspect of human sexual activity. Is it universal among human cultures? Does it occur in other species?Pete Fowler, Southland, New Zealand

On a beach walk last year in northern Mayo, Ireland, I thought I had found a spectacular fossil seaweed. Close inspection showed that instead it was a kind of photo image of a sargassum-type weed on a Dalradian metavolcanic rock. There were other examples nearby, all at the mean high water level and all of the same species of weed on the same rock type. I took the rock home and, a year on, the image is still crisp (see photo). However, all attempts to imitate the conditions have failed. Do readers have any idea of how the image occurred, and are there any other instances of natural photoetching?Tony Legg, St Martins, Jersey

While swimming near the island of Gozo, off Malta, I came upon thousands of jellyfish in the shallows. Among them was this astounding life form. Is it a deep-sea creature, a kind of jellyfish or a congenital abnormality?Ian Dingwall, Solothurn, Switzerland

While working at a factory that produces carbon powder, I noticed I had made a large black thumbprint on one of my sandwiches. This set me wondering why bread, or for that matter potatoes, rice and sugar, which are mostly carbon, are not black.Douglas Thompson, Holywell, Flintshire, UK.

Cats all seem to like fish, so why are they unwilling to swim?Tom Lorkin Beckenham, Kent, UK

On a field course in the foothills of the French Alps in July 2006, some of our students noticed a seemingly intact duck egg in a small pond. It clearly contained something moving. When we broke the shell we found three live minnows (Phoxinus phoxinus) inside (see Photo). Do any readers know for sure how the fish ended up here?Matthew Cobb and Henry McGhie, University of Manchester, UK

There are billions of birds worldwide, so why is it that you rarely, if ever, see a dead one?Maurice Boland, REM FM Radio, Marbella, Spain

I have two female dogs whose urine was killing patches of grass all over my lawn. On my mother's advice I started feeding them tomato ketchup and, sure enough, the dead patches stopped appearing. Why does this work, and is it harming my dogs?Jim Landon, Swindon, Wiltshire, UKThe following answers were selected and edited by New Scientist staff. You can add your replies in the comments section below.The urine acts as a liquid fertiliser, but can produce nitrogen overload where the puddle of urine is deepest. This "burns" the grass, creating a brown patch in the lawn.Towards the outside of the puddle, where less nitrogen has been applied, there can be a fertilising effect leading to a ring of luxuriant, greener grass. The urine of dogs and bitches does not differ much but, while dogs tend to deliver small samples of urine to mark their territory, bitches tend to empty their bladders entirely, causing more harm.Urine is slightly acidic, but so is tomato ketchup, so it does not neutralise the urine as some people believe. Instead, the salt content of tomato ketchup, juice or sauce makes dogs drink more, diluting the nitrogen in their urine.Be aware that increased salt intake can cause problems with existing kidney or heart conditions, so if you must tinker with your dogs' diet, consider reducing the protein content instead. This will also reduce the nitrogen content of their urine, and should be fine for all but the most active of dogs. Better still would be to train your dogs to urinate in a designated place or follow them out of the house with a hose pipe or watering can to dilute their urine.Mike Follows, Willenhall, West Midlands, UK

Scientists have worked out the evolutionary basis for many behaviours such as altruism and jealousy. However, over the millennia, people have been unbelievably and gratuitously cruel to each other. The evolutionary advantage of this is not obvious. So what is the biological basis of human cruelty?Brian Kavanagh, Maidstone, Kent, UKThe following answers were selected and edited by New Scientist staff. You can add your replies in the comments section below.There may not be an evolutionary advantage, at least not any longer. It is more likely a throwback from the past. Not so very long ago in evolutionary terms, humans lived as hunter-gatherers in small groups. Indeed, many such groups still remain, in the remnants of tropical rainforests, for example.There is safety in numbers and individuals can specialise in what they are good at, knowing that other essential tasks will be taken care of by their companions. However, although in-group individuals may be loving and caring towards each other, there is open hostility towards any out-group that competes for territory and food resources. Neanderthals (Homo neanderthalensis) were one group that lost out to humans (Homo sapiens) in such past conflicts. However, to speak of cruelty in an evolutionary context seems inappropriately anthropomorphic, despite the fact that we are discussing human beings.Homo sapiens evolved in parallel with the other great apes, in which we can see similar "cruel" behaviours. Even within their own groups, great apes are not necessarily loving and caring. They nevertheless gain advantage from living in a group, through increased security and the sharing of tasks. Such hunter-gatherers have to carve out and defend a territory for themselves in order to survive. In consequence, outside groups are a threat to their existence. A group of apes has its hierarchy, and a conflict between two groups is a threat not only to survival, but to the dominance of the males in a particular group. The conflict is resolved by either driving off the rival group or by killing all the rival males in the group and assimilating the females and young. Whichever group wins the conflict is obviously the fittest in terms of survival. That's what it's all about in the end, cruel or not.Modern humans behave in a similar fashion. It is quite clear even today that humans belonging to one group can easily be influenced to see humans of another group as being subhuman and inferior, enabling their extermination without mercy (see "They made me do it", New Scientist, 14 April, p 42). This is true even when the differences are over beliefs, rather than over limited resources.The Americas were recently colonised by Europeans, who pushed the Native Americans aside, and killed any that resisted. Hitler's motivation was just the same in the second world war. He envisaged a greater German territory, stretching to the Urals, into which the "superior" Aryan race could grow. The process repeats itself regularly, most recently in the Balkans and Darfur.Terence Hollingworth, Blagnac, FranceOnly one word is needed to answer this question - power. Let's leave women out of it. Very few women, if any, have reached the heights of cruelty of Nero or Caligula - or Saddam Hussein for that matter. The would-be alpha male must trample over all opposition, must create fear to keep himself in power and must keep enough of his cronies loyal. He thus obtains access to a great number of women, the best in material goods and food, and assures himself of the largest possible number of healthy descendants. Other methods of the cruel alpha male may include reducing the number of offspring of rivals, who might compete with his own children.Valerie Moyses, Bloxham, Oxfordshire, UKOne of your previous correspondents seeks to exclude women from the discussion of the survival of unbelievable and gratuitous cruelty as a human trait. They have not been so dominant in the history of large and dramatic behaviours, perhaps because they have seldom been in a position to initiate them. When they are in such a situation we see such exceptions as the 16th-century Hungarian serial killer Elizabeth Báthory, and ancient Roman females of the family that produced her role models, Caligula and Nero.

Today, in the everyday world, wannabe alpha females can create just as much nastiness as males, if not more. As a primary schoolteacher I have to deal with bullying. In 40 years, boy bullies, as opposed to boys who sometimes bully, have been rare. Female bullying is much more common, much more difficult to deal with, and delivers much more long-lasting, though less visible damage. "You can be my friend if you promise never to speak to her ever again," is the style of approach girls use.

I did ask the hangers-on once why they wanted to be the friend of someone who had demanded they dump another friend. They had no answer, even expressing disbelief that I should ask, despite being upset by the situation. Girls bullied by other girls are excluded, made to feel of no value, and have been driven to suicide. Females cannot be excused complicity in the production of cruelty.

I don't know why the trait of cruelty survives, but I do know why the trait of compliance survives. Survival depends on not being cast out from the central group. And while that trait persists, so do the queen bees.

Penelope Stanford, Greenhithe, Kent, UK