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

Can insects get fat?The following answers were selected and edited by New Scientist staff. You can add your replies in the comments section below.If, by getting fat, the questioner means obese, the answer is no. All insects undergo some sort of metamorphosis, passing through a larval stages before becoming an adult. The adult, or "imago", stage is relatively short-lived and very often adult insects do not feed at all. Mayflies (of the order Ephemeroptera) and many silk moths (of the family Saturniidae) are some examples. They neither have the time nor the inclination to feed and get fat.Those imagos that can feed are constrained by their inflexible exoskeleton. They have no means to expand this to take on excess fat. Incidentally, carcasses of death's head hawk moths that have been stung to death are often found in beehives, where they have made a vain attempt to feed. Their proboscis is too short to enable them to extract nectar from flowers, but long enough for them to consume honey if the bees in the hive would let them.The larval stages are similarly constrained. To grow they have to moult their exoskeleton periodically. Internal fluid pressure splits the exoskeleton and the insect expands into a new one it has grown underneath, which remains flexible just long enough to accommodate their increased size. They cannot keep doing this indefinitely; each species is limited to a predetermined number of moults. If they find abundant nourishing food, they will go through their moults quickly. If the opposite is the case, or if their food contains limited nourishment they will take a relatively long time to finish growing. Either way, once the moults are completed, they begin metamorphosis into the adult stage. They do not continue to get fatter and fatter and, in fact, the reverse can be true - those larvae unable to find sufficient food may begin metamorphosis early, skipping one or two moults. This produces a normal, if somewhat smaller than average imago.However, insects do store up a great deal of fat at the larval stage. Silk moths are generally large insects. Because they do not feed as adults they must have considerable fat reserves to enable the males to track down a female and mate and, in the case of the females, to produce and lay a large number of eggs, while sustaining their metabolism for a week or more. But this is the normal situation, they are not, in any way, obese.

Terence Hollingworth, Blagnac, France

Insect life cycles do not lend themselves to the concept of obesity as we know it. Most species accumulate food as fast as they can, but once they have enough, they enter their next stage of life, or reproduce and die. There are exceptions, but few can afford to get too fat - anything that interferes with their bodily function prevents reproduction, so what they cannot use, they dump. For example, sap-sucking insects get far too much sugar from plant sap, but instead of becoming uselessly fat, most dump the excess as honeydew or convert it into waxy armour. Using hormones to prevent insects from maturing may make them larger and fatter, but prevents their breeding.Still, healthy insects do accumulate fat. Their internal "fat bodies" are special organs crucial for storage, hormonal control, metabolism, growth, overwintering, fuel for travelling, yolk production for eggs and so on. Accordingly, many insects, such as locusts and termites, though not technically obese, are prized as fat-rich foods. As you may have seen on TV, termite queens of most species accumulate huge fat stores to support their role as virtually continuous egg-laying machines.

Jon Richfield, Somerset West, South Africa

With my colleague James Marden I have described (among other symptoms) infection-associated obesity in a dragonfly species. Infected dragonflies show an inability to metabolise fatty acids in their flight muscles and so build up lipids in their thorax, leading to a 26 per cent increase in thoracic fat content. The suite of symptoms caused by this infection includes decreased flight performance and reproductive success in male dragonflies (Proceedings of the National Academy of Sciences, vol 103, p 18805).

Ruud Schilder, School of Biological Sciences, University of Nebraska, Lincoln, Nebraska, US

I have just returned from a wonderful trip to the Galapagos Islands, where I observed the courting and nesting of blue-footed and red-footed boobies. What evolutionary advantage, if any, do their conspicuously coloured feet provide?Christina Bulawa, by email, no address supplied

A guest question from one of the Farmer's Weekly blogs:Why did this squirrel electrocute itself on power lines, when flocks of birds routinely sit on them for hours without any problems?Tim Relf, UK

Why do we like shiny things?Jonah Lawton, London, UK

If aliens wanted to create new human pet breeds using only selective breeding, what traits would they find easiest or hardest to alter and what kind of timescales would be involved? Would we be easier or more difficult to breed selectively than, say, dogs?Gerry Walsh, Maynooth, Kildare, Ireland

I get two bottles of milk delivered to my house each day. One, containing whole milk, ha a silver foil top, whereas the other, containing semi-skimmed milk, has a silver top overprinted with red stripes. Based on observation over several years, the local magpie population will often try to peck at and remove the striped top but hardly ever attack the plain silver foil top Have other readers observed magpies or other birds beings so discerning and is there a scientific explanation for it?tbcThe Last Word explores the science of everyday things. Both the questions and the answers are provided by the smartest people we know – you, the New Scientist users. You can post your answers in the comments under each blog post. Please keep on topic and concise.You can browse recent Last Word questions by topic: human body, domestic science, planet Earth, weather, Animals, plants, Environment, Technology and Transport. Or you can search all of the Last Word back to 1994 by adding your keywords to this search. You can also buy the latest book (US or UK).You can't post questions online just yet. For questions, and answers you want to send direct to us, please use email (lastword at newscientist.com, or use the button above right), post (Last Word, New Scientist, 84 Theobald’s Road, London, WC1X 8NS, UK), or even fax (+44 20 7611 1280).The writers of all answers published in our print magazine receive a cheque for £25 (or the US$ equivalent), so we'll need your email address to contact you. We reserve the right to edit for clarity and style. Reed Business Information Ltd reserves all rights to reuse the question and answer material submitted in any medium or format.