杏吧原创

Canine comforts

Dogs often greet people by jumping up and licking their faces. Why?

鈥� The main way in which a dog, wolf or jackal puppy begs for food to be regurgitated is to lick the face of its parents or another adult from beneath, particularly just below the muzzle. In common with juvenile or begging gestures in many species, this behaviour lingers on in adult dogs as a sign of submission or appeasement. You may see it, for instance, when one dog is being submissive to another and begging it to be friendly.

A dog鈥檚 face is the focus of its sensory, expressive and food-oriented behaviour, and domestic dogs have transferred these associations from their own faces, positioned at the front of a horizontal body, to ours, sitting atop a vertical one. By trying to lick human faces a dog is expressing its recognition of our superior social status and inviting us to be friendly.

Angus Martin

Camberwell, Victoria, Australia

鈥� Dogs tend to jump up and lick the faces of people and other dogs that enter their dens 鈥� now usually a kitchen or garden. This is down to retained ancestral behaviour and the permanent infantile state in which pet dogs are typically kept.

When young wolf pups are beginning to enjoy the delights of fresh meat in addition to their mother鈥檚 milk, they rely on her to bring the kill back to the den. The wolf carries the meal back in her stomach, and at the den is greeted by bundles of hungry fluff that signal their desire to eat by licking her face. She promptly responds by regurgitating the food, serving up a warm meal.

Today鈥檚 pet dogs continue to exhibit puppyish behaviour with their den 鈥減arent鈥�, nowadays a human, who provides food, grooming, comfort and play. Jumping up to greet you after you have been away 鈥渉unting鈥�, perhaps at the pet shop, is simply a dog鈥檚 way of requesting a tasty regurgitated meal. It is, however, easy to train dogs. My bouncy spaniel has learned that sitting quietly when I return magically makes my pockets regurgitate large chunks of hot dog.

Jacqueline Boyd

Aberdeen, UK

Tree total

I鈥檝e just returned from a trip to Cornwall where I spent a great deal of time watching the gulls out of my hotel window. They seemed happy to settle on any surface except the trees in the hotel garden. I never saw a gull alight on any part of any tree, even though the tree species differed. Why don鈥檛 gulls land on trees?

鈥� Gulls cannot perch on twigs and branches because their webbed feet, which are specially adapted for life on and around water, cannot grip rounded objects. Also, gulls are pretty large, heavy birds, so the smaller twigs and branches that usually make up the easily accessible outer parts of many trees are probably not substantial enough to support a gull. The stronger, broader tree limbs tucked away within the canopy are less accessible to a relatively large bird that needs open space to land and take off.

For the reasons mentioned above, gulls also don鈥檛 perch on overhead wires, preferring flatter, larger surfaces such as the tops of lamp posts 鈥� especially if my car is parked beneath.

Mike Thomson

Bridgend, Mid Glamorgan, UK

鈥� A local car park here is illuminated by large, flat light fittings connected to poles by narrow horizontal bars. They are favourite perches for both seagulls and crows.

The seagulls invariably stand on the light fittings because, I presume, their flat, webbed feet cannot grip a narrow bar. The crows invariably perch on the bars, perhaps because their claws would slide uncomfortably on the light fittings.

Fiona Vincent

University of St Andrews, Fife, UK

鈥� Might I suggest the questioner:

1) looks at a gull鈥檚 feet

2) obtains a pair of flippers as used by scuba divers

3) tries standing on the branch of a handy tree while wearing the flippers?

Kevin Luff

Douglas, Isle of Man, UK

See hear

I watch cricket and football matches on television with the sound turned down so I can listen to the commentary on the radio. Can anyone explain why there is about a half-second delay between what I hear and what I see?

鈥� You hear the radio first because the signal does not have to travel as far from its transmitter to your radio as the TV signal does from its satellite to your television. If we assume that the distance from the radio transmitter to your radio is 50 kilometres, the time taken for the signal, which travels at the speed of light (approximately 300,000 kilometres per second), to travel from the transmitter to the radio is virtually zero.

Compare this with the satellite signal which must be sent from the TV unit at the match up to a geostationary satellite and back down to Earth again. Because geostationary satellites orbit around 35,900 kilometres above the Earth, the round trip is 71,800 kilometres long. Again this signal is travelling at the speed of light and therefore takes approximately 0.24 seconds to complete its journey. This is one reason for the delay.

John Williams

By email, no address supplied

鈥� You are probably watching the sports coverage on a digital television platform 鈥� either via satellite, cable or terrestrial digital 鈥� but listening to a standard analogue radio station on a normal FM/AM radio.

One of the advantages of digital TV is that broadcasts can be compressed by computer algorithms so that more channels fit into the available bandwidth. However, compression and the subsequent decompression process which takes place in your set-top box take a short time to complete. Because the radio broadcast is analogue and not compressed, you hear the broadcasts in almost real time.

That means there is a 1 or 2-second gap in coverage between the different platforms. This, added to the answer above, accounts for the delay.

Nick Clark

London, UK

This week鈥檚 question

Heavy rain

While admiring a huge cloud between Bournemouth and the Isle of Wight off England鈥檚 south coast recently, I wondered if there was any way of measuring its mass. Potential rainfall from a cloud can be estimated using radar, but can we actually calculate how much it weighs?

R. B. Mannion

Bournemouth, Dorset, UK