Why are frogs not seen in seas?

Why are frogs not seen in seas?

Frogs and toads belong to the class amphibia (they live in water and on land).One of the biggest problems faced by these animals is Osmoregulation. When they are in water they face the same problem as freshwater fishes; they have to expel excess water from their body. When on land they have to conserve water.

They do not replace water by intentional drinking nor do they have the impermeable skin characteristic of other tetapods. Above all their kidneys are not capable of producing hypertonic urine.

Amphibians limit their water loss by their behaviour that reduces exposure to desiccating conditions. When they come to land foraging during night, they lose water and this is compensated by water uptake across the skin. Paradoxically, the skin, the most important source for dehydration is also the most important structure for rehydration.

The frogs and toads have specialized patches of skin called seat patches on their abdomen and thighs which can absorb water very quickly allowing them to rapidly rehydrate. They flatten their body on moist surfaces and absorb water through seat patches.

Neurohypophyseal(ADH) and adrenocortical hormones have special roles in regulating water and salt transfer across epithelial membranes of tetrapods. Such effects are seen in the skin and urinary bladder of frogs and toads.

Amphibians on the whole are intolerant of saline condition. Their kidneys are not capable of producing hypertonic urine. Only amphibians and mammals excrete urea. Had the amphibians not inherited the Ornithine-urea cycle from their crossopterygian ancestors, they would not have succeeded in the movement onto dry land.

Source: The Hindu

Do mammals other than humans have menstrual cycles?

Do mammals other than humans have menstrual cycles?

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Mammals other than primates do not menstruate and their sexual cycle is called an estrous cycle. In mammals the sexual activity of the male is more or less continuous but in most species the sexual activity of the female is cyclic. Most of the time the female avoids the male and repulses his sexual advances. Periodically, however, there is an abrupt change in behavior and the female seeks out the male, attempting to mate. These short episodes of heat or estrus are so characteristic that sexual cycle in mammalian species that do not menstruate, is named the estrous cycle. The change in female sexual behavior is brought on by a rise in the circulating estrogen level. Rabbit and ferret come into heat and remain estrous until pregnancy or pseudo pregnancy results. In these species ovulation is due to a neuroendocrine reflex. 

Stimulation of the genitals and other external stimulation at the time of copulation provokes release from the pituitary of gonadotropin that makes the ovarian follicle to rupture.In captivity, monkeys and apes mate at any time. But in the wild, females accept the male more frequently at the time of ovulation. In rats the underlying endocrine events are essentially the same as those in the menstrual cycle, but the first day of the cycle is the first day of bleeding, while day one of an estrous cycle is the first day of heat.

Source: thehindu.com

How Do Snakes Shed Their Skin Every Month?

How Do Snakes Shed Their Skin Every Month?

Skin of snakes consists of outer epidermis and inner dermis. The inner dermis (soft, fibrous and pliable) contains pigment cells that give characteristic colour patterns. The outer epidermis keratinised layer (stratum corneum) made up of horny scales, resists normal wear and tear and protects from injury.

The Keratinised layer is shed sloughed every 60-70 days or earlier or 3-4 times a year. In young ones and some species the rate of casting may be higher or only once in a year.

The snake, normally hiding just before casting, is lethargic. New epidermis is grown beneath the old skin and secretes a thin layer of fluid between old and new skin. The fluid secretion makes the snake almost if not completely blind.

Shedding starts by opening and stretching the mouth and rubbing it on surrounding available objects until the old skin at the edges of lips begins to split. The process of rubbing continues till the skin is shed in one piece from head to tail or some times the skin will break up and come off in pieces.

The skin, turned inside out like a tight glove being pulled off from hand, is a perfect cast of the snake including eye scales. This process takes nearly half an hour or more.

The cast is completely devoid of colour as the pigment cells remain in the dermis which is never shed. The shedded skin is transparent and brittle. After casting the snake becomes fresh, alert, active and bright. The sloughting indicates growth of snake and it is affected by factors such as health, environmental temperature and amount of food taken.

Published in The Hindu on Jan 31, 2002.

Why Do Rats Have Long Tails?

Why Do Rats Have Long Tails?

Answer 1:

Rats use their tails to balance and also to help maintain body temperature. The rat has specialised blood vessels, which control the amount of blood flowing to its tail. Blood flowing into the furless skin of the tail carries heat, which is then radiated out of the body.

The rat can direct anywhere between 0.1 and 10 per cent of its cardiac output to the tail. Under ideal conditions, it can get rid of up to 20 per cent of its metabolic heat in this way. Conversely, restricting blood flow to the tail reduces heat loss in cold weather.

It is interesting to note that the length of the rat's tail is partly determined by the temperature of the environment in which it is raised.

Radiating heat through the skin plays a major part in regulating body temperature in humans as well.

Answer 2: 

Observing three rats suggests that the long tail — usually around the same length as the body — is used for balancing. Although rats cannot grasp objects with their tails, they do seem able to control the muscles along the tail's length with considerable precision.

If a rat is running along a narrow surface such as a rope, the tail constantly flicks from side to side to steady them, in the same way tightrope walkers at the circus use a pole to help them stay on the wire. A cat walking along a fence does the same thing with its tail.

When rats are eating, they perch on the edge of the food bowl, which is about 20 cm from the floor of the cage, grasping it with their back paws and eating with their front paws.

In this position their rear ends stick out quite a long way over the side of the bowl so that it looks as if they should fall off backwards.

They manage to stay balanced by curling their tails under the bowl. This moves their centre of gravity below its back paws, keeping it balanced. The rat's centre of gravity is lower too, so it is more stable and less likely to fall.

As an experiment, a food bowl was moved to about 10 cm above the floor of the cage, which is about the length of a rat's tail. Instead of curling their tails under the bowl when they are feeding, the rats stiffened their tails and pointed them straight down so that they touched the floor, propping themselves up with this "third leg."

Published in The Hindu on Dec 20, 2001.