Word of the day: altricial

I have always been fascinated by the fact that some animals – for example camels, giraffe, rhinos, cows – give birth to miniature, almost fully functioning tiny versions of themselves. A baby giraffe has eyelashes, bright eyes and can run around shortly after birth. It is, essentially, a mini giraffe.

Others, such as kittens, and puppies, are born blind, deaf, and totally helpless. This is called being altricial.

When you look at newborn puppies they appear to be so helpless and incomplete. The idea that they are born functionally deaf (with their ear canals closed) and blind (with their eyelids tightly closed) seems to make no sense. Why would evolution allow any animal to be born in a state where two of its major senses are not functioning, and will not begin work well enough to provide useful information to the animal for a couple of weeks?

The answer lies in the dim past when the various species were developing and starting to discover the most adaptive way to prosper in the world. As mammals evolved each species, in effect, had to make a choice. This choice involved deciding upon the strategies for reproduction and development that would give that species and its offspring the best chance to survive in their particular environment. The alternative selected also had to fit with the usual patterns of behavior that the animals engage in to stay alive. Since mammals give birth to live babies the choice is whether to have a long pregnancy and produce fully formed and functional offspring, or to have a short pregnancy and produce immature, partly formed offspring that are helpless and take a lot of care.

At one extreme we have animals like deer and cattle. For example the gestation period (pregnancy) for a cow is nine months. A newborn calf weighs 25 to 45 kilograms (55 to 99 lb) and its brain is fully formed. In terms of sense organs it can see and hear efficiently. Most importantly it can run well enough to keep up with the herd. Biologists refer to species that produce relatively mature and mobile offspring as precocial from the word “precocious” which refers to the characteristic of exceptionally early development or maturity as in a precocious child who shows mental maturity beyond what we would expect for is age in years. Obviously, for a species where your ability to run away from predators can determine you whether you survive, being born mature is a necessity.

The various canine species, including dogs, represent the other extreme. In the wild, canines survive by hunting. Carrying a litter of puppies will slow the female and make it difficult to catch faster moving prey and to do her part in the coordinated hunting of the pack. That means that getting the pups out of the womb and onto the ground quickly is an advantage. In addition, between hunts (which can be spaced days apart) there is not much to do, so the female has time to care for helpless infants. When she is out pursuing food the pups can be safely stored in a den.

The gestation period in dogs is short, only about two months-58 to 63 days on average-however the trade off is that the puppies are quite helpless. Biologists refer to species that produce immature dependent offspring as altricial and the word is derived from the Latin root meaning “to nurse, to rear, or to nourish.” It refers to the need for young these species to be fed and taken care of for a long period of time.

Many of the puppy’s critical organs, including its brain, are not fully formed and they will spend several weeks developing rapidly. The same is true of the eyes. So the reason that puppies are born with their eyelids tightly shut is because the eye itself is still developing and is extremely fragile. It needs the protection that the closed eyelids provide to serve as a barrier to protect immature optical system from potential damage from foreign objects, such as dirt or grit, or even pathogens. Also exposure to too bright a light at this time might possibly damage the still delicate photoreceptors and optic mechanisms.

Most puppies begin to open their eyes at about two weeks of age, however even then the eyes are not fully developed and functioning perfectly. It will take several more weeks before their eyes mature and their eyesight begins to approach normal.

In the same way that the puppies are born with their eyes closed they are born effectively deaf because their ear canals are closed. Relative silence is important for the developing ears since sounds involve changes in pressure that mechanically move structures in the mature ear. If you force the pup’s ears to respond to sound inputs before the puppy’s fragile auditory machinery is fully developed it could cause great damage to the basic apparatus needed to hear.
The ear canals begin to open at about the same time that the eyes open, however when they do open, the ears are much more fully formed than the eyes are at this point in time. Usually, within a week or so the puppy’s hearing will be fully useable and quite acute.

Safely ensconced in their den, with a caring mother present, except for a few hours every day or two, even though they are blind and deaf the pups are still safe, and will develop quickly enough so that within a few weeks they will be exploring their world.’

Author: Janet Carr

Fashion, beauty and animal loving language consultant from South Africa living in Stockholm, Sweden.

One thought

  1. Wow!, this post is exceptionally useful with understanding the early development of the precocial and altricial species of mammals. It provides crisp clarity with the early biological mechanics of adaptation and survival with their offspring. Highly informative Post. Great thanks for sharing the knowledge.

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