Giraffe and Evolution – Not just a long (neck) story

Feeding on acacia leaves
Feeding high up on acacia leaves

In the early 19th century, Lamarck proposed a theory of evolution by studying the behaviour of giraffes. He believed that giraffes evolved to have long necks as they began reaching for higher leaves on trees. He called this “change through use and disuse”. According to this theory, an organ or a character that is used more often becomes stronger and better. Therefore, over the course of history, giraffe’s neck got longer as it began stretching it a lot more than usual. Lamarck also proposed the “law of inheritance of acquired characteristics” according to which the improved characteristics of an organism are passed on from one generation to the next. These improved features persists and the disadvantageous features disappear.

The Lamarckian theory was eventually abandoned* as it could not explain the genetic basis for inheritance of acquired characteristics (traits obtained after birth due to environmental changes, accidents, use and disuse; these traits are not inheritable). Lamarckism also predicts that simpler life forms will disappear from the earth once organisms become more complex. While we see some organisms evolving into more complex systems with intricate functions, the simpler life forms like the single celled prokaryotic cells still exist to this day.

Darwin’s theory of evolution on the other hand, can account for the continued existence of the simpler life forms on earth. Darwin believed that complexity is a result of adaptation to the environment from one generation to the next. In the Origin of Species, Darwin proposed a theory of evolution driven by natural selection. According to this theory, there is variation seen amongst individuals. Certain environmental conditions favours certain variations and the species exhibiting these variations adaptsurvive. The unadapted species which do not exhibit the favoured variations do not survive and become extinct over generations of time.

Applying this to giraffes, the long neck species are considered dominant and have greater chances of survival during harsh drought conditions compared to the short neck species that have to rely on ground habitation for food. This of course is just one theory amongst many other. The long necks are also used to reach deep inside trees that other competing animals cannot reach, and is therefore more advantageous. One of the latest proposal is the theory of sexual selection. Male giraffes fight with other males by “necking” to compete for female partners. As it turns out, females prefer males with longer and stronger necks. Natural selection again, favours long neck males.

“Animal Autopsy”, a show on National Geography channel dug deep into giraffes – quite literally! – to explore the physiological and anatomical features of this intriguing mammal to unravel some of its evolutionary secrets.

During the autopsy, Richard Dawkins talks about one of the evolutionary disadvantages caused due to the long laryngeal nerve that starts off in the brain and ends in the larynx (which is in fact situated very close to the brain). This nerve runs all the way down the neck, loops around one of the arteries in the chest and returns to the larynx on top. Why does the nerve take such a long route when it can simply connect from the brain to the larynx directly without having to pass through the entire neck? Also, consider this – giraffes with a long necks must also bend much lower to drink water from the ground. The contracting of the muscles along with the tension in the elastic tissue in the neck utilises way more energy, and is considered to be another evolutionary flaw. Another interesting fact to chew upon is that due to its long neck, giraffes have to pump blood to the brain that is ~2.5 meters above the heart – against gravity – by using extremely high blood pressure. How does the long neck favour such distant positioning of the heart and the brain?

Biologists consider different perspectives to understand the evolutionary reasoning behind the advantages as well as the flaws caused due to the long necks of giraffes. As more pieces of this puzzles are put together, it is quite remarkable to think about the imperfections that are caused due to evolution. Imperfections that somehow adds up to the making of such a marvellous mammal on our planet.

*New research reveals the epigenetic basis for the inheritance of acquired characteristics. In the article “A Comeback for Lamarckian Evolution?”, Emily Singer of the Tufts University School of Medicine provides evidence for the largely abandoned Lamarckian theory of evolution. Read article on MIT tech review here.

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Understanding sexual parasitism and cannibalism

The mating ritual of the deep-sea anglerfish is one of the most bizarre in the animal kingdom. The female anglerfish are larger, reaching a length of around 10 centimeters, and the male fish are just a fraction of this and can be more than ten times smaller. These peculiar (read ugly) looking creatures live at a depth of 1 to 3 kilometers in the ocean.  In the darkest depths of the waters where food is scarce, finding a mate is problematic. It is estimated that 80% of the females never encounter a male in their lifetime (which is around 30 years). These fish have adopted a way as to resolve the issue of nutrient and mate acquisition through evolution.

Female deep-sea anglerfish with attached male. Photo: Dr. Theodore W. Pietsch, University of Washington
Female deep-sea anglerfish with fused male (circle). Photo: Dr. Theodore W. Pietsch, University of Washington

The larger female anglerfish release chemical factors known as pheromones into the water for the males to find them. Upon finding her, the male bites onto her skin (the ultimate love bite in the animal kingdom, if you ask me) and gets fused to her permanently! His internal organs degenerates – except for the gonads which are used to impregnate the female – and he now survives solely on the female’s blood vessels to acquire nutrients. He never has to hunt for food in his life. The female can fuse with multiple males (around 6 to 8) ensuring a fresh supply of sperm throughout her life! The males are reduced to a mere small lump of tissue on the female’s skin.

While the anglerfish’s mating ritual serves as a classic case of sexual parasitism, another interesting and even bizarre reproductive behavior is seen in Australian redback spiders, in which the males are consumed (yes, eaten!) by the females during copulation. A male redback spider performs a somersault behavior during sperm transfer and positions it’s abdomen on top of the female’s jaws. The female starts to feed on the male during the duration of copulation. One may ask, why would the male spider want to risk his life and sacrifice himself? The answer is simple: survival of the species through increased reproductive success.

A female Australian redback spider. Photo: Toby Hudson on Wiki Commons
A female Australian redback spider. Photo: Toby Hudson on Wiki Commons

Darwin in 1871, wrote The Descent of Man, and Selection in Relation to Sex, where he proposed sexual selection to explain how some traits evolve to give an advantage in the struggle for reproduction, but reduce the probability of survival. This is exactly what happens in Australian redback spiders. Sexual cannibalism is favored by selection because the cannibalized male spiders receive two paternity advantages. Through a series of behavioral experiments, Maydianne C. B. Andrade found that cannibalized males copulated for longer periods of time during which more eggs are fertilized (advantage #1). After consuming their first male, females reject subsequent males for copulation. This increases the paternity of the first male since the female only produces his off-springs and reduces the probability of her mating with a second male (advantage #2). This explains why sexual cannibalism is in fact, an adaptive strategy for male redback spiders since it increases their reproductive value.

These wild mating rituals of certain creatures may seem strange and even outrageous at first. However, one can appreciate these reproductive strategies when they’re understood in the light of evolution.