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Category: life history
Why do killer whales go through menopause?
Here’s yet another reason why humans are weird: menopause. During our 40s, women permanently lose the ability to have children, but continue to live for decades. In doing this, we are virtually alone in the animal kingdom. From a cold evolutionary point of view, why would an animal continue to live past the point when it could pass on its genes to the next generation? Or put it another way: why don’t we keep on making babies till we die? Why does our reproductive lifespan cut out early?
One of the most popular explanations, first proposed in the 1966, involves helpful grandmothers. Even if older women are infertile, they can still ensure that their genes cascade through future generations by caring for their children, and helping to raise their grandchildren.* There’s evidence to support this “grandmother hypothesis” in humans: It seems that mothers can indeed boost their number of grandchildren by stepping out of the reproductive rat-race as soon as their daughters join it, becoming helpers rather than competitors.
Now, Emma Foster from the University of Exeter has found similar evidence among one of the only other animals that shows menopause: the killer whale.
The two-genome waltz: how the threat of mismatched partners shapes complex life
Two people are dancing a waltz, and it is not going well. One is tall and the other short; one is graceful, the other flat-footed; and both are stepping to completely different rhythms. The result is chaos, and the dance falls apart. Their situation mirrors a problem faced by all complex life on Earth. Whether we’re animal or plant, fungus or alga, we all need two very different partners to dance in step with one another. A mismatch can be disastrous.
Virtually all complex cells – better known as eukaryotes – have at least two separate genomes. The main one sits in the central nucleus. There’s also a smaller one in tiny bean-shaped structures called mitochondria, little batteries that provide the cell with energy. Both sets of genes must work together. Neither functions properly without the other.
Mitochondria came from a free-living bacterium that was engulfed by a larger cell a few billion years ago. The two eventually became one. Their fateful partnership revolutionised life on this planet, giving it a surge of power that allowed it to become complex and big (see here for the full story). But the alliance between mitochondria and their host cells is a delicate one.
Both genomes evolve in very different ways. Mitochondrial genes are only passed down from mother to child, whereas the nuclear genome is a fusion of both mum’s and dad’s genes. This means that mitochondria genes evolve much faster than nuclear ones – around 10 to 30 times faster in animals and up to a hundred thousand times faster in some fungi. These dance partners are naturally drawn to different rhythms.
This is a big and underappreciated problem because the nuclear and mitochondrial genomes cannot afford to clash. In a new paper, Nick Lane, a biochemist at University College London, argues that some of the most fundamental aspects of eukaryotic life are driven by the need to keep these two genomes dancing in time. The pressure to maintain this “mitonuclear match” influences why species stay separate, why we typically have two sexes, how many offspring we produce, and how we age.
Pregnant plesiosaur with giant foetus hints at caring parents
In 1987, Charles Bonner discovered the fossilised bones of a large sea reptile on his family ranch. It was a flipper-limbed plesiosaur, probably Polycotylus, and one of many such fossils recovered from Logan County in Kansas. But this specimen was special – there was a smaller one inside it. This plesiosaur was pregnant.
Dinosaurs grew fast, had teen pregnancies and died young
This is an old article, reposted from the original WordPress incarnation of Not Exactly Rocket Science. I’m travelling around at the moment so the next few weeks will have some classic pieces and a few new ones I prepared earlier.
For some dinosaurs, the best strategy was to grow fast and breed early. New fossil evidence suggests that at least three species, including celebrities like Tyrannosaurus and Allosaurus, were having sex in their teens. In this way, their pace of growth and maturity was closer to that of modern birds and mammals than it would be to a reptile scaled-up to the same size.
Orangutans are masters of conserving energy
Between office jobs, motorised transport, the Internet and television, it’s never been easier to be inactive. Many humans in Western countries are masters at conserving energy but in the rainforests of Borneo and Sumatra, there is an animal that would put hardened couch potatoes to shame – the orang-utan. These apes are no slackers – they lead active lives in the jungle canopy. But relative to their size, they still use up less energy than any other mammal except for sloths.
Herman Pontzer from Washington University, who made the discovery, thinks that orangutans have evolved to live life in the slow lane because they can’t be sure of a steady food supply. They mostly eat fresh fruit and, being large animals, they need lots of it. But rainforests are chaotic places where ripe fruit can disappear quickly, unpredictably and for a long time. If orangutans aren’t getting any fuel, they have to minimise the amount of energy they spend, so that they don’t starve to death. And they’re very good at it.
Short lives, short size – why are pygmies small?
This article is reposted from the old WordPress incarnation of Not Exactly Rocket Science.
For decades, anthropologists have debated over why pygmies have evolved to be short. Amid theories about their jungle homes and lack of food, new research suggests that we have been looking at the problem from the wrong angle. The diminutive stature of pygmies is not a direct adaptation to their environment, but the side-effect of an evolutionary push to start having children earlier.
Andrea Migliano at the University of Cambridge suggests that pygmies have opted for a ‘live fast, die short’ strategy. Their short lives gives them very limited time as potential parents, and they have adapted by becoming sexually mature at a young age. That puts a brake on their pubescent growth spurts, leaving them with shorter adult heights.
Pygmies are technically defined as groups of people whose men are, on average, shorter than 155cm (or 5 feet and an inch for the Imperial-minded). Strictly speaking, the word is restricted to several ethnic groups of African hunter-gatherers, like the Aka, Efe and Mbuti. But the world is surprisingly replete with shorter-than-average groups who also bear the colloquial moniker of pygmies, including some from Brazil, Bolivia, South-East Asia and Papua New Guinea.
The earlier explanations for a short stature worked for some of these groups, but they could never account for all of them. Some scientists suggested that smaller people move more easily through dense jungles, but some pygmies live outside forests. Other theorised that they could maintain their body temperature more easily, but many live in cool and dry climes.
One of the more popular theories put forward by Jared Diamond suggested that small people are more resilient to starvation and malnourishment when food becomes scarce. But this can’t be the whole story for Africa groups like the Turkana and Massai manage to be some of the tallest people on Earth despite facing similarly unstable food supplies!
A life in the trees is a longer one
An assortment of tree-living mammals
In The Descent of Man, Darwin talked about the benefits of life among the treetops, citing the “power of quickly climbing trees, so as to escape from enemies”. Around 140 years later, these benefits have been confirmed by Milena Shattuck and Scott Williams from the University of Illinois.
By looking at 776 species of mammals, they have found that on average, tree-dwellers live longer than their similarly sized land-lubbing counterparts. Animals that spend only part of their time in trees have lifespans that either lie somewhere between the two extremes or cluster at one end. The pattern holds even when you focus on one group of mammals – the squirrels. At a given body size, squirrels that scamper across branches, like the familiar greys, tend to live longer than those that burrow underground, like prairie dogs.
These results are a good fit for what we already know about the lives of fliers and gliders. If living in the trees delays the arrival of death, taking to the air should really allow lifespans to really take flight. And so it does. Flight gives bats and birds an effective way of escaping danger, and they have notably longer lives than other warm-blooded animals of the same size. Even gliding mammals too tend to live longer than their grounded peers.
The evolution of animal personalities – they're a fact of life
Animals have distinct personalities and temperaments, but why would evolution favour these over more flexible and adaptible mindsets? New game theory models show that animal personalities are a natural progression from the choices they make over how to live and reproduce.
Any pet owner, wildlife photographer or zookeeper will tell you that animals have distinct personalities. Some are aggressive, others are docile; some are bold, others are timid.
In some circles, ascribing personalities to animals is still a cardinal sin of biology and warrants being branded with a scarlet A (for anthropomorphism). Nonetheless, scientists have consistently found evidence of personality traits in species as closely related to us as chimpanzees, and as distant as squid, ants and spiders.
These traits may exist, but they pose an evolutionary puzzle because consistent behaviour is not always a good thing. The consistently bold animal could well become a meal if it stands up to the wrong predator, or seriously injured if it confronts a stronger rival. The ideal animal is a flexible one that can continuously adjust its behaviour in the face of new situations.
And yet, not only do personality types exist but certain traits are related across the entire animal kingdom. Aggression and boldness toward predators are part of a general ‘risk-taking’ personality that scientists have found in fish, birds and mammals.
Max Wolf and colleagues from The University of Groningen, Netherlands, have found a way to explain this discrepancy. Using game theory models, they have shown that personalities arise because of the way animals live their lives and decide when to reproduce.
The heavy cost of having children
While philosophers and poets muse on the meaning of life, natural selection casts a dispassionate eye on the whole affair. From the viewpoint of evolution, there is only one thing that matters – that we survive long enough to pass our genes on to the next generation, as many times as possible. And from the viewpoint of evolution, we are not doing a very good job.
Birth rates in several countries around the world – the UK, Japan, China – are falling dramatically. Women are having fewer children and they are having them later, close to the end of their fertile period. But the fact that women undergo menopause at all seems strange, and the reasons for this reproductive expiry date has long puzzled biologists. There doesn’t seem to be any obvious benefit to ending a woman’s child-bearing potential with many years or decades to spare. Nor is menopause a symptom of our healthy modern lives – even in traditional societies, women often survived long past this point.
The favoured idea is that women retire early from child-bearing for the same reasons that athletes retire from their sports at a young age – their bodies cannot handle the strain. Childbirth is a taxing process for a woman and at some point, it becomes too risky for mother and child. Scientists have suggested that menopause is an evolutionary respite from the burdens of having children. Now, Dustin Penn at the Austrian Academy of Science and Ken Smith from the University of Utah have found compelling evidence to support this idea.
Going strong at 100 – extreme lifespans don't mean extreme disability
People in most parts of the world are living longer and longer, thanks to great leaps in medicine and sanitation over the last century. But these growing life expectancies bring with them a sense of unease. The biggest worry is the possibility that medical advances are artificially prolonging life with little regard for its quality. Old age, after all, brings with it an increased risk of chronic diseases such as cancer, as well as both physical and mental decline.
This is not just a moral question, but an economic one too. The “oldest-old” are the fastest growing demographic in Western countries. If this expanding part of the population is indeed becoming more and more dependent on care from relatives or the state, the costs to society will start to skyrocket. But new research from Denmark suggests that this grim vision of the future is a fictitious one.
Kaare Christensen and colleagues from the University of Southern Denmark found that the proportion of elderly Danes who manage to remain independent holds steady at about 30-35 percent between the ages of 90 to 100. Individual people certainly risk losing their independence as they get older but the unhealthiest ones tend to pass away earlier despite improvements in medicine. This means that from society’s point of view, exceptional long-life won’t lead to exceptional levels of disability.
The scale of Christensen’s study is unprecedented. It exploited the fact that Denmark has kept a record of everyone living in it since 1968. Each person is assigned with a 10-digit identification number that links all their information across official registries. In 1998, Christensen’s team used this resource to contact every one of the 3,600 people who were born in Denmark in 1905 and were still alive.
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