Animals do the most amazing things. Read about them in this series by Janaki Lenin.
When we age, our senses become dull, and our muscles aren’t as coordinated as they used to be. Cells are programmed to die as we get older. More than any other organ, the brain is most susceptible to damage when the body is unable to produce enough antioxidants to neutralise free radicals. This is one of the main causes for worsening cognition.
In a recent study, scientists from the US, France, and Australia examined the influence of social status on age-related behaviour. They chose Pheidole dentata, a kind of big-headed ant found in North America, as their subject.
Ysabel Giraldo, who is the lead author, says the study grew out of conversations she had with her dissertation advisor Professor James Traniello of Boston University, who is the senior author of this paper. “We discussed the work of Dr. Robert Friedlander, a physician at Brigham and Women’s Hospital in Boston, on the role of cell death in human neurodegenerative diseases, such as Alzheimer’s. Since cellular mechanisms involved in cell death are highly conserved [retained in similar form in different species] among animals, we wondered what was going on in ant brains.”
Eusocial insects, like honey bees and ants, are unique subjects for ageing research. They divide labour by caste. Workers tend larvae and queens, forage outside the nest, and take care of household activities. Soldiers defend the nest against intruders. While all the queens do is churn out eggs.
A paper published on 27 January shows queens of bees and ants produce a chemical that makes workers sterile.
Although queens and workers are genetically similar, they have dramatically different lifespans. Queens live far longer than sterile workers. Since they live all their lives inside nests or hives, they are protected from the vicissitudes of life. However, workers have to brave the big nasty world outside their nests and often die from predation, parasites, disease, or environmental exposure.
No one knows the longevity of workers of the ant Pheidole dentata in the wild. Although mortality of workers that forage outside the nest is high, they can live up to 140 days in laboratories.
How do old workers behave? If they age like us, they probably can’t smell well – their primary sense – totter on six legs, and don’t recognise their compatriots as sharply as younger ones. They may also shirk their nursing duties or at least perform them slowly.
The research team collected ant nests from the wilds of Florida and brought them into the lab. The researchers knew the ages of the young by monitoring them from the time they hatched. They set various tasks for workers of four age groups: 20-22, 45-47, 95-97, and 120-122 days old.
They observed how often the workers carried, fed, and cared for the larvae. Whether young or old, the workers looked after the offspring with the same attention.
The scientists laid artificial scent trails and watched if the workers could sense the trails and how well they could follow them. By measuring how accurate they were, how long they walked, and how much time they took, the scientists were able to assess the ants’ performance. The results would show if they could forage efficiently.
No matter what the workers’ age, they had no trouble following trails with accuracy. In fact, the oldies followed trails for longer distances and were more active than youngsters.
Then the researchers tethered a fruit fly within an enclosure and observed the workers’ predatory behaviour. Did they flare their mandibles, attack half-heartedly, or attack ferociously? Or did they ignore them? Elderly workers had no trouble in attacking the poor fly.
Next, the experimenters used advanced molecular techniques to examine cell death in the brains of young and old ants. They found few dead cells in the area that processes information, learning, and memory. Nor was there a decline in the density of synapses, so the ants’ cognitive abilities were high.
In elderly humans, the levels of two brain chemicals, serotonin and dopamine, plummet. Low levels of serotonin causes memory loss and inability to learn, while dopamine affects physical movement as well as learning. However, old ant workers showed no dip in these chemicals.
What do all these results mean? Age has no effect on ant behaviour. Old workers had just an acute sense of smell as younger ones. They were limber and strong enough to walk great distances foraging for food. Back in the nest, they are better nurturers of larvae, nimbly cleaning eggs and feeding larvae. These workers work till the last weeks of their life.
“This study goes beyond what we could show in honey bees in our study,” Olav Rueppell of University of North Carolina at Greensboro told The Wire. “Thus, I applaud them for the thorough combination of behaviour, neuroanatomy, and neurophysiology. If I understood the methods description correctly, the experimental design was strictly longitudinal, which results in some problems with the interpretation. If you follow a cohort and test phenotypes [observable characteristics as the creatures interact with the environment] at multiple time points while the cohort is ageing, you cannot control for changes in test conditions or social conditions that may also change over time (in addition to the age of the subjects). The authors controlled these conditions as best as possible.
There is another problem, the issue of demographic selection. The older, surviving individuals may be the best individuals of the initial cohort, while the weaker individuals died at younger ages. So, if one doesn’t test the same individuals repeatedly, this selective mortality can explain why older individuals are as ‘fit’ as the younger ones. However, destructive sampling for measuring brain anatomy or physiology does not allow for repeated measures of the same individuals. So, in sum I think this study was designed as well as possible.”
If geriatric workers show no signs of senility, do they suddenly roll over on their backs and die?
“Interestingly, we really don’t know what the ants die from in the lab,” says Giraldo, who studied ageing in ants for her PhD. “In fact, even researchers who study Drosophila [fruit fly], which is the best studied insect, don’t know what kills the flies. So, it really is an open question as to what the ants die from. I will note that our colonies are collected in the wild, and so it is possible that they arrive with some parasites or other diseases, although we only used ants from colonies that appeared healthy. Our study was not designed to test cause of death, and it would certainly be difficult to perform ant autopsies, but it would be fascinating to examine this question in detail.”
Are these big-headed ants unique in dodging senility? “We have no reason to believe that this ant is unique among social insects,” replies Giraldo. “It’s social organisation is fairly typical of many ant species. However, because no other studies have examined a large suite of behaviours and neurobiological metrics in known age social insect workers, we really do not have the data to say one way or another.”
Although worker ants perform complex tasks, their brains are tiny. Individual insects don’t need to burn up energy storing information that they process collectively, nor do workers reproduce. Instead, the authors suggest the ants may channel their energy to providing molecular protection against neurological and behavioural decline.
In one study led by Professor Rueppell, honey bees show no functional decline with ageing. However, in another study, honey bee workers that foraged showed a decline in olfactory performance compared to bees that performed tasks within the hive. The decline appeared to be related to their tasks more than age.
Rueppell explains why there is a discrepancy. “Honey bee ageing does not depend on chronological age, but more on behavioural and physiological status. In-hive workers, and probably also queens, age very little, while outside foragers age in both demographic (increasing mortality rates) and functional (lower performance) senses. This is presumably due to a combination of environmental and internal factors.”
Giraldo points out that despite similarities, there are major differences between honey bees and ants.
“Notably, honey bees progress through tasks as they age,” says Giraldo.
As young workers, honey bees start out with nursing duties, and when they become older, they graduate to foraging outside the hive. “Most ants exhibit much more fluid age-based division of labor,” says Giraldo. “In particular, P. dentata workers add tasks as they age. Very young workers are in fact poor nurses. Mature workers, more than 20 days old, perform brood care, forage, and other colony tasks. In short, the social organisation is different between honey bees and ants. Also, honey bees are aerial whereas ants are terrestrial. Foraging bees experience significant wing damage after many foraging flights and this difference in ecology could lead to differences in ageing patterns.”
Rueppell says the study’s findings are significant. “It is a great confirmation that social evolution in insects has tremendous consequences for individual processes by shifting the emphasis of natural selection from the individual to the colony. This creates unique scenarios that are really valuable for testing general biological theories. For ageing and senescence, this study reaffirms that the clock is not necessarily ticking away for each and every living system. So, the inevitability of ageing, senescence, and death seems a little less inevitable.”
He adds, more research is needed. “The value of studying a wide variety of organisms to gain insights into general biological processes that also affect human health cannot be overstated.”
Ants may not have dodged death, but remaining spry as spring in the evening of one’s life is enviable.
The study was published in the journal Proceedings of the Royal Society B: Biological Sciences on January 6, 2016.
Janaki Lenin is the author of My Husband and Other Animals. She lives in a forest with snake-man Rom Whitaker and tweets at @janakilenin.