At first glance, the star of the new study may not seem particularly impressive: only a few centimetres long, Platynereis dumerilii is a species of polychaete worm that can be found in temperate to tropical coastal waters around the globe; if your goal is to find outstanding animal personalities, surely there are better suited candidates. But that wasn’t the primary goal of the study, which experts from the AWI, the Max Perutz Labs in Vienna, the Universities of Vienna and Oldenburg, and the Katholieke Universiteit Leuven in Belgium contributed to. First and foremost, the researchers were interested in the internal clocks that dictate countless organisms’ daily rhythms.

“Biological timing is important at a number of levels,” explains Kristin Tessmar-Raible, a biologist at the AWI. “The ecological ties between species depend just as much on it as they do on biochemical processes at the cellular level.” But how do organisms’ internal clocks react when human beings warm the climate or use artificial light to turn night into day? “When it comes to marine organisms, we still know very little,” says Sören Häfker, the study’s main author. In this regard, rhythms are especially important in their lives: temperature, available light and food, and various other factors change throughout the day, and the organisms have to respond accordingly. They adapt their behaviour, metabolism, and genetic activity to these external rhythms.

However, it remains unclear whether they’ll be equally successful at doing so in the future. And when their internal clocks no longer match their environment, it can become a matter of survival. “As such, we need a much better understanding of how the rhythms of the oceans are changing and what it will mean for individual species and populations,” the biologist stresses — which means there’s a wealth of reasons to take a closer look at the daily behaviour of Platynereis dumerilii. In fact, for chronobiology, which focuses on organisms’ internal clocks, this distant relative of the dew worm has become one of the most important model species.

In past experiments, the team had noticed how the worms had quite disparate daily rhythms. Among human beings, it’s a familiar phenomenon: early birds rarely turn into night owls, and vice versa. But what about in marine polychaete worms? Are their behavioural differences just random variations or do they also have a personal tact? To find out, the group systematically observed the worms’ daily activities when there was a new moon. What they saw: some individuals became active at exactly the same time every night. In turn, others appeared to be arrhythmic “couch potatoes” that were only occasionally active — plus, there were various “shades of grey” between these two extremes. When the same worms were observed again several weeks later, their behaviour remained largely unchanged: once a couch potato, always a couch potato. “We were very surprised to see how reproducible the individual behavioural rhythms were,” says Tessmar-Raible. “This shows us that even worms have tiny, rhythmic personalities, so to speak.”

More individuality = more resiliency

To gain further insights into these behavioural differences, the group systematically compared the genetic activity in the heads of worms prone to particularly rhythmic and arrhythmic behaviour. Surprisingly, they found that the daily internal clock worked perfectly fine in all specimens, even the arrhythmic “couch potatoes,” and that the number of genes with rhythmic activity was nearly as high as in the “punctual” worms. The wide range of strategies they employ could offer the worms an evolutionary edge, as the experts surmise. After all, they live in a coastal environment with highly variable conditions; as such, lifestyle A might be the best choice for a given spot, while not far away, lifestyle B might be a better fit. In addition, this form of individuality could make them more resilient to major anthropogenic changes — in a transforming world, this diversity increases the chances of at least some worms being able to cope with their new circumstances.

But the study doesn’t just offer new insights into marine rhythms; it also underscores the fact that the processes at work within a given organism aren’t necessarily reflected in its behaviour: even among the couch potato worms, the genetic activity follows a daily rhythm, even if it’s not externally recognisable. And that’s likely true not just for worms, but for human beings as well. “That’s why such findings are also exciting for fields like chronomedicine,” says Tessmar-Raible.

In recent years, there have been intensified and successful efforts to bear patients’ individual daily rhythms in mind in the context of treating them. But, just as with the worms observed, they consist of various components, ranging from behaviour to genetic activity, which can react differently to medications and the timing of when they are administered. Accordingly, especially when it comes to human beings, it is important for chronomedical analyses to consider several different levels — if even worms can be so individualistic, our species is likely no exception.

Muscle strength, especially hand grip strength, can indicate an individual’s physiological resources to protect against age-related diseases and disabilities, as well as their ability to cope with them. Age-related loss of muscle strength is individual and influenced not only by lifestyle but also by genetics.

The study revealed that individuals with a genetic predisposition for higher muscle strength have a slightly lower risk for common noncommunicable diseases and premature mortality. However, it did not predict better survival after acute adverse health events compared to the time before illness onset.

“It seems that a genetic predisposition for higher muscle strength reflects more on an individual’s intrinsic ability to resist and protect oneself against pathological changes that occur during aging than the ability to recover or completely bounce back after severe adversity,” says doctoral researcher Päivi Herranen from the Faculty of Sport and Health Sciences.

The research utilized a unique study population

Muscle strength is a multifactorial trait influenced by lifestyle and environmental factors but also by numerous genetic variants, each with a very small effect on muscle strength. In this study, the genetic predisposition for muscle strength was defined by constructing a polygenic score for muscle strength, which summarizes the effects of hundreds of thousands of genetic variants into a single score. The polygenic score makes it possible to compare participants with an exceptionally high or low genetic predisposition for muscle strength, and to investigate associations with inherited muscle strength and other phenotypes, in this case, common diseases.

“In this study, we were able to utilize both genetic information and health outcomes from over 340,000 Finnish men and women,” Herranen explains.

“To our knowledge, this is the first study to investigate the association between a genetic predisposition for muscle strength and various diseases on this scale.”

Further research on the effects of lifestyles is still needed

Information about the genetic predisposition for muscle strength could be used alongside traditional risk assessment in identifying individuals who are at particularly high risk of common diseases and health adversities. However, further research on the topic is still needed.

“Based on these results, we cannot say how lifestyle factors, such as physical activity, modify an individual’s intrinsic ability to resist diseases and whether their impact on health differs among individuals due to genetics,” Herranen notes.

The study utilized the internationally unique FinnGen dataset, compiled through the collaboration of Finnish biobanks. The dataset consisted of 342,443 Finns who had given their consent and provided a biobank sample. The participants were aged 40 to 108 years, and 53% of them were women. The diagnoses selected for the study were based on the leading causes of death and the most significant noncommunicable diseases in Finland. Selected diagnoses included the most common cardiometabolic and pulmonary diseases, musculoskeletal and connective tissue diseases, falls and fractures, mental health and cognitive disorders, cancers, as well as overall mortality and mortality from cardiovascular diseases.

The study is the second publication of Päivi Herranen’s doctoral thesis, which investigates how genetics and environmental factors affect biological aging, particularly the weakening of muscle strength and functional capacity with age. The research is part of the GenActive project, funded by the Research Council of Finland and the Juho Vainio and Päivikki and Sakari Sohlberg foundations. The project is led by Assistant Professor and Academy Research Fellow Elina Sillanpää. The research was conducted in collaboration with the Gerontology Research Center (GEREC), the Institute for Molecular Medicine Finland (FIMM), and the FinnGen research project.

“Chimpanzees and bonobos use aggression in different ways for specific reasons,” says anthropologist and lead author Maud Mouginot of Boston University. “The idea is not to invalidate the image of bonobos being peaceful — the idea is that there is a lot more complexity in both species.”

Though previous studies have investigated aggression in bonobos and chimpanzees, this is the first study to directly compare the species’ behavior using the same field methods. The researchers focused on male aggression, which is often tied to reproduction, but they note that female bonobos and chimpanzees are not passive, and their aggression warrants its own future research.

To compare bonobo and chimpanzee aggression, the team scrutinized rates of male aggression in three bonobo communities at the Kokolopori Bonobo Reserve (Democratic Republic of Congo) and two chimpanzee communities at Gombe National Park (Tanzania). Overall, they examined the behavior of 12 bonobos and 14 chimpanzees by conducting “focal follows,” which involved tracking one individual’s behavior for an entire day and taking note of how often they engaged in aggressive interactions, who these interactions were with, and whether they were physical or not (e.g., whether the aggressor engaged in pushing and biting or simply chased their adversary).

“You go to their nests and wait for them to wake up and then you just follow them the entire day — from the moment they wake up to the moment they go to sleep at night — and record everything they do,” says Mouginot.

To their surprise, the researchers found that male bonobos were more frequently aggressive than chimpanzees. Overall, bonobos engaged in 2.8 times more aggressive interactions and 3 times as many physical aggressions.

While male bonobos were almost exclusively aggressive toward other males, chimpanzees were more likely to act aggressively toward females. Chimpanzee aggression was also more likely to involve “coalitions” of males (13.2% vs. 1% of bonobo aggressions). The researchers think that these coalitions might be one reason why aggression is less frequent among chimpanzees. Altercations involving groups of males have the potential to cause more injuries, and within-community fighting could also weaken the group’s ability to fight off other groups of chimpanzees. Bonobos don’t have this issue because most of their disputes are one on one, they have never been observed to kill one another, and they are not thought to be territorial, which leaves their communities free to bicker among themselves.

For both chimpanzees and bonobos, more aggressive males had greater mating success. The researchers were surprised to find this in bonobos, which have a co-dominant social dynamic in which females often outrank males, compared to chimpanzees, which have male-dominated hierarchies in which male coalitions coerce females into mating.

“Male bonobos that are more aggressive obtain more copulations with females, which is something that we would not expect,” said Mouginot. “It means that females do not necessarily go for nicer males.”

These findings partially contradict a prevailing hypothesis in primate and anthropological behavior — the self-domesticating hypothesis — which posits that aggression has been selected against in bonobos and humans but not chimpanzees.

The researchers were not able to assess the severity of aggressive interactions in terms of whether they resulted in wounds or injuries, but this is data that they hope to collect in future. They also want to compare aggressive behavior in other groups of chimpanzees and bonobos as it’s possible that behavior varies between communities and subspecies.

“I’d love to have the study complemented with comparable data from other field sites so we can get a broader understanding of variation within and between species,” says Mouginot.