In this week’s blog post, we’re climbing the ranks to understand strategic growth in invertebrates! Discussing their paper: “Game over: Conflict resolution through strategic growth in an invertebrate”, author Pooja Radhakrishnan reveals how a marine annelid worm establishes a size hierarchy to compete against individuals they interact with. Pooja helps us understand how social competition can drive the evolution of adaptive mechanisms such as strategic growth modulation! Beyond the microscope, Pooja reminds us that life has its peaks and valleys, and to never lose your sense of humour as you go through both
About the paper
The faster you grow, the faster you climb the ranks. While this might bring back memories of the friendly Amway rep trying to recruit your unsuspecting mum on a lazy summer afternoon, that’s actually not too far from what we found in our recent study. Strategic growth is a kind of socially mediated, rank-specific growth. Instead of following a fixed, predetermined growth trajectory, which is typical of most animals including humans, individuals adjust their growth in relation to their nearest-size rivals or to changes in the social structure. This form of context-dependent growth has been best documented in group-living species where body size determines social rank. Higher-ranking individuals have priority access to reproduction and other resources, while lower-ranking individuals remain in the group because they have few outside options and group membership is still advantageous.
Growth then becomes a competitive tool and the ‘size hierarchy’ is an emergent outcome of individuals walking a social tightrope: grow just enough to stay ahead of your nearest rival, but not so much that you threaten the individual above you and invite aggression. Visually, the social group often looks like a set of organ pipes in a cathedral.
In our paper, the study species is a tiny marine worm Ophryotrocha puerilis with little paintbrush-like bristles at the end of each body segment (and yes, taxonomists would be very upset if you called them feet). A fully grown adult is only about the size of a grain of rice (anything but the metric system), and it lives in colonies tucked into narrow spaces, like the gaps between mussel clusters. It’s considered an “opportunistic coloniser” because it can thrive in environments from eutrophic harbours to ephemeral habitats such as fallen whale carcasses (except, apparently, in perfect laboratory conditions during a very important experiment).
One particularly fascinating detail is that it changes sex from male to female as it grows older and larger, which once made it so popular as a model species for sex allocation studies that researchers called it the “Drosophila of the polychaetes”. But for all its early scientific fame, this little worm quietly slipped out of the spotlight for over 30 years. When I joined Prof. Maria Cristina Lorenzi’s annelid lab for my PhD, it felt like the perfect opportunity to bring it back into focus.
What began as a simple experiment to understand what triggers sex change soon led us to something far more unexpected: evidence of strategic growth.
About the research
Until now, strategic growth had only been documented in vertebrates with well-defined social hierarchies such as clownfish and meerkats. We wanted to know whether a tiny marine worm could really show the same kind of socially tuned growth. Specifically, we wanted to know:
Do size hierarchies consistently emerge among social partners?
Do individuals adjust their growth depending on their subsequent social rank?
Does the onset of reproduction change social dynamics?
We exposed juvenile worms to identical conditions in terms of food, temperature and early life but raised them in different social environments: isolation, in pairs and triplets of size-matched juveniles, or exposed to larger adults. We followed their growth for about two months and fitted growth curves to extract simple descriptors of each worm’s growth trajectory, such as the timing, intensity and duration of its growth spurt and its final body size.
As expected, worms modulated their growth in response to the size and number of rivals in their social environment, and these adjustments changed their social rank in the group. Within a week, a clear size hierarchy had emerged. In pairs, one worm showed a steeper, longer growth spurt while its partner’s ended early, as if conceding the contest with a weary “Fine, you win”. In triplets, all three juveniles seemed to put in equal effort at the start, but their growth spurts tapered off at different times, sorting them into distinct ranks.
Interestingly, we also observed some rank-specific behaviours. For instance, juveniles housed with much larger males often stayed along the underside of the water surface, as if to say “don’t notice me, I’m definitely not worth attacking”. You can think of it like the flatmate who cooked eggs in a cast-iron pan and left it in the sink all weekend, then spent the next three days timing his kitchen visits to avoid everyone. We know what you did, Kyle.
It’s important here to remember that these worms start life as males and later change sex to female as they grow larger, so size does not only affect social rank, it also determines phenotypic sex. Since females have greater reproductive success, size-matched juveniles with an equal opportunity to become female are effectively competing to attain the female role by establishing a size hierarchy that clearly differentiates future females from males.
In pairs, once the larger worm changed sex and became female and the pair settled into a stable male-female partnership, the size gap softened. We think this is because in a monogamous pair, the reproductive interests of the two align and conflict eases.
In triplets, however, the presence of an extra-pair male changed the dynamics and the onset of reproduction stabilised the hierarchy. This suggests that individuals actively regulate their growth relative to rivals to mitigate reproductive conflict, and the size hierarchy is maintained as a result of these strategic adjustments.
At this point, a reasonable sceptic might say: “Maybe they’re just stressed when they’re together. Isn’t this just a density effect?”
Me, internally: [nervous Tobias Fünke noises]
Out loud, though, the answer is, we checked.
The key prediction of strategic growth is not the presence of size differences per se, but their consistent emergence among initially size-matched individuals under uniform, ad libitum feeding conditions.
In our study, fast-growing individuals in pairs and triplets grew faster than worms in isolation, even though the latter had no competition for food. So growth here isn’t limited by resource availability, but is shaped by social competition to claim the top rank. It’s a biological pyramid scheme: the more social partners there are in the group, the greater the potential reproductive success for the eventual female, and the greater the cost of remaining male because of male-male competition.
Overall, fast-growing worms in triplets had a longer growth spurt than their counterparts in pairs. This suggests that, beyond simply growing fast to attain the top rank (a “grow as fast as possible” strategy), these worms fine-tune their growth rates in response to the number of competitors and the growth trajectories of rivals (a “grow faster than the fastest close competitor” strategy).
Evidence of strategic growth in an invertebrate points to a broader pattern of convergent evolution in social behaviour. Very different animals, from fish and mammals to tiny worms, seem to have arrived at similar solutions when access to reproduction is limited to the largest individuals and walking away from the group is dangerous. However, maintaining such a hierarchy implies a kind of “I punish you, you punish your subordinate” monitoring chain that may be unsustainable beyond a certain group size. My inclination, from observing populations in the lab, is that beyond a certain group size threshold they may split into smaller social subgroups contained within mucous trails, but this has yet to be formally tested.
This study also suggests that sophisticated cognitive abilities are not a prerequisite for these kinds of social hierarchies. Sensitivity to simple cues such as relative size, resident versus intruder, familiar versus unfamiliar may already be enough to produce surprisingly complex social outcomes. I’m curious whether, if we looked at other invertebrates, we would see similar rules of competitive, conflict-averse growth modulation.
About the author
My path into science was not exactly conventional. I have a bachelor’s degree in Hotel Management, and my early career in hospitality was all about FIFO systems and pavlovas, not fitness landscapes and p-values. I still occasionally wake up in a cold sweat hearing a chef yell, “The consommé is cloudy!” across the kitchen. Everything changed about seven years ago when I picked up a popular science book on evolutionary biology. One of life’s small ironies is that I did eventually end up in Paris, the city of gastronomy, not to go to pastry school, but to study the private life of a tiny marine worm.
I am fascinated by how animals perceive and respond to social information and make dramatic adjustments to both phenotypic and life-history traits in response. Outside research, I wish I could say I spend my evenings reading Proust in the original French, but it’s far more likely you’ll find me on the Lidl app hunting for coupons. I do, however, love cooking, photographing tiny critters, and taking the bus to unknown places.
If I had one piece of advice for my younger self, it would be this: “put the MBA brochure down, you’re going to be a researcher. Yes, research. Yes…worms. Oh don’t be rude, they’re actually very cute.” More seriously though, I’d tell myself that life moves in peaks and valleys, so try to keep a sense of humour when things don’t go as planned, and make the most of good luck when it (inevitably) comes your way. Of course, there will be days when the desk rejections make the Amway life look oddly appealing, but don’t let self-doubt make you shrink to the point of stagnation. Strategic growth, after all, is a bit like learning to read the room and knowing when to stand tall, when to stay small, and when to pull a Kyle.
The author, Pooja Radhakrishnan (Credit: Pooja Radhakrishnan)
The author, Pooja Radhakrishnan (Credit: Pooja Radhakrishnan)
Leave a comment