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Behavior

Lobsters fighting antenna and claw over burrows

Paper: Sbragaglia V, Leiva D, Arias A, García JA, Aguzzi J, Breithaupt T (2017) Fighting over burrows: the emergence of dominance hierarchies in the Norway lobster (Nephrops norvegicus). J Exp Biol:jeb.165969

Introduction

It’s no secret that animals can sometimes be real bullies, especially when they’re trying to establish themselves as the dominant member of their group. Giraffes will whack their necks together in a fight for the best female. Elephant seals engage in bloody battles to determine who will be the mating male. Young male lions have to challenge a sitting dominant male for the right to mate with the lionesses. Clearly, these animals are all fighting over the same thing – mating.

But, there are other things that animals can and will fight for. In the case of the Norway lobster, the reward for being dominant isn’t necessarily the right to mate – instead, it’s a place to live. Norway lobsters live in burrows for most of their days, coming out at dusk or at night to forage for food, and staying inside when there are predators around (Figure 1). A good burrow for a lobster is one with a lot of space and often with rocks to conceal the front entrance from a predator’s eyes. The dominant lobster always has the best of those burrows, and sometimes, evicts less dominant lobsters from their burrows. That way, a dominant lobster can have more than one burrow, or more places to hide if a predator comes along.

Figure 1 – Norway lobster in its burrow. Source: Sublime Scuba Photography.

But, how do these lobsters determine their social hierarchy? Valerio Sbragaglia and his team, the researchers in this study from Germany, wanted to figure that out. They knew that most animals determine their dominance hierarchies by interacting with each other repeatedly, and the animals “remember” who wins and who loses. Then, after a few more fights, the animals don’t need to fight anymore – they know who will win each fight based on how the previous ones went.

That may sound easy, but it takes a lot of energy to be the dominant animal. In this case, the alpha lobster has to prove its worth a bunch of times against other lobsters, which costs energy and comes with a risk of injury. However, the benefits far outweigh the costs – dominant lobsters get the best shelters and, since they’ve already established themselves as the alpha lobster, don’t have to leave their shelter to fight anymore.

Methods

To learn more about how these hierarchies are formed, the researchers watched the behaviors of four equally-sized male lobsters in the lab as they fought over four equally-sized burrows over a five-day period. They recorded the lobsters 24 hours a day to see all of their fights, all of their evictions, and to see how much time each lobster spent outside its burrow.

They analyzed the videos with that behavioral data and used it to rank each lobster – alpha, beta, gamma, and delta. They even were able to weight the ranking to make the ranking more precise. For example, if the delta lobster unexpectedly beat the alpha lobster in a fight, the delta lobster got more “points” than if it had beaten the gamma lobster.

Results and Importance

They found that the social hierarchies developed gradually over the five-day period and became more stable as time went on. The lobsters started on day 1 on a level playing field, but by the time day 5 rolled around, it was very clear to the lobsters who was the top lobster. The hierarchies were also very polarized, which meant that there was a big difference between the delta lobster and the alpha lobster – i.e., the alpha lobster was fully established as the best of them.

With the polarization in the hierarchy, the number of fights or evictions also decreased. For example, on day 1 there were many fights as the lobsters tried to figure out which of them was the alpha, but by the time they hit day five, they already knew who was in charge and didn’t have to fight anymore (Figure 2).

Figure 2: Over the 5 days of the experiment (x axis), the number of fights (y axis) decreased as the lobsters figured out which was the alpha lobster. Modified from Sbragaglia et al. 2017.

 

 

 

 

 

 

 

 

 

 

The researchers were also counting the number of evictions that each lobster won or lost, since not every fight ended in an eviction. The alpha lobsters were the ones doing the most evicting, as can be expected, and also were evicted infrequently. The delta lobsters, on the other hand, almost never won evictions but were evicted a lot by the others. As a result, the lower the lobster was on the hierarchy, the less time it spent in its shelter – and the more vulnerable to predation it would be (Figure 3).

Figure 3: A) Evictions won by each lobster, ranked from 1 (alpha) to 4 (delta). The alpha lobster won evictions way more often than the delta lobster. C) Conversely, the delta lobster (4) suffered a lot of evictions at the claws of the other lobsters. Source: Sbragaglia et al. 2017

Clearly, shelter is very important to these lobsters – so much so that they fight for it! Shelters help lobsters avoid predation, since their main predator (codfish) isn’t able to see them in their burrows. The most powerful lobsters are able to avoid predation by keeping their shelter, yes, but also by evicting other lobsters from their shelters – that way, the most powerful lobsters are controlling more than one shelter each. That way, when they leave their regular burrow to forage, they’ll be able to hide in another burrow if a predator comes along.

The other side of the story is that the alpha lobsters aren’t leaving their burrows as often, which helps them avoid predation as well. They’re not engaging as much with other lobsters (since they already established their dominance) and they aren’t getting evicted by other lobsters, so their only reason to leave is to forage. This helps the lobsters save energy and avoid predation as well.

These results have the potential to impact fisheries of the Norway lobster. Right now, agencies for the fishery assess the abundance of the lobsters by counting the number of intact burrows that they see in a given area. The assumption is that one burrow means that one lobster lives there. However, in light of these results, the dominant lobsters may be controlling more than one burrow, and we could be overestimating how many of these lobsters are in the population. Of course, more needs to be done to figure that out for sure, but this study was a great start!

Engage: What other resources do animals fight over? What are some unique fighting mechanisms among animals?

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