This blog we are going back to foraging strategies, but with a focus on just one species: the bottlenose dolphin (Tursiops truncatus). This species shows behavioral plasticity, or a change in behavior in response to stimuli (like changing environmental conditions). These intelligent dolphins are able to modify, or even create a new behavior in order to capture their prey, and these methods vary depending on where they live and what prey they have available to them. Let’s read about a few of the coolest (we think) bottlenose dolphin foraging strategies.
In the northern Gulf of Mexico some bottlenose dolphins target catfish. Now the main problem with this is that the head of a marine catfish is formidable, including highly fused skull bones and rigid, venomous dorsal and pectoral spines. So if you were to try and eat one, it might not go well. In stranding records there are dolphins with trauma from catfish spines and they were found embedded in the tongue, mandible, pharynx, larynx, esophagus, lymph nodes, lungs, diaphragm, stomach, liver, pancreas, spleen and intestines, and for some of those cases the spines were a contributing factor to the cause of death. So obviously there is a risk, but at the same time the dolphins are trying to eat them. So how to get around those pesky spines? Well, as the Queen of Hearts (from Alice in Wonderland) says, “off with their heads!”. The dolphins apparently work to decapitate the catfish. They have been observed pursuing the catfish and rotating on their long axis as they attempt to capture the fish, lunging out of the water with the fish perpendicular in their rostrum or grasped tail first (which is thought to be a possible method to break the fish apart), and fish tossing (thought to help severe the head with a quick whiplash motion). Interestingly the researchers didn’t observe an actual beheading, but with the foraging behaviors occurring in the vicinity of floating catfish heads (eww!), and the fact that the stranding data show that ingestion likely occurs, you have a pretty strong case of a really interesting, new behavior.
Article can be found here.
We now move to the Bahamas where bottlenose dolphins here are digging for fish in the sandy bottom, leaving craters in their wake. This is a very interesting behavior that I got to see first-hand while researching these dolphins with the Wild Dolphin Project. The dolphins often search the sandy bottom moving their head side to side, oriented downward and you can hear the echolocation clicks. They would then stop, increase their click rate and dive head first into the sand (like that myth that ostriches stick their heads in the sand)! Sometimes we would see the dolphins higher in the water column, rotating their heads in a circle (with no audible sound), before zooming down to a particular spot in the sand and digging. They can dig all the way up to their pectoral fins, before shimming back out of the sand with a tasty fish or eel. Now the behavior gets its name from what they leave behind – a dolphin head-sized crater in the sand. When a group of dolphins do this together, the sandy bottom resembles a lunar landscape, with craters everywhere. The really interesting part of this story is the fact that sonar is not supposed to be able to see through sound (ours certainly can’t) – but obviously the dolphins are able to see where the fish lie in the sand (sometimes a fair distance down). No wonder humans are interested in understanding more about how dolphin sonar works – way better than anything we make! Another interesting point is that the spotted dolphins that live in the same area feed on the bottom, but on different fish on or just below the surface (like flounder), and have not been seen crater feeding. So it is just the bottlenose that are able to access these buried treasures; perhaps this is a way to divvy up the resources in a shared habitat.
A population of bottlenose dolphins found in the Florida Keys focuses on a method of feeding known as mud plume feeding. First observed in 1999, this population of dolphins suspend sediment in the water column to trap fish for easy capture. Similar to the idea of bubble net feeding in humpbacks, one dolphin in a small pod circles a small school of forage fish in shallow water, beating its tail in the mud to kick up the sediment and create a wall of mud enclosure around the school. The dolphins will then lunge into the mud enclosure and capture the fish that are trapped at the water’s surface inside the mud plume. Curiously, most lunges by these dolphins exhibit orientations of the animals right side facing the seabed. It is hypothesized that due to the asymmetry of the esophagus of the dolphin having an enlarged right side, positioning this way may enable easier swallowing of larger prey with the help of gravity. Another theory, based from a captive study finding dolphins prefer using their right eye primarily, suggests this orientation is preferred for quick observation and decision making by the animal. Watch dolphins mud-plume feeding here!
A population of bottlenose dolphins in Shark Bay, Western Australia has been observed using sponges for foraging, a first observation of tool use in this species. The dolphins of this region will tear off a small piece of basket sponge and wrap it around their rostrum. The animals will then sift through the debris on the seabed with this sponge, kicking up benthic fish to prey on. It is believed these dolphins use this sponge to protect their sensitive rostrums from the sharp rocks/coral bits, spiny animals, or other abrasive objects on the seafloor. Though this population also feeds on fish in the water column, the dolphins dig in the seafloor to locate the hidden fish that lacks a swim bladder organ. Normally, these dolphins can locate their prey with echolocation bouncing off their swim bladders. Without this organ to help find with echolocation, the dolphin thus must dig. Why does this population use this energetically costly method rather than the typical open water foraging in this area? One thought is this benthic fish are more nutritious than those found in the open water. Intriguingly, more females than males exhibit this behavior, passing it on to more daughters than sons, indicating a cultural significance to the activity. It is thought that since males are more wide ranging in activities (such as finding a mate), they may not focus on this specific behavior as much as the females, as it not essential to their survival.
Although bottlenose dolphins around the world are known to eat octopus, the ones in Bunbury, Western Australia have been particularly well-studied for this behavior. So how exactly does a dolphin consume an octopus?! Well, turns out these dolphins have figured out the perfect formula to capture and eat octopus – here’s how it works. Firstly, the dolphins bite off the head. They then shake the body and toss the octopus repeatedly on the surface of the water until its arms have been tenderized and the suction cups can no longer function. This step is particularly important, as the suction cups can still latch on to things even once the head of the octopus has been removed! The dolphin then continues tossing and slamming the octopus body onto the water to break it up into swallowable chunks. Although the dolphins seem to have got this behavior down, it is quite a risky food choice as octopus have been known to put up a fight: there are cases of dolphins being asphyxiated by octopus, as well as octopus latching onto a dolphin’s back, blowholes, and genitals!
One example of this is the Bunbury dolphin Gilligan, who died in 2015. Upon conducting a necropsy, scientists found an octopus halfway down the dolphin’s throat. It seems the octopus had suctioned on to Gilligan’s larynx, blocking the airways, and both predator and prey died. Gilligan was the first dolphin who was known to have died from octopus-asphyxiation. Read more about his story here. This also highlights why it’s important that the dolphins toss octopus so repeatedly – it’s thought that Gilligan didn’t disable his prey sufficiently, and so the prey was still alive enough to fight back!
Many of you may have seen this behavior on Blue Planet or Planet Earth – it is pretty dramatic! Strand-feeding happens on the coastal waterways and mudflats of South Carolina and Georgia, and involves bottlenose dolphins chasing fish up onto the bank before launching themselves after the fish, literally stranding themselves in the process.
The dolphins work as a team to corral a group of fish or shrimp (they love mullet!) onto a sandbar or beach. The dolphins then stop and rush at the bait ball, creating a tidal wave that pushes the fish or shrimp onto shore. The dolphins then keep going and launch themselves onto the shore – interestingly almost always on their right side – where they grab at the now-stranded fish or shrimp. When they’ve gotten all they can, the dolphins will wriggle themselves free and back into the water.
As the banks and shorelines are typically muddy or wet in these areas the dolphins stand a fairly good chance of being able to work themselves free, but every once and a while an individual will get stuck, so this is definitely another risky foraging strategy! This cooperative behavior is taught by mothers to their offspring, making it another example of cultural transmission in cetaceans.