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Behavior

Making Room for New Neighbors: Observing Penguin Foraging

Gentoo & Adélie penguins

Left: Gentoo Penguin, Morris, D. (2005) commons.wikimedia.org/wiki/Pygoscelis-gentoo Right: Adélie Penguin, Mandemaker, A. (2005) commons.wikimedia.org/wiki/Pygoscelis_adeliae

Cimino, M. A., Moline, M. A., Fraser, W. R., Patterson-Fraser, D. L., & Oliver, M. J. (2016). Climate-driven sympatry may not lead to foraging competition between congeneric top-predators. Scientific Reports. doi:10.1038/srep18820

 

Background:

Climate change is expected to throw different species together as habitats become less hospitable and organisms seek out areas that better suit their needs—that is, if that’s something they can do. Shifts in one species’ home range can impact its population in many ways. If, for example, the move causes an overlap in habitat with another species relying on an identical food source, one might expect increased competition to negatively impact either or both populations. Perhaps modeling would show us how this overlap would play out, but models are only as good as the information on which they are based. Indeed, often new evidence comes to light that can alter our predictions of what factors will affect populations in significant ways. This incidence is exactly what has happened thanks to recent behavioral observations of Adélie and Gentoo penguins off the West Antarctic Peninsula.

Both Adélie and Gentoo penguins feed primarily on the same food: krill. The changing climate and predictions of decreasing sea ice have caused some conservation concerns, since increasing overlaps of these species’ ranges would result in increased competition for morsels of krill—at least, that’s what the models and principles of competitive exclusion say. Thanks to Megan Cimino and colleagues and the collection of observations via autonomous underwater vehicles (AUVs), a small group of tagged Adélie and Gentoo penguins have shown they’re more than capable of “sharing” this food source—actively partitioning their food niches by modifying where they hunt.

 

Methods & Results:

Typically, observations of penguin foraging behavior have been limited to data collected from tow nets and other sampling gear. These types of data are snapshots in time and space and don’t provide continuous or high-resolution data on how foraging can be impacted by multiple factors. For this study, researchers tagged a small number of animals for a period of three weeks and surveyed areas around the West Antarctic Peninsula (Fig. 1) with Remote Environmental Monitoring UnitS (REMUS) AUVs during different tidal cycles. The REMUS sweeps acoustically detected shoals of krill at varying depths and provided researchers with coordinates to reconstruct areas of high prey density; the penguin tags recorded dive number, depth, duration, and location.

Screen Shot 2016-01-12 at 11.05.04 AM

Figure 1: Location along the West Antarctic Peninsula showing tracked penguins and AUV surveys (A&C) and penguin home ranges as well as dense and diffuse krill shoals (B&D); these surveys were taken during diurnal or semidiurnal tidal periods, meaning these areas experience either only one or two sets of high and low tides per day, respectively.

Both Adélie and Gentoo penguins are capable of diving to depths around 150 m in search of food, although Adélie penguins typically forage at <50 m while Gentoo penguins are consistently found foraging at a depth of <100 m. These patterns were seen throughout the study at the center of each species’ home range; however, when researchers examined areas of range overlap, they found that Gentoo penguins were diving deeper for longer periods of time, vacating the shallower waters and leaving them available for the Adélie penguins. (Fig. 2) This sounds rather magnanimous, and the scientists were quick to point out that it might all come down to energy expenditure. Gentoo penguins are naturally larger and would therefore achieve neutral buoyancy at deeper depths. Once neutrally buoyant, they would have more energy to devote to foraging instead of controlling depth. It is also possible the foraging tactics of Adélie penguins were sufficient for obtaining food in shallower waters, or that prey at deeper depths was absent, negating the need for deeper diving.

The biggest caveat to the study was the small sample size. (Overall, only 10 Adélie penguins and 8 Gentoo penguins were tagged, although individual dives were counted separately for each penguin.) The authors recognize this as a drawback to making sweeping statistical claims. However, the fact that any Gentoo penguin entering an area occupied by foraging Adélie penguins exhibited the behavioral shift of increasing foraging depth is intriguing. Of course, the overarching foraging trends of each species cannot be determined by observations made during one breeding season.

Figure 2: Depth profiles showing where krill shoals and number of foraging penguins was highest. Note box K where Gentoo penguins were noticed at higher densities at greater depths than their Adélie cousins. Interestingly, this behavior did not occur during semidiurnal tides (box L).

Figure 2: Depth profiles showing where krill shoals and number of foraging penguins was highest. Note box K where Gentoo penguins were noticed at higher densities at greater depths than their Adélie cousins. Interestingly, this behavior did not occur during semidiurnal tides (box L).

Big Picture:

Cimino’s team was right to point out the caveats to this study, but their findings also shed light on how stilted current models can be when it comes to the Antarctic ecosystem. The habitat is inhospitable towards scientific efforts much of the year, making it difficult to get a clear picture of all the factors influencing food webs and life histories. It is also costly and impractical to run continuous surveys. But every so often, like with these penguins, we get a glimpse of another type of data point for our models that just might make the difference. In this case, it seems these flightless birds may have enough behavioral flexibility to prove our predictions wrong. So I ask of you, my OceanBites audience: what other kinds of behaviors might allow other species to adapt to changing environmental conditions?

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