Introduction
The king penguin (Aptenodytes patagonicus; hereinafter KP) is a pelagic species with circumpolar distribution and with reproductive colonies on the sub-Antarctic islands between parallels 45° and 55°S (Bost et al. Reference Bost, Delord, Barbraud, Cotte, Péron, Weimerskirch, Borboroglu and Boersma2013, Pütz et al. Reference Pütz, Trathan, Pedrana, Collins, Poncet and Luethi2014). The largest population of KPs in the south-west Atlantic sector is found on the South Georgia Islands, which are the southernmost breeding colonies (Bost et al. Reference Bost, Delord, Barbraud, Cotte, Péron, Weimerskirch, Borboroglu and Boersma2013). Vagrant individuals have been reported in southern latitudes such as Laurie Island in the South Orkney Islands, Elephant Island and King George Island (25 de Mayo Island) in the South Shetland Islands and Neko Harbour on the Antarctic Peninsula (Coria et al. Reference Coria, Montalti, Rombola, Santos, García Betoño and Juarez2011, Bost et al. Reference Bost, Delord, Barbraud, Cotte, Péron, Weimerskirch, Borboroglu and Boersma2013, Petry et al. Reference Petry, Basler, Vallis and Krüger2013, Juáres et al. Reference Juáres, Negrete, Mennucci, Perchivale, Santos, Moreira and Coria2014, Borowicz et al. Reference Borowicz, Forrest, Wethington, Strycker and Lynch2020). Regardless of the breeding site, in summer the reproductive adults feed on or close to the Antarctic polar front (Scheffer et al. Reference Scheffer, Bost and Trathan2012), and in autumn/winter they make long trips to the marginal ice zone, where the availability of prey is higher (Pütz et al. Reference Pütz, Trathan, Pedrana, Collins, Poncet and Luethi2014). Recently, breeding and hatching attempts have been reported south of their historical range, suggesting expansion of their latitudinal limits (Petry et al. Reference Petry, Basler, Vallis and Krüger2013, Juáres et al. Reference Juáres, Negrete, Mennucci, Perchivale, Santos, Moreira and Coria2014, Borowicz et al. Reference Borowicz, Forrest, Wethington, Strycker and Lynch2020).
At the beginning of the century, the global population of KPs decreased as a result of anthropogenic actions. Currently, the largest KP populations are located on the Crozet Islands, the Kerguelen Islands, the South Georgia Islands, Heard Island and Macquarie Island (Bost et al. Reference Bost, Delord, Barbraud, Cotte, Péron, Weimerskirch, Borboroglu and Boersma2013). KPs have returned to former areas of presence, such as in the Malvinas Islands/Falkland Islands and Heard Island (Pistorius et al. Reference Pistorius, Baylis, Crofts and Pütz2012). This recovery translates into competitive pressure for space; the increase in population density and the greater proximity to the coast caused the displacement of pairs to the periphery of the colonies, with consequent increases in their latitudinal extensions of distribution (Chamaillé-Jammes et al. Reference Chamaillé-James, Guinet, Nicoleau and Argentier2000).
The number of breeding pairs observed on the Antarctic Peninsula, particularly on Elephant Island and the South Shetland Islands, is part of a phenomenon to be analysed, as we are not yet sure what environmental factors constrain KP breeding (Petry et al. Reference Petry, Basler, Vallis and Krüger2013, Juáres et al. Reference Juáres, Negrete, Mennucci, Perchivale, Santos, Moreira and Coria2014, Reference Juáres, Ferrer, Coria and Santos2017, Borowicz et al. Reference Borowicz, Forrest, Wethington, Strycker and Lynch2020).
San Martín Station (68°07'S, 67°08'W) is located in the west of the Antarctic Peninsula (WAP) and could represent a potential habitat for KPs in the immediate future. Following Cristofari et al. (Reference Cristofari, Xiaoming, Bonadonna, Cherel, Pistorius and Le Maho2018), San Martín Station has appropriate conditions for colonization: 1) free of ice and reasonably flat, 2) with sites within the maximum feeding distance and 3) low concentrations of sea ice during winter. Our objective is to document the southernmost occurrence record reported to date for the KP species. The high dispersal capacity of KPs, together with the observed trends of some environmental variables in the WAP, imply that southern locations such as San Martín Station could be suitable for the presence of this species in the future.
Observations
Study area
San Martín Station is located on Barry Islet, Marguerite Bay, WAP (68°07'S, 67°08'W; Fig. 1). The annual average maximum temperature is -1.5°C and the annual average minimum temperature is -8.5°C. The average annual wind intensity is 17.6 km/h, reaching gusts of more than 200 km/h. The islet is ~350 m long and ~230 m wide.
Location of the study area (upper left map), Marguerite Bay, west of the Antarctic Peninsula (lower left map) and San Martín Station, Barry Islet, Antarctica (right picture by Gastón Jacob).
King penguin sighting
On 3 February 2021, the station's scientific staff recorded the appearance of a KP specimen on Barry Islet. It was an immature individual, with dull and brown head and dorsal areas, a dull upper breast and a slightly dull bill stripe (Fig. 2). The plumage colour patterns suggest that it was a 1 or 2 year-old individual as from the third year KPs acquired their adult plumage.
King penguin (Aptenodytes patagonicus) juvenile at San Martín Station during summer 2021. a. General appearance; b. detail of head; c. close-up of the head, neck and chest. Photographs taken by Leandro Nahuel Solis Etchegoin and Rodrigo Azpilcueta.
The animal was apparently in good health. It had no injuries, moved freely and was very curious and sociable with humans. At times it was not in sight, suggesting that it was probably on feeding trips in the area. The penguin stayed on Barry Islet and was last seen on 27 March.
Discussion
Our work reports a KP sighting at the southernmost point of the distribution limits of the species. Its presence could be coupled to processes derived from population growth and from adaptive responses to climate change, resulting in increases in the environmental and trophic niches of the species. It was suggested that the arrival of the KP specimens reported at Stranger Point, South Shetland Islands, may have originated from the colonies of the South Georgia Islands (Juáres et al. Reference Juáres, Negrete, Mennucci, Perchivale, Santos, Moreira and Coria2014). This implies a linear route of ~1,500 km. However, in the case of the San Martín Station KP, its immigration could have started in the South Shetland Islands (King George/25 de Mayo Island or Elephant Island), where breeding events and population growth have been recorded (Petry et al. Reference Petry, Basler, Vallis and Krüger2013, Juáres et al. Reference Juáres, Ferrer, Coria and Santos2017, Gryz et al. Reference Gryz, Gerlée and Korczak-Abshire2019, Borowicz et al. Reference Borowicz, Forrest, Wethington, Strycker and Lynch2020). This specimen was recorded from February to March.
The KP record at the San Martín Station could suggest the expansion of the reproduction range of this species, however, the reasons for the presence of KPs in the Antarctic region continue to be speculative. Foraging site fidelity allows KPs to exploit areas with available and predictable prey; therefore, South Shetland Islands-recorded adults probably forage in Antarctic Polar Front waters or even further south (Scheffer et al. Reference Scheffer, Bost and Trathan2012, Petry et al. Reference Petry, Basler, Vallis and Krüger2013). The frequency of observations outside the southern limits of KPs are increasing; these isolated immigration processes could favour population growth and the expansion of future environmental and trophic niches (Petry et al. Reference Petry, Basler, Vallis and Krüger2013, Juáres et al. Reference Juáres, Ferrer, Coria and Santos2017, Borowicz et al. Reference Borowicz, Forrest, Wethington, Strycker and Lynch2020).
The Antarctic Peninsula region, particularly WAP, is warming faster than any other region in the Southern Hemisphere, and the Antarctic Polar Front is predicted to double its southwards shift by 2100 (Péron et al. Reference Péron, Weimerskirch and Bost2012). Faced with scenarios of environmental and physical variability, penguin species have responded by changing their distribution patterns and breeding ranges considerably (Gorman et al. Reference Gorman, Erdmann, Pickering, Horne, Blum and Lucas2010). KPs could show us the effects of climate change directly through its impacts on the environmental tolerance thresholds of the species and indirectly through the expanding population densities and trophic resources for the species. In this sense, there is limited knowledge regarding the adaptive responses of KPs to new environmental scenarios derived from climate change (Petry et al. Reference Petry, Basler, Vallis and Krüger2013). Because the effects of climate change are cumulative, wildlife will experience these impacts over several generations. Due to the scarce information available, it is the responsibility of scientists to record these events in order to allow effective and consistent responses to present and future environmental scenarios.
Acknowledgements
We thank Milton Percy Plasencia Linares, Héctor Ochoa and winter scientists Leandro Nahuel Solis Etchegoin and Rodrigo Azpilcueta from San Martín Station for their valuable contributions to carrying out this research and Gastón Jacob for contributing photographs of San Martín Station. JLO thanks the postgraduate programme in Ecology and Conservation of UNEMAT - Nova Xavantina. We thank the crew of the icebreaker Almirante Irizar for the operational facilities and logistical assistance provided. The permit for this work was granted by the Dirección Nacional del Antártico (Environmental Office). The Instituto Antártico Argentino provided financial and logistical support. We also thank the two reviewers and the editors for their helpful comments and suggestions on improving the manuscript.
Author contributions
All authors conceived and designed the investigation, performed fieldwork and laboratory work, analysed the data, contributed materials, reagents and analysis tools and wrote the paper.
