Every year, Magellanic penguins (Spheniscus magellanicus) become stranded along the coasts of northern Argentina, Uruguay and southern Brazil and of the thousands of stranded penguins, 75 per cent are female. A new study published on 7 January in Current Biology has now shed some light on why females are stranded more often than males (1).
Typically, Magellanic penguins breed until late February and start their annual migration north in April, travelling 1000 km from northern Argentina to southern Brazil. However, instead of returning to Patagonia between mid-September and mid-October, many penguins are becoming stranded on the northern coast of South America. Many of these stranded penguins are reported to be sick or tired, and some have evened washed up dead. More females are known to become stranded than males but due to lack of information on penguin behaviours outside of the breeding season, the reasons for this female-biased stranding remains largely unknown. So, researchers from Japan and Argentina set out to determine why female penguins are more likely to become stranded than males.
In 2017, researchers led by Takashi Yamamoto from the Institute of Statistical Mathematics in Japan placed tracking devices ― called geolocators ― on 14 penguins (8 males and 6 females) to find out where they go during the non-breeding period after leaving Cabo dos Bahía in Argentina in April and start migrating north. Interestingly, they found differences in behaviours between males and females. In particular, females venture farther north and exhibited other behavioural differences too. For example, males were found to dive deeper underwater than females ― around 59 metres compared to 35 metres.
Based on the findings, the authors of the study have suggested several reasons for these difference, such habitat preferences or size differences between male and female birds resulting in differences in their ability to withstand the northward-flowing waves of the ocean. The smaller, lighter females may prefer warmer temperatures and shallower waters. Competition for food may be driving some penguins to migrate farther away, and climate change and pollution may also play a role. However, the authors also suggest that penguins may use up more energy on their journey further north, which could be leaving them exhausted.
Sex-related differences are important since stranded penguins or those that travel further may be more exposed to potential threats, and therefore at higher risk of death. For example, in Brazil, the penguins are at higher risk due to both natural and anthropogenic threats ― threats caused by human activity ― such as water pollution due to oil drilling and marine transport and fishery-related activities, such as bycatch and depletion of prey species. This could skew the sex ratio, potentially affecting the entire Magellanic penguin population.
The authors highlight the need for a better understanding of the long-term spatial domains of Magellanic penguins and populations, and indeed other species, throughout annual cycles, including any differences between sexes that are potentially associated with different mortality risks. The findings will hopefully be used to promote species conservation action to prevent the number of stranded penguins from increasing.
(1) Yamamoto, T. et al. Female-biased stranding in Magellanic penguins. Current Biology (2019). DOI: 10.1016/j.cub.2018.11.023