26 April 2025

Introduction

This report presents an analysis of morphological and demographic data collected from three species of Antarctic penguins (Pygoscelis spp.) — Gentoo, Chinstrap, and Adelie (n = 344) — recorded on Dream, Biscoe, and Torgersen islands within the Palmer Archipelago, Antarctica. Body mass, flipper length, bill length, and bill depth were collected from 168 male and 165 female penguins, between 2007 and 2009 by Dr. Kristen Gorman. The aim of this research was to investigate ecological sexual dimorphism and explore interspecific variation in morphological traits in relation to geographic distribution. It is important to note that the dataset reflects only the penguins that were measured during the field study and does not represent a full census of penguin populations on each island.

Penguin Population Distribution

A total of 168 penguins were recorded on Biscoe Island, 124 on Dream Island, and 52 on Torgersen Island during the study period. Adelie penguins were the only species recorded across all three islands, with Dream Island accounting for the largest proportion of the sampled Adelie population (n = 54; 36.8%). Chinstrap penguins were observed exclusively on Dream Island (n = 68), comprising 73.8% of all penguins recorded on the island. Likewise, Gentoo penguins were only recorded on Biscoe Island (n = 92), representing 54.8% of the island’s sampled population. Sex was relatively evenly distributed across all islands, with Torgersen Island showing a slight female majority (51.1%).

Body Mass and Flipper Length

Mean body mass across all species was 4.3 kg (±0.04 kg SEM). Gentoo penguins (5.1 ±0.05 kg) were significantly heavier than both Adelie (3.7 ±0.04 kg; P ≤ 0.0001) and Chinstrap penguins (3.7 ±0.05 kg; P ≤ 0.0001). Males were significantly heavier than females across all species. Similarly, flipper length was greatest in Gentoos at 216 mm (±0.60 mm SEM), significantly longer than Chinstrap penguins (196 ±0.87 mm; P ≤ 0.0001), who in turn had longer flippers than Adelie penguins (190 ±0.54 mm; P ≤ 0.0001). As with body mass, flipper length was significantly greater in males than in females across all species.

These interspecific differences likely reflect adaptations to distinct foraging strategies and ecological niches. Gentoo penguins, for example, are known to dive deeper and forage in more benthic environments than Adelie and Chinstrap penguins, which may explain their larger body size and longer flippers—traits that improve diving efficiency and propulsion underwater. The consistent sexual dimorphism observed in both body mass and flipper length is also well-documented in seabirds and is typically linked to differences in reproductive roles, foraging behavior, or intra-species competition. The morphological variation observed here supports the idea that both ecological and sexual selection pressures shape penguin body structure in a species- and sex-specific manner.

Given the similarity in species differences for both body mass and flipper length, we examined the relationship between these two traits. A strong positive correlation was observed, with flipper length significantly associated with body mass (R² = 0.76), confirming that heavier penguins tend to have longer flippers.

Bill Length and Depth

Bill length and depth are important morphological traits that play key roles in foraging strategy, prey selection, and species differentiation among penguins. These traits are typically measured using calipers from the base of the bill to the tip (length) and from the top to the bottom at the widest point (depth).

Among the three species, Chinstrap penguins have the greatest mean bill length (48.8 ±0.4 mm SEM), which is significantly longer than Gentoo penguins (47.6 ±0.29 mm; P ≤ 0.01), and both were substantially longer than the bills of Adelie penguins (38.8 ±0.22 mm; P ≤ 0.0001). A similar pattern was observed for bill depth, with Chinstrap penguins also having highest mean depths (18.4 ±0.14 mm SEM). However, Adelie penguins have comparable bill depths (18.3 ±0.10 mm; P > 0.05), and both species have significantly thicker bills than Gentoo penguins (15.0 ±0.09 mm; P ≤ 0.0001). As with other morphological traits, males exhibited significantly longer and larger bills than females across all species (P ≤ 0.001), highlighting consistent sexual dimorphism in penguin bill morphology.

Given the disparity in bill morphology between species — with Chinstrap penguins exhibiting the greatest mean bill depth and Gentoo penguins the longest bills — it is perhaps unsurprising that no significant relationship was observed between bill measurements when assessed across species (R² = 0.06). However, when examined within species, moderate positive correlations were found between bill length and depth in both Chinstrap and Gentoo penguins (R² = 0.43 for both), suggesting coordinated growth of these traits within individuals of each species. In contrast, Adelie penguins exhibited only a weak association between these two bill dimensions (R² = 0.15), indicating greater morphological variation or potentially different ecological or developmental constraints within this species.

These findings suggest that while bill length and depth are not consistently associated across Pygoscelis species, intra-species patterns of covariation may reflect functional or ecological specialisation. For example, a stronger correlation between bill dimensions in Chinstrap and Gentoo penguins may be linked to more specialised feeding strategies or consistent prey types, whereas the weaker relationship in Adelies could reflect a broader or more variable diet. Further analysis incorporating diet composition or foraging behaviour could help clarify the ecological drivers of these morphological patterns.

Interannual Variation in Body Mass

Given the potential influence of climatic variability on food availability and foraging success in Antarctic ecosystems, we assessed whether penguin body mass varied across the study years (2007–2009). Changes in sea ice extent, ocean temperatures, and prey distribution can all affect penguin body condition; thus, identifying any interannual shifts is important for interpreting ecological trends. While detailed climate data for these specific years were not directly assessed here, regional studies suggest that the late 2000s were characterised by moderate climatic variability, without extreme temperature anomalies.

Overall, body mass remained relatively stable across years for most species and sexes. The only notable exception was observed in female Gentoo penguins, whose mean body mass increased significantly from 4.6 kg (±0.07 kg SEM) in 2007 to 4.8 kg (±0.04 kg SEM) in 2009 (P = 0.04). This modest but significant increase may reflect improved foraging conditions during these years, consistent with Gentoos’ flexibility in diet and habitat use compared to more specialised species such as Adelies. As Gentoo penguins are known to be less dependent on sea ice and can adapt their foraging strategies in response to environmental changes, their body condition may be more responsive to short-term fluctuations in marine productivity.

Conclusion

This study highlights the complex interplay between ecological pressures, species-specific adaptations, and sexual dimorphism in shaping penguin morphology within the Palmer Archipelago. The strong interspecific differences observed in body mass, flipper length, and bill dimensions likely reflect adaptations to distinct foraging strategies and habitat use. In contrast, the more subtle intraspecific patterns, such as the increase in female Gentoo body mass over time, suggest that some species may be more responsive to short-term environmental variability. These findings underscore the importance of morphological flexibility in a rapidly changing climate.

The data presented herein, provides a historical baseline for penguin morphology. Given ongoing environmental change in the Antarctic region, particularly shifts in sea ice dynamics and prey availability, repeating these measurements today would offer important opportunities to assess whether penguin morphology and body condition have shifted over time. Future research should also integrate long-term ecological monitoring with fine-scale dietary and foraging behaviour studies to better understand how environmental fluctuations shape morphological and demographic trends. Incorporating satellite tracking, stable isotope analysis, and prey field assessments could offer deeper insights into the ecological drivers of morphological variation. As climate change continues to reshape Antarctic ecosystems, understanding the links between morphology, behaviour, and environmental variability will be critical for predicting species resilience and guiding conservation strategies.

Data Availability

This research was conducted as part of the Palmer Station Long Term Ecological Research Program. These data were accessed via the Environmental Data Initiative (EDI) Data Portal and are freely available under a CC0 (“No Rights Reserved”) license in accordance with the Palmer Station Data Policy. All data processing and statistical analyses were conducted using R (version 4.4.3 for Windows), with the data retrieved via the palmerpenguins package (Horst, Hill, & Gorman, 2020), available on CRAN.

References

Horst AM, Hill AP, Gorman KB (2020). palmerpenguins: Palmer Archipelago (Antarctica) penguin data. doi:10.5281/zenodo.3960218, R package version 0.1.0, https://allisonhorst.github.io/palmerpenguins/.