DISCUSSION

Physiological status of GSL pups during the 2024 ENSO event

The present study provides an integrated clinical assessment of GSL pups during the 2024 El Niño event and compares these findings with those of sympatric GFS pups using morphometric, biochemical, and hematological parameters. Sampling coincided with a weak-to-moderate ENSO phase and reduced rookery-level chlorophyll-a concentrations relative to 2023, suggesting lower marine productivity during the study period. This environmental context is relevant because Galapagos pinnipeds are known to be sensitive to El Niño-driven oceanographic variability, although the magnitude of ecological effects differs among events [21].

Morphometric characteristics and body condition

No significant sex-related differences were detected in body weight, standard length, axillary girth, or body condition in GSL pups. These findings indicate morphometric similarity between females and males during early postnatal development and are consistent with previous studies showing that sexual size dimorphism in otariids generally becomes more apparent later in life [28, 29].

Interspecific comparisons revealed clearer differences. Both female and male GSL pups exhibited higher body condition values than GFS pups, indicating better relative body condition during the study period. Body condition indices are widely used as size-adjusted indicators of nutritional status and maternal investment in marine mammals [30, 31]. These interspecific differences may be related, at least in part, to contrasting foraging ecologies. GSLs typically forage in more coastal environments and consume mainly small epipelagic fish, whereas GFSs exploit more pelagic habitats and show a greater contribution of cephalopods to their diet [32]. In addition, GSLs have been reported to exhibit trophic flexibility during ENSO conditions, which may facilitate adjustments in prey use when marine productivity declines [33].

El Niño-associated environmental variability can reduce prey availability and increase maternal foraging effort, with potential downstream effects on pup growth and nursing frequency [20, 34, 35]. However, despite these constraints, the morphometric indicators observed in GSL pups remained relatively stable. This pattern may reflect the moderate intensity of the 2024 ENSO event or short-term physiological resilience, although sampling bias toward apparently healthy individuals should also be considered.

Serum biochemical profiles

The serum chemistry results revealed a partially similar pattern. GSL pups had higher GLOB and TP concentrations than GFS pups in both sexes, paralleling the interspecific differences observed in body condition. Serum protein levels in marine mammals may vary with nutritional status, hydration, immune activity, and inflammatory processes [10, 11, 36]. Therefore, the higher GLOB and TP concentrations observed in GSL pups may reflect broader interspecific differences in physiological condition, immune stimulation, or both. Nevertheless, because these markers are influenced by multiple biological processes, their interpretation should remain cautious.

In contrast, differences in ALB, BUN, and GLU were restricted to a single sex and should therefore be interpreted conservatively. ALB and BUN concentrations were higher in female GFS pups, whereas GLU concentrations were higher in male GFS pups. Because these analytes can be influenced by recent feeding, fasting duration, hydration, and short-term metabolic variation, such differences may reflect transient physiological states rather than stable species-level patterns [37].

Hematological characteristics

A similarly cautious interpretation applies to the hematological findings. Most leukocyte differential values overlapped between species, indicating broadly comparable hematological profiles. The few significant differences observed, including higher hematocrit values in GSL males and higher monocyte and eosinophil values in GFS males, may reflect variation in immune maturation, inflammatory status, or parasite exposure rather than consistent interspecific divergence. Previous studies in otariid pups have shown that parasitism and immune development can influence hematological parameters during early life [38, 39].

Comparison with other otariid species

Comparisons with published data from other otariid pups further support the overall interpretation of the present findings. Most biochemical and hematological analytes fell within ranges previously reported for other otariid species, suggesting that the clinical profiles of both Galapagos pinnipeds remained broadly within expected physiological limits. Nevertheless, ALP and TB showed some divergence from certain published datasets, with both species exhibiting relatively lower ALP and higher TB values. These differences should be interpreted with caution. Variation among studies may reflect differences in age class, nutritional status, health status, sampling context, environmental conditions, or analytical methodology rather than clear biological separation. Although lower ALP and higher TB values could be compatible with differences in metabolic or nutritional status, the present data do not allow a specific causal interpretation [11, 40–43].

Study limitations

Several limitations should be considered when interpreting this study. First, this was a cross-sectional assessment conducted during a single breeding season, which limits inference regarding temporal variability in physiological parameters. Hematological and biochemical values in marine mammals may vary across seasons, years, and life-history stages according to differences in diet, development, and metabolic demands.

Second, sample size limitations are inherent to studies of free-ranging marine mammals, particularly in remote and protected environments such as the Galapagos archipelago. Logistical constraints, animal welfare considerations, and restricted access to rookeries can limit the number of animals sampled and reduce statistical power.

Third, the study focused on apparently healthy pups accessible within breeding rookeries. Consequently, the reported values likely represent relatively healthy individuals and may underestimate the full range of health variability within the population.

Fourth, interpretation of clinical pathology in wildlife is constrained by the limited availability of species-specific reference intervals and methodological differences among studies. Variability in analytical platforms, laboratory procedures, and sample handling may influence reported values and complicate direct interstudy comparisons.

Fifth, capture and restraint may have influenced some blood analytes, particularly GLU concentrations and leukocyte-related variables, despite efforts to minimize handling time and disturbance.

Implications for conservation and health monitoring

Despite these limitations, this study provides the first direct comparison of GSL and GFS pup health during an active ENSO event. GSL pups exhibited higher relative body condition and higher GLOB and TP concentrations than GFS pups, whereas most hematological variables were similar between species. These findings suggest species-specific differences in physiological responses to reduced marine productivity, while also indicating broadly comparable clinical status across most measured parameters.

The baseline data generated in this study provide a useful foundation for long-term monitoring, early detection of nutritional stress, and conservation-oriented health assessment of Galapagos pinnipeds under increasingly variable oceanographic conditions.

Future perspectives

The present study evaluated morphometric and standard clinical pathology parameters but did not assess pathogen exposure, endocrine responses, or chronic stress biomarkers. Future studies integrating parasitological, immunological, and endocrine markers, together with longitudinal monitoring during both ENSO and non-ENSO periods, would provide a more comprehensive understanding of pinniped health under changing oceanographic conditions.