ECOPHYSIOLOGY

Tracing Energy Flow Within and Among Organisms

I use bulk tissue and compound-specific stable isotope analyses to study ecophysiology, namely the transfer/remobilization of nutrients during fasting or reproduction in free-ranging marine and terrestrial organisms (Lübcker et al. 2020a, b).

In less than two decades, stable isotope analysis has become a ubiquitous tool to trace ecological processes, yet ecophysiologists have not fully realized the power of this method.

We know that the isotope values of animals are often influenced by more than just ingested resources and that physiological processes associated with key biochemical pathways imprint on the isotopic composition of tissues (Lübcker et al. 2020a, b, 2021). Furthermore, failure to consider physiology may lead to erroneous ecological inferences about energy transfer among organisms that is often used to study species interactions and food web structure (Lübcker et al. 2016, 2017).

I am developing a novel amino acid isotope approach to trace nitrogen balance in response to reproduction and nutrient allocation in birds, reptiles, sea turtles, seals, and baleen whales (Busquets Vass et al. in prep; Lübcker et al. in prep). For example, I am pioneering a technique using carbon and nitrogen isotope values of amino acids in pectoral muscle of females and yolk of their eggs to quantify the reliance on income versus capital breeding at the compound level in a guild of migratory geese that breed in the Arctic in collaboration with Dr. Keith Hobson (Western University, Ontario). Arctic ecosystems are rapidly changing and are ideal locations to study the adaptive mechanisms used by animals in response to shifts in biotic (resource availability/phenology) and abiotic (temperature) conditions.

A new tool for identifying nitrogen balance of free-ranging mammals

It is notoriously difficult to monitor the nutritional status of free-ranging animals over time, particularly those living in the oceans. Whiskers and naturally shed tissues, such as hair or feathers, can be sampled using minimally invasive approaches and are increasingly used for stable isotope-based dietary studies.

FIGURE FROM LUBCKER ET AL. 2021. NITROGEN (Δ15N) ISOTOPE VALUES MEASURED IN TEMPORALLY OVERLAPPING, PAIRED HAIR (CIRCLES) AND CHRONOLOGICALLY SUB-SAMPLED WHISKERS (LINES) OF YOUNG-OF-THE-YEAR (YOY; N = 16 INDIVIDUALS) AND ADULT FEMALE (AF; N = 4) SOUTHERN ELEPHANT SEALS. THE MEAN (±SD) EPIDERMIS Δ15N VALUES OF N = 5 DIFFERENT ADULT FEMALES ARE ALSO SHOWN FOR COMPARISON. SOLID LINES: FITTED LOESS SMOOTHERS WITH ASSOCIATED STANDARD ERROR (GRAY SHADING); VERTICAL GRAY RECTANGLE: PORTION OF THE WHISKER GROWN CONCURRENTLY WITH HAIR SYNTHESIS DURING THE MOLT FAST; PURPLE DASHED LINES: POSITION ALONG THE WHISKERS THAT REFLECTS AT SEA FORAGING WITH THE PLOT POSITION (X-AXIS) SET TO ZERO, SIGNIFYING THE END OF THE MOLT FAST (LÜBCKER ET AL. 2020A). THE HAIR AND EPIDERMIS BULK TISSUE ISOTOPE VALUES WERE OVERLAID WITH THE WHISKER Δ15N VALUES TO ILLUSTRATE THE POSITION ALONG THE LENGTH OF THE WHISKERS WHERE THE Δ15N VALUES WERE MOST COMPARABLE. THE HAIR AND EPIDERMIS Δ15N VALUES RESEMBLED THE WHISKER Δ15N VALUES DURING A KNOWN CATABOLIC STATE.
Molting southern elephant seal, Mirounga leonina

Between November and February each year, southern elephant seals (SES) haul out ashore to undergo a ‘catastrophic’ molt. Because the shed hair is potentially synthesized while these animals are fasting, it is possible that the stable isotope values measured in the hair do not provide an accurate reflection of the isotope values of their prey. We found that both the bulk tissue and amino acid δ15N values values of hair, epidermis, and whiskers of SES are adversely influenced by the physiological changes associated with fasting. This was similarly observed in the portion of their whiskers synthesized while on land (Lübcker et al. 2020). See link for more information.

Moreover, fasting also resulted in a large increase in δ15N values for most glucogenic amino acids and a simultaneous depletion of alanine. This finding enabled us to accurately predict (74%) the nutritional status of these animals (fasting versus foraging). While this study adavnces the use of amino acid stable isotopes to observed physiological procesess, it is also evident that physiological factors have an important influence on tissue δ15N values. Not taking this into account can lead to erroneous bulk tissue or amino acid isotope-based reconstructions of foraging habits.