Physiological tipping points in the relationship between foraging success and lifetime fitness of a long-lived mammal

Reproductive success and offspring survival decline for female elephant seals past prime age

Maternal age can influence reproductive success and offspring fitness, but the timing, magnitude and direction of those impacts are not well understood. Evolutionary theory predicts that selection on fertility senescence is stronger than maternal effect senescence, and therefore, the rate of maternal effect senescence will be faster than fertility senescence. We used a 36-year study of northern elephant seals (Mirounga angustirostris) to investigate reproductive senescence. Our dataset included 103,746 sightings of 1203 known-age female northern elephant seals. We hypothesized that fertility (maternal reproductive success), offspring survival and recruitment into the breeding population, and male offspring production would decline with advanced maternal age. Furthermore, we hypothesized that older females would shorten their moulting haul out to allow for more time spent foraging. We found evidence for both fertility and maternal effect senescence, but no evidence for senescence impacting offspring recruitment or sex ratio. Breeding probability declined from 96.4% (95% CI: 94.8%-97.5%) at 11 years old to 89.7% (81.9%-94.3%) at 19 years old, and the probability of offspring survival declined from 30.3% (23.6%-38.0%) at 11 years old to 9.1% (3.2%-22.9%) at 19 years old. The rates of decline for fertility and maternal effect senescence were not different from each other. However, maternal effect senescence had a substantially greater impact on the number of offspring surviving to age 1 compared to fertility senescence. Compared to a hypothetical non-senescent population, maternal effect senescence resulted in 5.3% fewer surviving pups, whereas fertility senescence resulted in only 0.3% fewer pups produced per year. These results are consistent with evolutionary theory predicting weaker selection on maternal effect than fertility senescence. Maternal effect senescence may therefore be more influential on population dynamics than fertility senescence in some systems.

Elephant seals as ecosystem sentinels for the northeast Pacific Ocean twilight zone

The open ocean twilight zone holds most of the global fish biomass but is poorly understood owing to difficulties of measuring subsurface ecosystem processes at scale. We demonstrate that a wide-ranging carnivore-the northern elephant seal-can serve as an ecosystem sentinel for the twilight zone. We link ocean basin-scale foraging success with oceanographic indices to estimate twilight zone fish abundance five decades into the past, and into the future. We discovered that a small variation in maternal foraging success amplified into larger changes in offspring body mass and enormous variation in first-year survival and recruitment. Worsening oceanographic conditions could shift predator population trajectories from current growth to sharp declines. As ocean integrators, wide-ranging predators could reveal impacts of future anthropogenic change on open ocean ecosystems.

Maximizing biological insights from instruments attached to animals

Instruments attached to animals ('biologgers') have facilitated extensive discoveries about the patterns, causes, and consequences of animal behavior. Here, we present examples of how biologging can deepen our fundamental understanding of ecosystems and our applied understanding of global change impacts by enabling tests of ecological theory. Applying the iterative process of science to biologging has enabled a diverse set of insights, including social and experiential learning in long-distance migrants, state-dependent risk aversion in foraging predators, and resource abundance driving movement across taxa. Now, biologging is poised to tackle questions and refine ecological theories at increasing levels of complexity by integrating measurements from numerous individuals, merging datasets from multiple species and their environments, and spanning disciplines, including physiology, behavior and demography.

Tracking individual animals can reveal the mechanisms of species loss

As biodiversity loss continues, targeted conservation interventions are increasingly necessary. Stemming species loss requires mechanistic understanding of the processes governing population dynamics. However, this information is unavailable for most animals because it requires data that are difficult to collect using traditional methods. Advances in animal tracking technology have generated an avalanche of high-resolution observations for a growing list of species around the globe. To date, most research using these data has focused on questions about animal behavior, with less emphasis on population processes. Here, we argue that tracking data are uniquely poised to bring powerful new insights to the urgent, global problem of halting species extinctions by revealing when, where, how, and why populations are changing.

Balancing Act: An Interdisciplinary Exploration of Trade-offs in Reproducing Females

Trade-offs resulting from the high demand of offspring production are a central focus of many subdisciplines within the field of biology. Yet, despite the historical and current interest on this topic, large gaps in our understanding of whole-organism trade-offs that occur in reproducing individuals remain, particularly as it relates to the nuances associated with female reproduction. This volume of Integrative and Comparative Biology (ICB) contains a series of papers that focus on reviewing trade-offs from the female-centered perspective of biology (i.e., a perspective that places female reproductive biology at the center of the topic being investigated or discussed). These papers represent some of the work showcased during our symposium held at the 2024 meeting of the Society for Integrative and Comparative Biology (SICB) in Seattle, Washington. In this roundtable discussion, we use a question-and-answer format to capture the diverse perspectives and voices involved in our symposium. We hope that the dialogue featured in this discussion will be used to motivate researchers interested in understanding trade-offs in reproducing females and provide guidance on future research endeavors.

Two decades of three-dimensional movement data from adult female northern elephant seals

Northern elephant seals (Mirounga angustirostris) have been integral to the development and progress of biologging technology and movement data analysis, which continue to improve our understanding of this and other species. Adult female elephant seals at Año Nuevo Reserve and other colonies along the west coast of North America were tracked annually from 2004 to 2020, resulting in a total of 653 instrument deployments. This paper outlines the compilation and curation process of these high-resolution diving and location data, now accessible in two Dryad repositories. The code used for data processing alongside the corresponding workflow is available through GitHub and Zenodo. This data set represents 3,844,927 dives and 596,815 locations collected from 475 individual seals with 178 repeat samplings over 17 years. We anticipate that these data will stimulate further analysis and investigation into elephant seal biology and aid in developing new analytical approaches for large marine predators.

Overcoming the pitfalls of categorizing continuous variables in ecology, evolution and behaviour

Many variables in biological research-from body size to life-history timing to environmental characteristics-are measured continuously (e.g. body mass in kilograms) but analysed as categories (e.g. large versus small), which can lower statistical power and change interpretation. We conducted a mini-review of 72 recent publications in six popular ecology, evolution and behaviour journals to quantify the prevalence of categorization. We then summarized commonly categorized metrics and simulated a dataset to demonstrate the drawbacks of categorization using common variables and realistic examples. We show that categorizing continuous variables is common (31% of publications reviewed). We also underscore that predictor variables can and should be collected and analysed continuously. Finally, we provide recommendations on how to keep variables continuous throughout the entire scientific process. Together, these pieces comprise an actionable guide to increasing statistical power and facilitating large synthesis studies by simply leaving continuous variables alone. Overcoming the pitfalls of categorizing continuous variables will allow ecologists, ethologists and evolutionary biologists to continue making trustworthy conclusions about natural processes, along with predictions about their responses to climate change and other environmental contexts.

Individual variation in life-history timing: synchronous presence, asynchronous events and phenological compensation in a wild mammal

Many animals and plants have species-typical annual cycles, but individuals vary in their timing of life-history events. Individual variation in fur replacement (moult) timing is poorly understood in mammals due to the challenge of repeated observations and longitudinal sampling. We examined factors that influence variation in moult duration and timing among elephant seals (Mirounga angustirostris). We quantified the onset and progression of fur loss in 1178 individuals. We found that an exceptionally rapid visible moult (7 days, the shortest of any mammals or birds), and a wide range of moult start dates (spanning 6-10× the event duration) facilitated high asynchrony across individuals (only 20% of individuals in the population moulting at the same time). Some of the variation was due to reproductive state, as reproductively mature females that skipped a breeding season moulted a week earlier than reproductive females. Moreover, individual variation in timing and duration within age-sex categories far outweighed (76-80%) variation among age-sex categories. Individuals arriving at the end of the moult season spent 50% less time on the beach, which allowed them to catch up in their annual cycles and reduce population-level variance during breeding. These findings underscore the importance of individual variation in annual cycles.

A grazer's niche edge is associated with increasing diet diversity and poor population performance

The core-periphery hypothesis predicts niche cores should be associated with greater survivorship, reproductive output and population performance rates than marginal habitats at niche edges. However, there is very little empirical evidence of whether niche centrality influences population trends in animals. Using the Cape mountain zebra (Equus zebra zebra) as a model system, we evaluated whether niche centrality is associated with population trends, resource availability and diet across a core-periphery gradient. Population growth rates and density progressively declined towards niche peripheries. Niche peripheries were resource-poor and Cape mountain zebra consumed more phylogenetically diverse diets dominated by non-grass families. In core habitats they consumed grass-rich diets and female reproductive success was higher. This combination of spatial niche modelling and functional ecology provides a novel evaluation of how bottom-up resource limitation can shape species distributions, population resilience and range change and can guide conservation management.