Do intraspecific life history patterns follow interspecific predictions? A test using latitudinal variation in ringed seals

Abstract

Mammals adapted to unpredictable and low‐energy environments often evolve a “bet‐hedging” life history strategy characterized by less costly reproductive outputs over a longer and slower‐growing life. In contrast, species adapted to more predictable (i.e., low variation) and higher energy environments may evolve greater fecundity over a shorter and faster‐growing life. We tested whether this known interspecific pattern also occurs within a species. We compared life history traits of the ringed seal (Pusa hispida) in the Canadian High Arctic to those closer to the southern limit of the species' circumpolar distribution. We found that northern seals grew slower than southern seals (Brody growth coefficient), achieved a greater asymptotic body weight (82 and 69 kg vs. 74 and 54 kg female and male, respectively), reached sexual maturity later (6.1 years vs. 4.5 years), had lower fecundity (1.8 years vs. 1.3 years interbirth interval), longer average lifespan (5 years vs. 3 years median age), and greater movements (1,269 vs. 681 km). Mating systems also likely differed with northern seals showing morphological evidence of a promiscuous mating system with potential sperm competition as indicated by greater relative testes size. The northern region was also characterized by more unpredictable environmental timing of seasonal events, such as spring sea ice breakup. Life history variation between the intraspecific groups of seals appears to agree with interspecific patterns and provides a better understanding of how species' life history parameters shift in concert with environmental conditions.

Comparing temporal patterns in body condition of ringed seals living within their core geographic range with those living at the edge

“…Other biological measures could be used to assess the predictions of greater variability associated with populations occupying peripheral range with suboptimal habitat. Movement studies, although challenging, have provided evidence of seasonal dive behavior (Gjertz et al 2000, Crawford et al 2019 and could be used to compare regions (Yurkowski et al 2016b, Ferguson et al 2019. In addition, analysis of seal tissues for dietary biomarkers (e.g.…”

“…To describe seal population differences between the core (High Arctic) and peripheral (Hudson Bay) populations, we summarize average adult (> 10 yr) length (cm), weight (kg), and age (yr). In addition, we provide the estimate of female age of sexual maturity as an informative life-history variable that assists in comparing the two populations (Ferguson et al 2019). Average age of adult seals older than 10 yr to minimize bias associated with hunter selection.…”

“…c Age of sexual maturity was determined following the methods of Demaster (1978). See Ferguson et al (2019) for details of statistical calculation method.…”

“…The majority of seals collected were adults but the peripheral region had more YOY relative to the core region (30 versus 21%). The season when most seals were collected shifted from May to September in the core region to June-January in the peripheral region (see Supplementary material Appendix Table A1 in Ferguson et al 2019). For both regions, more males than females were harvested (58% in core and 56% in peripheral; Chi-square test p-value = 0.90).…”

“…Ringed seals have a southern limit that varies around the Holarctic; thereby providing a good model to test biogeographic predictions. In the eastern Canadian Arctic, ringed seals live within a wide latitudinal range (55 to 90° latitude) where seasonal and interannual variation in body condition has been observed (Ferguson et al ). Ringed seals at the southern limit of their range in Hudson Bay have been shown to be smaller, both in length and mass, than seals found in core areas of the central Arctic, supporting both regional and latitudinal size differences (McLaren , Holst and Stirling , Krafft et al , Chambellant , Ferguson et al ).…”