Evolution of age at primiparity in pinnipeds in the absence of the quality–quantity trade‐off in reproduction

Abstract

Age at primiparity (AP) is a key life history trait which is crucial to the evolution of life history strategies. This trait is particularly interesting in pinnipeds (walrus, eared seals, and true seals), which are monotocous animals. Thus, the commonly observed trade‐off between offspring quality and quantity does not apply to this taxon. Therefore, comparative studies on the evolution of AP might shed light on other important evolutionary correlates when litter size is fixed. Using phylogenetic generalized least squares analyses, we found a strong negative and robust correlation between relative birth mass (mean pup birth mass as a proportion of mean adult female mass) and AP. Rather than trading‐off an early start of reproduction with light relative offspring mass, this result suggests that pinnipeds exhibit either faster (i.e., higher relative offspring mass leading to shorter lactation length, and thus shorter interbirth interval) or slower life histories and that an early AP and a heavy relative offspring mass co‐evolved into a comparatively fast life history strategy. On the other hand, AP was positively related to lactation length: A later start of reproduction was associated with a longer lactation length. Consequently, variation in AP in pinnipeds seems to be affected by an interplay between costs and benefits of early reproduction mediated by relative investment into the single offspring via relative birth mass and lactation length.

Spatio-temporal dynamics of a model for the effect of variable ages at reproduction

“…Using similar arguments as those in the proof of statement (b), we know that M − is globally attractive for (14). Then (15) implies that statement (c) holds. Moreover, if f is nondecreasing in [0, M + ], then we have M * = M ± , which implies that lim t→∞ u(t, x; φ) = M * .…”

“…For example, the age at which an organism starts to reproduce (i.e. age at recruitment or age at primiparity) is regarded as a very important demographic parameter defining the species population dynamics [7,9,15]. As an important species in ecology and epidemiology, Ixodes ticks are the main vector to transmit various tick-borne pathogens, including Lyme disease, babesiosis, anaplasmosis and tick-borne encephalitis [19].…”

“…Quantifying the potential role of heterogeneity within populations has been remaining an important objective in population ecology. There are trade-offs between early reproduction and delayed maturation [15]. For example, organisms start to reproduce early may have a possibly increased probability of realizing reproduction.…”