Reproductive success delays moult phenology in a polar mammal

Abstract

Animals can respond to dynamic environments through phenological plasticity of life history events; however, changes in one part of the annual cycle can diminish the success of subsequent life history events. Our aims were to determine the associations between reproduction and moult phenology across years and to quantify phenological plasticity across varying environmental conditions. We conducted demographic surveys of 4,252 flipper-tagged Weddell seals (Leptonychotes weddellii) in the Ross Sea, Antarctica during four austral summers. At each sighting, seals were assigned a moult code based on the visible presence of new fur and the start date of each animal's moult was back-calculated. Reproductive success and parturition dates were obtained for the breeding season prior to and following the moult. We found that successful reproduction delayed moult by 16 days relative to non-parturient females. Phenology of the intervening moult was indicative of previous reproductive dynamics but not predictive of subsequent reproductive outcomes. Across years, moult phenology varied by about two weeks and covaried strongly with sea ice break-out timing for all reproductive categories. Our findings suggest these polar mammals have some flexibility within the annual cycle that allows adjustment of moult phenology to fluctuating environmental conditions without compromising future reproductive success.

Figure 1

During surveys, each individual was assigned a moult code: 0 (unmoulted), 1 (head or dorsal stripe moulted), 2 (head and wide dorsal stripe moulted), 3 (flank starting to moult), or 4 (completely moulted).

Figure 2

Based on individual sightings, adult female Weddell seals were assigned a phenology category for three life history events: reproduction in Year 1, moulting in Year 1, and reproduction in Year 2. Early, mid, and late correspond to the distribution 0–25%ile, 25–75%ile, and 75–100%ile, respectively; Non-Parturient had previously pupped but were not parturient in a given year; and Non-Attendant had been seen during the Year 1 pupping and moulting season but were not seen during the Year 2 pupping season.

Figure 3

Moult sighting data from a theoretical animal with moult code n shown as numbers along the date axis. Each sighting is represented as a grey circle with moult code n shown. Mean moult stage durations τ_n were used to back-calculate a start date for each individual when the animal was not observed in a moult code n. A glossary of parameters is shown in the top panel.

Figure 4

Moult initiation dates across reproductive categories and years (panels; 2013 is the 2013 austral summer including the December 2013 – February 2014 moult) for Attendant individuals. Within each year, different letters denote significantly different moult initiation dates across reproductive categories (Tukey’s HSD, p < 0.05). During all study years, sexually mature females that did not produce a pup (Non-Parturients) moulted earlier than all other reproductive categories. On the contrary, sexually mature females that produced a pup (Parturients) tended to moult later than sexually immature females (Juvenile Females, significant difference in 2013, 2015, 2016), and Males (significant difference in 2013, 2016).

Figure 5

Proportion of seals in each moult category in Year 1 comprised of different reproductive categories from Year 1. Moult phenology was not independent of pupping phenology: the Early-Moulters category was predominated by Attendant Non-Parturient, whereas the Late-Moulters category was predominated by Attendant Parturients.