Intrinsic and extrinsic influences on standard metabolic rates of three species of Australian otariid

Abstract

The study of marine mammal energetics can shed light on how these animals might adapt to changing environments. Their physiological potential to adapt will be influenced by extrinsic factors, such as temperature, and by intrinsic factors, such as sex and reproduction. We measured the standard metabolic rate (SMR) of males and females of three Australian otariid species (two Australian fur seals, three New Zealand fur seals and seven Australian sea lions). Mean SMR ranged from 0.47 to 1.05 l O₂ min^-1, which when adjusted for mass was from 5.33 to 7.44 ml O₂ min^-1 kg^-1. We found that Australian sea lion mass-specific SMR (sSMR; in millilitres of oxygen per minute per kilogram) varied little in response to time of year or moult, but was significantly influenced by sex and water temperature. Likewise, sSMR of Australian and New Zealand fur seals was also influenced by sex and water temperature, but also by time of year (pre-moult, moult or post-moult). During the moult, fur seals had significantly higher sSMR than at other times of the year, whereas there was no discernible effect of moult for sea lions. For both groups, females had higher sSMR than males, but sea lions and fur seals showed different responses to changes in water temperature. The sSMR of fur seals increased with increasing water temperature, whereas sSMR of sea lions decreased with increasing water temperature. There were no species differences when comparing animals of the same sex. Our study suggests that fur seals have more flexibility in their physiology than sea lions, perhaps implying that they will be more resilient in a changing environment.

Keywords: Metabolic rate; otariid; sex; water temperature.

Figure 1:

Moulting, breeding and pupping time line of Australian fur seals, New Zealand fur seals and Australian sea lions and timetable of experiments conducted at three Australian marine facilities over 3 years. Shaded boxes indicate that trials were conducted during that month in the respective facility. RF1 is located in a temperate to sub-tropical region, RF2 is located in a sub-tropical region, and RF3 is located in a temperate region.

Figure 2:

Logarithm of metabolic rate while resting in water (SMR; in litres of oxygen per minute) as a function of the logarithm of body mass (in kilograms) for one female Australian fur seal (n = 13), one male Australian fur seal (n = 16), three male New Zealand furseals (n = 31), five female Australian sea lions (n = 68) and two male Australian sea lions (n = 26). The line plotted is the fitted equation: log(SMR) = −3.48 + 0.66log(mass).

Figure 3:

Median, interquartile range (box) and range (bars) of mass-specific standard metabolic rate (sSMR; in millilitres of oxygen per minute per kilogram) for an Australian fur seal male (black box, n = 1) and female (white box, n = 1) and New Zealand fur seal males (grey box, n = 3) during the moult, post-moult and pre-moult periods.

Figure 4:

Median, interquartile range (box) and range (bars) of mass-specific standard metabolic rate (sSMR; in millilitres of oxygen per minute per kilogram) for male (grey box, n = 2) and female (white box, n = 5) Australian sea lions over the course of the year.

Figure 5:

Relationship between mass-specific standard metabolic rate (sSMR; in millilitres of oxygen per minute per kilogram) and water temperature (WT; in degrees Celsius) for four individual fur seals. (A) Female Australian fur seal (sSMR = 1.12 + 0.21 × WT, logLik = −17.42, R² = 0.140, P = 0.207, n = 13). (B) Male Australian fur seal (sSMR = −9.70 + 0.59 × WT, logLik = −15.63, R² = 0.683, P < 0.001, n = 16). (C) Male New Zealand fur seal (sSMR = −5.99 + 0.45 × WT, logLik = −18.36, R² = 0.404, P = 0.011, n = 15). (D) Male New Zealand fur seal (sSMR = 1.95 + 0.29 × WT, logLik = −14.70, R² = 0.587, P = 0.003, n = 12).

Figure 6:

Relationship between mass-specific standard metabolic rate (sSMR; in millilitres of oxygen per minute per kilogram) and water temperature (WT; in degrees Celsius) for Australian sea lions. (A) Adult male (ASM1; sSMR = 9.79 − 0.16 × WT, logLik = −12.40, R² = 0.450, P = 0.017, n = 12). (B) Adult female (ASF4; sSMR = 9.26 − 0.12 × WT, logLik = −13.16, R² = 0.348, P = 0.034, n = 13). (C) Adult female (ASF2; sSMR = 12.67 − 0.20 × WT, logLik = −14.38, R² = 0.497, P = 0.011, n = 12). (D) Adult male (ASM2; sSMR = 7.96 − 0.12 × WT, logLik = −11.73, R² = 0.336, P = 0.038, n = 13).