The metabolic cost of incline locomotion in the svalbard rock ptarmigan (lagopus muta hyperborea): The effects of incline grade and seasonal fluctuations in body mass

SummaryIn a terrestrial environment animals must locomote over different terrain; despite this, the majority of studies focus on level locomotion. The influence moving up an inclined surface has on the metabolic cost of locomotion and the efficiency with which animals perform positive work against gravity is still not well understood. Generally speaking, existing data sets lack consistency in the use of grades, further compounded by differences between species in terms of morphology and locomotor gait. Here we investigated the metabolic cost of locomotion using respirometry in the Svalbard ptarmigan (Lagopus muta hyperborea). The Svalbard ptarmigan provides a unique opportunity to investigate the cost of incline locomotion as it undergoes a seasonal fluctuation in body mass, which doubles in winter, meaning the requirement for positive mechanical work also fluctuates with season. We demonstrate that at the same degree of incline, the cost of lifting 1 kg by 1 vertical metre remains relatively constant between seasons despite the large differences in body mass from summer to winter. These findings are consistent with the notion that positive mechanical work alone dictates the cost of lifting above a certain body mass. However, our data indicate that this cost may vary according to the degree of incline and gait.</jats:p

Cryptic impact: Visual detection of corona light and avoidance of power lines by reindeer

Assessing the impact of human development on animals is complicated by the fact that overt effects may have covert causes. Cryptic impacts (sensu Raiter et al. 2014) can arise where sensory stimuli to which species respond fall outside the human sensory range. Ultraviolet (UV) light, which is detected by a range of nonprimate mammals, is a potential example. We review evidence that dark-adapted eyes of reindeer–caribou Rangifer tarandus can detect light at 330–410 nm emitted by electrical corona on high-voltage power lines, which is necessarily barely visible to humans. Based on this, we suggest that the superior ability of Rangifer to detect corona UV light may partly account for the tendency of the animals to avoid power lines. Rangifer has UV-permissive ocular media that transmit approximately 15 times more corona light than human eyes. Retinal irradiance under full dilation is in the order of 7 times greater in Rangifer compared with humans. Seasonal transformation of the tapetum lucidum substantially increases retinal sensitivity in this species in winter. Threshold distances of detection of corona by Rangifer are in the order of hundreds of meters. Displays of corona may catch the animals' attention, and plume coronas, in particular, may induce the illusion of motion (the phi phenomenon), thereby falsely signaling the presence of potential predators. Both features are likely to increase wariness and cause animals to withdraw from the source of the stimulus. We suggest that spatial and temporal variability of corona contributes to substantial variation observed in the strength and persistence of avoidance responses in Rangifer at these structures. © 2016 The Authors. Wildlife Society Bulletin published by Wiley Periodicals, Inc. on behalf of The Wildlife Society

Where clocks are redundant: weak circadian mechanisms in reindeer living under polar photic conditions

Biological rhythms are a result of interplay between endogenous clocks and the ambient light–dark (LD) cycle. Biological timing in resident polar organisms presents a conundrum because these experience distinct daily LD cycles for only a few weeks each year. We measured locomotor activity in reindeer, Rangifer tarandus platyrhynchus (SR, n=5 and 6) and R. tarandus tarandus (NR, n=6), ranging freely at 78 and 70°N, respectively, continuously throughout 1 year using data loggers. NR, but not SR, are gregarious which enabled us to examine the integrated effects of differences in social organisation and the photic environment at two different latitudes on the organisation of activity. In both sub-species, ultradian bouts of activity and inactivity alternated across the 24-h day throughout the year. This pattern was modified by the LD cycle in NR but barely at all in SR. Periodogram analysis revealed significant ultradian rhythmicity in both subspecies; the frequency of daily cycles of activity increased from three per day in winter to nearly five in summer. We conclude that this increase, and a concomitant increase in the level of daily activity, reflected the seasonal increase in the animals’ appetite and the quality of their forage. Secondly, the combination, most evident in SR, of a weak photic response, weak circadian mechanisms and a weak social synchronization reduces the constraints of biological timing in an environment which is effectively non-rhythmic most of the year and permits expression of the basic ultradian pattern of ruminant activity. Third, the weaker 24-h rhythmicity in SR compared to NR indicates a latitudinal decrease in circadian organization and photic responsiveness in Rangifer.

Understanding sex differences in the cost of terrestrial locomotion

Little is known regarding the physiological consequences of the behavioural and morphological differences that result from sexual selection in birds. Male and female Svalbard rock ptarmigans (Lagopus muta hyperborea) exhibit distinctive behavioural differences during the breeding season. In particular, males continuously compete for and defend territories in order to breed successfully, placing large demands on their locomotor system. Here, we demonstrate that male birds have improved locomotor performance compared with females, showing both a lower cost of locomotion (CoL) and a higher top speed. We propose that the observed sex differences in locomotor capability may be due to sexual selection for improved male performance. While the mechanisms underlying these energetic differences are unclear, future studies should be wary when pooling male and female data

A Circadian Clock Is Not Required in an Arctic Mammal

SummarySeasonally breeding mammals use the annual change in the photoperiod cycle to drive rhythmic nocturnal melatonin signals from the pineal gland, providing a critical cue to time seasonal reproduction [1]. Paradoxically, species resident at high latitudes achieve tight regulation of the temporal pattern of growth and reproduction despite the absence of photoperiodic information for most of the year [2]. In this study, we show that the melatonin rhythm of reindeer (Rangifer tarandus) is acutely responsive to the light/dark cycle but not to circadian phase, and also that two key clock genes monitored in reindeer fibroblast cells display little, if any, circadian rhythmicity. The molecular clockwork that normally drives cellular circadian rhythms is evidently weak or even absent in this species, and instead, melatonin-mediated seasonal timing may be driven directly by photic information received at a limited time of year specific to the equinoxes

Gait kinematic parameters plotted against walking speed (<i>U</i>) in winter (blue) and summer (red) acclimated birds.

<p>A) Duty factor decreased with <i>U</i>, but never dropped below 0.5 across the speed range tested, B) The relative durations of the stance (<i>l</i>_stance, circles) and swing (<i>l</i>_swing, squares) phases. <i>l</i>_swing remained relatively unchanged across the speed range and was slightly reduced in winter birds. <i>l</i>_stance decreased curvi-linearly and was less in winter birds C) Stride frequency increased linearly with <i>U</i> and was 0.51Hz faster across the speed range during winter D) Stride length similarly increased linearly with <i>U</i> and was reduced in winter birds.</p