Increased Diurnal Activity Is Indicative of Energy Deficit in a Nocturnal Mammal, the Aardvark

Abstract

Shifting activity to cooler times of day buffers animals from increased heat and aridity under climate change. Conversely, when resources are limited, some nocturnal species become more diurnal, reducing energetic costs of keeping warm at night. Aardvarks (Orycteropus afer) are nocturnal, obligate ant-and termite-eating mammals which may be threatened directly by increasing heat and aridity, or indirectly by the effects of climate change on their prey. We hypothesised that the minimum 24-h body temperature of aardvarks would decline during energy scarcity, and that aardvarks would extend their active phases to compensate for reduced resource availability, possibly resulting in increased diurnal activity when aardvarks were energetically compromised. To measure their thermoregulatory patterns and foraging activity, we implanted abdominal temperature and activity data loggers into 12 adult aardvarks and observed them for varying durations over 3 years in the Kalahari. Under non-drought conditions, aardvarks tightly controlled their 24-h body temperature rhythm (mean amplitude of the 24-h body temperature rhythm was 1.8 ± 0.3 • C during summer and 2.1 ± 0.1 • C during winter) and usually were nocturnal. During a summer drought, aardvarks relaxed the precision of body temperature regulation (mean 24-h amplitude 2.3 ± 0.4 • C) and those that subsequently died shifted their activity to progressively earlier times of day in the weeks before their deaths. Throughout the subsequent winter, the aardvarks' minimum 24-h body temperatures declined, causing exaggerated heterothermy (4.7 ± 1.3 • C; absolute range 24.7 to 38.8 • C), with one individual's body temperature varying by 11.7 • C within 8 h. When body temperatures were low, aardvarks often emerged from burrows during daytime, and occasionally returned before sunset, resulting in completely diurnal activity. Aardvarks also shortened their active periods by 25% during food scarcity, likely to avoid energetic costs incurred by foraging. Despite their physiological and behavioural

Morphological evaluation of the orbit, eye tunics, eyelids, and orbital glands in young and adult aardvarks Orycteropus afer, Pallas, 1766 (Tubulidentata: Orycteropodidae): Similarities and differences with representatives of the Afrotheria clade

“…According to Pettigrew et al (2010), Walls (1942) distinguished five different eye types based on the specializations of the eye and retina, that is, in addition to the three above‐mentioned animal types, strictly nocturnal animals that are only active at night, and crepuscular animals that are active at dawn or dusk twilight periods. Weyer et al (2020), in their latest work on the ant habitat, hypothesize that aardvarks may become increasingly diurnal when they were more compromised energetically. Macroscopic measurements of the eyeball carried out in the examined young and adult aardvark specimens were compared with data from other Afrotherian mammals (Bapodra et al, 2010; Jennings & Rathbun, 2001; Kern et al, 1993; Lewis, 1983; Mass et al, 1997; Mass & Supin, 2007; Piggins et al, 1983), which revealed that the eyeball was larger in the O. afer and the four‐toed elephant‐shrew than that in sirenias, Asian elephants, and the Neurotrichus genus.…”

“…The presence of a separate tapetum lucidum influences the light-sensitivity capability in a species. The choroidal tapetum lucidum fibrosum present in the aardvark developed as an adaptive trait to function in a nocturnal lifestyle, although, according to Weyer et al (2020), it is possible that the aardvark also functions as a diurnal animal. Our histological observations revealed the presence of a choroidal tapetum lucidum fibrosum in young and adult aardvarks, which is similar in its structure to that found in cows, sheep, and goats (Ollivier et al, 2004).…”

Behavioral responses of two small‐sized rodents, Phyllotis vaccarum and Abrothrix andina, to energy challenges of high‐altitude habitats in the Andes Mountains

“…These results do not corroborate our expectations that, at higher altitudes, the animals would expand the margins of their territory in order to fulfill their energy requirements. However, such demands could be supplied by temporarily extending their active phase through increased activity during daylight hours (Weyer et al, 2020). In this sense, and independently of their predominant temporal habits, the diurnal activity of both species was significantly higher in animals at 3100 m compared to those at 2300 m altitude; and such increase did not occur in detriment of nocturnal activity but in addition to it.…”

Estimating the distribution and habitat suitability for aardvarks (Orycteropus afer) in Kruger National Park, South Africa

“…The use of surface water has been observed and has been proposed to be an important resource for aardvarks under some circumstances (Kerley & Tompkins, 2018), but other reviews of this topic suggest that aardvarks typically obtain water from their prey (Taylor et al, 2018;Taylor & Skinner, 2003). Rey et al, (2017) observed high starvation rates of aardvarks during drought, presumably because of the effects of drought on ant and termite abundance and availability, and concluded that climate change increasingly may impact aardvark survival and distribution by increasing aridity, as did Weyer et al, (2020). Thus, establishing sable dans les sols.…”