Male echidna must stay on the move to find females before other males do (Image Credit: JKMelville, CC BY-SA 3.0, Image Cropped)
Energetics meets sexual conflict: The phenology of hibernation in Tasmanian echidnas (2019) Nicol et al., Functional Ecology, https://doi.org/10.1111/1365-2435.13447

The Crux

Seasonality (i.e. the change in season throughout the course of the year) has huge impacts on the lives of animals that live in temperate habitats. The change in season is associated with changes in food availability, and as such some animals hibernate through the tough winter months and wait until the food and warmer weather comes back. Another aspect of an animal’s life impacted by seasonality is the breeding season, as animals living in temperate habitats must time their breeding around the winter months, while animals in tropical habitats can breed year-round.

Within a single species the timing of hibernation may be affected by the different energetic and reproductive needs of the different sexes. Females may start hibernating later than males because they have to store more energy for their pregnancy and lactation, while males may emerge from hibernation earlier than females to establish territories and increase their chance of mating. Tasmanian echidnas (Tachyglossus aculeatus) exhibit markedly different hibernation patterns among the sexes, and the authors of today’s study wanted to know if these differences are due to where they live or whether they are inherent to the species itself.

What They Did

For the study population the authors had data from a continuous 18-year study of wild Tasmanian echidnas. Individual animals were fitted with radio trackers and temperature loggers to monitor their behavior over the course of the study. Changes in temperature allowed the authors to track the start and end of the hibernation period, while loss of signal (or the discovery of a dead animal) was used to monitor the survival over time.

The authors measured the masses of individual echidnas, start and end of the hibernation period, and breeding events. They then used these data in models to calculate if these factors affected the length, start, and end of the hibernation period for the different sexes.

Did You Know: Hibernation vs. Torpor

Taking a period of rest during the harder winter months is an adaptive strategy used by many different animals, but there are two different kinds commonly employed: hibernation and torpor. Torpor involves short periods of inactivity, lowered body temperature, and a markedly lower metabolic rate, all brought about by changes in the temperature and food available to the animal. Hibernation, on the other hand, lasts longer than torpor, with both the temperature and metabolic rate falling far below that of an animal in torpor. Also, unlike torpor, hibernation is induced by changes in the amount of daylight and hormones.

What They Found

Females and males had slightly different hibernation periods, with the males both beginning and ending their hibernation before the females did. Interestingly, successful mating events were associated with earlier emergence from hibernation for both males and females. Although the duration of hibernation was not affected by body mass, males that were heavier at the start of their hibernation emerged earlier than lighter males. There was no relationship between hibernation duration or emergence and mass for females. Over the course of the 18-year study 18 females and 7 males died, but it was not possible to determine the cause of death for most of the animals. Survival was higher for males than females on average.

Problems

The data sets used in this study were unfortunately not the same year to year, due to the fact that every echidna was not always accessible during each year. Additionally, the methods used over the course of the 18-year study changed and evolved (as you would expect as the researchers got better at measuring/figured out better ways to capture certain patterns). This is less an issue and more of a “what could have been”, as there were not consistent measures over the entire study, and as such certain patterns or dynamics may not have been detected over the long study period.

So What?

The results of this study have shown that the unusual dynamics of echidna hibernation is a product of the unique habitat that they live in, and less due to an inherent property of the species itself, as echidna in other habitats do not show these same hibernation patterns. Long-term studies like this one are essential for detecting patterns in nature, especially those of (relatively) long-lived animals like the echidna.