Food restriction delays seasonal sexual maturation but does not increase torpor use in male bats

Abstract

Balancing energy budgets can be challenging, especially in periods of food shortage, adverse weather conditions and increased energy demand due to reproduction. Bats have particularly high energy demands compared to other mammals and regularly use torpor to save energy. However, while torpor limits energy expenditure, it can also downregulate important processes, such as sperm production. This constraint could result in a trade-off between energy saving and future reproductive capacity. We mimicked harsh conditions by restricting food and tested the effect on changes in body mass, torpor use and seasonal sexual maturation in male parti-coloured bats (Vespertilio murinus). Food-restricted individuals managed to maintain their initial body mass, while in well-fed males, mass increased. Interestingly, despite large differences in food availability, there were only small differences in torpor patterns. However, well-fed males reached sexual maturity up to half a month earlier. Our results thus reveal a complex trade-off in resource allocation; independent of resource availability, males maintain a similar thermoregulation strategy and favour fast sexual maturation, but limited resources and low body mass moderate this latter process.

Male bats respond to adverse conditions with larger colonies and increased torpor use during sperm production

“…Our study was conducted in June and July of 2016 and 2017 in the Polish part of the Białowieża Primaeval Forest (52°45′8′′ N, 23°52′45′′ E). This overlapped with sexual development of male parti-coloured bats (Hałat et al 2018;Komar et al 2020).…”

“…We observed colonies in Białowieża Primaeval Forest from the beginning of June until the 9th of July. This period fully overlapped with the development of reproductive tissues, as confirmed by the progressive filling of the epididymes (Krutzsch 2000;Safi 2008;Hałat et al 2018;Komar et al 2020). Colony size ranged from 46 to just a few or even single individuals during this period (Hałat et al 2018).…”

“…This is probably due to the fact that our study only covered the early period of sperm production (see also Fig. 3, laboratory data from Komar et al 2020). The heterothermy index was significantly higher at lower ambient temperatures (LMM, P < 0.001, Fig.…”

“…Insectivorous bats are a good model for investigating these processes, as their expensive mode of locomotion puts them under high energetic pressure (Alexander 2002). Especially during energetically costly reproduction (Lane et al 2010) bats have to increase caloric intake while they reduce use their main energy saving strategy, torpor, as it interferes with foetal development, milk, and sperm production (Racey and Swift 1981;Kurta and Kunz 1988;Entwistle et al 1998;Wilde et al 1999;Dietz and Kalko 2006;Komar et al 2020). High blood testosterone level further prevents torpor use in reproductive males (Speakman and Thomas 2003).…”

“…It has been hypothesized that males form colonies to avoid torpor during spermatogenesis (Safi 2008). They would then profit from social thermoregulation, similar to the females, as torpor slows down sperm production (Entwistle et al 1998) and even food restricted males avoid torpor during this time (Komar et al 2020). In addition, males may profit from increased foraging success through passive information transfer during the energetically demanding spermatogenesis (Safi 2008).…”