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

Abstract

Changes in environmental conditions can have strong energetic effects on animals through limited food availability or increased thermoregulatory costs. Especially difficult are periods of increased energy expenditures, such as reproduction. Reproductive female bats from the temperate zone often aggregate in maternity colonies to profit from social thermoregulation to reduce torpor use and buffer the effects of poor conditions. The much rarer male colonies may form for similar reasons during testes development. Male colonies thus allow us to study the influence of environmental conditions on energy budget and colony size, without the confounding effects of parental care. We remotely monitored skin temperature and assessed colony size of male parti-coloured bats Vespertilio murinus during summer, and correlated those variables with environmental conditions and food availability (i.e. insect abundance). As we had hypothesized, we found that colony size increased at colder temperatures, but decreased at low wind speeds. Also as predicted, torpor use was relatively low, however, it did increase slightly during adverse conditions. Male sociality may be an adaptation to adverse environmental conditions during sexual maturation, but the pressure to avoid torpor during spermatogenesis may be lower than in pregnant or lactating females.

Energy allocation shifts from sperm production to self-maintenance at low temperatures in male bats

“…Free-ranging colonies of male V. murinus break up after reaching sexual maturation stage 2 36 , 42 , 52 , 53 . After this stage, according to our results, the need for maintaining high body temperature is lessened.…”

“…We extrapolated from our results that bats from the T 25°C treatment would not reach sexual maturity until late September, if continued to be kept at 25 °C. Thus, bats exposed to constant elevated temperature would reach sexual maturity more than one month after wild individuals 36 , 53 or captive ones maintained under naturally fluctuating ambient temperatures and provided with high amounts of food 26 (Fig. 4 ).…”

“…Another suggested strategy to optimize energy budget during spermatogenesis is to become social. The males of some species of bats form temporary bachelor colonies during summer 35 , probably to benefit from more efficient foraging on ephemeral insects and or social thermoregulation 36 . Additionally, social thermoregulation reduces heat loss.…”

“…Finally, if conditions are harsh, males may still use torpor (e.g. 26 , 36 – 38 ) but only during the coldest time of the day, when the cost of heat production is the highest 36 , 37 . This compromise should promote survival, but at the cost of reduced or slowed-down sperm production 31 , 39 , 40 .…”

“…It is challenging to disentangle the effects of ambient temperature and food availability on the pace of sexual maturation in free ranging animals, but especially on insectivorous male bats. Insect activity, and thus food availability covary with weather conditions and affect bats' behaviour 36 , 41 , but the direct effect of temperature on sexual maturation remains untested. In this study, we experimentally tested the trade-off between energy allocated to thermoregulation and to seasonal sexual maturation in male parti-coloured bats, Vespertilio murinus .…”