Why saving energy can clash with making babies
Torpor is a controlled drop in body temperature and metabolism that lets a small bird or mammal shut down costly processes and ride out a stretch of low food, cold, or water shortage [1]. Reproduction is the opposite kind of demand: pregnancy, lactation, and sperm production are among the most energy-hungry things an animal does. Because both are regulated through the same metabolic machinery, the two pull on each other, and mammals show a two-way relationship in which reproductive state changes torpor patterns while heavy torpor use changes the reproductive axis [3].
The conflict is easiest to see in bats. In big brown bats, males used torpor at night more often, entered deeper torpor, and stayed in it longer than females during both pregnancy and lactation, while females rarely skipped foraging [2]. The researchers read this as evidence that torpor carries a higher fitness cost for reproductive females: slowing metabolism is thought to prolong gestation and slow the growth of pups, shrinking the window before hibernation [2]. Males, with less to lose on that schedule, can afford to save energy this way.
When torpor helps rather than hinders
Torpor is not only a winter survival trick. In spring and summer it can conserve energy and water, support reproduction or growth when resources are limited, and cut down on foraging time, which also reduces exposure to predators [1]. That is the case where torpor helps: the energy saved is worth more than the delay it introduces, and the animal has room in its schedule to absorb that delay.
Seasonal timing matters here. Torpor expressed in spring or summer is usually shallower and shorter than winter torpor, partly because temperatures are higher and partly because of seasonal flexibility in how the body regulates it [1]. Some species even flip the usual pattern: subtropical nectar-feeding blossom-bats and desert spiny mice use more frequent and pronounced torpor in summer than in winter, tracking the seasonal availability of nectar or water [1]. For them, the energy crunch comes with the heat, not the cold.
Reproductive animals often cut back on torpor
The clearest sign that torpor can hinder reproduction is that reproductive animals often reduce it. Free-ranging male common noctule bats used torpor extensively when non-reproductive, but wild reproductive males showed the expected reduction in torpor use, presumably to support spermatogenesis [4]. Notably, captive males in the same study kept using torpor through the day even during reproduction, so the laboratory did not reproduce the wild pattern [4]. That gap is a caution: metabolic measurements taken in captivity may not reflect what free-ranging animals actually do at different life stages [4].
The broader picture is that torpor and reproduction are not simply incompatible. Species with seasonal torpor can adapt their timing, and the balance is struck differently across mammals and birds rather than following one rule [5]. What varies is the cost of the delay: where young must be grown and fattened before a hard season, slowing down is expensive; where food or water is the binding constraint, the savings can outweigh the lost time [1][2].
Why there is no single answer for a species
Seasonal torpor is messier than the textbook picture of hibernating all winter. Some hibernators stay in torpor nearly year-round, others from late summer to spring, and only a minority are strictly winter hibernators; daily torpor in particular is diverse and typically less seasonal than hibernation [1]. Because the challenges differ by region, habitat, and diet, the same energy-saving tool can be a help in one place and a hindrance in another [1].
For a reader trying to predict the outcome, the useful question is not whether torpor is good or bad for reproduction but which constraint binds harder. If the animal is racing a deadline, as reproductive female big brown bats appear to be, torpor is something to use sparingly [2]. If the animal is short on fuel or water, torpor buys the time and resources that reproduction needs [1][5]. The underlying signaling that sets this balance is still being worked out, and current reviews weigh competing hypotheses rather than settle them [3].
Sources used in this answer
Seasonal Expression of Avian and Mammalian Daily Torpor and Hibernation: Not a Simple Summer-Winter Affair†
Seasonal torpor use varies widely across birds and mammals, and spring/summer torpor can offer advantages including energy and water conservation and facilitation of reproduction or growth under limited resources.
Sex differences in the use of daily torpor and foraging time by big brown bats (Eptesicus fuscus) during the reproductive season
Male big brown bats used torpor and deep torpor more often and longer than reproductive females during pregnancy and lactation, consistent with higher fitness costs of torpor for reproductive females.
Biological Mechanisms Balancing Torpor and Reproduction in Mammals
Mammals show a bidirectional relationship in which reproductive state influences torpor patterns and frequent torpor use in turn alters the reproductive axis.
Contrasting Torpor Use by Reproductive Male Common Noctule Bats in the Laboratory and in the Field
Free-ranging reproductive male common noctule bats reduced torpor use as predicted, while captive reproductive males continued using torpor through the day, showing laboratory and field torpor behavior can diverge by life stage.
Torpor during reproduction in mammals and birds: dealing with an energetic conundrum
Torpor and reproduction in mammals and birds involve energetic trade-offs, with benefits such as increased survival and costs such as extended reproductive periods, rather than simple incompatibility.