Live fast, don't die young: Survival-reproduction trade-offs in long-lived income breeders

Higher and bigger: How riparian bats react to climate change

The altitudinal distribution of animals and changes in their body size are effective indicators of climate change. Bats are sensitive to climate change due to their dependence on temperature during critical life stages. However, long-term studies documenting responses over extended periods are rare. We present a 24-year investigation of Myotis daubentonii, a riparian bat known for altitudinal sexual segregation, along a river course in Central Italy. While males occupy the entire river course, females are confined to downstream warmer areas supporting successful reproduction due to improved foraging site productivity. In 2000, females were absent above 900 m a.s.l in our study area. We hypothesise that a) this altitude threshold is now higher, due to thermal gradient changes along the river course; and b) thermoregulatory costs for reproductive females have declined, leading to increased energy investment in offspring and subsequent generational growth in bat body size. Confirming our hypotheses, females exhibited a 175-m upward shift in altitude limit. Furthermore, we found a concurrent increase in body size (but not condition). Temperatures increased in the 24 years, likely allowing females to extend their range to higher elevations and favouring an increase in newborn body mass. Riparian vegetation remained unchanged, excluding habitat quality changes as the cause for the observed responses. The rapid female elevation rise might imply future disruption of established social structures, altering intra- and intersexual competition for roosts and food. Given the global decline in insect populations, larger bats might face future difficulties in finding food to sustain their body size, increasing mortality. However, the full impact of such changes on bat fitness remains unexplored and warrants further investigation, including other bat populations. This knowledge is crucial for informing conservation in the face of ongoing climate change and preserving the ecosystem services bats deliver in riparian ecosystems.

Artificially raised roost temperatures lead to larger body sizes in wild bats

Climate warming has major consequences for animal populations, as ambient temperature profoundly influences all organisms' physiology, behavior, or both.1 Body size in many organisms has been found to change with increased ambient temperatures due to influences on metabolism and/or access to resources.2,3,4,5,6 Changes in body size, in turn, can affect the dynamics and persistence of populations.7 Notably, in some species, body size has increased over the last decades in response to warmer temperatures.3,8 This has primarily been attributed to higher food availability,3 but might also result from metabolic savings in warmer environments.9,10 Bechstein's bats (Myotis bechsteinii) grow to larger body sizes in warmer summers,11 which affects their demography as larger females reproduce earlier at the expense of a shorter life expectancy.12,13 However, it remains unclear whether larger body sizes in warmer summers were due to thermoregulatory benefits or due to increased food availability. To disentangle these effects, we artificially heated communal day roosts of wild maternity colonies over four reproductive seasons. We used generalized mixed models to analyze these experimental results along with 25 years of long-term data comprising a total of 741 juveniles. We found that individuals raised in heated roosts grew significantly larger than those raised in unheated conditions. This suggests that metabolic savings in warmer conditions lead to increased body size, potentially resulting in the decoupling of body growth from prey availability. Our study highlights a direct mechanism by which climate change may alter fitness-relevant traits, with potentially dire consequences for population persistence.

Brood size in an uncertain world

Reproduction in an uncertain world is fraught. The consequences of investing in too many offspring in a resource poor season can be disastrous but so too is missing the opportunity of a resource rich year. We consider a simple population and individual growth model and use Lyapunov exponents to find analytical results for the optimum brood size under stochastic environmental conditions. We show that if the environment shows dramatic changes between breeding seasons choosing a smaller brood size is more likely to be successful but the best strategy is to synchronize your reproduction to the food availability. Finally, we show that if the cost of having offspring is high it can be better to live in a highly varying world with a plastic strategy that synchronizes to the environment than to live in a deterministic world with a constant strategy, a finding with implications for invasive species and climate change.

Long-term field study reveals that warmer summers lead to larger and longer-lived females only in northern populations of Natterer's bats

Animals often respond to climate change with changes in morphology, e.g., shrinking body size with increasing temperatures, as expected by Bergmann's rule. Because small body size can have fitness costs for individuals, this trend could threaten populations. Recent studies, however, show that morphological responses to climate change and the resulting fitness consequences cannot be generalized even among related species. In this long-term study, we investigate the interaction between ambient temperature, body size and survival probability in a large number of individually marked wild adult female Natterer's bats (Myotis nattereri). We compare populations from two geographical regions in Germany with a different climate. In a sliding window analysis, we found larger body sizes in adult females that were raised in warmer summers only in the northern population, but not in the southern population that experienced an overall warmer climate. With a capture-mark-recapture approach, we showed that larger individuals had higher survival rates, demonstrating that weather conditions in early life could have long-lasting fitness effects. The different responses in body size to warmer temperatures in the two regions highlight that fitness-relevant morphological responses to climate change have to be viewed on a regional scale and may affect local populations differently.

Global warming leads to larger bats with a faster life history pace in the long-lived Bechstein's bat (Myotis bechsteinii)

Whether species can cope with environmental change depends considerably on their life history. Bats have long lifespans and low reproductive rates which make them vulnerable to environmental changes. Global warming causes Bechstein’s bats (Myotis bechsteinii) to produce larger females that face a higher mortality risk. Here, we test whether these larger females are able to offset their elevated mortality risk by adopting a faster life history. We analysed an individual-based 25-year dataset from 331 RFID-tagged wild bats and combine genetic pedigrees with data on survival, reproduction and body size. We find that size-dependent fecundity and age at first reproduction drive the observed increase in mortality. Because larger females have an earlier onset of reproduction and shorter generation times, lifetime reproductive success remains remarkably stable across individuals with different body sizes. Our study demonstrates a rapid shift to a faster pace of life in a mammal with a slow life history.

Warming summers across a 25-year study are linked to larger body sizes in female bats, leading to a switch from a slow-reproducing, long-lived species to a faster pace of life.

Long-term field studies in bat research: importance for basic and applied research questions in animal behavior

Animal species differ considerably in longevity. Among mammals, short-lived species such as shrews have a maximum lifespan of about a year, whereas long-lived species such as whales can live for more than two centuries. Because of their slow pace of life, long-lived species are typically of high conservation concern and of special scientific interest. This applies not only to large mammals such as whales, but also to small-sized bats and mole-rats. To understand the typically complex social behavior of long-lived mammals and protect their threatened populations, field studies that cover substantial parts of a species’ maximum lifespan are required. However, long-term field studies on mammals are an exception because the collection of individualized data requires considerable resources over long time periods in species where individuals can live for decades. Field studies that span decades do not fit well in the current career and funding regime in science. This is unfortunate, as the existing long-term studies on mammals yielded exciting insights into animal behavior and contributed data important for protecting their populations. Here, I present results of long-term field studies on the behavior, demography, and life history of bats, with a particular focus on my long-term studies on wild Bechstein’s bats. I show that long-term studies on individually marked populations are invaluable to understand the social system of bats, investigate the causes and consequences of their extraordinary longevity, and assess their responses to changing environments with the aim to efficiently protect these unique mammals in the face of anthropogenic global change.

Selection for male stamina can help explain costly displays with cost-minimizing female choice

Abstract

In many species, male lifespan is shorter than that of females, often attributed to sexual selection favouring costly expression of traits preferred by females. Coevolutionary models of female preferences and male traits predict that males can be selected to have such life histories; however, this typically requires that females also pay some costs to express their preferences. Here we show that this problem diminishes when we link coevolutionary models of costly mate choice with the idea of stamina. In our model, the most successful males are those who can combine high attendance time on a lek — or, more generally, tenacious effort in their display time budgets — with high viability such that they are not too strongly compromised in terms of lifespan. We find that an opportunistic female strategy, that minimizes its costs by mating with highly visible (displaying) males, often beats other alternatives. It typically resists invasion attempts of genotypes that mate randomly in the population genetic sense, as well as invasion of stricter ways of being choosy (which are potentially costly if choice requires e.g. active rejection of all males who do not presently display, or risky travel to lekking sites). Our model can produce a wide range of male time budgets (display vs. self-maintenance). This includes cases of alternative mating tactics where males in good condition spend much time displaying, while those in poor condition never display yet, importantly, gain some mating success due to females not engaging in rejection behaviours should these be very costly to express.

Significance statement

In many species, males spend much time and energy on displaying to attract females, but it is not always clear what females gain from paying attention to male displays. The tradition in mathematical models attempting to understand the situation is to assume that random mating is the least costly option for females. However, random mating in the population genetic sense requires females to behave in a manner that equalizes mating success between displaying and non-displaying males, and here we point out that this is biologically unlikely. Opportunistically mating females can cause males to spend much of their time budgets displaying and will shorten male lifespans in a quality-dependent manner.

Novel passive detection approach reveals low breeding season survival and apparent lactation cost in a critically endangered cave bat

Capture-mark-recapture/resight (CMR) methods are used for survival-rate studies, yet are often limited by small sample sizes. Advances in passive integrated transponder (PIT) technology have enabled passive detection or ‘resight’ of marked individuals using large antennas with greater read-ranges than previously possible. We used passively-detected resight data and CMR models to study survival rates of the southern bent-winged bat Miniopterus orianae bassanii, a critically endangered, cave-dwelling bat. Over three years, we used PIT-tagging to monitor 2966 individuals at the species’ largest breeding aggregation, using daily detection data (> 1.6 million detections) to estimate seasonal survival probabilities, structured by age, sex and reproductive status, and parameterise population projection matrices. This has hitherto been impossible using traditional CMR methods due to disturbance risk and low recapture rates. Bats exhibited lowest apparent seasonal survival over summer and autumn, particularly for reproductive females in summer (when lactating) and juveniles in autumn (after weaning), and high survival in winter. Lowest survival rates coincided with severe drought in summer–autumn 2016, suggesting that dry conditions affect population viability. Under all likely demographic assumptions, population projection matrices suggested the population is in deterministic decline, requiring urgent action to reduce extinction risk. Passively-collected resight data can now be used in combination with CMR models to provide extensive, robust information for targeted wildlife population management.

Precipitation during two weeks in spring influences reproductive success of first-year females in the long-lived Natterer's bat

Bats are characterized by low reproductive rates in contrast with most of other small mammals. This makes their populations vulnerable when inclement environmental conditions such as cold and rainy weather impair the reproductive success of females. The fine-scale effect of weather on bats, however, remains largely unknown. Using a sliding window analysis approach on an 18-year individualized dataset on six Natterer's bat (Myotis nattereri) colonies, we investigated the effect of fine-scale weather conditions on age-specific reproductive success. We found that increased precipitation during a short time window in spring strongly reduced the probability of successful reproduction of first-year (FY) females. Our data suggest that this time window is concomitant with implantation or early pregnancy, before substantial investment into embryo development. In addition, larger FY had higher reproductive success, suggesting that reproduction may be condition dependent in young females. Reproductive success of older females was not affected by either weather or individual parameters. Our results show that changes in precipitation pattern may compromise the reproductive success of FY females. Further studies are needed to better understand the impact of weather conditions on reproductive success in long-lived bats under climate change scenarios.

Exploring the role of life history traits and introduction effort in understanding invasion success in mammals: a case study of Barbary ground squirrels

Invasive species-species that have successfully overcome the barriers of transport, introduction, establishment, and spread-are a risk to biodiversity and ecosystem function. Introduction effort is one of the main factors underlying invasion success, but life history traits are also important as they influence population growth. In this contribution, we first investigated life history traits of the Barbary ground squirrel, Atlantoxerus getulus, a species with a very low introduction effort. We then studied if their invasion success was due to a very fast life history profile by comparing their life history traits to those of other successful invasive mammals. Next, we examined whether the number of founders and/or a fast life history influences the invasion success of squirrels. Barbary ground squirrels were on the fast end of the "fast-slow continuum", but their life history was not the only contributing factor to their invasion success, as the life history profile is comparable to other invasive species that do not have such a low introduction effort. We also found that neither life history traits nor the number of founders explained the invasion success of introduced squirrels in general. These results contradict the concept that introduction effort is the main factor explaining invasion success, especially in squirrels. Instead, we argue that invasion success can be influenced by multiple aspects of the new habitat or the biology of the introduced species.

Phenology of reproductive condition varies with age and spring weather conditions in male Myotis daubentonii and M. nattereri (Chiroptera: Vespertilionidae)

We examine the extent to which intrinsic and extrinsic factors influence reproductive phenology in male bats at the population level. Using data from thirteen breeding seasons (2006-2018), encompassing the reproductive histories of 1546 Myotis daubentonii and 530 M. nattereri males, we compare rates of sexual maturation and the temporal distribution of phases of spermatogenesis between juvenile (born that season) and adult (born in previous seasons) males. We found that (i) higher proportions of M. daubentonii (50.81%) than M. nattereri (12.85%) became sexually mature as juveniles, (ii) the proportion of juveniles in reproductive condition per annum was influenced by spring weather conditions, (iii) in both species males that reached puberty as juveniles had higher body mass, on average, than immature juveniles, (iv) older males (aged ≥4 years old) commenced spermatogenesis earlier than young adult males (aged 1-3 years old), whilst juveniles that commenced spermatogenesis did so later in the year than adults, in both species, and (v) M. daubentonii commenced and completed spermatogenesis earlier than M. nattereri in the equivalent age class. Our findings suggest that selection pressure exists for early mating readiness and synchronisation with female receptivity.

Male long-distance migrant turned sedentary; The West European pond bat (Myotis dasycneme) alters their migration and hibernation behaviour

During autumn in the temperate zone, insectivorous male bats face a profound energetic challenge, as in the same period they have to make energy choices related to hibernation, mating and migration. To investigate these energetic trade-offs, we compared the body mass of male and female pond bats (Myotis dasycneme) through the summer season, characterized the known hibernacula in terms of male or female bias, and subsequently compared their population trend during two study periods, between 1930–1980 and 1980–2015. Towards the end of summer, males began losing weight whilst females were simultaneously accumulating fat, suggesting that males were pre-occupied with mating. We also found evidence for a recent adaptation to this energetic trade-off, males have colonised winter roosts in formerly unoccupied areas, which has consequently led to a change in the migration patterns for the male population of this species. As male bats do not assist in raising offspring, males have ample time to restore their energy balance after hibernation. Our results suggest that choosing a hibernacula closer to the summer range not only decreases energy cost needed for migration, it also lengthens the mating season of the individual male. Our findings have important conservation implications, as male and female biased hibernation assemblages may differ critically in terms of microclimate preferences.

Live fast, don't die young: Survival-reproduction trade-offs in long-lived income breeders

Trade-offs between survival and reproduction are at the core of life-history theory, and essential to understanding the evolution of reproductive tactics as well as population dynamics and stability. Factors influencing these trade-offs are multiple and often addressed in isolation. Further problems arise as reproductive states and survival in wild populations are estimated based on imperfect and potentially biased observation processes, which might lead to flawed conclusions. In this study, we aimed at elucidating trade-offs between current reproduction (both pregnancy and lactation), survival and future reproduction, including the specific costs of first reproduction, in long-lived, income breeding small mammals, an under-studied group. We developed a novel statistical framework that encapsulates the breeding life cycle of females, and accounts for incomplete information on female pregnancy and lactation and imperfect and biased recapture rates. We applied this framework to longitudinal data on two sympatric, closely related bat species (Myotis daubentonii and M. nattereri). We revealed the existence of several, to our knowledge previously unknown, trends in survival and breeding of these closely related, sympatric species and detected remarkable differences in their age and costs of first reproduction, as well as their survival-reproduction trade-offs. Our results indicate that species with this type of life history exhibit a mixture of patterns expected for long-lived and short-lived animals, and between income and capital breeders. Thus, we call for more studies to be conducted in similar study systems, increasing our ability to fully understand the evolutionary origin and fitness effects of trade-offs and senescence.