Current Reproductive Output Research Articles

A dynamic state variable model suggests a stronger effect of age than individual energetic state on reproductive allocation in burying beetles

Evolutionary fitness is determined by how an organism allocates energy, or other limited resources, to reproduction during its lifetime. For iteroparous organisms, two alternative patterns of lifetime reproductive allocation are terminal investment and reproductive restraint. Terminal investment maximizes an individual’s current reproductive output by allocating all available resources to current reproduction at the cost of future reproduction. In contrast, the reproductive restraint strategy allocates the individual’s resources toward future survival and reproductive events. We used dynamic state variable modeling to investigate the conditions under which the burying beetle, Nicrophorus orbicollis, would balance between reproductive restraint and terminal investment over their lifetime. Our model provides a formal extension, specific to burying beetle biology, of the dynamic terminal investment threshold conceptual model. For young females, we show that delayed reproduction and reproductive restraint are the optimal tactic for all individuals except for those in the highest condition. However, as age increases, terminal investment becomes the optimal tactic over an increasingly broader range of individual conditions. Surprisingly, manipulation of a variety of factors, such as survival rate, resource availability, and metabolic costs, causes only minor changes in the general pattern observed. We suggest that in burying beetles, and other similar organisms, age plays a dominant role in determining the pattern of reproductive allocation over a lifetime. Individual energetic condition is important in changing the boundaries between alternative reproductive strategies, but it does not change the overall pattern of dominance of delayed reproduction or reproductive restraint at early ages and dominance of terminal investment with increasing age.

Reproducing in a changing world: combined effects of thermal conditions by day and night and of water constraints during pregnancy in a cold‐adapted ectotherm

Climate change will continue to increase mean global temperatures with daily minima increasing more than daily maxima temperatures. Altered rainfall patterns due to climate change will also disrupt water availability for terrestrial organisms already facing climatic warming. To explore how organisms may adjust to changes in multiple, concurrent climate‐related environmental conditions, we manipulated day and night temperatures as well as water availability during gestation in female common lizards Zootoca vivipara, a cold and wet adapted species facing climatic changes notably in populations located on the warm margin. We jointly manipulated temperature (hot or cold) independently during the daytime and nighttime as well as water availability (± ad libitum access to water) throughout pregnancy and quantified the effects on maternal traits (morphology, physiology and phenology) and reproductive output. Overall, we found that higher day or night temperatures decreased gestation length and increased energetic demands. Higher temperatures during the day, coupled with water restrictions, increased dehydration and water restrictions affected ability to allocate resources but had no impact on reproductive output. While high temperatures during the day were beneficial to current reproductive output and success, high temperatures during the night had the opposite effect. Our results suggest that high nighttime temperatures can dramatically increase the burden on pregnant mothers already constrained by heavy resource and water investment. This could provide a mechanistic explanation for the long‐term declines of warm‐margin populations in this species.

Lack of trade-offs in host offspring produced during fecundity compensation.

Host-parasite coevolution may result in life-history changes in hosts that can limit the detrimental effects of parasitism. Fecundity compensation is one such life-history response, occurring when hosts increase their current reproductive output to make up for expected losses in future reproduction due to parasitic infection. However, the potential trade-offs between this increase in quantity and the quality of offspring have been relatively unexplored. This study uses the trematode, Schistosoma mansoni, and its snail intermediate host, Biomphalaria glabrata, to better understand how this host life-history response, fecundity compensation, impacts host reproduction. Measures of host reproductive output as well as offspring hatching success and survival were collected to assess the reproductive consequences of infection. Infected snails exhibited fecundity compensation by increasing the number of eggs laid and the overall probability of laying eggs compared to uninfected snails. Parental infection status did not play a significant role in hatching or offspring survival to maturity. Offspring from a later reproductive bout demonstrated a higher hatching success rate. Overall, the lack of an apparent trade-off between quantity and quality of offspring suggests that infected parental snails invest more resources towards reproduction not only to increase reproductive output, but also to maintain the fitness of their offspring, possibly at the expense of their own longevity.

Effects of Mustelid gammaherpesvirus 1 (MusGHV-1) Reactivation in European Badger (Meles meles) Genital Tracts on Reproductive Fitness.

Reactivation of latent Gammaherpesvirus in the genital tract can lead to reproductive failure in domestic animals. Nevertheless, this pathophysiology has not received formal study in wild mammals. High prevalence of Mustelid gammaherpesvirus 1 (MusGHV-1) DNA detected in the genital tracts of European badgers (Meles meles) implies that this common pathogen may be a sexual transmitted infection. Here we used PCR to test MusGHV-1 DNA prevalence in genital swabs collected from 144 wild badgers in Ireland (71 males, 73 females) to investigate impacts on male fertility indicators (sperm abundance and testes weight) and female fecundity (current reproductive output). MusGHV-1 reactivation had a negative effect on female reproduction, but not on male fertility; however males had a higher risk of MusGHV-1 reactivation than females, especially during the late-winter mating season, and genital MusGHV-1 reactivation differed between age classes, where 3–5 year old adults had significantly lower reactivation rates than younger or older ones. Negative results in foetal tissues from MusGHV-1 positive mothers indicated that cross-placental transmission was unlikely. This study has broader implications for how wide-spread gammaherpesvirus infections could affect reproductive performance in wild Carnivora species.

Experimental facilitation of heat loss affects work rate and innate immune function in a breeding passerine bird.

The capacity to get rid of excess heat produced during hard work is a possible constraint on parental effort during reproduction [heat dissipation limit (HDL) theory]. We released hard-working blue tits (Cyanistes caeruleus) from this constraint by experimentally removing ventral plumage. We then assessed whether this changed their reproductive effort (feeding rate and nestling size) and levels of self-maintenance (change in body mass and innate immune function). Feather-clipped females reduced the number of feeding visits and increased levels of constitutive innate immunity compared with unclipped females but did not fledge smaller nestlings. Thus, they increased self-maintenance without compromising current reproductive output. In contrast, feather clipping did not affect the number of feeding visits or innate immune function in males, despite increased heat loss rate. Our results show that analyses of physiological parameters, such as constitutive innate immune function, can be important when trying to understand sources of variation in investment in self-maintenance versus reproductive effort and that risk of overheating can influence innate immune function during reproduction.

Oxidative status and fitness components in the Seychelles warbler

Summary Oxidative damage, caused by reactive oxygen species during aerobic respiration, is thought to be an important mediator of life‐history trade‐offs. To mitigate oxidative damage, antioxidant defence mechanisms are deployed, often at the cost of resource allocation to other body functions. Both reduced resource allocation to body functions and direct oxidative damage may decrease individual fitness, through reducing survival and/or reproductive output. The oxidative costs of reproduction have gained much attention recently, but few studies have investigated the long‐term consequences of oxidative damage on survival and (future) reproductive output under natural conditions. Using a wild population of the cooperatively breeding Seychelles warbler (Acrocephalus sechellensis), we tested the prediction that high levels of reactive oxygen species, or high antioxidant investments to avoid oxidative damage, have fitness consequences because they reduce survival and/or reproductive output. We found that individuals with higher circulating non‐enzymatic antioxidant capacity had a lower probability of surviving until the next year. However, neither current reproductive output, nor future reproductive output in the surviving individuals, was associated with circulating non‐enzymatic antioxidant capacity or oxidative damage. The negative relationship between antioxidant capacity and survival that we observed concurs with the findings of an extensive comparative study on birds, however the mechanisms underlying this association remain to be resolved. A lay summary is available for this article.

Patterns of nest reuse in forest raptors and their effects on reproductive output

AbstractThe presence of old nest structures can be an influential resource in reuse patterns and reproductive output for some birds. We used 15‐year territorial occupancy data referring to the booted eagle Aquila pennata (a trans‐Saharan migrant) and the common buzzard Buteo buteo (a sedentary species in southeastern Spain) to analyse old nest effects in territorial settlement patterns (new territories, new establishments in old territories and reoccupancies), to describe the patterns of nest building versus nest reuse and to test whether nest building is costly in terms of current reproductive output. The results indicated that the rates of reoccupancy and new establishments in old territories were higher than the rates of creating new territories for both booted eagles (74.13, 23.35 and 2.52%, respectively) and common buzzards (58.25, 38.84 and 2.91%, respectively). When breeding pairs settled in old territories, we observed a noticeably lower pattern of nest building than nest reuse both in booted eagles (10.03 vs. 89.97%) and common buzzards (8.00 vs. 92.00%). The nest‐building rate by booted eagles was significantly lower in reoccupancies than in new establishments in old territories. Reproductive output for each species was not increased by nest reuse, although breeding success and productivity were significantly higher when newly established booted eagles constructed new nests than when reusing old nests. Our findings provides an interesting view on how forest raptors use old nests as important resources, probably taking them as location cues for nesting site selection and suggesting that unused nest sites should be left undisturbed since they could attract breeding raptor pairs in future years.

Live or let live: life‐history decisions of the breeding female Monteiro's Hornbill Tockus monteiri

Life‐history theory maintains that long‐lived species such as hornbills (Bucerotiformes) maximize lifetime fecundity by ensuring adult survival, rather than investing in current reproductive output. Unusually, female Tockus hornbills are sealed into the nest cavity during breeding, placing a large burden on the male who feeds her and the nestlings for up to 9 weeks. Early departure to help feed young may improve nestling survival, but could reduce female survival if moult is incomplete. We investigated whether departure time of the female Monteiro's Hornbill Tockus monteiri is more strongly related to measures of female fitness or juvenile condition. Departure was independent of nestling development and growth, instead depending on female state. At departure, variability in condition between females was smaller than at any other stage, implying a strong stabilizing selection on optimal female departure condition. Similarly, moult showed signs of being completed, and variability in the state of moult between females was lowest at departure. Both predictions were met in favour of the hypothesis that Monteiro's Hornbills would base the timing of their departure from the nest on maximizing their own survival. Thus, as predicted, future reproductive potential takes precedence over current reproductive output in a trade‐off decision in this species.

Life‐history consequences of partial‐moult asymmetry

Summary Experiments have shown that asymmetry in flight feathers reduces flight performance. Large birds moult only part of their flight feathers during one season. We explore the asymmetry in the pattern of partial moult of flight feathers. We compared the partial‐moult pattern of all 10 primary and the 10 outermost secondary feathers on the right vs. the left wing of 327 Ural owl females. We counted the number of asymmetrically moulted feather positions. The number of feathers moulted affects the potential asymmetry in partial moult. We used bootstrapping to correct for this dependency. Females moulted especially their primary feathers, but also their secondary feathers, more symmetrically than expected by chance. Ural owls moulted fewer feathers after breeding than after not breeding, but the moult pattern was not more asymmetric than expected by chance. Partial‐moult asymmetry did not correlate with current reproductive output (laying date), but the asymmetry in primaries was associated with a reduced probability to survive to the next breeding season. Developmental homeostasis in partial moult appears to be under stabilizing survival selection. Partial‐moult asymmetry may link current reproductive expenditure to the next season and may serve as a tool for biomonitoring populations of large birds.

The relationship between parental age and reproductive effort in the California Gull (Larus californicus)

Reproductive effort was compared among California gulls ranging in age from 3 to 18 years old. Results indicated that reproductive effort increased with parental age. Older parents (11–18 years old) invested more in foraging effort and in defense of offspring than did younger parents (<10 years old), and did so over a greater period of time. Increases with age in the efficiency of parenting could not be demonstrated. Increased levels of reproductive effort, therefore, appeared to be the main factor responsible for age-related increases in breeding success. The higher levels of reproductive effort of old parents were associated with higher mortality. Thus, data support the prediction that a cost-benefit system of risk of mortality vs reproductive gain operates in this population. Higher levels of reproductive effort can increase an individual's reproductive success; however, it also increases the likelihood that it will die. Therefore, for the young parent, high risks of mortality involved in current production would be too costly (in terms of loss of future reproductive success) to justify the benefit of high current reproductive output. Parents achieve the highest likelihood of maximizing life-time reproductive output by increasing reproductive effort with age as the cost of mortality diminishes. Data testing the relationship between parental age and reproductive effort demonstrate the existence of age-related variation in reproductive investment. They also demonstrate that the concept of reproductive value proposed by Fisher (1930) has validity as a framework which supports much of our theory on life history evolution.

Demographic strategies in fluctuating populations of small rodents.

A graphic model for individual selection determined by the logarithmic growth rates, dN i /N i ·dt, is developed for studying optimal demographic strategies at different phases of microtine cycles. In a density-independent situation (no crowding) selection leads to maximization of total life-time reproductive output (or equivalently, the Malthusian parameter, m) at the expense of competitive (contest type) abilities. In a density-dependent situation (crowding), selection leads to maximization of contest type competitive abilities at the expense of reproductive output. These two modes of selective pressure are called r- and α-selection. r-selection is presumed to occur during the increase phase of a cycle. As the habitat becomes crowded, α-selection takes over and is assumed to be extreme at high densities.The characteristics of r- and α-strategists are found to be similar to those of the docile and aggressive forms presumed in Chitty's theory for fluctuating populations. Literature supports the attributes predicted by the theory. I argue that sensitivity to density-independent factors is higher in the α-strategy. On the basis of a graphic model, I show that the α-strategists' high sensitivity to extrinsic factors will account for the crash in microtine cycles.On the basis of these theoretical considerations, Chitty's theory for fluctuating populations is interpreted to imply that interactions between intrinsic and extrinsic (random) factors will result in cycles. A graphic model for this interpretation of Chitty's theory is discussed. The heterogeneity of the habitat is an important aspect.According to theory, dispersal of pregnant females is explained as an adaptation leading to increased current reproductive output. This behaviour is presumed to dominate numerically during the increase phase of a cycle, a prediction supported in literature.