Mass loss in incubating Eurasian dotterel: adaptation or constraint?

Body mass loss is frequently observed in breeding birds: whether this is an adaptive response to a change in the relative value of body stores and locomotion performance or a consequence of energetic constraint is still in debate. The male alone cares for most nests of the Eurasian dotterel Charadrius morinellus , although females assist at a proportion of nests. Energetic costs are probably high in the dotterel's arctic-alpine environment and uniparental care restricts the foraging time available to meet these costs, so that incubating dotterel may have to fuel themselves partly using body stores. Nesting male dotterel lost 7.8% of their mass through the incubation period but were 6.8% heavier during periods of high food abundance. Males that were assisted in incubation by a female were 6.7% heavier than uniparental males. We conclude that, since dotterel were heavier when energetic constraints were lifted, mass loss through incubation was principally a consequence of energetic constraint, rather than adaptive mass optimisation.     

Incubation capacity and clutch size determination in two calidrine sandpipers: a test of the four-egg threshold

Several groups of vertebrate taxa, including shorebirds, are unusual in that they produce a fixed number of offspring. The aim of this study was to examine whether the incubation capacity of western sandpipers (Calidris mauri) and semipalmated sandpipers (C. pusilla) limits their maximum clutch size to four eggs. Experimental enlargement of clutch size had no effect on rates of nest abandonment, nest attendance or loss of body mass by incubating sandpipers. The duration of incubation was significantly longer for enlarged five-egg nests, and there were trends towards increased partial clutch loss and asynchrony at hatch, but overall hatching success was unaffected by experimental egg number. I conclude that small, calidrine sandpipers with biparental care are able to compensate for an additional egg in an enlarged nestbowl, despite the constraints of conically shaped eggs and two brood patches. Possibly, shorebirds do not lay more than a fixed clutch size of four eggs because selection on factors acting during egg production or brood-rearing is more important in regulating offspring number. Received: 20 June 1996 / Accepted: 30 September 1996     

Effects of parental body condition and size on reproductive success in a tenebrionid beetle with biparental care

Abstract. 1. The beetle Parastizopus armaticeps (Coleoptera: Tenebrionidae) inhabits the Kalahari desert of southern Africa, constructs breeding burrows after rainfall, and shows extensive biparental care. Previous work has shown that it is predominantly male size, not female size, that determines breeding success; however, in the field these beetles show size assortative mating. This might obscure or override effects of female size on reproduction. Moreover, the inaccessibility of the breeding burrows makes it impossible to test effects of female and male size on offspring development and survival before adulthood. 2. To disentangle the effects of male and female length, body mass, and body condition on reproductive success, males and females were paired randomly in small breeding cages in the laboratory (n = 887 breeding pairs). The construction of the breeding cages allowed a clear view of the brood chamber contents at each stage in offspring development. Larva, pupa, and imago numbers and development were monitored daily, and imago mass at hatching from the pupa (hatchlings), offspring mass, and offspring body length at complete exoskeleton melanisation (juveniles) were determined. 3. There was a weak positive correlation between body condition and body length for females only. Breeding chronology was related to male body condition: males in better condition were fast to start and finish a breeding bout. Males in better condition produced heavier hatchlings and juveniles, and larger‐sized males produced larger‐sized juveniles. In contrast, numbers of larvae and juveniles produced were determined mainly by female length and body condition: larger females in better condition hatched more larvae and produced more offspring. 4. The results suggest that male size and condition will be the most important determinant of reproductive success under relatively dry conditions, when burrow length is critical for reproductive success. Female size might be more important for the pair's reproductive success under wet breeding conditions, when burrow length is less critical for successful reproduction.     

Nest temperature, incubation period, and investment decisions of incubating wood ducks Aix sponsa

Incubating birds must balance their energetic demands with the time needed to provide care to developing embryos. Reduced care by incubating parents can result in longer incubation periods that increase predation risk and potentially influence neonate phenotype. We measured nest temperature, incubation period, and body mass dynamics of female wood ducks Aix sponsa , and used an information-theoretic approach to investigate effects of several explanatory variables on incubation period and thermal characteristics of nests. A model that included clutch size and standard deviation of nest temperature best explained the variation in incubation period. Parameter estimates indicated that incubation period increased with increases in clutch size and standard deviation of nest temperature. Next, we examined relationships between maternal effects and the standard deviation of the nest temperature. The best fitting model included initiation date of incubation. There was little support for including early incubation body mass of females, incubation constancy, and percent change in female body mass in the model. The parameter estimate showed that standard deviation of nest temperature declined as initiation date of incubation advanced. Female body mass at the start of incubation was not related to structural size suggesting that heavy females were in better physical condition than were light females. Heavy females nested earlier and lost more body mass during incubation than light females, but heavy females did not reduce variation in nest temperature to decrease the incubation period. The fact that early nesting females in good physical condition did not shorten incubation periods by keeping nest temperatures less variable could have been due to either energetic limitations or restraints. Experimental manipulations of incubation costs will be needed to distinguish between these hypotheses.     

Early body condition and hatching success in the snow petrel Pagodroma nivea

We examined how variations in parental quality affect the reproductive success of a long-lived seabird, the snow petrel (Pagodroma nivea). In particular, we focused on how prebreeding body condition (prebreeding body mass adjusted for structural size) influences the hatching success of male and female snow petrel. Condition in females, but not in males, had a significant effect on hatching success. Among breeding pairs, early body condition of pairs was not significantly related to their hatching success. Laying date had a significant effect on hatching success, but this was due to heavy snowfalls during the beginning of the laying period. We suggest that females in poor early condition would not be able to build up sufficient body reserves necessary for successful incubation during the energy-demanding egg formation period. Moreover males, being structurally larger, would have a higher fasting capacity. These results show that the hatching success of the snow petrel is clearly influenced by female condition and suggest that effects of variations in environmental conditions may depend on body condition of individuals. However, the year of the study appeared to be an unusually poor year for reproduction, which may be why female body condition appears to be important. Accepted: 24 June 1998     

Variation in the adult body mass of Wilson’s storm petrels Oceanites oceanicus during breeding

Mass loss of breeding birds might be due to the physiological stress of breeding or it could be an adaptation to lower the costs of flight to the feeding areas. We examined the natural variation in the adult body mass of Wilson’s storm petrels Oceanites oceanicus on King George Island, South Shetland Islands over four breeding seasons. During two seasons, the prey abundance was high, while it was poor during the other two seasons. Only breeding birds were sampled; the fluctuations in mass were similar among males and females. During incubation, the mass of the adults was high in good seasons and low in poor seasons. Thus, body mass during incubation was determined by energetic constraints. However, during chick feeding, adults lost mass in the good seasons but gained mass in the poor seasons, suggesting that mass loss during chick rearing is not primarily caused by stress, but is regulated adaptively. Adults in poor conditions may buffer against unpredictable food supply by increasing their own body mass, even at the expense of the chick. Reduced body condition at the beginning of the breeding season was associated with reduced egg volumes and late laying, suggesting that the initial body condition influenced the level of investment in the current breeding attempt.     

State-dependent male mating tactics in the grey seal: the importance of body size

The purpose of this study was to examine the importance of bodysize and body composition as determinants of conditional matingtactics exhibited in male grey seals. We combined behavioralobservations with measures of energy expenditure and successon 42 known-age individuals during the breeding seasons of 1997–2001at Sable Island, Canada. Males with a large body mass arrivedat the breeding grounds with more body fat and body energy andcatabolized less body protein than smaller males. Males consumed1.9 ± 0.2 MJ day^–1, and those with a smaller percentageof body fat had higher rates of food energy intake. The amountof body energy on arrival was positively correlated with theduration of the breeding period. Males that exhibited the primarymating tactic of consortship were heavier, had absolutely morebody fat and body energy, and sustained breeding longer thanthose males that did not exhibit the primary tactic. Amongstall males, body mass showed a quadratic relationship with thenumber of female consorts mated and the estimated number ofpups sired. Thus, intermediate-sized males mated with the mostconsorts and achieved the highest success. Intermediate bodysize may be optimal during breeding due to greater agility inmale combat. Body size was an important determinant of matingtactics used by male grey seals. A large body size providedan energetic advantage of greater endurance while an intermediatebody size may provide greater competitive ability in acquiringconsortships.     

Variation in body mass dynamics among sites in Black Brant Branta bernicla nigricans supports adaptivity of mass loss during moult

Birds employ varying strategies to accommodate the energetic demands of moult, one important example being changes in body mass. To understand better their physiological and ecological significance, we tested three hypotheses concerning body mass dynamics during moult. We studied Black Brant in 2006 and 2007 moulting at three sites in Alaska which varied in food availability, breeding status and whether geese undertook a moult migration. First we predicted that if mass loss during moult were simply the result of inadequate food resources then mass loss would be highest where food was least available. Secondly, we predicted that if mass loss during moult were adaptive, allowing birds to reduce activity during moult, then birds would gain mass prior to moult where feeding conditions allowed and mass loss would be positively related to mass at moult initiation. Thirdly, we predicted that if mass loss during moult were adaptive, allowing birds to regain flight sooner, then across sites and groups, mass at the end of the flightless period would converge on a theoretical optimum, i.e. the mass that permits the earliest possible return to flight. Mass loss was greatest where food was most available and thus our results did not support the prediction that mass loss resulted from inadequate food availability. Mass at moult initiation was positively related to both food availability and mass loss. In addition, among sites and years, variation in mass was high at moult initiation but greatly reduced at the end of the flightless period, appearing to converge. Thus, our results supported multiple predictions that mass loss during moult was adaptive and that the optimal moulting strategy was to gain mass prior to the flightless period, then through behavioural modifications use these body reserves to reduce activity and in so doing also reduce wing loading. Geese that undertook a moult migration initiated moult at the highest mass, indicating that they were more than able to compensate for the energetic cost of the migration. Because Brant frequently change moult sites between years in relation to breeding success, the site‐specific variation in body mass dynamics we observed suggests individual plasticity in moult body mass dynamics.     

The importance of energetic versus pelvic constraints on reproductive allocation by the eastern fence lizard (Sceloporus undulatus)

In ectothermic species, females often produce larger eggs in colder environments. Models based on energetic constraints suggest that this pattern is an adaptation to compensate for the slower growth of offspring in the cold. Yet, females in cold environments also tend to be larger than females in warm environments. Consequently, thermal clines in egg size could be caused by pelvic constraints, which stem from the inability of large eggs to pass through a small pelvic aperture. Models based on energetic constraints and models based on pelvic constraints predict similar relationships between maternal size and egg size. However, pelvic constraints should produce these relationships both within and among populations, whereas energetic constraints would not necessarily do so. If pelvic constraints are important, we might also expect small females to compensate by producing eggs that are relatively rich in lipids (i.e. high energy density). The present study aimed to assess whether energetic or pelvic constraints generate geographical variation in egg size of the lizard Sceloporus undulatus . Pelvic width is very highly correlated with body length in S. undulatus , making maternal size a suitable measure of pelvic constraint. Although maternal size and egg mass (dry and wet) covaried among populations, these variables were generally not related within populations. Energetic density of eggs tended to increase with decreasing egg mass (dry and wet), but this relationship was strongest in populations where no relationship between maternal size and egg mass was observed. Our results do not support the pelvic constraint model and thus indicate energetic constraints play a greater role in generating geographical variation in egg size.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 91 , 513–521.     

Reproductive success and the energetic cost of parental care in male smallmouth bass

Direct field measurements of the energetic expenditure on parental care and within-nest reproductive success of individual male smallmouth bass Micropterus dolomieui were determined by measuring the change in total body mass as well as by total body electroconductivity analysis (TOBEC™). With TOBEC, the change in total body lean mass of the same live individual was measured non-destructively at the beginning and end of the parental care period. Lean mass was the primary source of energy utilized during parental care indicating starvation and potential loss of future reproduction. Individual loss in lean mass was related positively to reproductive success suggesting that the energy expended during parental care does affect individual fitness.     

Parameters of Mating Effort and Success in Male European Ground Squirrels,Spermophilus citellus

Behavioural and physiological changes during the reproductive period were documented in male European ground squirrels including: initiated conflict, scent marking, locomotion and range size, as well as testis size, scrotal pigmentation, testosterone levels and changes in body mass. Spring emergence dates in adult males varied but preceded those of females. Gonadal maturation was complete in adult males at emergence. Most yearlings did not mature and emerged with females. After a quiescent premating phase, male range, range overlap, conflict rates and locomotion increased at female emergence. The synchrony of females, their small ranges and short latencies to ovulation produced a highly male-biased operational sex ratio (OSR) and potential for scramble competition polygyny. In contrast to other ground squirrel species, each female apparently mated with only a single male. Scramble competition did appear to occur, in that males tried to contact as many females as often as possible before copulation. Females placed high temporal prerequisites on males. Costs of scrambling were documented by body mass loss during mating, which covaried with the number of ‘acquired’ females. Testicular regression occurred earlier, and fattening, moult and onset of next hibernation were delayed in successful males. The determinant factor in these costs was found to be mass loss intensity during the main mating period. Surprisingly, this factor was best related to feeding changes and not to locomotion or aggression. Time budget constraints perhaps imposed by the female, with compounding influences of exercise and stress effects of conflict, then negatively affected ensuing seasonal activities. The individual changes may adversely affect individual survivability and/or future reproductive success.     

Changes in body mass and feeding behaviour in male harbour seals, Phoca vitulina, in relation to female reproductive status

Loss of body mass has been used as an index of the cost of reproductive effort in mammals. We studied changes in body mass of male harbour seals, Phoca vitulina , during the breeding season on Sable Island, Nova Scotia, Canada. Individually marked subadult ( n = 21) and adult ( n = 22) males were captured at approximately six-day intervals throughout the breeding season. Adult males weighed an average of 108 kg ± 5·6 (S.E.) at initial capture, whereas subadults weighed an average of 76 ± 6·6 kg. The rate of mass loss by adult males did not differ from zero during the pre-mating period (i.e. the period without receptive females), but differed significantly from zero (-0·91 ± 0·007 kg/day) during the mating period (i.e. the period with receptive females). By contrast, the rate of mass change of subadults did not differ from zero in either period, indicating that food availability was unlikely to be responsible for the observed changes in adults. Adult males lost up to 24% of body mass during the breeding season. Examination of sera for the presence of chylomicrons (i.e. evidence of recent feeding) also suggested that adults stopped feeding during the mating period, while subadults did not. These results suggest that reproduction represents a significant energetic cost to adult male harbour seals.     

Sexual dimorphism and sexual segregation in foraging strategies of northern giant petrels, Macronectes halli, during incubation

Giant petrels ( Macronectes spp.) are the most sexually dimorphic of all seabirds. We used satellite-tracking and mass change during incubation to investigate the influence of sexual size dimorphism, in terms of the intersexual food competition hypothesis, on foraging and fasting strategies of northern giant petrels at South Georgia. Females foraged at sea whereas males foraged mainly on the South Georgia coast, scavenging on seal and penguin carcasses. Foraging effort (flight speed, distance covered, duration of foraging trips) was greater for females than for males. In contrast, foraging efficiency (proportionate daily mass gain while foraging) was significantly greater for males than for females. Females were significantly closer to the desertion mass threshold than males and could not compensate for the mass loss during the incubation fast while foraging, suggesting greater incubation costs for females than for males. Both sexes regulated the duration and food intake of foraging trips depending on the depletion of the body reserves. In males the total mass gain was best explained by mass at departure and body size. We suggest that sexual segregation of foraging strategies arose from size-related dominance at carcasses, promoting sexual size dimorphism. Our results indicate that sex-specific differences in fasting endurance, contest competition over food and flight metabolic rates are key elements in maintenance of sexual size dimorphism, segregating foraging strategies and presumably reducing competition between sexes.     

Double-nesting behaviour and sexual differences in breeding success in wild Red-legged Partridges Alectoris rufa

Double-nesting behaviour, a rare breeding system in which females lay in two nests, one incubated by herself and the other one by her mate, could be considered an intermediate stage in the evolutionary trend from biparental to uniparental care of single clutches. We examined the occurrence and success of double-nesting behaviour in Red-legged Partridges Alectoris rufa in Central Spain. Clutch size and hatching success were recorded, as well as the variation in these between years and between incubating sexes. Participation in incubation was higher for females (94.76%) than males (41.0%), and the proportion of incubating males varied markedly between years, with no incubating males in one dry year and approximately 50% of males incubating in other years. There was significant variation among years and between sexes in laying date, clutch size and hatching success. Clutch size decreased with later laying date in males and females. The probability of clutch loss to predation differed between sexes, being much higher for nests incubated by females. Our results suggest that both rainfall and predation influence the occurrence and success of double-nesting.     

Do energetic demands constrain incubation scheduling in a biparental species?

The high energetic demands of incubation in birds may be animportant ecological factor limiting the evolution of life-historytraits, such as clutch size. In biparental species, however,the demands of incubation may not be a major constraint becausethere may always be sufficient feeding time available for theoff-duty bird to regain energy used during an incubation bout.We investigated whether the energetic demands of incubationconstrain optimum incubation bout length in a biparental incubatorby decreasing the energetic demands of incubation. We put aninsulated cup around the lining of semipalmated sandpiper nestsso that the rate of cooling of eggs was reduced by 21%. Semipalmatedsandpipers responded by increasing their mean incubation boutlength of around 11.1 h by about 10%. Bout lengths in unmanipulatednatural nests became longer as hatch approached (incubationstage), and this was independent of weather. Bout lengths mayhave decreased with increasing rainfall and were independentof time of day. The results suggest that bout length in semipalmatedsandpipers is constrained by their cumulative energetic expenditureduring an incubation bout, and this is determined partly bythe high costs of steady-state incubation. The results alsosuggest that the incubating bird determines the bout lengthrather than the returning bird. Semipalmated sandpipersmay havemaximized incubation bout length to minimize changeovers duringincubation because these probably increasepredation risk. Selectionto minimize the frequency of changeover may then be a factorcontributing to the evolution ofbiparental care and life-historytraits in semipalmated sandpipers.     

Costs of large communal clutches for male and female Greater Rheas Rhea americana

The breeding system of the Greater Rhea Rhea americana is almost unique among birds as it combines harem polygyny and sequential polyandry, with communal egg-laying and uniparental male care. In this species, large communal clutches (more than 30 eggs) are rare and have a lower hatching success than smaller clutches. Here we analyse the proximate causes of hatching failures and the costs of large communal clutches (and therefore the costs of extensive polygyny) for males and females. We evaluated if length of the nesting period, egg viability, egg losses during incubation and male parental activity at the nest were affected by clutch size. We also evaluated if chicks hatched from large clutches have a lower survival during the first 2 months after hatching. Large clutches had longer nesting period and lower hatching success, mainly as a result of bacterial contamination of the eggs and increased hatching asynchrony. In addition, large clutches tended to lose more eggs as a result of accidental breakage or predation. Male activity at the nest and chick survival were not related to clutch size. Low hatching success, nest predation risk and energetic costs associated with large clutches penalize females that join large harems and males that accept additional eggs into the nest.     

Body Mass and not Wing Length Predicts Territorial Success in a Tropical Satyrine Butterfly

The males of numerous butterfly species fight with conspecific rivals to possess mating territories. Although there is little consensus on the nature of fight costs or on what traits favor victory, a recent analysis suggests that size may be of minor importance. However, data are inconsistent, and wing length, the metric that has been most widely used for expressing size in butterfly field studies, may not provide a reliable index of muscle mass or other traits that determine resource holding potential. Here we investigate the influence of wing length, body mass and wing wear on the territorial success of a neotropical satryine butterfly, Paryphthimoides phronius. In a removal experiment, original residents were heavier than replacement males that took over territories in the following minutes. Winners of naturally occurring contests were also heavier than losers after correcting for weight loss since weighing. Although wing length correlated significantly with body mass and body mass decreased with age, neither wing length nor wing wear (a surrogate for age and body condition) contributed to explaining territorial success. As body mass may be related to traits such as muscle mass, fat stores, and age, no conclusion is possible on how mass translates into success in disputes. However, because wing length is unrelated to residency status in P. phronius, whereas mass is both a reliable predictor and performs significantly better than wing length, our results may help explain the frequent failure of wing length to predict territorial success in butterfly field studies.     

Seasonal variation in reproductive success and post-nuptial moult of blue tits in southern Europe: an experimental study

Post-nuptial moult and reproductive success were studied in relation to timing of breeding in blue tits, Parus caeruleus, breeding in southern Europe. A group of experimentally delayed pairs was created by removing first clutches, thereby inducing late repeat clutches. Reproductive success and post-nuptial moult of delayed pairs were compared with both control pairs that bred early and unmanipulated late-breeding pairs. Delayed pairs fledged fewer young and with a lower body mass than control pairs. However, the number of fledged young and fledgling mass did not differ between delayed and late-breeding pairs. These results were more consistent with the date hypothesis, and it is concluded that the timing of breeding and reproductive success may be causally related in the blue tit. This study reveals a harmful effect of relaying on female body mass at the end of the nestling period. Therefore, females apparently pay the costs of relaying, since a reduction in body mass during the nestling period may be accompanied by a lowered survival probability. Delayed and late-breeding males often began moulting while still feeding young, but neither control males nor females from the three study groups did so. These results support the view that timing-related energy constraints on breeding may be important causes of a seasonal decline in reproductive success at different latitudes. Received: 15 March 1999 / Accepted: 19 July 1999     

Body mass patterns of Little Stints at different latitudes during incubation and chick-rearing

Due to the ‘double‐clutch’ mating system found in the arctic‐breeding Little Stint Calidris minuta, each parent cares for a clutch and brood alone. The resulting constraint on feeding time, combined with the cold climate and a small body size, may cause energetic bottlenecks. Based on the notion that mass stores in birds serve as an ‘insurance’ for transient periods of negative energy balance, but entail certain costs as well, body mass may vary in relation to climatic conditions and stage of the breeding cycle. We studied body mass in Little Stints in relation to breeding stage and geographical location, during 17 expeditions to 12 sites in the Eurasian Arctic, ranging from north Norway to north‐east Taimyr. Body mass was higher during incubation than during chick‐rearing. Structural size, as estimated by wing length, increased with latitude. This was probably caused by relatively more females (the larger sex) incubating further north, possibly after leaving a first clutch to be incubated by a male further south. Before and after correction for structural size, body mass was strongly related to latitude during both incubation and chick‐rearing. In analogy to a similar geographical pattern in overwintering shorebirds, we interpret the large energy stores of breeding Little Stints as an insurance against periods of cold weather which are a regular feature of arctic summers. Climate data showed that the risk of encountering cold spells lasting several days increases with latitude over the species’ breeding range, and is larger in June than in July. Maintaining these stores is therefore less necessary at southern sites and during the chick‐rearing period than in the incubation period. When guarding chicks, feeding time is less constrained than during incubation, temperatures tend to be higher than in the incubation period, reducing energy expenditure, and the availability of insect prey reaches a seasonal maximum. However, the alternative interpretation that the chick‐tending period is more energetically stressful than the incubation period, resulting in a negative energy balance for the parent, could not be rejected on the present evidence.     

Impaired flight ability during incubation in the pied flycatcher

During the breeding season, many female passerine birds increase in body mass before egg laying, maintain a relatively high body mass during incubation, and then drop back to the original level during the chick-rearing period. The post-hatching reduction in body mass, which can be as large as 10–20%, has been suggested to represent an adaptive mass loss to reduce wing loading, thereby increasing parental flight efficiency when chicks have hatched and have to be fed. Here we present the first study of changes in flight ability from incubation to chick rearing in birds. Wild female pied flycatchers Ficedula hypoleuca flew more slowly during incubation than during chick rearing; a 7% reduction in body mass after the chicks had hatched was associated with a 10% increase in vertical take-off speed. Furthermore, the flight muscle size of the females tracked the reduction in wing load, suggesting that muscle size was adaptively reduced when no longer needed. Since incubation-feeding by males reduces the time females have to spend outside the nest foraging, our results suggest that in addition to increasing female nutritional status and reducing incubation time, incubation-feeding will also reduce predation risk during the period when females face impaired flight ability.