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Hatching asynchrony in birds is considered an adaptation to facilitate brood reduction because under conditions of food scarcity, the smallest nestling usually dies soon after hatching, thereby minimizing parental effort. However, in species with extreme hatching asynchrony, the last hatchlings paradoxically experience a very low probability of survival and death can take so long that it can hardly be considered an adaptation. Here, we propose and experimentally tested a new adaptive hypothesis explaining the brood reduction paradox, namely the "Male Manipulation Hypothesis". Our hypothesis suggests that by inducing asynchronous hatching, females increase the feeding requirements of the brood, which will induce males to increase provisioning effort. In addition, females may extend the period of male manipulation by feeding the smallest nestling just enough to sustain life. Our study showed that male common blackbirds ( Turdus merula) increased their effort (i.e., number of food items per hour) in experimental asynchronous broods compared to synchronous broods, while females reduced their contribution, as predicted by the hypothesis.

Sibling cannibalism is relatively common in nature, but its evolution in birds and certain other vertebrates with extended parental care had been discarded. Here, however, we demonstrate its regular occurrence in two European populations of the Eurasian hoopoe (Upupa epops) and explore possible adaptive and non-adaptive explanations. Results showed that sibling cannibalism was more frequently detected in Spain (51.7%) than in Austria (5.9%). In these two populations, the hoopoes laid similar clutch sizes, resulting in similar fledging production, but hatching failures were more frequent in the northern population. Consequently, having more nestlings condemned to die in the southern population may explain the higher incidence of sibling cannibalism. In accordance with this interpretation, hatching span and failure, but not breeding date, explained the probability of sibling cannibalism in the Spanish hoopoes, while all three variables predicted brood reduction intensity. Furthermore, experimental food supply reduced the probability of sibling cannibalism, but not the intensity of brood reduction. Finally, females allocated fewer resources to the smallest nestlings when they were going to starve, but not necessarily when they were going to be used as food for their siblings. These results suggest that hoopoes produce extra eggs that, in the case of reduced hatching failure and food scarcity, produce nestlings that are used to feed older siblings. These findings provide the first evidence that sibling cannibalism occurs regularly in a bird species, thus expanding our evolutionary understanding of clutch size, hatching asynchrony, parent-offspring conflict, infanticide, and sibling cannibalism in the animal kingdom.

In animals with uniparental care, the quality of care provided by one sex can deeply impact the reproductive success of both sexes. Studying variation in parental care quality within a species and which factors may affect it can, therefore, shed important light on patterns of mate choice and other reproductive decisions observed in nature. Using Syngnathus typhle, a pipefish species with extensive uniparental male care, with embryos developing inside a brood pouch during a lengthy pregnancy, we assessed how egg size (which correlates positively with female size), male size, and water temperature affect brooding traits that relate to male care quality, all measured on day 18, approximately 1/3, of the brooding period. We found that larger males brooded eggs at lower densities, and their embryos were heavier than those of small males independent of initial egg size. However, large males had lower embryo survival relative to small males. We found no effect of egg size or of paternal size on within-pouch oxygen levels, but oxygen levels were significantly higher in the bottom than the middle section of the pouch. Males that brooded at higher temperatures had lower pouch oxygen levels presumably because of higher embryo developmental rates, as more developed embryos consume more oxygen. Together, our results suggest that small and large males follow distinct paternal strategies: large males positively affect embryo size whereas small males favor embryo survival. As females prefer large mates, offspring size at independence may be more important to female fitness than offspring survival during development.

In animals with uniparental care, the quality of care provided by one sex can deeply impact the reproductive success of both sexes. Studying variation in parental care quality within a species and which factors may affect it can, therefore, shed important light on patterns of mate choice and other reproductive decisions observed in nature. Using Syngnathus typhle, a pipefish species with extensive uniparental male care, with embryos developing inside a brood pouch during a lengthy pregnancy, we assessed how egg size (which correlates positively with female size), male size, and water temperature affect brooding traits that relate to male care quality, all measured on day 18, approximately 1/3, of the brooding period. We found that larger males brooded eggs at lower densities, and their embryos were heavier than those of small males independent of initial egg size. However, large males had lower embryo survival relative to small males. We found no effect of egg size or of paternal size on within-pouch oxygen levels, but oxygen levels were significantly higher in the bottom than the middle section of the pouch. Males that brooded at higher temperatures had lower pouch oxygen levels presumably because of higher embryo developmental rates, as more developed embryos consume more oxygen. Together, our results suggest that small and large males follow distinct paternal strategies: large males positively affect embryo size whereas small males favor embryo survival. As females prefer large mates, offspring size at independence may be more important to female fitness than offspring survival during development.

In animals with uniparental care, the quality of care provided by one sex can deeply impact the reproductive success of both sexes. Studying variation in parental care quality within a species and which factors may affect it can, therefore, shed important light on patterns of mate choice and other reproductive decisions observed in nature. Using Syngnathus typhle, a pipefish species with extensive uniparental male care, with embryos developing inside a brood pouch during a lengthy pregnancy, we assessed how egg size (which correlates positively with female size), male size, and water temperature affect brooding traits that relate to male care quality, all measured on day 18, approximately 1/3, of the brooding period. We found that larger males brooded eggs at lower densities, and their embryos were heavier than those of small males independent of initial egg size. However, large males had lower embryo survival relative to small males. We found no effect of egg size or of paternal size on within-pouch oxygen levels, but oxygen levels were significantly higher in the bottom than the middle section of the pouch. Males that brooded at higher temperatures had lower pouch oxygen levels presumably because of higher embryo developmental rates, as more developed embryos consume more oxygen. Together, our results suggest that small and large males follow distinct paternal strategies: large males positively affect embryo size whereas small males favor embryo survival. As females prefer large mates, offspring size at independence may be more important to female fitness than offspring survival during development.

In animals with uniparental care, the quality of care provided by one sex can deeply impact the reproductive success of both sexes. Studying variation in parental care quality within a species and which factors may affect it can, therefore, shed important light on patterns of mate choice and other reproductive decisions observed in nature. Using Syngnathus typhle, a pipefish species with extensive uniparental male care, with embryos developing inside a brood pouch during a lengthy pregnancy, we assessed how egg size (which correlates positively with female size), male size, and water temperature affect brooding traits that relate to male care quality, all measured on day 18, approximately 1/3, of the brooding period. We found that larger males brooded eggs at lower densities, and their embryos were heavier than those of small males independent of initial egg size. However, large males had lower embryo survival relative to small males. We found no effect of egg size or of paternal size on within-pouch oxygen levels, but oxygen levels were significantly higher in the bottom than the middle section of the pouch. Males that brooded at higher temperatures had lower pouch oxygen levels presumably because of higher embryo developmental rates, as more developed embryos consume more oxygen. Together, our results suggest that small and large males follow distinct paternal strategies: large males positively affect embryo size whereas small males favor embryo survival. As females prefer large mates, offspring size at independence may be more important to female fitness than offspring survival during development.

In animals with uniparental care, the quality of care provided by one sex can deeply impact the reproductive success of both sexes. Studying variation in parental care quality within a species and which factors may affect it can, therefore, shed important light on patterns of mate choice and other reproductive decisions observed in nature. Using Syngnathus typhle, a pipefish species with extensive uniparental male care, with embryos developing inside a brood pouch during a lengthy pregnancy, we assessed how egg size (which correlates positively with female size), male size, and water temperature affect brooding traits that relate to male care quality, all measured on day 18, approximately 1/3, of the brooding period. We found that larger males brooded eggs at lower densities, and their embryos were heavier than those of small males independent of initial egg size. However, large males had lower embryo survival relative to small males. We found no effect of egg size or of paternal size on within-pouch oxygen levels, but oxygen levels were significantly higher in the bottom than the middle section of the pouch. Males that brooded at higher temperatures had lower pouch oxygen levels presumably because of higher embryo developmental rates, as more developed embryos consume more oxygen. Together, our results suggest that small and large males follow distinct paternal strategies: large males positively affect embryo size whereas small males favor embryo survival. As females prefer large mates, offspring size at independence may be more important to female fitness than offspring survival during development.

In animals with uniparental care, the quality of care provided by one sex can deeply impact the reproductive success of both sexes. Studying variation in parental care quality within a species and which factors may affect it can, therefore, shed important light on patterns of mate choice and other reproductive decisions observed in nature. Using Syngnathus typhle, a pipefish species with extensive uniparental male care, with embryos developing inside a brood pouch during a lengthy pregnancy, we assessed how egg size (which correlates positively with female size), male size, and water temperature affect brooding traits that relate to male care quality, all measured on day 18, approximately 1/3, of the brooding period. We found that larger males brooded eggs at lower densities, and their embryos were heavier than those of small males independent of initial egg size. However, large males had lower embryo survival relative to small males. We found no effect of egg size or of paternal size on within-pouch oxygen levels, but oxygen levels were significantly higher in the bottom than the middle section of the pouch. Males that brooded at higher temperatures had lower pouch oxygen levels presumably because of higher embryo developmental rates, as more developed embryos consume more oxygen. Together, our results suggest that small and large males follow distinct paternal strategies: large males positively affect embryo size whereas small males favor embryo survival. As females prefer large mates, offspring size at independence may be more important to female fitness than offspring survival during development.

A parent's nutritional state may influence its ability to provide care to offspring and ability to handle infections. In the broad-nosed pipefish, Syngnathus typhle, males care for their offspring by brooding the developing embryos in a brood pouch, providing nutrients and oxygen, resembling a pregnancy. Here, we demonstrate that the nutritional state of pregnant males covaries with their own survival during a selective event. Brooding males surviving a Vibrio sp. infection were in a significantly better nutritional state, as estimated by their hepatosomatic index. Furthermore, a higher nutritional state of the brooding male correlated with a lower embryo mortality, while feeding treatment (low vs. high) had no effect on male survival, nutritional state or embryo mortality. Finally, males brooding heavier embryos also showed a lower embryo mortality. This may reflect a maternal effect (if large eggs result in higher embryo survival), a paternal effect (if higher provisioning of male care promotes both embryo growth and survival), or a combination thereof (males caring more for large embryos). The results demonstrate the importance of a good nutritional state for a caring parent when their immunity is challenged.

A parent's nutritional state may influence its ability to provide care to offspring and ability to handle infections. In the broad-nosed pipefish, Syngnathus typhle, males care for their offspring by brooding the developing embryos in a brood pouch, providing nutrients and oxygen, resembling a pregnancy. Here, we demonstrate that the nutritional state of pregnant males covaries with their own survival during a selective event. Brooding males surviving a Vibrio sp. infection were in a significantly better nutritional state, as estimated by their hepatosomatic index. Furthermore, a higher nutritional state of the brooding male correlated with a lower embryo mortality, while feeding treatment (low vs. high) had no effect on male survival, nutritional state or embryo mortality. Finally, males brooding heavier embryos also showed a lower embryo mortality. This may reflect a maternal effect (if large eggs result in higher embryo survival), a paternal effect (if higher provisioning of male care promotes both embryo growth and survival), or a combination thereof (males caring more for large embryos). The results demonstrate the importance of a good nutritional state for a caring parent when their immunity is challenged.

A parent's nutritional state may influence its ability to provide care to offspring and ability to handle infections. In the broad-nosed pipefish, Syngnathus typhle, males care for their offspring by brooding the developing embryos in a brood pouch, providing nutrients and oxygen, resembling a pregnancy. Here, we demonstrate that the nutritional state of pregnant males covaries with their own survival during a selective event. Brooding males surviving a Vibrio sp. infection were in a significantly better nutritional state, as estimated by their hepatosomatic index. Furthermore, a higher nutritional state of the brooding male correlated with a lower embryo mortality, while feeding treatment (low vs. high) had no effect on male survival, nutritional state or embryo mortality. Finally, males brooding heavier embryos also showed a lower embryo mortality. This may reflect a maternal effect (if large eggs result in higher embryo survival), a paternal effect (if higher provisioning of male care promotes both embryo growth and survival), or a combination thereof (males caring more for large embryos). The results demonstrate the importance of a good nutritional state for a caring parent when their immunity is challenged.

A parent's nutritional state may influence its ability to provide care to offspring and ability to handle infections. In the broad-nosed pipefish, Syngnathus typhle, males care for their offspring by brooding the developing embryos in a brood pouch, providing nutrients and oxygen, resembling a pregnancy. Here, we demonstrate that the nutritional state of pregnant males covaries with their own survival during a selective event. Brooding males surviving a Vibrio sp. infection were in a significantly better nutritional state, as estimated by their hepatosomatic index. Furthermore, a higher nutritional state of the brooding male correlated with a lower embryo mortality, while feeding treatment (low vs. high) had no effect on male survival, nutritional state or embryo mortality. Finally, males brooding heavier embryos also showed a lower embryo mortality. This may reflect a maternal effect (if large eggs result in higher embryo survival), a paternal effect (if higher provisioning of male care promotes both embryo growth and survival), or a combination thereof (males caring more for large embryos). The results demonstrate the importance of a good nutritional state for a caring parent when their immunity is challenged.

A parent's nutritional state may influence its ability to provide care to offspring and ability to handle infections. In the broad-nosed pipefish, Syngnathus typhle, males care for their offspring by brooding the developing embryos in a brood pouch, providing nutrients and oxygen, resembling a pregnancy. Here, we demonstrate that the nutritional state of pregnant males covaries with their own survival during a selective event. Brooding males surviving a Vibrio sp. infection were in a significantly better nutritional state, as estimated by their hepatosomatic index. Furthermore, a higher nutritional state of the brooding male correlated with a lower embryo mortality, while feeding treatment (low vs. high) had no effect on male survival, nutritional state or embryo mortality. Finally, males brooding heavier embryos also showed a lower embryo mortality. This may reflect a maternal effect (if large eggs result in higher embryo survival), a paternal effect (if higher provisioning of male care promotes both embryo growth and survival), or a combination thereof (males caring more for large embryos). The results demonstrate the importance of a good nutritional state for a caring parent when their immunity is challenged.

In fish, fecundity correlates with female body size and egg-tending males often eat small broods. Therefore, small females may prefer to spawn in nests that already contain many eggs, to ensure the brood is as large as possible. In contrast, large females may prefer nests with few eggs, if high egg number or density has a negative effect on egg survival, or if there are drawbacks of spawning last in a nest. To test the hypothesis that female body size affects nest (and male mate) choice, using the sand goby (Pomatoschistus minutus), we allowed small and large females to choose between two males that were matched in size - one guarding a small clutch and the other a large clutch, respectively. We recorded where females spawned (measure of female preference), the combined brood size, male courtship, egg care and nest building. We also quantified the effect of brood size and egg density on egg survival in a separate data set. Although the combined broods did not exceed the small brood sizes that are at risk of being eaten, both small and large females preferred to spawn in nests with smaller clutch sizes. This preference could not be explained by more courtship or male parental effort, nor by reduced survival of larger or denser broods. Instead, our result might be explained by females avoiding the danger of cannibalism of young eggs by males or the risk of reduced egg health associated with being near the nest periphery., AuthorCount:2

The occurrence of male pregnancy in the family Syngnathidae (seahorses, pipefishes, and sea dragons) provides an exceptionally fertile system in which to investigate issues related to the evolution of parental care. Here, we take advantage of this unique reproductive system to study the influence of maternal body size on embryo survivorship in the brood pouches of pregnant males of the broad-nosed pipefish, Syngnathus typhle. Males were mated with either two large females, two small females, a large then a small female, or a small then a large female. Our results show that offspring survivorship depends on an interaction between female body size and the number of eggs transferred by the female. Eggs of larger females deposited in large numbers are more likely to result in viable offspring than eggs of smaller females laid in large numbers. However, when females deposited smaller numbers of eggs, the eggs from smaller females were more likely to produce viable offspring compared to those from larger females. We found no evidence that this result was based on mating order, the relative sizes of competing females, or egg characteristics such as dry weight of eggs. Additionally, male body size did not significantly influence the survivorship of offspring during brooding. Our results suggest that the factors underlying offspring survivorship in pipefish may be more complex than previously believed, with multiple factors interacting to determine the fitness of individual offspring within the broods of pregnant males.

The occurrence of male pregnancy in the family Syngnathidae (seahorses, pipefishes, and sea dragons) provides an exceptionally fertile system in which to investigate issues related to the evolution of parental care. Here, we take advantage of this unique reproductive system to study the influence of maternal body size on embryo survivorship in the brood pouches of pregnant males of the broad-nosed pipefish, Syngnathus typhle. Males were mated with either two large females, two small females, a large then a small female, or a small then a large female. Our results show that offspring survivorship depends on an interaction between female body size and the number of eggs transferred by the female. Eggs of larger females deposited in large numbers are more likely to result in viable offspring than eggs of smaller females laid in large numbers. However, when females deposited smaller numbers of eggs, the eggs from smaller females were more likely to produce viable offspring compared to those from larger females. We found no evidence that this result was based on mating order, the relative sizes of competing females, or egg characteristics such as dry weight of eggs. Additionally, male body size did not significantly influence the survivorship of offspring during brooding. Our results suggest that the factors underlying offspring survivorship in pipefish may be more complex than previously believed, with multiple factors interacting to determine the fitness of individual offspring within the broods of pregnant males.

No environment is truly constant in time. As a result, animals have evolved multiple adjustments to cope with such fluctuations. However, the allocation of ffort to costly activities that imply long-term commitments, such as breeding, may be extremely challenging when future resources change constantly and unpredictably, a context that has received little investigation. To fill this gap, we studied the breeding response by a wetland-dependent raptor, the black kite Milvus migrans, to within and between-years fluctuations in resource availability (inunda- tion levels). The breeding performance of the population was decomposed into reproductive components expressed in a sequence of successive tasks along the breeding cycle (e.g. timing of laying, clutch size, hatching success, brood reduction). Variation in each component was related to resource levels observed at different key dates of the season in order to test whether and when population-level reproduction was adjusted to available resources. Along a 22-year time-series, inundation levels fluctuated unpredictably within and among years, and mostly affected the later components of kites’ reproduction, such as hatching success and the incidence of brood reduction, which were the main determinants of the population yearly breeding output. Results were consistent with multiple adjustments to cope with uncertainty. As the season progressed and resources became easier to assess, a bet-hedging waiting strategy based on a conservatively small, invariant and asynchronous clutch gave way to real-time resource-track- ing mechanisms mediated by progressive adjustments to current prey availability, so that population-level breeding rates were determined and tuned to resources rather late in the season. Such adjustments were the likely outcome of the interaction between parental tactics and environmental constraints. Behavioural flexibility, such as dietary opportunism, probably promoted further resistance to resource oscillati

It is well known that many animals with placenta-like structures provide their embryos with nutrients and oxygen. However, we demonstrate here that nutrients can pass the other way, from embryos to the parent. The study was done on a pipefish, Syngnathus typhle, in which males brood fertilized eggs in a brood pouch for several weeks. Earlier research has found a reduction of embryo numbers during the brooding period, but the fate of the nutrients from these 'reduced' embryos has been unknown. In this study, we considered whether (i) the brooding male absorbs the nutrients, (ii) siblings absorb them, or (iii) a combination of both. Males were mated to two sets of females, one of which had radioactively labelled eggs (using C-14-labelled amino acids), such that approximately half the eggs in the brood pouch were labelled. This allowed us to trace nutrient uptake from these embryos. We detected that C-14-labelled amino acids were transferred to the male brood pouch, liver and muscle tissue. However, we did not detect any significant C-14-labelled amino-acid absorption by the non-labelled half-siblings in the brood pouch. Thus, we show, to our knowledge, for the first time, that males absorb nutrients derived from embryos through their paternal brood pouch.

We studied the factors that led to brood reduction in 1192 nest/years of 143 territories of the Spanish imperial eagle during the period 1988–2003 and we carried out an innovative supplementary feeding programme in perches near nests in order to mitigate nestling mortality and improve fledging rate. Chick mortality affected 9.3% of hatchlings and 13.9% of the nests monitored. Sibling aggression was the most important factor in brood reduction (54.8% mortality of chicks in the nest), and supplementary feeding limited the process of sibling aggression. In addition, we compared the fledging rate in nests with supplementary food to that of non-supplemented control nests. Supplementary food increased the number of fledglings per brood (1.56 vs 0.72 in the control nests). This increase was repeated in subadult pairs (1.57 vs 0.53 in the control nests) and occurred both in high-quality territories (2.32 vs 1.36) and in low-quality territories (1.35 vs 0.54). Our results also suggest that supplementary feeding does not appear to have any effect on the provisioning rate of either males or females or on the wild prey. Food supplementation led to the recovery of breeding success after a period of loss due to a reduction of prey as a consequence of viral haemorrhagic disease. We conclude that the Spanish imperial eagle’s breeding output is food-limited during the nestling period and that food taken to the nest regulates sibling aggression. Supplemental feeding when food is scarce, or under emergency conditions, can be considered a good management tool for the conservation of this endangered species.

The evolution of parental care includes typical caring behaviour, such as protecting or feeding offspring. Sibling rivalry is anthropogenic viewed as conflicting behaviour, and can ultimately lead to brood reduction. In this study we have used object oriented programming to simulate different scenarios in this sibling rivalry conflict. A number of hypotheses have been proposed to explain the complex interactions occurring during brood reduction, but few simulation models successfully combines it with a hypothesis necessary to describe an ESS. In our solution we present a simple experimental simulation for brood reduction, where each sibling acts as an autonomous agent that has the ability to initiate actions for cooperation and competition against other siblings. Agents have a limited set of actions that can be activated during the onset of some specific environmental conditions, such as prey density, different climate variables etc. Parameters for food distribution are determined on a basis of a former known theory for maximizing inclusive fitness. In the computer simulations we have studied effects on sizes of siblings and fitness measures with varying degree of hatching patterns and prey density for siblings within the artificial brood. Results from the experimental simulations demonstrate interesting similarities with brood reduction in a real world setting, such as in raptorial birds that practice facultative siblicidal behaviour. Agents within the artificial brood respond with stronger conflicting behaviour whenever resources are limited and simulating increased hatching asynchrony lead to a lower survival rate (because of increased size hierarchy) of siblings within the simulated brood.

We tested the hypothesis that mother birds counterbalance the negative effects of hatching asynchrony for later-hatched chicks by increasing the yolk androgen concentrations in consecutive eggs of their clutch. In doing so, they may adaptively tune each offspring’s competitive ability and, thus, growth and survival. However, evidence in support of this hypothesis is contradictory. The yolk concentrations of maternal androgens in the eggs of black-headed gulls increase significantly with the laying order of the eggs in a clutch. We experimentally tested the functional consequences of this increase on chick development under natural conditions by injecting eggs with either an oil or androgen solution. We created experimental clutches in which androgen levels either stayed constant or increased with laying order while controlling for differences in egg quality by using only first-laid eggs. We then compared development, growth and survival between these broods. Androgen treatment enhanced embryonic development because androgen-treated eggs hatched half a day earlier than controls, while their size at hatching was similar to oil-treated controls. Androgen treatment did not increase chick survival, but it enhanced growth. Androgen-treated, third-hatched chicks had a higher body mass and longer legs than third-hatched chicks that hatched from oil-treated eggs. At the same time, growth of first chicks (which were all oil treated) was reduced by the presence of two androgen-treated siblings, suggesting that yolk androgens enhance the competitive ability of later-hatched chicks. Our results support the hypothesis that transfer of different amounts of androgens to the eggs of a clutch is a mechanism by which mothers maximize their reproductive output.

Hatching asynchrony commonly induces a size hierarchy among siblings and the resultant competition for food between siblings can often lead to starvation of the smallest chicks within a brood. We created herring gull (Larus argentatus) broods with varying degrees of hatching synchrony by manipulating the timing of incubation while maintaining the originally laid eggs. The degree of hatching asynchrony affected sibling size hierarchy at the time of hatching of the last-hatched ”c-chick.” In unmanipulated broods, there was no disadvantage of being a c-chick. However, when asynchrony was experimentally increased, we found reduced survival of the c-chick only in the exaggerated asynchronous experimental group. The effects were observable only during the first 10 days of chick life. We recorded no cases of the chicks dying of starvation. Furthermore, behavioral observations indicated that there was no sibling competition, and no selective feeding of larger sibs in the study colony. We propose that the observed lower survival rates of c-chicks in exaggerated asynchronous broods resulted from their lesser motor abilities, affecting their chances of escaping predators. Fledging success for the whole colony was generally high and almost half of all pairs fledged all three chicks, which is indicative of a good feeding environment. We argue that normal hatching asynchrony is a favorable solution in a good feeding environment, but that increased asynchrony reduces breeding success. We do not view asynchrony in the herring gull as an adaptation for brood reduction and propose instead that it may come about because there has been selection for incubation to start before clutch completion.

1. In the pipefish Syngnathus typhle sex roles are reversed, as females compete for access to males. In this species males provide all brood care (i.e. are 'pregnant') and female reproductive rate exceeds the reproductive rate of males. Consequently females are limited by access to mates and male reproductive success is limited by male brooding ability and/or mate quality. Thus, phenomena like brood reduction and a trade-off between number and weight of newborn may be expected in males. 2. In this paper I demonstrate the following in males of S. typhle: (a) the initial weight of the egg (received from the female) positively influenced the weight of newborn; (b) in the male's brood pouch, number of newborn frequently is less than number of eggs initially received (brood reduction), and the extent of this decrease in brood size positively influenced the weight of newborn; (c) a trade-off between number and weight of newborn was demonstrated in males from field samples and in large experimental males (independent of brood reduction), but not in small males that presumably allocated resources to several other demands (condition, growth, etc.); (d) paternal length, per se, did not affect the weight of newborn. 3. The results indicate that in S. typhle, male reproductive success is limited by their own brooding ability and influenced by the egg size received (indirectly quality of mate), which may be expected in a situation of sex-role reversal.