Parental care is any parental behaviour that benefits the offspring, frequently at the cost of survival or further mating opportunities for the carers1; therefore it should evolve when the carers’ benefits outweigh the costs2. Parents provide no direct care in most species, yet females, males or both parents of different species across many lineages exhibit a great diversity of care behaviours3,4. The documented costs of male care in both vertebrates and invertebrates include increased risk of predation, parasitism or infection5,6,7,8,9, reduced mobility and foraging time5, leading to loss of body mass and condition10,11,12, loss of potential mating opportunities13, and in some species reduced survival, for example, (ref. 14). Given these costs, why do males care? Theoretical models and empirical studies show that males may trade-off costly care for a greater certainty or degree of paternity, reducing the level of care when female promiscuity is high3,15,16,17. Alternatively, when future mating opportunities are scarce, males might do better to care for their current offspring, regardless of paternity levels18. Most studies on the evolution of male care focus on the direct costs and benefits for the male, such as increased certainty of paternity, and the evolutionary relationship between male care and mating system3,17,19,20,21,22. While males invest a considerable amount of time and energy in caring, whether and how this leads to possible benefits to females and offspring is much less well understood2, particularly in species where females already care for the offspring (that is, biparental care). Quantifying these benefits, such as increased offspring survival and growth rates or female fecundity, is important because they could in turn increase the male’s inclusive fitness and lead to evolutionary feedback between male care and life history traits3. Here, we investigate hypotheses that relate life history traits and male care at a large comparative scale in mammals, a taxon with obligatory female care and in which male care is also present in ∼10% of species2,19.

By providing an energetic contribution towards offspring rearing through costly care, such as provisioning dependent offspring or carrying heavy offspring, care by helpers, including the male, may allow females to redirect more resources into reproduction and in turn increase female reproductive success and/or offspring growth rates2,20,23. Males and additional carers may also enable females to spend more time foraging and gain more resources for current or future offspring2. The hypothesis that care by other individuals allows females to increase their reproductive output has been proposed mostly in the context of allocare (care by either the male or other individuals) for species with female care, such as birds and mammals (‘load-lightening’ hypothesis24; see also Woodroffe and Vincent for male care2). Support for this hypothesis in relation to male care specifically is found in burying beetles (Nicrophorus sp.), where caring males help provisioning the offspring, and allow females to reduce their parental effort in the current brood and greatly increase their future brood mass25. Furthermore, female California mice (Peromyscus californicus) wean more offspring per reproductive bout and reproduce more frequently when males care26,27, indicating that females cannot meet the energetic demands of rearing more numerous and larger litters alone. Thus, it appears that the presence of male care has a ‘load-lightening’ effect similar to that observed in species, such as meerkats (Suricata suricatta)28 and grey crowned babblers (Pomatostomus temporalis)29, where females helped by other carers can substantially increase their reproductive output24. However, whether male care is consistently associated with higher female fecundity across species is still poorly understood as the majority of studies addressing this question focus only on a few model species.

Large-scale comparative approaches are well suited to unravel the generality of patterns and processes30, but most comparative work on male care concerns primarily its evolutionary relationship with mating systems, for example, (refs 19, 20). The few comparative studies that test, at least partially, the hypothesis that male care associates with female fecundity focus on mammals. These studies find that litters are larger in species where females are helped by alloparents (males and/or other individuals)31,32, and that breeding frequency is higher in socially monogamous mammals in which males provision or carry the offspring19, a result also found in primates with allocare31. However, it is unclear whether a larger litter associates specifically with male care or care by other individuals, as previous studies do not separate care according to the identity of the carer, although benefits and costs of care may differ between the male and alloparents. Likewise, it is important to identify at which stage of reproduction male care is more likely to associate with higher female fecundity. Lactation is the most energetically demanding period of reproduction for a female mammal, with daily energy expenditure increasing by up to four times33,34,35. Male care may thus allow females to gain or save energy that can be (re)invested in more or better quality milk20,23, which in turn may result in a shorter lactation2,23 and lead to shorter interbirth intervals; if so, the documented association between frequency of breeding and male care19,31 is mediated by a reduction in the duration of lactation2. Alternatively, by caring for the offspring post-weaning, males may allow females to invest more time foraging, regain body condition more quickly and mate sooner, regardless of the duration of lactation27. Discriminating between these scenarios and identifying the relevant male care behaviour at a given stage of reproduction is fundamental because it helps to pinpoint the mechanism that underlies the evolutionary associations between male care and life history traits, and the possible evolutionary feedback between them.

Regardless of whether a higher frequency of breeding is achieved through male care post-weaning or by enabling females to wean the offspring sooner, higher female reproductive rates benefit the male only if he mates with the same female over more than one breeding event. This appears to be the case in mammals as recent comparative studies conclude that the evolution of social monogamy precedes the evolution of male care and is evolutionarily associated with it19,20. This evolutionary relationship may be especially relevant in long-lived species, as greater female fecundity over a longer lifespan could further help compensate for the loss of potential mating opportunities that should be experienced by monogamous caring males. Whether longer-lived species are more likely to exhibit male care is, however, unknown.

Like the care by other helpers, male care may also benefit the offspring by enhancing their survival to independence through protection against predators and/or by increasing offspring growth rates or size at independence2,36. Consistent with this hypothesis, zebra finch offspring (Taeniopigia guttata) have faster growth rate37 and snow bunting fledglings (Plectrophenax nivalis) are larger38 when raised by two parents than by one parent alone. Moreover, a non-phylogenetic study in carnivores finds that females have greater milk energetic output and offspring have higher growth rates in species where males or other individuals provision females and offspring23. However, we still do not know to what extent male care associates with greater offspring growth rates across mammals, and whether this leads to heavier offspring, which enjoy greater survival.

Although comparative studies cannot rule out the possibility that male care evolves in species where female fecundity or offspring growth are higher, the limited available experimental and field evidence in mammals and other organisms suggests that the absence of males caring for the offspring can have detrimental effects on both female fecundity, for example, (refs 25, 27) and offspring survival39,40,41,42. Yet, we currently lack a clear and comprehensive picture of how life history traits associate specifically with male care in any animal taxon at a large comparative scale, which is necessary to help direct future efforts aimed at disentangling cause and effect of the evolution of male care. Furthermore, previous comparative studies often analyze male care together with care by other individuals, but to what extent male care and care by others exhibit the same costs and benefits, and associations with life history traits, is unknown. In addition, previous comparative work considers only a subset of all male care behaviours under the assumption that some are more costly (for example, carrying and provisioning, most common in primates and carnivores)19,31. However, behaviours often regarded as less expensive, such as grooming and huddling with the offspring (most frequent in rodents), may entail substantial fitness costs for the male10,11,43,44, while allowing females to forage for longer periods and gain more resources for reproduction. Thus, identifying whose care—by the male or by other helpers—and which specific behaviour associates with life history traits is fundamental to understanding how and why male care evolves, as the benefits and costs of care are likely to vary in relation to the identity of the carer and the behaviour performed. Finally, assessing whether specific life history traits are evolutionary associated with male care also requires that the correlated evolution between life history traits is accounted for, as ignoring it may lead to misleading conclusions, as shown in (refs 45, 46).

Here, we compile the largest and most detailed dataset of male care behavior in mammals until now, and test the hypothesis that increased female fecundity and offspring fitness related traits are associated with male care using phylogenetic comparative methods. From this hypothesis, we test the predictions that species with male care exhibit (i) shorter lactation and/or gestation, (ii) more frequent and/or larger litters and (iii) larger neonates and/or weanlings. We consider both a broad definition of male care and each of the most frequently observed male care behaviours in mammals; provisioning the offspring, carrying, huddling and grooming. We also investigate provisioning reproducing females by the male, as this behaviour could indirectly benefit the offspring as well as the female. Using phylogenetic generalized least squares models (PGLS) to account for species’ shared ancestry47,48, we build multi-predictor models where the dependent continuous variable is a life history trait of interest and male care is a binary independent variable. After accounting for allometry, the correlated evolution between life history traits49 and other potential confounding variables, we demonstrate that fecundity is higher in species with male care, but the way this is achieved is complex and varies across orders and the nature of male care. Litters are larger in species in which males provision reproducing females, especially carnivores, while frequency of breeding is higher in species with carrying, mostly primates, due to a reduction in lactation time. Lactation time is also shorter in carnivores where males provision reproducing females.