Dependent Offspring Research Articles

Foraging plasticity in a highly specialized carnivore, the endangered black-footed ferret

The extirpation of black-footed ferrets (Mustela nigripes) from the wild resulted from the rangewide decline of prairie dogs (Cynomys spp.) brought about by poisoning campaigns, the arrival of an exotic disease, and habitat loss. It is widely accepted that ferrets are an obligate, near monophagous, dietary specialist of prairie dogs and that high-density prairie dog colonies are necessary for effective recovery. To test the extent to which ferrets are dietary specialists, we measured the stable isotopic values of 321 ferrets of known age and sex as well as of their potential prey (e.g., prairie dogs, mice, ground squirrels, and rabbits). Our results confirmed that prairie dogs are the most common diet item for ferrets, although ferrets possessed greater foraging plasticity than previously reported, consuming substantial quantities of other species. The degree to which ferrets were specialized on prairie dogs differed between age–sex groups. Adult male and juvenile ferrets had equivalent diets, with prairie dogs constituting nearly 75% of their assimilated diet. In contrast, adult females obtained over one third of their diet from other species, notably mice. However, female ferrets appeared to have provisioned prairie dogs to their dependent offspring. Conservation of ferrets, one of North America’s most endangered mammals, will require prairie dogs, not just as prey, but also for the prey-rich habitat that their colonies provide.

Developing and Disseminating Financial Guidelines for Retirement Planning

Financial planning and decision making for retirement are challenging tasks. The effects on retirement finances of expected health-care costs, tax-based preferences for retirement saving, retirement benefits, housing values, and asset allocation mean that many retirees must play the roles of—or at least assess the advice from—doctor, accountant, realtor, and stockbroker. Retirees’ financial planning efforts are often hampered by lack of basic financial literacy, confusing or misleading financial products, high levels of debt, and the additional burdens of dependent adult offspring or advanced-age parents. There is substantial evidence that older households often lack financial literacy and often make poor financial choices, which can result in real economic hardship. The authors discuss criteria for developing and disseminating a single, comprehensive graphic to retirees and near-retirees to help bolster financial acumen and security. <b>TOPICS:</b>Retirement, social security, risk management

Infanticide as a male reproductive strategy has a nutritive risk effect in brown bears

Behavioural strategies to reduce predation risk can incur costs, which are often referred to as risk effects. A common strategy to avoid predation is spatio-temporal avoidance of predators, in which prey typically trade optimal resources for safety. Analogous with predator–prey theory, risk effects should also arise in species with sexually selected infanticide (SSI), in which females with dependent offspring avoid infanticidal males. SSI can be common in brown bear (Ursus arctos) populations and explains spatio-temporal segregation among reproductive classes. Here, we show that in a population with SSI, females with cubs-of-the-year had lower quality diets than conspecifics during the SSI high-risk period, the mating season. After the mating season, their diets were of similar quality to diets of their conspecifics. Our results suggest a nutritive risk effect of SSI, in which females with cubs-of-the-year alter their resource selection and trade optimal resources for offspring safety. Such risk effects can add to female costs of reproduction and may be widespread among species with SSI.

Burying beetles

Burying beetles

Effect of housing and husbandry practices on adrenocortical activity in captive Canada lynx (Lynx canadensis)

AbstractIn recent years, there has been an increasing focus on the study and assessment of animal welfare in captive settings, such as zoological gardens and aquaria. Canada lynx (Lynx canadensis) are a relatively common species in zoos, yet are known to exhibit frequent reproductive problems in captive environments. We provide an exploratory analysis of housing and husbandry factors that are associated with patterns of adrenocortical activity in lynx. Adrenocortical activity was assessed using the non-invasive technique of monitoring faecal glucocorticoid metabolites (FGM). First, we calculated baseline FGM values for each individual and controlled for sex, age class, and reproductive status. The residual values were used to determine how levels of adrenocortical activity correlated with institutional husbandry practices. Second, we compared the occurrence of FGM peaks to events and disturbances recorded by keepers. Our results highlighted that adrenocortical activity is strongly correlated with: (i) the size of the enclosure; (ii) the number of hiding locations available; and (iii) the social environment. Based on our findings, we recommend that lynx should generally be housed alone (unless with dependant offspring or temporarily paired up for mating purposes), in larger enclosures and with the provision of several species-appropriate hiding locations.

Brief communication: Female fecal androgens prior to the mating season reflect readiness to conceive in reproductively quiescent wild macaques

An extensive body of research exists on androgen secretion in males. Although androgens are also known to drive patterns of female reproductive physiology and behavior, little attention has been paid to these "male" hormones in females. Here, we examined female fecal androgen excretion in strictly seasonally breeding wild Assamese macaques (Macaca assamensis) and tested the hypothesis that changes in premating season fecal androgens vary with female readiness to resume ovarian cycling and thus to conceive. Across 2 years, 155 fecal samples were collected from seven reproductively quiescent (i.e., acyclic) females during the premating season months, August and September. Samples were analyzed for immunoreactive epiandrosterone (iEA), a major fecal metabolite of testosterone in macaques. Changes in iEA concentrations during the premating season were significantly negatively associated with (1) the presence of dependent offspring, i.e., lactating females showed a smaller change, and (2) with the timing of conceptions in the mating seasons, i.e., females that conceived early exhibited the greatest changes. These results provide strong evidence for a relationship between changes in androgens during the premating season and female readiness to conceive.

Hibernation and seasonal fasting in bears: the energetic costs and consequences for polar bears

Global warming has the potential to reduce arctic sea ice and thereby increase the length of summer–fall fasting when polar bears (Ursus maritimus) lose access to most marine mammals. To evaluate the consequences of such changes, we compared the cost of fasting by polar bears with hibernation by brown bears (U. arctos), American black bears (U. americanus), and polar bears and made projections about tissue reserves polar bears will need to survive and reproduce as fasts become longer. Hibernating polar bears expend energy at the same rate per unit mass as do brown bears and black bears. However, daily mass losses, energy expenditures, and the losses of lean mass are much higher in fasting, active polar bears than in hibernating bears. The average pregnant polar bear living around Hudson Bay during the 1980s and 1990s could fast for 10.0 6 2.3 months (¯ X 6 SD), and the average lactating female with cubs born during the preceding winter could fast for 4.2 6 1.9 months. Thus, some pregnant or lactating females with lower levels of body fat content were already approaching or beyond the constraint of being able to produce cubs and survive the required 8 months of fasting if producing new offspring or 4 months if accompanied by older offspring. Pregnant or lactating females and their dependent offspring have the most tenuous future as global warming occurs. Thus, we predict a significant reduction in productivity with even modest increases in global warming for polar bears living in the very southern part of their range and are concerned about more northern populations depending on their ability to accumulate increasing amounts of fat.

Size-biased allocation of prey from male to offspring via female: family conflicts, prey selection, and evolution of sexual size dimorphism in raptors

In birds with bi-parental care, the provisioning link between prey capture and delivery to dependent offspring is regarded as often symmetric between the mates. However, in raptors, the larger female usually broods and feeds the nestlings, while the smaller male provides food for the family, assisted by the female in the latter part of the nestling period, if at all. Prey items are relatively large and often impossible for nestlings to handle without extended maternal assistance. We video-recorded prey delivery and handling in nests of a raptor with a wide diet, the Eurasian kestrel Falco tinnunculus, and simultaneously observed prey transfer from male to female outside the nest. The male selectively allocated larger items, in particular birds and larger mammals, to the female for further processing and feeding of nestlings, and smaller items, in particular lizards and smaller mammals, directly to the nestlings for unassisted feeding. Hence, from the video, the female appeared to have captured larger prey than the male, while in reality no difference existed. The female's size-biased interception of the male's prey provisioning line would maximize the male's foraging time, and maximize the female's control of the allocation of food between her own need and that of the offspring. The male would maximize his control of food allocation by capturing smaller prey. This conflict would select for larger dominant females and smaller energy-efficient males, and induce stronger selection the longer the female depends on the male for self-feeding, as a proportion of the offspring dependence period.

Malicious Motherhood: Instance of Infanticide by a Female Barn Swallow

Abstract Infanticide, the killing of dependent offspring, often provides direct or indirect fitness benefits to the perpetrator. Infanticide in socially monogamous systems, like that in many passerine birds, is typically performed by males in an attempt to gain access to potential mates. We observed infanticide by a female Barn Swallow (Hirundo rustica) in North America and, to the best of our knowledge, this is the first documentation of this behavior. A female pecked and threw out nestlings belonging to a neighboring pair of swallows. There were no obvious fitness benefits gained by this female, thus established evolutionary explanations are not applicable. Further investigations into the frequency of female infanticide, easily mistaken for predation, should be pursued to better assess the selective pressures driving this behavior.

A Migratory Northern Ungulate in the Pursuit of Spring: Jumping or Surfing the Green Wave?

The forage-maturation hypothesis (FMH) states that herbivores migrate along a phenological gradient of plant development in order to maximize energy intake. Despite strong support for the FMH, the actual relationship between plant phenology and ungulate movement has remained enigmatic. We linked plant phenology (MODIS-normalized difference vegetation index [NDVI] data) and space use of 167 migratory and 78 resident red deer (Cervus elaphus), using a space-time-time matrix of "springness," defined as the instantaneous rate of green-up. Consistent with the FMH, migrants experienced substantially greater access to early plant phenology than did residents. Deer were also more likely to migrate in areas where migration led to greater gains in springness. Rather than "surfing the green wave" during migration, migratory red deer moved rapidly from the winter to the summer range, thereby "jumping the green wave." However, migrants and, to a lesser degree, residents did track phenological green-up through parts of the growing season by making smaller-scale adjustments in habitat use. Despite pronounced differences in their life histories, we found only marginal differences between male and female red deer in this study. Those differences that we did detect pointed toward additional constraints on female space-use tactics, such as those posed by calving and caring for dependent offspring. We conclude that whereas in some systems migration itself is a way to surf the green wave, in others it may simply be a means to reconnect with phenological spring at the summer range. In the light of ubiquitous anthropogenic environmental change, understanding the relationship between the green wave and ungulate space use has important consequences for the management and conservation of migratory ungulates and the phenomenon of migration itself.

Pride Diaries: Sex, Brain Size and Sociality in the African Lion (Panthera leo) and Cougar (Puma concolor)

The purpose of this study was to examine if differences in social life histories correspond to intraspecific variation in total or regional brain volumes in the African lion (Panthera leo) and cougar (Puma concolor). African lions live in gregarious prides usually consisting of related adult females, their dependent offspring, and a coalition of immigrant males. Upon reaching maturity, male lions enter a nomadic and often, solitary phase in their lives, whereas females are mainly philopatric and highly social throughout their lives. In contrast, the social life history does not differ between male and female cougars; both are solitary. Three-dimensional virtual endocasts were created using computed tomography from the skulls of 14 adult African lions (8 male, 6 female) and 14 cougars (7 male, 7 female). Endocranial volume and basal skull length were highly correlated in African lions (r = 0.59, p < 0.05) and in cougars (r = 0.67, p < 0.01). Analyses of total endocranial volume relative to skull length revealed no sex differences in either African lions or cougars. However, relative anterior cerebrum volume comprised primarily of frontal cortex and surface area was significantly greater in female African lions than males, while relative posterior cerebrum volume and surface area was greater in males than females. These differences were specific to the neocortex and were not found in the solitary cougar, suggesting that social life history is linked to sex-specific neocortical patterns in these species. We further hypothesize that increased frontal cortical volume in female lions is related to the need for greater inhibitory control in the presence of a dominant male aggressor.

Mechanisms and fitness effects of antibacterial defences in a carrion beetle

Parents of many species care for their offspring by protecting them from a wide range of environmental hazards, including desiccation, food shortages, predators, competitors, and parasites and pathogens. Currently, little is known about the mechanisms and fitness consequences of parental defences against bacterial pathogens and competitors. Here, we combine approaches from microbiology and behavioural ecology to investigate the role and mechanistic basis of antibacterial secretions applied to carcasses by parents of the burying beetle Nicrophorus vespilloides. This species rears its larvae on vertebrate carcasses, where larvae suffer significant fitness costs due to competition with bacterial decomposers. We first confirm that anal secretions produced by parents are potently bactericidal and that their effects are specific to gram-positive bacteria. Next, we identify the source of bacterial killing as a secreted lysozyme and show that its concentration changes throughout the breeding cycle. Finally, we show that secreted lysozyme is crucial for larval development, increasing survival by nearly two-fold compared to offspring reared in its absence. These results demonstrate for the first time that anal secretions applied to carrion is a form of parental care and expand the mechanistic repertoire of defences used by parent insects to protect dependent offspring from microbial threats.

Cross-fostering reveals that among-brood differences in ornamental mouth coloration mostly reflect rearing conditions in nestling house sparrows

Dependent offspring use specialized traits to attract parental care. In birds, this includes morphological ornaments (e.g. colourful plumage or mouthparts) that are associated with nestling condition and shape the allocation of parental care. Ornament expression often differs among broods, even after differences in individual condition are accounted for statistically. Understanding how this variation arises is important for understanding the information content of these signals, their functional importance, and their evolution. The present study used a cross-fostering experiment to assess the relative contributions of parental effects to among-brood differences in the mouth coloration of nestling house sparrows, specifically the carotenoid-richness, overall brightness, and ultraviolet (UV) coloration of rictal flanges. The expression of carotenoid-based coloration was explained by synchronous breeding, nest-of-rearing and nest-of-origin. Brightness and relative UV intensity, however, were explained only by synchronous breeding, and there was substantial unexplained variation in all three colour parameters. Among-brood variation in mouth coloration, then, may primarily contain information about the environment in which offspring are reared. At the individual level, ontogenetic changes in the carotenoid-richness and brightness of flanges positively reflected mass gain (a proxy for food intake). Larger and yellower chicks gained more mass, consistent with parental preferences for these traits. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 106, 169–179.

Nonlinear effects of group size on the success of wolves hunting elk

Despite the popular view that social predators live in groups because group hunting facilitates prey capture, the apparent tendency for hunting success to peak at small group sizes suggests that the formation of large groups is unrelated to prey capture. Few empirical studies, however, have tested for nonlinear relationships between hunting success and group size, and none have demonstrated why success trails off after peaking. Here, we use a unique dataset of observations of individually known wolves (Canis lupus) hunting elk (Cervus elaphus) in Yellowstone National Park to show that the relationship between success and group size is indeed nonlinear and that individuals withholding effort (free riding) is why success does not increase across large group sizes. Beyond 4 wolves, hunting success leveled off, and individual performance (a measure of effort) decreased for reasons unrelated to interference from inept hunters, individual age, or size. But performance did drop faster among wolves with an incentive to hold back, i.e., nonbreeders with no dependent offspring, those performing dangerous predatory tasks, i.e., grabbing and restraining prey, and those in groups of proficient hunters. These results suggest that decreasing performance was free riding and that was why success leveled off in groups with .4 wolves that had superficially appeared to be cooperating. This is the first direct evidence that nonlinear trends in group hunting success reflect a switch from cooperation to free riding. It also highlights how hunting success per se is unlikely to promote formation and maintenance of large groups. Key words: Canis lupus, carnivore, cooperation, free riding, group hunting, group living, interference, predation, sociality, wolf. [Behav Ecol]

Use of glacial and terrestrial habitats by harbor seals in Glacier Bay, Alaska: costs and benefits

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 429:277-290 (2011) - DOI: https://doi.org/10.3354/meps09073 Use of glacial and terrestrial habitats by harbor seals in Glacier Bay, Alaska: costs and benefits Gail M. Blundell1,*, Jamie N. Womble2,3, Grey W. Pendleton1, Shawna A. Karpovich4, Scott M. Gende3, Jason K. Herreman5 1Division of Wildlife Conservation, Alaska Department of Fish and Game, Juneau, Alaska 99811-0024, USA 2Department of Fisheries and Wildlife, Marine Mammal Institute, Oregon State University, Newport, Oregon 97365 USA 3Glacier Bay Field Station, National Park Service, Juneau, Alaska 99801 USA 4Division of Wildlife Conservation, Alaska Department of Fish and Game, Fairbanks, Alaska 99701-1599 USA 5Department of Zoology and Physiology, University of Wyoming, Laramie, Wyoming 82071 USA *Email: gail.blundell@alaska.gov ABSTRACT: Among pinnipeds, harbor seals Phoca vitulina have the broadest distribution (a 34° to 50° range in latitudes in the Pacific and Atlantic regions, respectively) and are found in a diversity of habitats. Harbor seals in Alaska, USA, similar to Arctic pinnipeds in many respects, rely upon glacial ice for pupping, mating, and molting. Just as climate change affects Arctic sea ice, tidewater glaciers are rapidly retreating in Alaska, reducing ice availability for harbor seals. An increased understanding of glacial vs. terrestrial harbor seals may reveal information important to conservation of harbor seals and Arctic pinnipeds, as effects of climate change continue. We compared foraging distances, activity budgets, diet, and body condition for seals captured at glacial and terrestrial sites in Glacier Bay, Alaska. Foraging strategies and activity budgets of seals using glacial ice differed substantially from seals using terrestrial sites. Glacial seals traveled significantly farther to forage (≥40 vs. <5 km) and spent more time hauled out than terrestrial seals (26 vs. 11 to 16%). Diets of glacial seals were higher in pelagic fishes compared to diets of terrestrial seals that foraged primarily on intertidal/demersal fishes. Body condition of seals was similar between habitats (p ≥ 0.09) and suggests that costs of longer foraging trips for glacial seals may be offset by obtaining higher quality diets of pelagic fishes, which may allow seals to spend more time hauled out. During the brief lactation period, more time hauled out could result in more time available for the transfer of energy from adult females to dependent offspring. KEY WORDS: Foraging behaviour · Activity budget · Body condition · Habitat use · Stable isotopes · Time-depth recorder · Phocid · Pinniped Full text in pdf format PreviousNextCite this article as: Blundell GM, Womble JN, Pendleton GW, Karpovich SA, Gende SM, Herreman JK (2011) Use of glacial and terrestrial habitats by harbor seals in Glacier Bay, Alaska: costs and benefits. Mar Ecol Prog Ser 429:277-290. https://doi.org/10.3354/meps09073 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 429. Online publication date: May 16, 2011 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2011 Inter-Research.

Fitness consequences of maternal rearing strategies in warthogs: influence of group size and composition

Cooperatively breeding species are defined by the presence of individuals who help in rearing the offspring of others. This seemingly altruistic behaviour has been difficult to define and the help provided has not always resulted in a reproductive advantage to the recipient. We examine maternal rearing strategies in the common warthog, Phacochoerus africanus, a facultative, cooperative breeder that displays variation in the number of reproductive and non-reproductively aged individuals in a group. We compare rearing strategies in adult females to assess whether group size or group composition increases the production and survival of group offspring. We found that although the number of offspring observed in groups with multiple adult females was larger than the number of offspring observed in groups with only one adult female, the average number of offspring observed per female was similar. Additionally, maternal rearing strategy had no effect on the survival of dependant offspring and variation in offspring survival could not be explained by either the number of reproductive or non-reproductive individuals per offspring. Taken together, these results suggest that the differences in maternal rearing strategies may not benefit dependent offspring, but we suggest that grouping between reproductive females may increase the survival of yearlings through group augmentation.

Spousal violence and paternal disinvestment among Tsimane' forager‐horticulturalists

We develop and test a conceptual model of factors influencing the likelihood of physical wife abuse. The paternal disinvestment model emphasizes that spousal conflict over resource use results from men's attempts to increase individual fitness at a cost to the family (e.g., through pursuit of extramarital affairs). We propose that men use violence to control women's responses to the diversion of resources away from the family: to quell women's objections to male disinvestment, maintain women's parental investment, and to dissuade women from pursuing relationships with other men. Interviews were conducted among men and women to determine rates of violence and demographic and behavioral covariates. Structural equation modeling and generalized estimating equations analyses were used to test predictions derived from the model. We also collected data on frequent complaints in marriage and women's perceptions of arguments precipitating violence. Over 85% of women experienced physical wife abuse (n = 49). Indicators of paternal disinvestment positively covary with indicators of marital strife and with rates of wife abuse. The wife's age, matrilocal residence, and presence of joint dependent offspring decrease the likelihood of violence through direct and indirect routes. Wife abuse is linked to the importance of paternal investment in human families, and is a means by which men control women's responses to a dual reproductive strategy of familial investment and pursuit of extramarital sexual relationships. This framework is more general than traditional sociological and evolutionary perspectives emphasizing patriarchy and men's sexual jealousy, respectively.

Differential prey use by male and female cougars in Washington

AbstractMale and female predators are often assumed to have the same effects on prey. Because of differences in body size and behavior, however, male and female predators may use different species, sexes, and ages of prey, which could have important implications for wildlife conservation and management. We tested for differential prey use by male and female cougars (Puma concolor) from 2003 to 2008 in Washington State. We predicted that male cougars would kill a greater proportion of larger and older prey (i.e., adult elk [Cervus elaphus]), whereas females would kill smaller and younger prey (i.e., elk calves, mule deer [Odocoileus hemionus]). We marked cougars with Global Positioning System (GPS) radio collars and investigated 436 predation sites. We located prey remains at 345 sites from 9 male and 9 female cougars. We detected 184 mule deer, 142 elk, and 17 remains from 4 other species. We used log‐linear modeling to detect differences in species and age of prey killed among cougar reproductive classes. Solitary females and females with dependent offspring killed more mule deer than elk (143 vs. 83, P &lt; 0.01), whereas males killed more elk than mule deer (59 vs. 41, P &lt; 0.01). Proportionately, males killed 4 times more adult elk than did females (24% vs. 6% of kills) and females killed 2 times more adult mule deer than did males (26% vs. 15% of kills). Managers should consider the effects of sex of predator in conservation and management of ungulates, particularly when managing for sensitive species. © 2011 The Wildlife Society.

Stable nitrogen and carbon isotope discrimination between juveniles and adults in an income-breeding small mammal (Peromyscus maniculatus)

We investigated the stable nitrogen- and carbon-isotope compositions of blood, liver, muscle and hair of income breeding deer mice (Peromyscus maniculatus) to determine the extent to which stable isotope compositions of mothers and offspring differed. We found small differences between the δ15N and δ13C values of dependent offspring and adult tissues (by a magnitude of −0.8‰ to 1.1‰ for 15N and 0.3–0.9‰ for 13C), and limited 15N discrimination between juvenile tissues and milk, which explains the small mother-offspring trophic effects in isotopic composition. Discrimination of 15N between offspring and adults was greater than expected but smaller than known for capital breeding mammals, suggesting that different biochemical pathways for milk production and processing may affect the discrimination of 15N in these systems.

Cuckoos versus hosts in insects and birds: adaptations, counter-adaptations and outcomes

Avian parents and social insect colonies are victimized by interspecific brood parasites-cheats that procure costly care for their dependent offspring by leaving them in another species' nursery. Birds and insects defend themselves from attack by brood parasites; their defences in turn select counter-strategies in the parasite, thus setting in motion antagonistic co-evolution between the two parties. Despite their considerable taxonomic disparity, here we show striking parallels in the way that co-evolution between brood parasites and their hosts proceeds in insects and birds. First, we identify five types of co-evolutionary arms race from the empirical literature, which are common to both systems. These are: (a) directional co-evolution of weaponry and armoury; (b) furtiveness in the parasite countered by strategies in the host to expose the parasite; (c) specialist parasites mimicking hosts who escape by diversifying their genetic signatures; (d) generalist parasites mimicking hosts who escape by favouring signatures that force specialization in the parasite; and (e) parasites using crypsis to evade recognition by hosts who then simplify their signatures to make the parasite more detectable. Arms races a and c are well characterized in the theoretical literature on co-evolution, but the other types have received little or no formal theoretical attention. Empirical work suggests that hosts are doomed to lose arms races b and e to the parasite, in the sense that parasites typically evade host defences and successfully parasitize the nest. Nevertheless hosts may win when the co-evolutionary trajectory follows arms race a, c or d. Next, we show that there are four common outcomes of the co-evolutionary arms race for hosts. These are: (1) successful resistance; (2) the evolution of defence portfolios (or multiple lines of resistance); (3) acceptance of the parasite; and (4) tolerance of the parasite. The particular outcome is not determined by the type of preceding arms race but depends more on whether hosts or parasites control the co-evolutionary trajectory: tolerance is an outcome that parasites inflict on hosts, whereas the other three outcomes are more dependent on properties intrinsic to the host species. Finally, our review highlights considerable interspecific variation in the complexity and depth of host defence portfolios. Whether this variation is adaptive or merely reflects evolutionary lag is unclear. We propose an adaptive explanation, which centres on the relative strength of two opposing processes: strategy-facilitation, in which one line of host defence promotes the evolution of another form of resistance, and strategy-blocking, in which one line of defence may relax selection on another so completely that it causes it to decay. We suggest that when strategy-facilitation outweighs strategy-blocking, hosts will possess complex defence portfolios and we identify selective conditions in which this is likely to be the case.