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While for example a wolf pack provides protection to a sick wolf, increasing its chance of survival, there is a risk of infecting other members of the pack, decreasing their total chance of survival.

Do animals have mechanisms for preventing that? For example the sick member leaving its group temporarily?

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  • $\begingroup$ Diversity in immune genes within the population could be something. $\endgroup$ – AlexDeLarge Jul 6 '16 at 16:24
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Three proposed mechanisms:

  1. altruistic self-removal (i.e., diseased individuals leave the group to prevent transmission to kin) has been documented in eusocial insects (ants and bees); Rueppell et al 2010 J. Evol. Biol. say:

altruistic self‐removal by sick social insect workers to prevent disease transmission is expected under most biologically plausible conditions. The combined theoretical and empirical support for altruistic self‐removal suggests that it may be another important kin‐selected behaviour and a potentially widespread mechanism of social immunity.

  1. social distancing/avoidance: This is a commonly used term in human infectious disease epidemiology for the practice of avoiding contact with infected people. I don't know of any great references, but Loehle 1995 Ecology "Social Barriers to Pathogen Transmission in Wild Animal Populations" says:

Data on animal behavior in this regard [social avoidance] are sketchy. Sick individuals in cattle herds do seem to be isolated somewhat from the group (L. R. Rittenhouse, Department of Range Science, Colorado State University, and Ray Strickland, University of Maryland, personal communication 1988), though whether by chance (failing to keep up with the group due to listlessness), by voluntary isolation, or by avoidance on the part of others is not clear. Edwards (1988) found a higher degree of investigatory behaviors but a lower degree of touching between other group members and mice infected with Trichinella spiralis, though this disease is not transmissible by contact. This example would fit the proposed behaviors perfectly. More study is needed.

  1. seasonal migration has been proposed as a way for group-living organisms to temporarily leave their habitat, allowing the built-up parasite burden in the environment to diminish before they return; it may also have a group-level effect of filtering out diseased individuals.

Folstad et al. 1991 Can. J. Zool. Parasite avoidance: the cause of post-calving migrations in Rangifer?

Intensities of warble fly larvae, Hypoderma tarandi (L.), were examined in slaughtered reindeer (Rangifer tarandus tarandus L.) from different summer grazing areas of Finnmark County, northern Norway. To test the hypothesis that larval abundance decreases with increase in post-calving migration distance (i.e., distance from calving grounds), herds with differing migration distances were sampled. The prevalence of infection in the total sample of 1168 animals was 99.9%. The study revealed significant differences in larval abundance among herds from different summer grazing areas. Herds with post-calving migrations have significantly lower larval abundances than herds remaining on or near the calving grounds for the whole summer. Between-herds variation in abundance of H. tarandi larvae is assumed to reflect differing densities of the infective stage (adult flies) on the herds' summer ranges. Larval abundance in a herd is in turn negatively correlated with the distance between the main larval shedding areas (i.e., calving grounds) and the areas of greatest transmission (i.e., summer pastures). These results are discussed in relation to transmission of other parasites common to Rangifer and suggest that this host's post-calving migration may be a behavioural adaptation that reduces levels of parasitic infections.

Loehle 1995 also discusses this (qualitatively/speculatively):

A more subtle effect results from migration. Consider a species that migrates seasonally some large distance. An area that becomes fouled and unsanitary during one season will become largely clean again by the following year. In between, nest and skin parasites such as ticks and fleas will have nothing to eat and will be greatly reduced in number. For example, wide spacing in rabbits results in reduced flea numbers (Mohr 1963) for these same reasons. Excrement piles will decompose between seasons and wash away. Significant benefits could thus accrue to migratory species, even though long distance migratory behavior is not likely to have arisen from this cause alone. (It may result from predator avoidance [Fryxell et al. 1988], or more commonly be due to weather and food supply factors.) Abandonment of rookeries during the non-mating season might, however, be the result of disease avoidance. Another consequence of migratory behavior is the effect on internal parasites ... Consideration of epidemiology thus leads to some unexpected benefits of migratory behavior.

Hall et al. 2014 J. Anim. Ecol. "Greater migratory propensity in hosts lowers pathogen transmission and impacts" explore a theoretical model of migration effects.

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Almberg ES, Mech LD, Smith DW, Sheldon JW, Crabtree RL (2009) A Serological Survey of Infectious Disease in Yellowstone National Park’s Canid Community. PLoS ONE 4(9): e7042. doi:10.1371/journal.pone.0007042

The authors found that the majority of Yellowstone wolves had been exposed to a number of different viruses (evidence from antibodies in blood samples).

They also found evidence for historical outbreaks of canine distemper virus in wolves, coyotes and red foxes in 1999 and 2005. These outbreaks correlated with peaks in wolf pup mortality.

This evidence suggests that wolves have no mechanisms for avoiding the spread of these viral diseases. Perhaps periodic episodes of increased mortality are the price that has to be paid for a longer-term 'herd immunity'?

Edit - in response to OP comment below:

The data indicate very high levels of seropositivity to the viruses. I would say that 'not 100% successful' is an understatement. But I must admit that I am way out of my area of expertise here!

From the abstract:

We found high, constant exposure to canine parvovirus (wolf seroprevalence: 100%; coyote: 94%), canine adenovirus-1 (wolf pups [0.5-0.9 yr]: 91%, adults [>or=1 yr]: 96%; coyote juveniles [0.5-1.5 yrs]: 18%, adults [>or=1.6 yrs]: 83%), and canine herpesvirus (wolf: 87%; coyote juveniles: 23%, young adults [1.6-4.9 yrs]: 51%, old adults [>or=5 yrs]: 87%) suggesting that these pathogens were enzootic within YNP wolves and coyotes.

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  • $\begingroup$ That's very interesting. But I think the conclusion is not completely correct. I'd rather conclude that "based on the evidence, if wolves have mechanisms for avoiding the spread of those diseases, they're not 100% successful." It could be that they have such mechanisms, and without them, the mortality would be even higher. The last sentence is an intriguing point. Perhaps from evolutionary perspective it's better to let the whole pack/herd become infected and if it survives, all the survivors will be immune. $\endgroup$ – Petr Pudlák Dec 3 '12 at 10:10
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(This answer is still a work in progress, but I'll work more on it eventually...)

It's not terribly controversial to say that infected hosts may be more likely to be eaten by predators. However, at least one author (Smith Trail et al 1980; full reference below) has suggested that this could be adaptive; in some sitautions there could be a net gain to inclusive fitness if, by 'submitting to' predation, infected hosts are able to reduce the force of infection acting upon close relatives.

(Full reference: Smith Trail DR. 1980 Behavioral interactions between parasites and hosts: host suicide and the evolution of complex life cycles. American Naturalist 116, 77-91)

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