The expected value of wild animal welfare is framed by age-specific survivorship

This post sum­ma­rizes work con­ducted at both Wild An­i­mal Ini­ti­a­tive and An­i­mal Ethics. The first part can be found here and con­tinues here. A pre-print pa­per (cur­rently un­der re­view) can be found here.


The doc­u­men­tary se­ries “Our Planet” opens with a flam­ingo chick whose legs have be­come caked with salt from the mud flats. The young bird can’t keep up with the rest of their flock and is left to die. At the same time, other chicks and healthy adults seem to be liv­ing rea­son­ably con­tented lives, able to find food and over­come other challenges. It seems plau­si­ble that an adult flam­ingo has lived a life char­ac­ter­ized more by plea­sure than suffer­ing. This chick, though – and some pro­por­tion of all flam­in­gos who have been born – never got to ex­pe­rience their best years.

As part of an on­go­ing pro­ject to un­der­stand the welfare of wild an­i­mals, I an­a­lyzed age-spe­cific mor­tal­ity rates and con­sid­ered how they might re­late to welfare, in­tro­duc­ing a new con­cept for un­der­stand­ing the lives of wild an­i­mals: welfare ex­pec­tancy. Welfare ex­pec­tancy can serve as a frame­work for weigh­ing up the differ­ent lev­els of well-be­ing an­i­mals might ex­pe­rience over the course of their lives, helping to model the welfare con­se­quences of in­ter­ven­tions and nat­u­ral pres­sures, such as pre­da­tion, that may dis­pro­por­tionately af­fect an­i­mals of par­tic­u­lar ages.

Welfare expectancy

To un­der­stand the bal­ance of plea­sure and suffer­ing in na­ture, we need to un­der­stand what pro­por­tion of an­i­mals ex­pe­rience differ­ent welfare-rele­vant out­comes. In the face of the di­ver­sity of in­di­vi­d­ual ex­pe­riences wild an­i­mals may have, we need a way to tally them up in or­der to as­sess the over­all welfare of a pop­u­la­tion. Ex­pected value does this by tak­ing the sum of the value of each out­come mul­ti­plied by its prob­a­bil­ity. For ex­am­ple, to calcu­late the life ex­pec­tancy of a pop­u­la­tion (i.e. the ex­pected value of lifes­pan), one would mul­ti­ply the pro­por­tion of in­di­vi­d­u­als who die at a cer­tain age by the num­ber of years they lived and sum this across all pos­si­ble lifes­pans.

Lifes­pan is a blunt way of quan­tify­ing welfare out­comes, be­cause two an­i­mals may have very differ­ent ex­pe­riences and yet die at the same age. How­ever, within the same species, the fre­quency of differ­ent lifes­pans is likely to re­flect com­mon challenges as­so­ci­ated with spe­cific stages of life. Since only liv­ing an­i­mals are ca­pa­ble of ex­pe­rienc­ing welfare, lifes­pan is effec­tively an up­per bound on the amount of af­fec­tively pos­i­tive or nega­tive ex­pe­rience an an­i­mal can ac­crue.

If av­er­age welfare lev­els are con­stant through­out life, then life ex­pec­tancy is the only welfare-rele­vant met­ric we can de­rive from pat­terns of age-spe­cific mor­tal­ity. How­ever, welfare likely does vary with age, as ju­ve­niles, sub-adults, re­pro­duc­tive adults and senes­cent an­i­mals face differ­ent lev­els and forms of dis­ease, com­pe­ti­tion, pre­da­tion and en­vi­ron­men­tal hard­ship. This po­ten­tial for vari­a­tion calls for a dis­tinct con­cept of welfare ex­pec­tancy.

Con­sider a species for which welfare is poor in early life, but high in adult­hood. If the prob­a­bil­ity of sur­viv­ing early life is high, then the life­time ex­pected value of welfare for an in­di­vi­d­ual born into that pop­u­la­tion may be high, be­cause most in­di­vi­d­u­als are go­ing to have a chance to live out their best years in adult­hood. If the prob­a­bil­ity of sur­viv­ing early life is low, then most in­di­vi­d­u­als will only live to ex­pe­rience the ju­ve­nile pe­riod of poor welfare. Con­versely, in a species where welfare is higher in early life than in adult­hood (e.g. due to good parental care), the net welfare of even a short-lived an­i­mal could be rel­a­tively high.

We are profoundly un­cer­tain about whether most an­i­mals’ lives are dom­i­nated by plea­sure or suffer­ing, or even how to go about weigh­ing these up. There­fore, it may be pru­dent to con­cen­trate on a mea­sure of “rel­a­tive welfare ex­pec­tancy” (RWE), rep­re­sent­ing the nor­mal­ized welfare ex­pec­tancy of a pop­u­la­tion di­vided by its life ex­pec­tancy. For a fixed life ex­pec­tancy, the high­est welfare ex­pec­tancy is achieved by max­i­miz­ing the pro­por­tion of an­i­mals liv­ing to ex­pe­rience the best years of life while min­i­miz­ing the pro­por­tion ex­pe­rienc­ing the worst years.

Con­fi­dent ap­pli­ca­tion of the welfare ex­pec­tancy con­cept will re­quire em­piri­cal data on val­ues of age-spe­cific welfare, which are cur­rently scarce for wild an­i­mal pop­u­la­tions. A plau­si­ble work­ing hy­poth­e­sis, how­ever, is that the av­er­age welfare ex­pe­rienced by an an­i­mal of a given age is pro­por­tional to their prob­a­bil­ity of sur­viv­ing that pe­riod of life. The jus­tifi­ca­tion for this is that the same fac­tors which lead to mor­tal­ity (e.g. dis­ease, vuln­er­a­bil­ity to preda­tors, com­pe­ti­tion for food) have been shown to lead to chronic stress and poor phys­i­cal con­di­tion.

Age-spe­cific mortality

The vast ma­jor­ity of an­i­mals live very short lives; not only in ab­solute terms, but also rel­a­tive to the longest-lived mem­bers of their species. In fact, of the pop­u­la­tions an­a­lyzed in the pa­per as­so­ci­ated with this pro­ject, av­er­age lifes­pans were on av­er­age 16% of a species’ max­i­mum lifes­pan, with only 5% of pop­u­la­tions hav­ing life ex­pec­tan­cies >33% of their max­i­mum. Im­por­tantly, this rep­re­sents an av­er­age across pop­u­la­tions, not across in­di­vi­d­u­als. Be­cause short-lived species tend to be more pop­u­lous, lives in na­ture are likely to be cut short far more of­ten than these num­bers sug­gest. Depend­ing on how welfare varies with age in their re­spec­tive species, es­pe­cially short-lived in­di­vi­d­u­als will be miss­ing out on a great deal of pos­i­tive and/​or nega­tive ex­pe­rience.

Pat­terns of age-spe­cific mor­tal­ity are too di­verse to as­sign uni­ver­sal clas­sifi­ca­tions to large tax­o­nomic groups. For ex­am­ple, even some in­sect species have rel­a­tively high rates of ju­ve­nile sur­vivor­ship. How­ever, while age-spe­cific pat­terns vary, an­i­mals of cer­tain groups, such as the ray-finned fishes (act­inoptery­gii), do have con­sid­er­ably shorter life ex­pec­tan­cies and lower av­er­age an­nual sur­vival rates than oth­ers, in­clud­ing birds (aves) and mam­mals. Im­por­tantly, life ex­pec­tancy is only an av­er­age of the lifes­pans. As ex­plained above, not only the av­er­age, but the pre­cise dis­tri­bu­tion of lifes­pans is im­por­tant to con­sider be­cause of how it may cor­re­spond to the dis­tri­bu­tion of age-spe­cific welfare. For a de­tailed dis­cus­sion of some ex­am­ples of age-spe­cific mor­tal­ity pat­terns, see the post by An­i­mal Ethics or the pre-print manuscript.

For an an­i­mal to have an en­joy­able life on net, they must ex­pe­rience enough plea­sure to com­pen­sate for the pain of their death. Cause of death, and there­fore the du­ra­tion and pain of an an­i­mal’s ex­pe­rience of dy­ing, may also vary with age similarly to welfare. In a hy­po­thet­i­cal species, ju­ve­niles might be most likely to starve while adults are most likely to be pre­dated, with the rel­a­tive prob­a­bil­ities of these and other mor­tal­ity fac­tors shift­ing over a life­time. If the pain of death is a suffi­ciently strong fac­tor to negate some of the pos­i­tive welfare an an­i­mal might have ex­pe­rienced while al­ive, age-spe­cific vari­a­tion in the in­ci­dence of var­i­ous man­ners of death and their sever­ity could also be im­por­tant to ac­count for.


At the in­di­vi­d­ual level, welfare ex­pec­tancy unites two dis­tinct con­cepts: day-to-day qual­ity of life and the quan­tity of welfare ex­pe­rienced over an in­di­vi­d­ual’s life­time. How­ever, a similar quan­tity-qual­ity dis­tinc­tion ap­plies at the level of pop­u­la­tions, with welfare ex­pec­tancy ad­dress­ing the qual­ity side of the ar­gu­ment and quan­tity be­ing de­ter­mined by the pop­u­la­tion size. Ideally, a pop­u­la­tion should be man­aged in such a way that max­i­mizes its to­tal welfare ex­pec­tancy.

The field of welfare biol­ogy is at a very early stage, and lit­tle ded­i­cated work from the life sci­ences has been in­vested up un­til re­cently. While progress is still limited by the lack of em­piri­cal stud­ies of wild an­i­mal welfare, knowl­edge of age-spe­cific mor­tal­ity pat­terns and a pre­dic­tive un­der­stand­ing of pop­u­la­tion ecol­ogy will be es­sen­tial for con­tex­tu­al­iz­ing this in­for­ma­tion, as well as eval­u­at­ing and pri­ori­tiz­ing among in­ter­ven­tions that differ­en­tially af­fect var­i­ous age groups.