Life history classification


Un­der­stand­ing the life his­tory of an­i­mals is im­por­tant for un­der­stand­ing wild an­i­mal welfare, but has been un­der­stud­ied by an­i­mal welfare ad­vo­cates. In par­tic­u­lar, life his­tory gen­er­al­iza­tions have been used to claim that the lives of most wild an­i­mals are net nega­tive (see dis­cus­sion of this po­si­tion in Bren­nan 2017 ). How­ever, there are sev­eral meth­ods of life his­tory clas­sifi­ca­tion in use in ecol­ogy and evolu­tion­ary biol­ogy. The the­o­ret­i­cal foun­da­tions for r-K se­lec­tion referred by some ad­vo­cates have been dis­cred­ited, and in ad­di­tion some large species groups can­not be placed on this con­tinuum. How­ever, a re­lated form of this clas­sifi­ca­tion, fast-slow is still in use in the sci­ences. Tri­par­tite clas­sifi­ca­tion schemes seem to be more ap­pro­pri­ate for plant, in­sect and fish species, which do not eas­ily fit into a sin­gle axis. More gen­er­ally, large scale re­views usu­ally come to the con­clu­sion that a sin­gle com­pos­ite axis of vari­a­tion is not suffi­cient to ex­plain the wide range of life his­tory vari­a­tion. One very im­por­tant point is that all clas­sifi­ca­tion meth­ods are con­sid­ered con­tinu­ums; that is, most species will lie some­where in the mid­dle of axes of vari­a­tion rather than at the ex­tremes.

What is life his­tory?

Non-hu­man or­ganisms have a be­wil­der­ing ar­ray of modes of life. While some species have lives quite similar to our own, the ma­jor­ity are quite differ­ent. For ex­am­ple, some species have a similar body form through­out their lives, while other species, like but­terflies, have quite differ­ent body forms at differ­ent stages. Some, like hu­mans and aphids, are iteroparous (re­pro­duc­ing mul­ti­ple times dur­ing their life­time) oth­ers, like salmon and gi­ant Pa­cific oc­to­pus, re­pro­duce only once (semel­parous). Some species, like whales and dung bee­tles, have very few offspring while oth­ers, like sea tur­tles and rab­bits, have many. Some, like ter­mites, provide ex­ten­sive care for these offspring, but some, like harp seals, do not. Some species, like qua­hog clams, live a very long time, oth­ers, like mayflies do not. Some, like monarch but­terflies, travel long dis­tances, oth­ers stay in one lo­ca­tion at all times. Col­lec­tively, these var­i­ous as­pects of or­ganism life are termed life his­tory.

r- and K-se­lec­tion theory

Early on in ecol­ogy and evolu­tion­ary biol­ogy, efforts were made to both clas­sify and un­der­stand this large ar­ray of life his­tory. Scien­tists fo­cused on the fact that or­ganisms could al­lo­cate differ­ent amounts of en­ergy and time to sur­vival, growth, and re­pro­duc­tion de­pend­ing on se­lec­tion pres­sures. MacArthur and Wil­son (1967), ex­pand­ing on ideas from Dobzhan­sky (1950), coined the terms r and K-se­lec­tion while at­tempt­ing to ex­plain the se­lec­tion forces that de­ter­mined life his­tory trade­offs in un­crowded and crowded en­vi­ron­ments, re­spec­tively.

Th­ese terms, r and K-se­lec­tion, re­fer to the lo­gis­tic model of pop­u­la­tion growth (Fig 1). This sim­ple heuris­tic model sug­gests that the pop­u­la­tion growth rate is den­sity de­pen­dent, so that per cap­ita pop­u­la­tion growth rates are large and pos­i­tive when the pop­u­la­tion den­sity is low, and nega­tive when the pop­u­la­tion is large and ex­ceeds some nat­u­ral limit of en­vi­ron­men­tal re­sources (the car­ry­ing ca­pac­ity, K: see Lotka 1925). MacArthur and Wil­son (1967) equate r-se­lec­tion with strongly sea­sonal en­vi­ron­ments in tem­per­ate re­gions, where re­sources are pe­ri­od­i­cally available in great abun­dance, so that se­lec­tion favours great pro­duc­tivity. Since the pop­u­la­tion is at low val­ues rel­a­tive to the car­ry­ing ca­pac­ity of the en­vi­ron­ment and is grow­ing ex­po­nen­tially at rate r, this is termed r-se­lec­tion. K-se­lec­tion is defined as oc­cur­ring in rel­a­tively con­stant en­vi­ron­ments, char­ac­ter­ized by limited re­sources and strong com­pe­ti­tion like re­gions in the trop­ics, where the pop­u­la­tion is at its max­i­mum sus­tain­able size (or car­ry­ing ca­pac­ity), K. This en­vi­ron­ment is pre­dicted to favour se­lec­tion for effi­cient re­source use, strong com­pet­i­tive abil­ity, and preda­tor avoidance.

Figure 1: The re­la­tion­ship be­tween pop­u­la­tion den­sity, as pre­dicted by the lo­gis­tic equa­tion, and r- and K-se­lec­tion pres­sures.

Pi­anka (1970) noted that these types of se­lec­tion pres­sures were not re­stricted to the tem­per­ate and trop­i­cal zones and so ex­panded the scope of MacArthur and Wil­son’s ideas to many more species. He fur­ther com­mented that no or­ganism would ever be com­pletely r- or K-se­lected, and de­scribes an r-K con­tinuum where var­i­ous species would be placed in be­tween these two se­lec­tion ex­tremes. He sug­gests that ter­res­trial ver­te­brates tend to fall on the K-se­lec­tion side, while ter­res­trial in­sects tend to­ward the r-ex­treme, but notes that there are of course ex­cep­tions in each case (e.g., 17-year ci­cadas). He in­di­cates that aquatic or­ganisms do not fol­low this gen­er­al­iza­tion, and in par­tic­u­lar notes that fish span the en­tire r-K spectrum

Pi­anka then pro­vides a table of life his­tory traits that do not come di­rectly from MacArthur and Wil­son’s brief pre­sen­ta­tion, but rather rep­re­sent traits which he as­so­ci­ates with each of these ex­tremes (Table 1). Since ex­treme r-se­lec­tion would oc­cur in the pres­ence of freely available re­sources, when en­vi­ron­men­tal con­di­tions keep the pop­u­la­tion con­sis­tently lower than the car­ry­ing ca­pac­ity, even the most un­fit can ac­quire them. As a re­sult, it is more ad­van­ta­geous from an evolu­tion­ary point of view to pro­duce as many offspring as pos­si­ble, as quickly as pos­si­ble, re­gard­less of fit­ness. An ex­treme K-strate­gist lives in a sta­ble en­vi­ron­ment near the car­ry­ing ca­pac­ity K, and its se­lec­tion pres­sures are char­ac­ter­ized by com­pe­ti­tion and threat of star­va­tion be­cause of severely limited re­sources. Since there is not enough food to go around, the offspring’s suc­cess in com­pe­ti­tion is what is most im­por­tant. Here, wast­ing time pro­duc­ing nu­mer­ous offspring that are not as fit as pos­si­ble will doom the species to failure (see also Table 1).

Table 1: Char­ac­ter­is­tics of r- and K-se­lec­tion. Mod­ified from Pi­anka (1970).

The r-K se­lec­tion the­ory was pop­u­lar in the mid 1970s to 1980s (see Fig 2, and also Table 1 in Blute 2016 ) as a heuris­tic de­vice. It lost im­por­tance in the early 1990s, when it was crit­i­cized be­cause of a dis­par­ity be­tween the­o­ret­i­cal con­cepts and em­piri­cal stud­ies (e.g., Stearns 1992, Roff 1993). Prob­lems in­cluded the over­sim­plifi­ca­tion of life his­tory strate­gies along a sin­gle axis that com­bines both dis­tur­bance and re­source availa­bil­ity, and the fact that many species do not match the clas­sifi­ca­tion (Stearns 1992). For ex­am­ple marine tur­tles have both longevity and pro­lifi­cacy, and as a re­sult, the as­sump­tion of a trade-off be­tween r and K char­ac­ter­is­tics is not valid for these species. In ad­di­tion, the traits at­tributed to K-se­lec­tion by Pi­anka (1970) are not read­ily jus­tifi­able as a re­sult of a con­stant en­vi­ron­ment, but in­stead merely rep­re­sent a con­trast to traits at­tributed to r-se­lec­tion (Reznick et al. 2002). More­over, this sim­ple clas­sifi­ca­tion ap­proach ne­glected other im­por­tant fac­tors that de­ter­mine nat­u­ral se­lec­tion. In par­tic­u­lar, pat­terns of age-spe­cific mor­tal­ity, while in­cluded in Pi­anka’s origi­nal clas­sifi­ca­tion, were not re­ally con­sid­ered as an im­por­tant axis of se­lec­tion (Reznick et al. 2002), nor was dis­per­sal, nor abil­ity to weather harsh abiotic con­di­tions (Grime 1977).

Figure 2: The use of the terms r-se­lec­tion and K-se­lec­tion have de­clined in the ecol­ogy and evolu­tion­ary biol­ogy books (mea­sured as per­cent oc­cur­rence in pub­lished works), af­ter a peak in the 80s, al­though they have en­joyed a re­sur­gence in the psy­chol­ogy liter­a­ture (pro­duced by google ngram viewer).

Reznick et al. (2002) sug­gest that “de­mo­graphic the­ory” has re­placed the old r- and K-se­lec­tion ap­proach, and the fo­cus is now on de­ter­min­ing how spe­cific as­pects of life his­tory will af­fect se­lec­tion. For ex­am­ple, in­creased adult mor­tal­ity rates rel­a­tive to ju­ve­nile rates are pre­dicted to fa­vor geno­types that in­vest in cur­rent rather than fu­ture re­pro­duc­tion, with early ma­tu­rity and high fe­cun­dity, even if this in­vest­ment comes at a cost of also pro­duc­ing smaller offspring (Stearns 1992; Roff 2002). Con­versely, when ju­ve­nile stages ex­pe­rience high mor­tal­ity rates rel­a­tive to adult mor­tal­ity rates, larger offspring may be more ro­bust to un­favourable con­di­tions and ma­ter­nal in­vest­ment in larger, but po­ten­tially fewer offspring are favoured (Stearns 1992; Roff 2002). Reznick et al. (2002) con­clude that the sim­ple r-K con­tinuum has given way to a mul­ti­tude of al­ter­na­tives that can only be cor­rectly ap­plied if a great deal is known about an or­ganism and its en­vi­ron­ment. How­ever some au­thors still seem fo­cused on sim­pler clas­sifi­ca­tion ap­proaches.


With the demise of r and K-se­lec­tion an­other life his­tory clas­sifi­ca­tion scheme a new but strongly re­lated con­cep­tual map arose in the liter­a­ture. This fast-slow con­tinuum is pri­mar­ily fo­cused on re­pro­duc­tive life his­tory traits (e.g., Sæther 1987). Reynolds (2003; cited in Jeschke & Kokko 2009) de­scribes a fast life his­tory as char­ac­ter­ized by early re­pro­duc­tion, short gen­er­a­tion time, short lifes­pan, small adult body size, small offspring size, and high fe­cun­dity, while a slow life his­tory has the op­po­site char­ac­ter­is­tics.

This fast-slow clas­sifi­ca­tion uses the vast vari­a­tion in body size among or­ganisms as a ba­sis for com­par­i­son. Some of the most ba­sic changes in life his­tory are due to the fact that it takes longer for larger or­ganisms to de­velop than it does for smaller or­ganisms, and small or­ganisms have faster metabolic rates. Thus, lifes­pans are longer for larger species. Jeschke and Kokko (2009) note that there are is­sues with this clas­sifi­ca­tion method in that the defi­ni­tion of fast and slow is not con­sis­tent among re­searchers. In par­tic­u­lar, some re­searchers have in­cluded adult body size as a co­vary­ing trait, while oth­ers fac­tor it out and ex­am­ine the resi­d­u­als of the traits. Dod­son (2007) defines the fast-slow con­tinuum as the place­ment of species along this scale af­ter the effects of body size are re­moved, and notes that species po­si­tion along this scale differs from that of body size (Dod­son 2007). There­fore, small short-lived species like hum­ming­birds may be grouped to­gether with large-bod­ied, long-lived pe­trals as both hav­ing low re­pro­duc­tive out­put, if one fo­cuses on re­pro­duc­tive effort per unit mass.

Of those anal­y­sis that have ex­am­ined the resi­d­u­als of body mass, there is ev­i­dence that there may not be a sin­gle axis of life his­tory vari­a­tion that fits most species. In a large multi-species anal­y­sis of over 2000 fish, mam­mal and bird species, Jeschke and Kokko (2009) con­clude that there is no uni­ver­sal fast-slow defi­ni­tion of life his­to­ries across these cat­e­gories. For ex­am­ple, the di­rec­tion of the re­la­tion­ship be­tween fe­cun­dity and other life-his­tory traits is the op­po­site in fish than in mam­mals. How­ever, even within rel­a­tively tightly re­lated groups there may be de­vi­a­tions from the clas­sifi­ca­tion. In their anal­y­sis of 267 mam­mal species, Bielby et al. (2007) note that some species show a mix­ture of fast and slow re­pro­duc­tive traits, and iden­tify two-axes of vari­a­tion to ex­plain the data. The au­thors note in ad­di­tion to their work, the foun­da­tional pa­pers in this area (e.g., Stearns 1983, Gaillard et al. 1989 ) also iden­ti­fied two axes of vari­a­tion: one that may be re­lated to fast-slow clas­sifi­ca­tion, and an­other re­lated to var­i­ous as­pects of the timing or en­ergy of re­pro­duc­tive effort. There­fore, there are ques­tions about the the re­al­ity of a sin­gle fast-slow con­tinuum of life-his­tory vari­a­tion. More­over, sev­eral stud­ies re­port vari­a­tion from fast to slow traits within sin­gle species across re­source gra­di­ents (e.g., Singh & Mishra 2016 , Tabek et al. 2018).

In re­sponse to these challenges, some re­searchers have fac­tored in the evolu­tion­ary his­tory of the ex­am­ined species as well as ease of re­source ac­qui­si­tion in var­i­ous en­vi­ron­ments to demon­strate that life his­tory is si­mul­ta­neously re­lated to body size (phys­i­cal con­straints), phy­logeny (evolu­tion­ary his­tory), and more re­cent se­lec­tion pres­sures as­so­ci­ated with “lifestyle” (see Dob­son 2012 for an overview). For ex­am­ple, in an anal­y­sis of over 600 pla­cen­tal mam­mals, Silby and Brown (2007) find that those species with lifestyles that lead lower pre­da­tion rates (flighted or ar­bo­real species) have lower re­pro­duc­tive out­put per unit mass. Nev­er­the­less, even with these ad­di­tional axes of vari­a­tion, it seems clear that the fast-slow clas­sifi­ca­tion ap­plies much more eas­ily to mam­mals and birds than to in­sects and fish. How­ever, much like r and K-se­lec­tion, there are still refer­ences to a sin­gle fast-slow axis of life his­tory vari­a­tion in the liter­a­ture (Net­tle & Franken­huis 2019).

Tri­par­tite clas­sifi­ca­tion schemes

Three-strat­egy frame­works re­solve some difficul­ties of a sin­gle axis of life his­tory clas­sifi­ca­tion (e.g., r-K se­lec­tion or fast-slow clas­sifi­ca­tion) for groups like fish and in­sects. There is the com­pe­ti­tion-dis­tur­bance-stress clas­sifi­ca­tion (Grime 1977) origi­nally based on plants, while for fish com­mu­ni­ties, Wine­mil­ler and Rose (1992) de­vel­oped the equil­ibrium-op­por­tunis­tic-pe­ri­odic model (Fig 3). Both have end­point strate­gies as­so­ci­ated with colon­is­ing vs. com­pet­i­tive abil­ity, but they differ in terms of the suite of at­tributes as­so­ci­ated with the third strat­egy. Life-his­tory mod­els with three strate­gies are gen­er­ally recog­nised as hav­ing more pre­dic­tive power than the two-strat­egy frame­works which can fail to recog­nise ad­di­tional axes of life-his­tory vari­a­tion (Stearns 1977; Wine­mil­ler and Rose 1992)

Grime (1977) de­scribed three life-his­tory strate­gies:com­pet­i­tive, stress-tol­er­ant or rud­eral (i.e., weedy) (Grime 1977; and Table 2 in Grime & Pierce 2012). This clas­sifi­ca­tion has the ad­van­tage of de­scribing species with life his­to­ries that al­low them to weather ex­tremely harsh en­vi­ron­ments. Tol­er­ant species are long-lived, with low growth and re­pro­duc­tive rates be­cause of the trade-off in en­ergy de­voted to defense and sur­vival. Com­pet­i­tive species are found in low dis­tur­bance en­vi­ron­ments and there­fore de­vote less en­ergy to re­pro­duc­tion and more re­source ac­qui­si­tion, while rud­eral species are found in low stress, dis­turbed en­vi­ron­ments with am­ple re­sources, and al­lo­cate much more en­ergy to re­pro­duc­tion. Similar tri­an­gu­lar con­tinu­ums of life-his­tory strate­gies were sub­se­quently pro­posed for in­sects (e.g., Greenslade 1983).

Work­ing with data from fish, Wine­mil­ler and Rose (1992) iden­tify a pe­ri­odic end­point in­stead of the stress-tol­er­ant strat­egy iden­ti­fied by Grime (1977). Pe­ri­odic species are char­ac­ter­ized by long lifes­pan, high fe­cun­dity, pe­ri­odic re­pro­duc­tion and low in­vest­ment in in­di­vi­d­ual propag­ules favoured in habitats with large‐scale en­vi­ron­men­tal vari­a­tion that in­fluences early life stage sur­vival (some­times called ‘bet hedg­ing’). Un­der this scheme, many trees, in­ver­te­brates and fishes would be clas­sified as pe­ri­odic strate­gists that have large in­ter­an­nual and spa­tial vari­a­tion in re­cruit­ment. This model, or similar tri­par­tite clas­sifi­ca­tions, have been ap­plied mostly to fishes (Wine­mil­ler 1992, Juan-Jordá et al 2013), a group that has ex­treme vari­a­tion in life his­tory at­tributes rel­a­tive to other an­i­mal groups. Flow­er­ing plants and arthro­pods are other groups that span large ar­eas within the three part con­tinuum iden­ti­fied by Wine­mil­ler and Rose (1992), but some groups, such as birds and mam­mals, oc­cupy small zones (Wine­mil­ler 1992).

Figure 3: Tri­par­tite axes de­scribing se­lec­tion pres­sure on life his­tory for clas­sifi­ca­tions from Grime (1977) and Wine­mil­ler and Rose (1992)

Trait-based approaches

Most re­cently, and some­what out­side the fo­cus of evolu­tion­ary se­lec­tion pres­sures, ecol­o­gists have de­scribed com­mu­ni­ties as com­posed of in­di­vi­d­u­als with differ­ing fea­tures. This ap­proach is pre­sented as an al­ter­na­tive to gen­er­al­iz­ing across species, which may in­clude large vari­a­tion in strate­gies be­tween in­di­vi­d­u­als and pop­u­la­tions. Such trait-based ecol­ogy seeks to ex­plain ecolog­i­cal pat­terns where in­di­vi­d­ual traits will in­clude fea­tures of life his­tory such as lifes­pan and re­pro­duc­tion de­tails, but also in­for­ma­tion about feed­ing, anatomy, so­cial dy­nam­ics etc (e.g., Table 2 mod­ified from Moretti et al. 2017). Such ap­proaches have been ap­plied pri­mar­ily to plants, but in­ter­est in us­ing these ideas for an­i­mals has steadily in­creased (e.g., Brousseau et al. 2018)

Table 2: Ex­am­ples of ter­res­trial in­ver­te­brate traits im­por­tant in re­spond­ing to the en­vi­ron­ment. Par­tial list mod­ified from Table 1 in Moretti et al. 2017.

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This es­say is a pro­ject of Re­think Pri­ori­ties. It was writ­ten by Kim Cud­ding­ton. Thanks to Ja­son Schukraft, Daniela Wald­horn , David Moss, Mar­cus Davis and Peter Hur­ford for com­ment. If you like our work, please con­sider sub­scribing to our newslet­ter. You can see all our work to date here.