A newly introduced giraffe population starts producing calves. Its numbers rise, year after year, and the release appears successful. Yet the early increase may partly reflect a temporary advantage: the founders are young, with many breeding years ahead of them. What happens when they age, and when their descendants increasingly breed within a small population, is a different test.
Derek Lee and colleagues’ 2020 population viability study examined that problem through simulations of giraffe translocations. Small founder groups could show encouraging growth for decades while remaining vulnerable over a century. Under the study’s moderate demographic and genetic assumptions, 30 females and three males were needed to meet its stricter persistence criterion. Preserving more than 95% of the source population’s genetic diversity required 50 females and five males.
These were conditional modelling results, not universal instructions for moving giraffes. Their importance lay in the distinction between two outcomes that can look similar early on: a population that is increasing now, and one with the demographic and genetic foundations to persist.
A deliberately idealized destination
The researchers modelled a hypothetical receiving site rather than a particular reserve. The site had a carrying capacity of 1,000 giraffes, giving the founder populations considerable room to grow. The analysis assumed no extra mortality from the capture and transfer process and no movement out of the receiving population.
Those choices isolated the questions the authors wanted to study: how founder numbers, their sex ratio, demographic rates and inbreeding effects influenced the future. They also made clear what the model did not evaluate. It did not establish that a real destination had enough food through every season, that all animals would survive handling, or that released giraffes would remain there.
Every founder was modelled as a two-year-old. The tested releases varied the number of females and males, producing 198 scenarios across different demographic and genetic conditions. Each scenario was repeated 1,000 times for 100 years, allowing random differences in births, deaths and inheritance to generate different outcomes.
The model defined extinction as fewer than two animals remaining. Genetic diversity was evaluated in populations still extant at the end of the simulation. A genetic result among survivors therefore answers a different question from the fraction of runs in which the population persisted at all.
The advantage of a young starting population
All founders began at the same young age. Some died before reaching the model’s breeding ages, producing an initial decline. As surviving founders and their offspring reproduced, numbers then increased during a long early phase.
That age structure was unlike a long-established population containing a broader balance of young, mature and old animals. The founders initially had relatively little old-age mortality ahead of them in the immediate future. Later, their cohort approached the end of its lifespan, and the temporary demographic advantage disappeared.
The model’s early population growth could therefore exceed the longer-term growth supported by its underlying survival and reproductive rates. This is a transient effect: a consequence of the starting age structure that does not continue indefinitely.
For a conservation project, the distinction is practical. A decade of births is evidence that animals reproduced. It does not, by itself, demonstrate that the population has reached a durable balance of reproduction, survival and age structure. The authors chose a century-long projection partly because the shorter-term increase could conceal the eventual consequences of a small founding population.
The simulation does not imply that every real release will decline on the same schedule. Real founders may differ in age, survival and reproductive histories. It demonstrates why interpreting early growth requires attention to who is growing old alongside the calves being born.
Small populations also lose inherited variation
The second delayed problem was genetic. Even when founders initially represented the source population well, a small isolated population could lose variation as only some individuals and some inherited variants contributed to later generations.
The model also included inbreeding depression: adverse effects associated with breeding among relatives. The researchers examined three possible levels of this genetic burden. Their lowest estimate used captive giraffe studbook data; the higher levels explored uncertainty about the burden that wild populations might carry.
Those inputs were not direct measurements of genetic effects in every wild giraffe population. They were alternative assumptions used to test how strongly the answers depended on an important unknown.
The two genetic processes are related but distinct. Genetic drift can remove variants by chance in a small population. Inbreeding can expose harmful effects when relatives reproduce. A population can become more numerous after its early years without automatically recovering the variation already lost. Later numerical growth is therefore not a guarantee that its original genetic representation has been preserved.
This is why the study treated persistence and genetic diversity as separate success criteria. A population still present after 100 years could have lost a substantial portion of the variation represented by the source.
Different definitions of success changed the answer
The authors tested both less than 5% and less than 1% probability of extinction over 100 years, alongside thresholds for retaining genetic diversity. The number of founders needed depended on which standard was chosen and on the assumed demographic and genetic conditions.
Under moderate demographic rates and a moderate genetic burden, the model’s threshold for less than 5% extinction probability was 20 females and two males. Tightening the criterion to less than 1% increased the founder requirement to 30 females and three males. Retaining more than 95% of the source genetic diversity raised it further, to 50 females and five males.
| Goal after 100 simulated years | Founders under the moderate assumptions |
|---|---|
| Less than 5% extinction probability | 20 females, 2 males |
| Less than 1% extinction probability | 30 females, 3 males |
| More than 95% genetic diversity retained | 50 females, 5 males |
The table is a comparison within one modelling framework. It is not a menu that guarantees those results at an actual site. Slower growth or greater inbreeding effects generally required more founders, while favourable conditions sometimes allowed smaller groups to meet a criterion.
It also shows why a statement such as “enough giraffes to survive” is incomplete. How much extinction risk is considered acceptable? How much genetic diversity should remain? Will immigration or later reinforcement be possible? The same starting group can meet one goal and miss another.
Our broader translocation guide explains the wider assessment process. This experiment addresses the narrower question that early field observations alone cannot resolve: how founder composition can shape the population long after the release has disappeared from immediate memory.
Which assumptions mattered most?
In a sensitivity analysis, the team changed one parameter at a time around a reference release of 30 females and three males. The largest effects came from increased first-year mortality and reduced reproduction.
Raising first-year mortality from 50% to 63% increased extinction probability by 11.4 percentage points and reduced retained genetic diversity by 10.6 percentage points. Reducing the reproductive parameter from 50% to 38% increased extinction probability by 10.2 percentage points and reduced genetic diversity by 10.5 percentage points.
These are contrasts between simulations, not measured effects of an intervention in the field. Their message is that the expected benefit of a founder group depends strongly on the production and survival of the next generation. If those rates are worse than assumed, the projected future becomes less favourable.
A smaller effect of changing adult mortality in this particular analysis does not establish that adult deaths are unimportant for giraffes. Sensitivity depends on the baseline values, changes tested, population structure and outcome being measured.
A later Bénoué modelling study, for example, examined an existing small population under different demographic assumptions. Occasional stipulated adult removals sharply worsened its simulated persistence, and adult female mortality was among the influential uncertain inputs. The studies are not competing measurements of one universal rule. They ask different management questions of differently constructed populations. Our Bénoué account follows why preventing losses can be critical even when they are infrequent.
The breeding assumptions were optimistic
The founder simulations allowed all males of breeding age an equal chance to mate. If a few males instead fathered most calves, the next generation would receive genes from a smaller effective set of fathers than the headcount suggested. That could accelerate genetic loss relative to the model.
The researchers also used early first-breeding ages—two for males and three for females—and explicitly described these as optimistic. Delayed reproduction would postpone contributions from the founders and could worsen long-term outcomes.
Neither issue is resolved by simply adding more locations to a tracking dataset or obtaining a more precise census. They require evidence about reproduction, parentage and survival. The authors identified the actual extent of inbreeding depression in wild giraffes and the distribution of male breeding success as important research gaps.
Immigration could also change the result. The simulations assumed isolation, whereas exchange with other populations could introduce genetic variation. Conversely, a fenced population without natural exchange might require continued management. The study did not calculate a universal schedule for that support.
Following the founders beyond their first calves
The value of this work is the longer view it adds to a promising early observation. A founder population can produce calves and increase while still carrying the demographic consequences of its unusual age structure and the genetic consequences of its small size.
Testing a real project against those mechanisms would mean following age-specific survival and reproduction, comparing source and introduced genetic diversity, and updating population projections as observations accumulate. Monitoring should also include the source population, because removing founders can affect the animals left behind.
The researchers’ numbers make the trade-offs visible; they do not replace assessment of the receiving landscape or the animals involved. Their most durable conclusion is that early growth deserves to be celebrated as an observed milestone, while the stronger claim of lasting success requires evidence that outlives the founders.
Sources
Lee, Derek E.; Fienieg, Elmar; Van Oosterhout, Cock; Muller, Zoe; Strauss, Megan; Carter, Kerryn D.; Scheijen, Ciska P. J.; and Deacon, Francois (2020). “Giraffe translocation population viability analysis.” Endangered Species Research, 41, 245–252. Paper and DOI. Open full paper.
Colston, Kane P. J.; Johnson, Caspian L.; Nyugha, Denis; Mengamenya Goué, Achile; and Penny, Samuel G. (2023). “Viability analysis of Kordofan giraffe (Giraffa camelopardalis antiquorum) in a protected area in Cameroon.” African Journal of Ecology, 61, 929–944. Paper and DOI.
