Flexible or opportunistic breeders mate whenever the conditions of their environment become favorable. Their ability and motivation to mate are primarily independent of day-length (

photoperiod Photoperiod is the change of day length around the seasons. The rotation of the earth around its axis produces 24 hour changes in light (day) and dark (night) cycles on earth. The length of the light and dark in each phase varies across the season ...

) and instead rely on cues from short-term changes in local conditions like rainfall, food abundance and temperature. Another factor is the presence of suitable breeding sites, which may only form with heavy rain or other environmental changes.WE Duellman, L Trueb. ''Biology of amphibians.'' The Johns Hopkins University Press, 1994, p. 20 Thus, they are distinct from

seasonal breeder Seasonal breeders are animal species that successfully mate only during certain times of the year. These times of year allow for the optimization of survival of young due to factors such as ambient temperature, food and water availability, and ch ...

s that rely on changes in day length to induce entry into

estrus The estrous cycle (, originally ) is a set of recurring physiological changes induced by reproductive hormones in females of mammalian subclass Theria. Estrous cycles start after sexual maturity in females and are interrupted by anestrous phas ...

and to cue mating, and continuous breeders like humans that can mate year-round. Other categories of breeders that perhaps can be subdivided under the heading "opportunistic" have been used to describe many species, such as many that are

anuran A frog is any member of a diverse and largely semiaquatic group of short-bodied, tailless amphibian vertebrates composing the order Anura (coming from the Ancient Greek , literally 'without tail'). Frog species with rough skin texture due to ...

s like frogs. These include sporadic wet and sporadic dry, describing animals that breed sporadically not always under favorable conditions of rain or lack thereof. Many opportunistic breeders are non-mammals. Those that are mammals tend to be small

rodents Rodents (from Latin , 'to gnaw') are mammals of the order Rodentia ( ), which are characterized by a single pair of continuously growing incisors in each of the upper and lower jaws. About 40% of all mammal species are rodents. They are n ...

. Since changes in season can coincide with favorable changes in environment, the distinction between seasonal breeder and opportunistic can be muddled. In equatorial climes, the change in seasons is not always perceptible and thus, changes in day length not remarkable. Thus, the tree kangaroo (Dendrolagus) previously categorized as a seasonal breeder is now suspected to be an opportunistic breeder.Martin R, Johnson P. ''Bennett's Tree-kangaroo.'' In: R. Straham, ed., Mammals of Australia, Washington, DC: Smithsonian Institution Press, 1995, pp. 307-8, Additionally, opportunists can have qualities of seasonal breeders. The

red crossbill The red crossbill or common crossbill (''Loxia curvirostra'') is a small passerine bird in the finch family Fringillidae. Crossbills have distinctive mandibles, crossed at the tips, which enable them to extract seeds from conifer cones and other ...

exhibits a preference (not a requirement) for long-day seasonality, but requires other factors, especially food abundance and social interactions, in order to breed. Conversely, food availability by itself incompletely promotes reproductive development.

Physiology

Opportunistic breeders are typically capable of

breeding Breeding is sexual reproduction that produces offspring, usually animals or plants. It can only occur between a male and a female animal or plant. Breeding may refer to: * Animal husbandry, through selected specimens such as dogs, horses, and rab ...

at any time or becoming fertile within a short period of time. An example is the golden spiny mouse where changes in dietary salt in its desert habitat due to rainfall appear to cue reproductive function. Increased levels of salinity in drying vegetation cause females to experience a reproductive hiatus. While reproduction is generally independent of photoperiod, animals can still experience reduced fertility with changes in day-length.

Partial list of opportunistic breeders

Mammals

* Golden spiny mouse *

Red kangaroo The red kangaroo (''Osphranter rufus'') is the largest of all kangaroos, the largest terrestrial mammal native to Australia, and the Largest mammals#Marsupials (Marsupialia), largest extant marsupial. It is found across mainland Australia, exce ...

* Tree kangaroo * Musk shrew * Cape spiny mouseFleming, P. A., and S. W. Nicolson.
Opportunistic breeding in the Cape spiny mouse (Acomys subspinosus)
" African Zoology 37.1 (2002): 101-105.

Birds

Zebra finch The zebra finches are two species of estrildid finch in the genus ''Taeniopygia'' found in Australia and Indonesia. They are seed-eaters that travel in large flocks. Species The species are: Previously, both species were classified as ...

* Crossbill *

Budgerigar The budgerigar ( ; ''Melopsittacus undulatus''), also known as the common parakeet, shell parakeet or budgie ( ), is a small, long-tailed, seed-eating parrot native to Australia. Naturally the species is green and yellow with black, scallop ...

* Princess parrot *

Blue-footed booby The blue-footed booby (''Sula nebouxii'') is a marine bird native to subtropical and tropical regions of the eastern Pacific Ocean. It is one of six species of the genus ''booby, Sula'' – known as boobies. It is easily recognizable by its dis ...

Amphibians

Frogs and toads including: * Long-thumbed frog *Southern spadefoot toad (

Scaphiopus ''Scaphiopus'' is a genus of North American amphibian commonly referred to as the North American spadefoots, southern spadefoots, or eastern spadefoot toads. They differ greatly from true toads (those of the family Bufonidae {{Cat main, Toad Thi ...

) *

African clawed frog The African clawed frog (''Xenopus laevis''), also known as simply xenopus, African clawed toad, African claw-toed frog or the ''platanna'') is a species of African Aquatic animal, aquatic frog of the family Pipidae. Its name is derived from the ...

Fowler's toad ''Anaxyrus fowleri'', Fowler's toad, is a species of toad in the family Bufonidae. The species is native to North America, where it occurs in much of the eastern United States and parts of adjacent Canada. It was previously considered a subspec ...

References

{{reflist Animal breeding Reproduction Fertility