There's a saying that children born into better families "have a running start" as they tend to grow up with better prospects. In the animal kingdom, this "running start" extends even further back to the time of birth. Of course, for animals, this concerns not their future accomplishments, but rather, it is a matter of life and death - when an animal is born into a time of resource scarcity, such as during winter when food is scarce due to the cold, its chances of survival are significantly reduced!
Mammals: Embryonic Diapause
As early as over 100 years ago, hunters in Europe observed a kind of roe deer that displayed an unusual gap between their mating and birthing times. It was later confirmed that this was due to the delayed development of the roe deer's blastocyst (embryonic cells during the earliest stage of pregnancy) in the mother's uterus. With the advancement of biological monitoring technology, biologists have discovered that many more mammals display similar situations. They delay the implantation of fertilised eggs in the uterus to postpone the growth of the embryo. Our national treasure, the Giant Panda, is among the animals that do this, employing the same method to control the timing of its foetal births.
Mammals trigger embryonic diapause or restart embryo growth by secreting different combinations of hormones. The period of diapause can last more than half a year without affecting embryonic development. Currently, over 130 mammals are known to exhibit embryonic diapause, with only the few belonging to tropical species that experience less climate variation. This suggests that diapause is primarily triggered by unfavourable environmental factors related to seasonal climates, and commonly associated with food scarcity and adverse weather conditions.
Furthermore, some nursing mammals secrete prolactin, a hormone that inhibits blastocyst development, within their bodies. It serves to prevent them from getting pregnant again during the parenting and lactation period, allowing the concentration of resources on caring for the already-born offspring.
Insects: Seasonal Diapause
Unlike mammals which can regulate their own body temperature, the activity and survival of insects depend more on environmental temperature, which is why many insects resort to diapause at different stages of their growth to avoid extreme cold or hot weather. In particular, the Lepidoptera order (butterflies and moths) are typical examples of insects that undergo diapause.
Biologists have found that Lepidoptera species can slow down the moulting process of nymphs and pupae by inhibiting the secretion of ecdysteroid hormones, allowing them to adjust for the optimal time for eclosion. Some butterfly species even develop different seasonal morphs due to the different durations of diapause. An example would be the local species Red Ring Skirt (Hestina assimilis), of which the generation that ecloses into butterflies in spring uses diapause to deliberately delay eclosion during the nymph stage, avoiding winter, a time when food is scarce. A select few of this generation exhibit a significantly different appearance, known as "form nigrivena", which is attributed, some say, to prolonged diapause.
Integrated study results indicate that insect diapause is primarily triggered by environmental signals such as photoperiod, temperature, and humidity. These signals enable insects to synchronise their growth pace with seasonal patterns. However, climate change is disrupting these interrelationships. The "Solar Terms and Butterflies" survey conducted by Green Power, with data spanning 18 years, shows that the patterns of butterfly emergence in recent years have been influenced by the frequency of extremely hot weather conditions, causing phenological mismatches that cannot be ignored (Click here to know more).
