Puberty Onset Influencing Factors - Anatomy & Physiology

()Map REPRODUCTIVE SYSTEM (Map) PUBERTY

Female Threshold Body Size

• Nutritional intake of the newborn is directed towards body maintenance.
• The priority of the neonate is to maintain vital physiological functions.
• Non-essential processes, such as reproduction are of low priority.
• As the neonate grows, energy consumption increases and the body mass increases with a relative decrease in body surface area.
• This allows a shift in metabolic expenditure so that other physiological functions can develop.
• The excess internal energy is converted to fat stores, and a priority is then placed on reproduction so the onset of puberty begins.
• Fat accumulation alone does not determine the onset of puberty, but it is an important regulator.
• All female mammals must reach a certain body size before the onset of puberty can be initiated.
• Metabolic signals affect production of GnRH.

Photoperiod

The month of birth will influence the onset of puberty, especially in seasonal breeders.

Sheep

• Short day breeders (begin oestrous cycles in response to short day length)
• Spring born lambs (February-March) reach puberty in the subsequent autumn (September-October) if they have adequate nutrition. The age at puberty is 5-6 months.
• Autumn born lambs do not reach puberty until 10-12 moths of age.
• This is because, seasonal cues (long days of spring/summer) delay reproductive cyclicity.
• Thus, oestrus in both spring and autumn born lambs is synchronised.
• This maximizes the chance that all ewes become pregnant in the fall and give birth in the spring when nutritional and environmental conditions are favourable.
• An adequate degree of nutrition ('fatness') is required before photoperiod can exert an effect.
• There are sex differences that also have an effect. Ram lambs reach puberty at ~10 weeks of age during summer, but ewe lambs born at the same time (spring) will not reach puberty until 25-35 weeks of age.

Heifers

• Heifers born in the autumn generally reach puberty before those born in the spring.
• Exposure to long photoperiods and warm temperatures during the second 6 months of life hastens the onset of puberty.
• The sequence of photoperiod exposure makes a difference. Exposure to short days during the first 6 months of life, followed by increasing day lengths in the second 6 months of life is associated with the earliest puberty. This is true of calves born in the autumn with exposure to natural photoperiods.
• Season of birth does not affect the onset of puberty in bull calves.

Bitch

• Little seasonality associated with the onset of puberty.

Environmental and Social Conditions

• Pheromonal substances in the urine act as a stimulus to enhance the onset of puberty.
• Females reaching puberty in the presence of a male (producing the pheromones) have a greater opportunity of becoming pregnant.
• Pubertal onset cannot be accelerated using pheromones in animals that have not achieved the appropriate metabolic body size to trigger hypothalamic responsiveness to Oestradiol (oestrogen).

• The size of the social group in which females are housed has an influence over pubertal onset.
• Gilts housed in small groups will have delayed pubertal onset compared to gilts housed in larger groups.
• The presence of a male in either visual or direct physical contact will hasten the onset of puberty in gilts.

• Visual and olfactory environmental information is perceived by sensory neurons of the optic and olfactory systems.
• Stimuli are processed by the central nervous system and converted into neural inputs to the GnRH neurons of the hypothalamus.
• The hypothalamus then gains the ability to produce high frequency and high amplitude pulses of GnRH at an earlier age.